Sustainable Agriculture Research and Education - Publications - Building Soils for Better Crops- Chapter 18 - Other Fertility Issues: Nutrients, CEC, Acidity and Alkalinity

The potential available nutrients in a soil, whether natural or added in
manures or fertilizer, are only in part utilized by plants
T.L. Lyon and E.O. Fippin, 1909

Although farmers understandably emphasize nitrogen and phosphorus because of the large quantities used and the potential for environmental problems additional nutrient and soil chemical issues remain important. In most cases, the overuse of these other fertilizers and amendments doesn't cause problems for the environment, but inappropriate use may waste money and reduce yields. There are also animal health considerations. For example, excess potassium in feeds for dry cows (between lactations) results in metabolic problems, and low magnesium availability to dairy or beef cows in early lactation can cause grass tetany. As with most other issues we have discussed, focusing on the management practices that build up and maintain soil organic matter will help eliminate many problems or, at least, make them easier to manage.

Potassium is one of the N-P-K "big three" primary nutrients needed in large amounts and frequently not present in sufficient quantities for plants. Potassium availability to plants is sometimes decreased when liming a soil to increase the pH by one or two units. The extra calcium, as well as the "pull" on potassium exerted by the new cation exchange sites (see discussion of CEC), contribute to lower potassium availability. Problems with low potassium levels are usually dealt with easily by applying muriate of potash (potassium chloride), potassium sulfate, or Sul-Po-Mag or K-Mag (potassium and magnesium sulfate). Manures also usually contain large quantities of potassium.

Magnesium deficiency is easily corrected if the soil is acidic by using a high-magnesium (dolomitic) limestone to raise soil pH (see discussion of soil acidity below). Otherwise, Sul-Po-Mag is one of the best choices for correcting such a deficiency.

Calcium deficiencies are usually associated with low pH soils and soils with low CECs. The best remedy is usually to lime and build up the soil's organic matter. However, some important crops, such as peanuts, potatoes and apples, commonly need added calcium. Calcium additions also may be needed to help alleviate soil structure and nutrition problems of sodic soils (see below). In general, if the soil does not have too much sodium, is properly limed and has a reasonable amount of organic matter, there will be no advantage to adding a calcium source, such as gypsum. However, soils with very low aggregate stability may sometimes benefit from the extra salt concentration associated with surface gypsum applications. This is not a calcium nutrition effect, but a stabilizing effect of the dissolving gypsum salt. Higher soil organic matter and surface residues should do as well as gypsum to alleviate this problem.

Sulfur deficiencies are common on soils with low organic matter. Some soil testing labs around the country offer a sulfur soil test. (Those of you who grow garlic should know that a good supply of sulfur is important for the full development of garlic's pungent flavor so garlic growers want to make sure there's plenty available to the crop.) Much of the sulfur in soils occurs as organic matter, so building up and maintaining good amounts of organic matter should result in sufficient sulfur nutrition for plants. Although reports of crop response to added sulfur in the Northeast are rare, it is thought that deficiencies of this element may become more common now that there is less sulfur air pollution originating in the Midwest. Some fertilizers used for other purposes, such as Sul-Po-Mag and ammonium sulfate, contain sulfur. Calcium sulfate (gypsum) also can be applied to remedy low soil sulfur. The amounts used on sulfur-deficient soils are typically 20 to 25 lbs. sulfur/acre.

Zinc deficiencies occur with certain crops on soils low in organic matter and in sandy soils or those with a pH near neutral. Zinc problