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PDF link to: Soil Testing Publication
| SOIL TESTING OFFERS MULTIPLE ADVANTAGES |
Soil is analyzed to help guarantee successful planting, to extend and prolong established plants, to help determine whether excessive or insufficient irrigation water is being applied and to help keep plants growing vigorously.
Plants in good healthy condition have the ability to tolerate some pests. However, when there are multiple problems, the ability of the plants to outgrow diseases, to remain green, and to survive periods of weather extremes declines rapidly. Soil testing is plant insurance.
OPTIMUM NUTRIENT CONCENTRATION
All nutrients are needed at moderate amounts. Excessive concentrations will cause toxicity while insufficient concentrations will stunt growth. One of the more frequent causes of mediocre growth is too much fertility or the addition of the incorrect product.
Waiting to analyze the soil until discoloration, burned leaves, wilting and diseases occur decreases the probability of successful corrections. Green plants does not necessarily mean that growth conditions are ideal. Green plants may have hidden hunger. Optimum balance of the thirteen essential mineral nutrients, irrigation and environmental factors allow for the proper growth and vigor of the plants. Moderate mineral deficiencies will not cause major changes in the appearance of the tissues, but they can substantially lower the vitality. The worst cases are multiple simultaneously. With critical deficiencies or nutrient toxicities, discoloration follows with a rapid decline of the plant.
Soil testing as a management tool is greatly under utilized. The most recent data for lawns and gardens are from 1987. The frequency of soil testing is the highest in the Southern States where it is one test per about 200 people per year. The per capita rate of soil testing is very low in the Southwestern States. California has a rate of one test per 2,091 people per year while Arizona has a rate of one test per 958 people per year. Ideally, every site should be tested every few years. The frequency of needed soil testing depends upon the amount of irrigation, the quality of irrigation water, the use of nutrients and amendments and the initial soil properties. Testing could help to prevent and solve many problems.
WHAT IS SOIL?
Soil is formed from the parent minerals contained in rocks. Through the influence of climate (rain, wind, heating, freezing etc.) and organisms the rocks weather. Simple plants like lichens and microorganisms use the minerals released in the weathering process and continue with the formation of soil. As organisms grow and die, organic matter accumulates which interacts with the mineral particles. Eventually, a horizon or profile of developed soil is generated which is called a topsoil.
Rain leaches the soluble minerals into the deeper soil profiles. The topsoil profile is called an “A horizon” while the soil profile which receives the minerals that are moved into the soil by water is called a “B horizon” or subsoil. Below the B horizon is the “C horizon” or the unweathered rock.
The properties of the topsoil depend mainly upon native vegetation and upon the amount of rain. These are a function of climate. Weathering of rocks releases salts which had previously been encapsulated, usually as part of the structure of the rock. In the desert and semiarid zones of the Southwest, the salt content of the soil is very high due to little leaching of the soils. The salts also impart an alkaline condition to the soil. Native plant species are desert shrubs which are tolerant of the local conditions. Due to poor plant growth, the accumulation of organic matter is generally low and the soil has poor physical properties. The soils are light colored.
The Great Plains has more rainfall than do the Western deserts; the salinity or salt level is decreased, but there is not an excessive amount of mineral leaching. The topsoil supports the growth of grasses. Organic matter accumulates and the soils have good tilth, are fertile and have a dark brown or black appearance due to the accumulation of organic matter.
As the amount of rain increases such as in the eastern regions of the country, the nutrient content of the topsoil is lower. The minerals have been leached into the deeper soil profiles. The soil is a gray-brown. The native species are broadleaf deciduous trees in high rainfall locations and the trees are needle-leaf trees in higher rainfall areas. Organic matter decomposes forming organic acids making acidic conditions. The acidity dissolves the nutrients which are leached into the groundwater by the rains.
Tropical soils near the surface are red because of the extremely low level of organic matter which unmask the presence of iron oxides which are very prevalent in soils. The fertility is extremely low and mostly what is available comes from the recycling of the nutrients from decaying vegetation and parasitic growth from plants growing on host plants. Weathering is rapid in the hot, humid conditions which releases some nutrients from the rocks.
Variations of the above conditions exist. Former marine sediments can be exposed in the normally alkaline west containing deposits of sulfur or iron sulfide which were formed from sulfate ions in the ocean. When exposed to the air, sulfuric acid is produced by oxidation leading to acidic soils. Also earth slides, erosion or grading can expose alkaline deposits from the “B” horizon in areas which are normally acidic.
Soils with the best physical properties exist with the highest level of soil organic matter. This occurs in the areas with moderate rain. The extremes of too little water and of too much rain decrease soil organic matter with a reduction in the tilth of the soil.
MODIFICATION OF SOIL PROPERTIES
The application of soil amendments and fertilizers can increase directly or indirectly the level of soil organic matter, increase the fertility of the soil and change the salt level and pH of the soil. Acidifying fertilizers such as ammonium sulfate and soil sulfur at high rates can make soil too acidic in normally alkaline conditions. Alkaline forming fertilizers such as calcium nitrate and potassium nitrate or the addition of excessive amounts of limestone can also cause growth inhibition.
It is surprising for most people to learn that plants have growth optimum conditions for nutrients; too much can be as bad as too little. When too much fertilizer is applied which is common for many sites, the rate of plant growth is decreased. Part of the inhibition is an induced deficiency of another nutrient caused by competition. Additionally, excessive nutrients increases the salt level of the soil which interferes with the moisture absorption by most plants.
In some cases, the soils are very resistant to change. This can be the situation for the arid and semiarid climates of the Southwest where limestone is present — it is extremely difficult to acidify the soil. Plant growth problems exist in plants species (acid-loving plants) adapted to acidic soils when they are grown under alkaline conditions. Iron deficiency recognized by yellow leaves with narrow green veins is caused by the limestone and bicarbonates. In extreme cases the yellow leaves are very small; totally white leaves are easily burned from saline conditions. Special iron products are available to correct these problems. The iron deficiency condition is so common in some locals that many people believe that some species are normally yellow when they are not.
GUIDANCE FROM SOIL TESTING
Soil Acidity – If the soil is too acidic, aluminum is dissolved causing a specific ion toxicity. The plant growth is stunted and the leaf coloration is sometimes deep green.
If the soil is too alkaline, some plant nutrients are unavailable causing a mineral deficiency. The source of these two problems can be the use of too much plant fertilizers of the incorrect type. A soil acidity (pH test) is required to know the soil acidity status.
Managing alteration in soil acidity with choice of nitrogen fertilizers – Fertilizer products are not interchangeable. Each product has a particular advantage and benefit over other materials. Use of the incorrect product will exacerbate problems while the correct on will enhance growth. For instance with nitrogen products, ammonium sulfate (21-0-0) will acidify the soil; ammonium nitrate (34-0-0) will be pH neutral if not over applied; calcium nitrate (15.5-0-0) will slightly increase the soil pH; urea (46-0-0) needs to be hydrolyzed before it is available. Nitrate nitrogen will supply soil oxygen. Ammonium nitrogen consumes oxygen when it is nitrified to nitrate.
Slow-release nitrogen materials also have certain benefits. Ureaformaldehyde (38-0-0) release nitrogen according to temperature and biological activities. IBDU releases in the presence of moisture and acidity, not according to plant growth. If coated products are broken, they become rapid-release.
Presence of limestone – If limestone (calcium carbonate or chalk) is present, acid-loving plants become iron deficient unless corrective measures are taken.
Lime Requirement – In areas of high rainfall, there are inadequate levels of potassium, calcium and/or magnesium due to the acidic soil. Tests for the required level of limestone or dolomite needed to raise the soil pH to a safe level are essential.
Excess Salts in the Soil – The term used by laboratories is salinity. If salts have excessively accumulated in the soil, many plants are unable to use the moisture in the soil and may have toxicity from sodium and/or chloride. A salinity test is required to determine if this is a problem. The salinity can be controlled by leaching unless soils have drainage problems. A soil high in salinity is called “saline.”
Excessive Sodium – Excessive sodium or a “sodic” soil most often has an elevated pH level. Soils high in pH values are suspect. Sodium can cause toxicity, but the more likely problem is soil compaction and poor drainage caused by the reaction of sodium on the clay.
Gypsum Requirement Excessive sodium can be corrected with the addition of gypsum. Another cause of high pH values is the presence of bicarbonates. Gypsum is also used to precipitate the excessive bicarbonates and lower the very high soil pH values. A laboratory test shows how much gypsum is needed.
Fertility Most plants require at least 16 nutrients. Three nutrients are supplied by the water and by the air (oxygen, hydrogen and carbon). Thirteen are mineral nutrients. If any one is too low, the plants will not grow. In some cases, too much fertilizers have been applied causing an adverse reaction. Too much phosphorus, for instance, inhibits the plant uptake of iron, manganese, zinc and copper causing induced deficiencies. The best method to determine if a problem is caused by a true deficiency or is an induced deficiency is soil testing. Soil analysis is used to assess the nutrient levels of the soil. Plant tissue testing is also used to ascertain which nutrients have reduced availability in the soil.
Toxicity – Soils may contain toxic metals. They either exist in the soil naturally or have been introduced as contaminants in amendments. Mined minerals and waste products are the frequent contaminant sources. These elements prevent plant growth. If a vegetable garden is to be grown and if the presence of heavy metals is suspected, the soil should be tested as a precaution for human poisoning. Lead can be present in urban soil at levels which do not injure plants but can accumulate in produce at levels which may harm humans. Excess levels of selenium and molybdenum are problems with wild life or cattle. Other common toxic elements are aluminum, cadmium, chromium, nickel, arsenic, silver, and vanadium.
Soil Compaction – Excessive compaction impedes root growth, impairs water penetration and reduces soil aeration. Reduced aeration hinders the absorption of nutrients. In addition, slow water penetration exasperates the problem. Soil compaction can be measured and corrected with soil conditioners. Their need can be detected with soil testing.
Conclusions
Soil testing is best done before planting. Since many of the nutrients do not move through the soil, they remain on the soil surface if they have been broadcasted over the soil. Unless the non-mobile nutrients are tilled into the soil, they are not readily available for the plants. The best time for incorporation into the soil is during soil preparation. Except for nitrogen, sufficient nutrient can be added to provide adequate minerals to last a decade or more.
If the conditions of the soil restrict plant root growth, it is by far easier to remedy the problems before planting. Of course, plants can be used as indicators of soil conditions. If plants wilt easily, grow poorly, give discoloration etc., observation of plant symptoms is one of the methods of performing fertility testing. The cost of using or losing plants is more expensive than using a laboratory for soil analysis. Even without discoloration, plant growth can be severely limited. For full utilization of soil testing data, plant tissue analyses is needed. The two types of data indicate how well and which nutrients are readily available as well as any possible impeded nutrient availability due to interactions.
Soil is a precious resource. Properly managed, it benefits all of us. With poor cultural practices, it becomes a liability. As with any asset, soil needs monitoring and evaluation for its best and most productive use as well as to make it most valuable to the owner. It is even wise to analyze soils from properties being considered for acquisition.
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PDF link to: Water Testing Publication
| WATER QUALITY EVALUATION FOR OPTIMUM VISUAL APPEARANCE AND EASIER MAINTENANCE |
Summary: Water quality has more meanings in the 1990s than in previous decades. Users now need to be concerned about water quality of the runoff and deep percolation water from the landscape site as well as the water quality of the irrigation water used on site. Due to drought and keen competition for water supplies, it is now necessary to use sources of water with high concentrations of potentially troublesome constituents than previously. New types of skilled management techniques are necessary to get the full advantage from the use of municipal reclaimed water and other sources of water for irrigation. To salinity and boron problems of yesteryear are now added new problems of phosphorus, nitrogen, certain heavy metals (especially nickel, copper, chromium, lead and zinc), pathogens and some organic substances. These extend the meaning of water quality. More than ever, users need the help of a qualified laboratory for safe and effective water management.
The Wonderful Uniqueness of Water
Oxygen is the most abundant element on earth; almost half of the weight of the earth is oxygen (47%). The majority of it is found combined with hydrogen to form water. Three-fourths of the surface of the earth is sea water with an average depth of about 13,000 feet with 360,000,000 cubic miles of water unfit for human and most plant needs. Annually, about 80,000 cubic miles of water are evaporated from the oceans, and about 15,000 cubic miles are evaporated from the lakes and land surfaces of the world. Total precipitation on the land surfaces is about 24,000 cubic miles of water where runoff and seepage through the soil is about 9,000 cubic miles of water. The annual flows of rivers in the USA are approximately 385 cubic miles or 1.3 billion acre feet.
Besides being ubiquitous, water is indispensable for life. An average person requires 5.5 pints of water a day to maintain the 71% water content of the body. Plants are less efficient with water needs; an average tree requires about 40 to 50 gallons per day to maintain a moisture content from 40 to 50%. Herbaceous plants and grasses contain 80% to 90% water and require an average of 60 gallons of water to produce one pound of dry matter.
Water is unique in its properties. Water swells when it freezes. It freezes lakes from the top down and not from the bottom up; it heaves soil and gradually wears down mountains. Water’s boiling point and melting point are extremely high compared with other molecules of similar size.
Water has a high cohesion or surface tension causing it to bead up. This property allows for its capillary rise or wicking action. Water flows through the soil as a liquid or vapor to plant roots to replenish the moisture depleted by the roots. Water moves up into the shoots of plants from the roots probably by capillary rise as it flows through the narrow passageways in the stems or trunk. Another remarkable property is its adhesion or wetting ability. Water will wet soil, cellulose, cotton, and other biological materials. These two properties of cohesion and wetting ability allow water to be stored in the fine capillaries of soil holding it against the force of gravity. In addition, water evaporates slower than other liquid substances with the same size molecule.
One of the most extraordinary properties of water is that of a solvent. Water is known as a polar chemical, meaning that one end of the molecule has a slight positive charge while the other end is slightly negative. This fact makes it an outstanding solvent which dissolves most inorganic chemicals or salts, and organic compounds containing oxygen or nitrogen such as sugars, alcohols, acids etc. Water is the medium in which all cells metabolize energy sources such as sugars and fats or produce energy storing compounds such as sugar in photosynthesis.
Water Carries Nutrients to Plants
Water is the medium in which mineral salts dissolve in the soil and are transported to the roots. As the relatively large volume of water is transpired though the leaves of plants, the salts are passively transported to the plant roots. The water adjacent to the roots, therefore, could be of low quality if not properly managed because of the removal of the moisture by the roots. When the ratio of the external concentration ions outside the roots to the internal concentration is large, ions leak into the roots. Healthy roots can discriminate within limits and keep certain ions from being absorbed.
Some nutrients are bound to the soil particles and are present in the soil moisture at very low levels while other nutrients are soluble at various concentrations in the soil water. Less soluble minerals dissolve as they are being depleted from the soil solution and move by diffusion to the roots which actively absorb them while consuming metabolic energy. As the root system becomes more developed, more nutrients can be absorbed from a larger volume of soil.
Roots actively transport nutrients across membranes against concentration gradients and into the roots by various means which require the metabolism of energy-containing compounds. The flow of salt into the roots can be independent of water flow, especially if there is no leakage. Metabolic needs require oxygen which must be present in the roots. Low oxygen levels are the cause for some of the poor plant growth in soils which are insufficiently aerated. The metabolism of dicot plants generally acidifies the soil releasing minerals while moncot plants instead produce transport molecules to dissolve minerals which then diffuse back to the roots.
Water Quality
Water quality used to mean how much salt and how much boron and occasionally how much bicarbonate was in the irrigation water. Today, there are expanded meanings for water quality, and we not only need to consider the effects on plant growth but also on the environment and the potability of water. One relatively new problem is that fresh water supplies are stressed to the limit (due to drought and increasing populations) and there is need for use of lower quality water including municipal waste water for irrigation. These require a more extensive look at water quality. There is much concern about nitrate and phosphate in the runoff water. For example, the characteristics of water that impart poor quality include the osmotic concentrations of dissolved substances as well as the concentration of individual substances which may be nutrients and/or substances which can result in specific toxicities. All need to be known and understood.
In arid and semi-arid areas, water quality needs to be carefully considered. Due to high temperature and low relative humidity, water evaporation is high. Salts are then concentrated in the remaining moisture. Warm soil surfaces especially with wind lose moisture from the soil; water then moves up to maintain an equilibrium of soil moisture. As this water moves to the soil surface through the capillary rise in soil, it brings salts to the surface. A higher level of salinity is deposited on the surface by the evaporating water. Besides the salts present in the soil, irrigation water may have high levels of total dissolved salts which add to the problem. One acre foot of Colorado River water has about 1 1/2 tons of salts. Annually, eight tons of salts could be added to each acre under irrigation. This is about 400 pounds of salt for each 1,000 square foot area. This salt must be removed and any negative conditions must be corrected. The water quality will determine the level of needed management to control the soil conditions and the plant palettes suitable for the conditions.
Soils with poor physical structure, such as those which have a surface crust, or which are compacted, or which have poor drainage, need better water quality than the soils with better properties. The poorer soils lose more water due to evaporation resulting in higher salinity levels and, at the same time, do not have sufficiently good leaching to remove the excess salts. Soils with good physical structure such as 50% compaction or less, 20% or more pore space when at maximum water holding capacity, and drainage rates of well over 2 inches per hour support the growth of plants better and can tolerate lower water quality in terms of osmotic concentrations. Soil conditioning in the soils having low soil organic matter (less than 5 to 7 percent) is needed for the best responses to fertilization and irrigation. In the last 10 years, new soil conditioners have been developed for imparting excellent soil physical properties. They are quite valuable for land reclamation and for removing excess salinity from soil and to maintain good drainage and aeration.
The amount of water needed for plant growth is variable and depends on water quality. Plants not stressed are more efficient is their use of water. Proper nutrition enables an increase in water-use efficiency. Less water is needed for the same amount of biomass production.
Salinity as an Osmotic Problem
Salinity is a qualitative term used for the state of saline water or saline soil that contain various amounts of salt. A salt is the combination of a cation (a positively charged metal ion) with an anion (a negatively charged ion such as ionized acids). For example, sodium bicarbonate and sodium hydroxide or lye react with hydrochloric acid forming sodium chloride or table salt. Many salts are possible such as sodium sulfate and calcium salts. As the salt level increases in water, the water molecules are held more firmly preventing the diffusion of water from the external solution into plant roots (this phenomenon is called osmosis). When the level of salinity is too high, the water is unavailable for the plantings and the water is unsuitable. Organic molecules such as sugar or the alcohol mannitol cause osmotic problems also.
The level of salinity increases in soil after irrigation with water containing salts. The water evaporates from the soil, and water is transpired from the soil through the plants. Both processes concentrate the remaining salts. Soil salinity increases with depth in the soil. At deeper depths in the soil more water has been removed by transpiration of the shallow and deep roots leading to increased salinity.
Plants vary in their ability to resist the osmotic aspect of salinity. Chart I gives the effects of salinity on a few common plants. The more sensitive plants start responding adversely to salinity in the soil of 1 millimho/cm (a measure of the ability of water to conduct electricity which in turn depends upon the salt concentration) by decreasing their growth. Plant problems start occurring with water containing about 0.75 millimho/cm. Above 3 millimho/cm the problems become severe. The degree of the problems depends upon the soil conditions and plant conditions. Healthy roots can restrict the flow of passive movement of salts into the plant. Application of water in excess of water loss from evaporation and transpiration (ET) to well drained soil will leach excess salts below the root zone. Methods for calculating the leaching fraction (the fraction of water needed in addition to the ET requirements) to keep excess salts out the root zone have been developed to guide the needed application of excess irrigation water.
Salinity is a qualitative term used for the state of saline water or saline soil that contain various amounts of salt. A salt is the combination of a cation (a positively charged metal ion) with an anion (a negatively charged ion such as ionized acids). For example, sodium bicarbonate and sodium hydroxide or lye react with hydrochloric acid forming sodium chloride or table salt. Many salts are possible such as sodium sulfate and calcium salts. As the salt level increases in water, the water molecules are held more firmly preventing the diffusion of water from the external solution into plant roots (this phenomenon is called osmosis). When the level of salinity is too high, the water is unavailable for the plantings and the water is unsuitable. Organic molecules such as sugar or the alcohol mannitol cause osmotic problems also.
The level of salinity increases in soil after irrigation with water containing salts. The water evaporates from the soil, and water is transpired from the soil through the plants. Both processes concentrate the remaining salts. Soil salinity increases with depth in the soil. At deeper depths in the soil more water has been removed by transpiration of the shallow and deep roots leading to increased salinity.
Plants vary in their ability to resist the osmotic aspect of salinity. Chart I gives the effects of salinity on a few common plants. The more sensitive plants start responding adversely to salinity in the soil of 1 millimho/cm (a measure of the ability of water to conduct electricity which in turn depends upon the salt concentration) by decreasing their growth. Plant problems start occurring with water containing about 0.75 millimho/cm. Above 3 millimho/cm the problems become severe. The degree of the problems depends upon the soil conditions and plant conditions. Healthy roots can restrict the flow of passive movement of salts into the plant. Application of water in excess of water loss from evaporation and transpiration (ET) to well drained soil will leach excess salts below the root zone. Methods for calculating the leaching fraction (the fraction of water needed in addition to the ET requirements) to keep excess salts out the root zone have been developed to guide the needed application of excess irrigation water.
| Chart ITolerance of Plants to Salinity (Millimho/cm) Threshold level Level of salinity where growth begins for 50% | ||||||||||||||||||||||||||||||
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| (under otherwise good soil conditions) |
Beans and strawberries are affected by salinity levels over 1 millimho/cm. All turf grasses can grow with salinity of 4 millimho/cm. Highland Bent and Kentucky Blue grass are the least tolerant. Alta Fescue is able to tolerate 7 and Creeping Bent can tolerate a salinity of 10. These levels of salinity decreased the growth 25%. Germination is more salt sensitive than is the vegetative growth. Lower levels of salinity are needed for germination.
Symptoms of excessive salts are leaves with necrotic or dead tissue on the margins or tips. The plants often extrude excess salts at the tips which causes tip burn and kills the tissues. Interestingly, some desert xeric plants actually thrive on low and medium amounts of salinity. Such plants can be used where problem waters and soils indicate.
Salinity and Permeability of Soil
Soil crumbs are cemented together with agents made from salts. If water contacting the soil has a very low salinity, i.e. rain or runoff water from snow pack off of mountains, the cements are dissolved causing the soil crumbs to disperse. This seals the soil and prevents water recharge into the root zone or rhizosphere. At salinity levels of less than 0.2 millimho/cm, this problem is severe in many soils. Salinity levels can be increased. In some cases, calcium sulfate salt known as gypsum is injected into irrigation water from a slurry tank to increase the salinity. Calcium is needed also for good water permeability.
Permeability of Soil Affected by Sodium
Well drained, productive soils have low levels of sodium bound to the clay particles (called exchangeable sodium since it can be made to dissolve). When the exchangeable sodium level increases due to irrigation with high sodium/low calcium water, the clay particles become dispersed and possess poor physical properties. Some clays swell when wet especially in the presence of sodium. And when the particles swell, the pores in the soil are narrowed causing a decrease in water percolation. The alkalinity increases from dissolved carbon dioxide which is produced from the roots and microorganisms (bicarbonate and carbonate are produced). Since the soil becomes sealed, the carbon dioxide is trapped and kept from escaping as a gas. Leaching is reduced keeping the alkaline bicarbonate and carbonate present. If the problem is not corrected, the soil can become alkaline. In this condition, the soils turn black from the soil organic matter which dissolves. Sodic soils or soils containing high levels or sodium have a sodium adsorption ratio (SAR which is a modified ratio of sodium to the sum of calcium and magnesium). Irrigation water containing an SAR of 3 to 5 or more significantly increase this problem. Frequently, an adjusted SAR is used. Since the bicarbonates and carbonates precipitate the calcium and magnesium as limestone and magnesium carbonate, the SAR is altered to reflect the decreased concentration of calcium and magnesium. Excellent soils contain about 70% calcium and 15% magnesium with only a few percent sodium. Sodic soils contain 15% or more sodium.
Bicarbonates
Bicarbonates and carbonates precipitate some of the micronutrients rendering them unavailable to plants. Iron deficiency is common because of this effect. Symptoms are yellow leaves with green veins. New growth is more affected than older growth. Bicarbonates also have a physiological affect on the roots reducing nutrient absorption. Problems start at around 75 parts per million. If it exceeds 150 parts per million (2.5 milliequivalents per liter), the water is probably not suitable. The problems are less severe if the water is applied by flooding because foliar absorption is more of a problem than root absorption. The bicarbonates and carbonates can be reduced in the water by treatment with gypsum to precipitate them or with sulfuric acid to neutralize them.
Salinity and Specific Ion Toxicities
Liebig’s old law of the minimum is not always correct. This law states that the most toxic constituent limits plant growth such as the weakest link of a chain determined the force needed to break it. The law implies that the factor in most limitation needs correction before responses to other factors are obtained. More recently, it has been determined that this is not correct in many cases. The toxicity effects are accumulative. For instance, if two factors were to lower the growth potential to 80 percent each, the overall effect would be 0.8 times 0.8 or 0.64 of optimum. Correction of both factors per the “Law of the Maximum” would give a response showing a 36% adjustment. The response is the synergistic and exceeds the sum of the individual responses if each were correct separately. All limiting factors due to poor water quality need to be considered in concert for the best growth improvement.
Chloride
Chloride at levels in water of less than 100 parts per million will be suitable for all needs. More of a problem occurs with sprinkler irrigation due to foliar absorption. Severe problems occur at levels over 350 parts per million in the soil solution because of root absorption. Chloride can lower the availability of nitrate uptake due to competition of the roots for uptake of ions.
Sulfate
Plants are comparatively insensitive to sulfate toxicity. When the level is about 3,000 parts per million of sulfate, plant growth would be adversely affected.
Boron
Boron is an essential element but the range between ideal and toxic concentrations is small. In general, the plants tolerant to boron are not also tolerant to salinity as shown in Chart II. Levels less than 0.5 parts per million are essential for plants sensitive to boron. For semi-tolerant plants, levels up to 1 part per million are allowable. For tolerant plants, 2.5 parts per million are satisfactory.
Sodium
Sodium is an essential nutrient for several plants species adapted to saline soils. Sodium has a sparing effect on potassium deficiency for other species. When potassium is low, sodium may substitute for some of the growth requirements. For other species, sodium is deleterious and can be toxic if excessive. This specific ion effect is independent of the associates anion and related to the sodium concentration in the leaves.
Metals – magnesium, lithium, zinc and heavy metals
Magnesium is essential for growth but sometimes its presence in water can cause problems. If the proportion of magnesium is higher than the calcium, then magnesium can induce a calcium deficient. Lithium is not essential but is present in arid soils. At levels over 3 parts per million, toxicity can occur.
Zinc as well as other heavy metals are also toxic. They effect the uptake of the essential micronutrients and can cause induced deficiencies. In addition, higher levels will cause general toxicity reactions.
Nutrient Balance
Inasmuch as plants respond mainly to the ratio of nutrients, a toxicity or deficiency can result when an element is not in the needed proportion. Some ions in irrigation water can upset the normal balances. Essential trace metals; iron, manganese, zinc and copper; are sometimes difficult to keep in proper proportions. Plants may have iron levels in the leaves reduced with excessively high zinc or copper levels in the soil. Some sources of water can increase the level of these metals in the soil. In addition, nonessential elements can inhibit the uptake of these four essential metals. Soils on the average contain appreciable levels of nonessential heavy metals. Fortunately, the metals are not easily dissolved by water. The more active elements such as calcium, lithium, sodium, etc. will dissolve in the soil moisture readily. The heavy metals do not easily dissolve and the actual available concentrations are much lower than the total concentrations. Acidic conditions, however, will dissolve them. Chart III gives the maximum safe levels of some essential and non-essential elements in water. In addition it lists the maximum levels in general along with the natural abundance of these elements and the ranges found in soils.
Sources of Water Affect Its Quality
Surface waters from rain have less dissolved salts than the water which seeps into the ground. As the water moves through the soil, salts dissolve. Seepage can flow into rivers gradually increasing the salinity of the water downstream. When the Colorado river water gets to Mexico, it is too saline for irrigation. The salinity comes from tributaries which drain large areas. Some low quality drainage water flows into it from farms while flash floods move salt from the desert into the river. In contrast rivers from granite mountains are almost salt free.
Ground water from wells can contain appreciable levels of salts and may have extremely poor water quality from the extracted minerals. Water from deep wells or geologic sources may have good quality while shallow renewable aquifers may have high salts from drainage of irrigated lands. Shallow well water may be unfit for human consumption.
Reclaimed Water
Reclaimed municipal water typically contains appreciable levels of boron probably mostly from borax laundry products. Sensitive plants will be affected. Sodium, chloride, bicarbonate and zinc are other general problems. The bicarbonate and carbonates are the by products of the oxidations of the organic constituents of the waste stream. Nitrogen levels are lowered during this process.
Zinc comes from galvanized pipes in homes etc. The zinc used for galvanization is contaminated with cadmium at about one percent. Recycled water containing industrial waste may have high levels of cadmium and other heavy metals such as nickel, lead and chromium. These severely lower water quality. Micronutrient fertilization problems need to take this into account. High soil pH values and liming of the soils can reduce these potential toxicities. Phosphorus when high in reclaimed water may be applied in excess of plant needs and precipitate the trace
elements. Plants can generally tolerate excess, nonessential elements better than can animals or humans. Water used for produce production should entail a complete analysis of potentially toxic elements. Fortunately, some potentially toxic elements are not translocated from the roots to the shoots in many plants.
A case study of common bermudagrass in Tucson was conducted with reclaimed water by Hayes et al. (Agronomy Journal, Vol. 82, 943-6 (1990)). Turfgrass appearance was better on the average with unfertilized reclaimed water than with fertilized potable water. Iron chlorosis was a problem with reclaimed water when nitrogen was applied due to the lower unavailability of iron caused by phosphorus when the turf demand was increased by the nitrogen. Sodium levels increased in the soil indicating the need for more amending with gypsum and perhaps need for more aerification.
Effect of Soil Types on Allowable Water Quality (Osmotic Quality)
Sandy soils allow the use of lower water quality since the leaching of the salts is rapid avoiding a large increase in soil salinity. Soils with lower percolation rates require better water quality because of the slow rate of leaching.
On the other hand, clay soils buffer or remove from the water undesirable ions. Clays have high absorption of metals which lower the availability and toxicity of metals.
How to use Poorer Quality Water
Some water quality factors can be improved through management practices guided by laboratory analyses. Salinity can not be lowered easily nor can boron or chloride be removed, but sodium problems can be reduced. The effect of bicarbonates and carbonates can be controlled.
Gypsum Requirement
Excessive sodium levels in low quality water, can be solved with the addition of calcium such as from gypsum. The calcium can lower the relative activity of sodium by dilution. Calcium can precipitate the bicarbonates and carbonates as well as supply sufficient calcium for plant needs. The gypsum requirement is the sum of these three needs. For good water quality, none is needed. For poor water quality, gypsum may be needed in the range of several pounds per 1,000 square feet for each inch of irrigation.
The gypsum requirement can be lowered if the bicarbonates and carbonates are removed from the water through acidification such as with sulfuric acid.
Leaching Requirement
Unless salts that have been added to the soil are removed, plant growth will decline because of an increase in salinity. Adequate leaching is required to maintain the salinity of the soil at tolerable levels. The amount of leaching increases with poorer soil conditions. The leaching requirement is the percentage of additional irrigation water which should be applied to the site in order to leach sufficient salts to avoid growth reduction of more than 20 percent. It is calculated from the water quality data and the desired maximum level of salinity depending upon the tolerance of the plants. Values can range from a few percent for good water quality to 50 percent for poor water quality.
Leaching of salts requires that water will flow through the soil profile. If the subsoil is not permeable for the salts to move below the root zone, drain line or other procedures are required for plant growth.
Water-soluble polymeric soil conditioner requirement
Highly permeable, well aerated soils are extremely important when marginal-quality water is used. Water-soluble soil conditioners can help create desirable characteristics in soil. Laboratory tests have been developed (see Soil Science, Volume 141, pages 390-394, 1986) to measure the amount of soil conditioners needed to improve the physical properties of soil. If soils are not readily permeable to water, leaching of salts will occur too slowly to be of benefit. A review of the water-soluble polymeric soil conditioners appeared in Soil Technology, volume 3, pages 1-8, 1990.
Analytical Guidance
Water quality assessment with help of a laboratory can properly guide the use of irrigation water. Since soil conditions affect water leaching and changes in water quality, they need to be evaluated through soil analysis. Plant analysis through tissue testing is the third component of the triad of proper site evaluation. The plant is the bioindicator of the overall interactions of the soil chemistry and soil physical limitations. No one of these parts of the triad should be ignored if successful landscapes are to be achieved.
How to read a Water Quality Report plus Definitions of Terms and Concepts
| acid – | a substance that releases hydrogen ions in solution. The substance which losses the hydrogen ion has a negative charge is called an anion. |
| acre foot – | the water required to cover one acre, 1 foot deep or 325,829 gallons. |
| anions – | negatively charged ions which frequently are nonmetals |
| base – | an alkaline chemical such as sodium hydroxide or calcium carbonate (limestone) which is the opposite of an acid. |
| cations – | positively charged ions which most frequently are metallic ions such as sodium and calcium. |
| ESP – | Exchangeable sodium percentage is the percent of sodium cations (metal ions) which are potentially soluble from the surface of the soil particles compared to all cations. Soils with an ESP of 15% or more are considered to be sodic. Exchangeable cations are those bound to soil particles which can be made to solublize or exchanged by each other. |
| ET – | Evapotranspiration is the sum of the moisture being lost due to evaporation from the soil and by transpiration from the plants. |
| ions – | molecules or atoms which has have positive or negative charge(s). |
| milliequivalents per liter – | a term used to quantify the number of anions or cations in solution. |
| pH – | the measurement of the degree of acidity. The sum of the hydrogen ions and hydroxide ions is 14 when they are measured in negative logarithms using molar concentrations. At pH 7, the hydrogen ions and the hydroxide ions (14-7=7) are equal and the solution is neutral. Acids contain more hydrogen ions than hydroxide ions. Thus acids are solutions having pH values less than 7. Alkaline solutions have pH values more than 7. Moderately acidic solutions are solution with pH values less than 5 and moderately alkaline solutions are those with pH values over 9. |
| ppm – | parts per million. The newer term is milligrams per liter. One liter of water weighs about one million milligrams. One ppm is about 1 milligram per liter (mg/l) of water. |
| salinity (millimho/cm) – | Also known as the electroconductivity or EC. When measured in water, it is called ECw. When measured in a saturation extract from soil, it is called ECe. The measurement is the reciprocal of the resistance of the solution to an electrical current. |
| salt – | the union of an anion or acid and an cation or base such as sodium chloride and calcium sulfate which dissolves in water and release the individual anions and cations |
| SAR – | Sodium Absorption Ratio. It is used as is the ESP to measure the ability of sodium in water to become fixed on the soil surface in soils. The ESP is used only for soil. |
| sodic – | soils with an excessive level of sodium. SAR values over 6 or ESP values of at least 15% |
| Total Dissolved Solids – | The amount of solids dissolved in water. Units are parts per million or milligrams per liter. |
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PDF link to: Plant Tissue Testing Publication
| PLANT TISSUE ANALYSIS COMPLEMENTS SOIL TESTING |
By Garn A. Wallace, Ph. D.
Various tools are available for enhancing growth and reducing plant problems. Each tool has it strength and limitations. Soil testing is not always 100% adequate in resolving plant nutritional needs. It is estimated by some to be only 75% accurate. Soil testing is valuable in resolving major problems but does not do well with minor adjustments nor does it perform as well in soils which have poor physical properties.
Soil tests can be valuable when calibrated for a specific plant in a specific soil. Since there are thousands of soil types and numerous plant species which differ in their responses to soils and nutrition, this is difficult.
The most efficient procedure to assess plant nutritional requirements is with the use of various combined analyses. Visual symptoms, the result of plant growth to different treatments as well as soil and tissue testing need to be used. It is best to have a second opinion before applying nutrients which can not be readily removed from the soil.
The micronutrients are needed in very low amounts. Boron for instance has a very fine line between optimum levels for good growth and the toxic level. Poorly buffered soils such as sandy soils can be adversely effected with the application of an essential trace metal. A little too much zinc or copper can induce an iron or manganese deficiency.
Since the root systems of plants assimilate the nutrients, the availability of nutrients present in the soil depends upon the size and status of the root system. A very invasive root system in an infertile, loose, friable soil can give good growth as well as a more restrictive root system in a fertile soil. Most nutrients move with the flow of water to the roots. Low levels of nutrients over a large root system are just as effective as higher levels of nutrients in a smaller system. The proper evaluation of soil when using soil testing should include fertility as well as physical evaluations. Otherwise, plants can be used as a bioassay to determine what is really available. The measurement of nutrient uptake by plants eliminates all the complex interactions of soil and gives a picture of what is available.
RESPONSE OF PLANT GROWTH TO NUTRITIONAL STATUS
Generally, as the concentration of a nutrient present in tissues increases, the growth rate is found to be faster as shown in figure 1. The greatest increase in growth occurs when the plant is highly deficient but not severely deficient. The curve is steepest in the highly deficient zone meaning that a small increase of a nutrient in the tissue gives a large increase in the growth rate. When mineral content of the tissue is sufficient, there is little change in the growth rate with additional fertilizers. As the concentration of nutrients increases even higher, toxicity occurs with a decreased growth rate.
The shape of the growth curve in the severely deficiency range is “C” shaped. As the growth rate becomes extremely slow, the production of biomass is too low to cause much dilution of the absorbed minerals. The concentration of the minerals could be on the curve in the moderately deficient range or on the lower curve in the severely deficient range. It is best to interpret the data in conjunction with regards to several elements. For example with iron deficiency, phosphorus and calcium are also depressed. It is wise to evaluate the results as a whole and not rely on absolute values.
Frequently, laboratories will use a critical value for an elements. Normally the “critical value” for the deficient nutrient level is the concentration present for 90% of the optimum growth rate. The “critical toxicity level” is the concentration present where the growth rate is depressed 10% from the optimum growth. More reliable recommendations are made within a range of concentrations.
Optimum growth occurs over a range of nutrient contents. Deficiency also occurs over a range of concentrations. In lieu of a “critical value”, it is more accurate to use a “critical nutrient range” for diagnostic needs. As with all organisms, variation occurs from individual to individual. What is optimum for one differs a little for another.
Guidance with the use of critical values can cause poor growth if multiple elements are deficient. If just one nutrient were slightly deficient and results in a 10% decrease in growth while all of the other nutrients were in the optimum range, total growth would be 90% of maximum if all cultural practices and other factors were also at optimum. Seldom are all factors perfect. If two nutrients were present at 90% of optimum levels, the total growth rate is 90% times 90% or 81% of optimum. For three factors at 90%, the result is 73% (90% x 90% x 90%) and 10 factors at 90% would be 35% of optimum. It is important to correct all factors to near 100% if the goal is to have good growth.
Some experts believe that the ratio of the concentration of one nutrient relative to the concentration of another element is more important than the absolute concentration of either. This concept has led to a method for the interpretation of multi-element analyses of plant nutrients often called by the acronyms such as DRIS or TEAM. The method has given some success. Its main advantage is that it organizes the nutrients into a series from mostly likely deficient to least likely deficient (or most toxic).
Tissue Analysis as a Forensic Tool
Some plants suffer from stress from excess salts which result in damaged leaves and roots. Necrotic leaves can be analyzed to determine the source of the stress. Excess salts in the plant are extruded on the margins of leaves causing a marginal burn. Analyses of leaves can determine the actual salts causing the problem.
Moisture stress can cause the nutrients to be recycled from leaves prior to leaf loss. These leaves lose nitrogen, potassium, phosphorus and carbohydrates. The leaves become thinner than normal. With a reduced tissue weight, the nonmobile elements remaining in the tissue will have high concentrations based upon tissue mass.
Toxicity from heavy metals such as nickel, chromium, cadmium, vanadium, arsenic, silver etc. can be detected in leaf tissue analyses. For some elements such as silver, nickel, chromium and others, the root system is a barrier to the movement of the metals into the upper parts of the plants. Root analysis is more reliable in detecting these possible problems.
Multiple Testing approach to Nutritional Status
Soil testing is helpful to determine broad problems such as salinity, acidity and major problems with deficiencies. Soil testing can not provide precise answers for nutrient needs. Soil testing data are used to predict that there may be a probability of a increased growth response to the application of a nutrient in question. It is not uncommon in relying on soil data predicting that a nutrient is low that one finds upon supplying an element in large amounts that there is no response to increased growth. Thus the test can be considered as inaccurate. In reality, the test is accurate but the interpretation is inadequate. The same problem occurs with tissue analysis.
When several different methods of assessing nutritional status are combined, the results are more dependable. Evaluation methods are soil testing, tissue testing, visual symptoms and responses to the testing of nutrient application – typically foliar application.
Soil properties can vary from spot to spot. The manner in which plants grow is the total sum of the differences for each spot. Soil testing may not give a true picture of the soil properties in a particular location but plant appearances reflect these problem areas.
Tissue content varies within the plant. Leaves from the same age vary. Also tissues change with age. The newest growth has higher levels of nitrogen, potassium and phosphorus than do older leaves. As the tissue ages, mobile elements such as nitrogen, potassium and phosphorus are transported to new tissues and the concentrations are lowered. Nonmobile elements such as calcium, magnesium increase with age. The selection of tissue for testing needs to be selective.
Visual Indications OF DEFICIENCIES
Visual interpretation of the nutritional status of plants can help diagnose problems. The following symptoms are helpful if only one element were deficient. With multiple deficiencies or toxicities in addition to deficiencies, the use of visual signs is difficult
| Nitrogen | Low nitrogen causes a pale green coloration. Since the nitrogen is mobile, new growth is greener than the older growth. |
| Iron | The old growth versus new growth symptoms for low iron are reversed for nonmobile elements such as iron. The newest growth is yellower for iron. In addition, iron deficiency, if not too deficient, will have green veins in the leaves with yellowness in between the veins. |
| Manganese | Manganese deficiency is similar to iron. However, the width of the green veins is greater. |
| Phosphorus | Phosphorus deficiency causes slow, weak growth. Newer leaves may be dark green while the older leaves have a purple pigmentation. |
| Potassium | Potassium deficient plants are sensitive to disease infestation. Older leaves will be as if they had been burned along the edges, a deficiency known as “scorch.” Plants deficient in potassium may become sensitive to ammonium toxicity. |
| Calcium | The growing tips of plants turn brown and die with calcium deficiency. Leaves curl and their margins turn brown with newly emerging leaves sticking together at the margins, leaving the expanded leaves shredded on their edges. |
| Zinc | Zinc deficiency causes a chlorosis of the interveinal areas of new leaves. The chlorosis is mosaic. With increasing severity of deficiency, growth is stunted and leaves die and abscise. Excess phosphorus can induce zinc deficiency. Excess zinc can induce iron deficiency. |
Tissue analysis is a valuable tool to aid those growing, establishing or maintaining vegetation. As with all tools, the proper ones are needed at the appropriate time. With the correct interpretation and recommendations, valuable plantings can be maintained for many years.
Garn A. Wallace, pH. D. earned his doctorate degree from UCLA in the Department of Biochemistry. He worked as a research biochemist in the Laboratory of Biomedical and Environmental sciences before forming Wallace Laboratories with Arthur Wallace, professor emeritus, Department of Agricultural Sciences [Plant Nutrition and Soil Science] UCLA. Jointly the Wallaces have over 600 publications in the fields of plant nutrition, soil science, microbiology, plant physiology, ecology, soil conditioners, mineral excesses, water relationship in plants, mineral toxicities etc. They are located at 365 Coral Circle, El Segundo, CA 90245, (310) 615-0116.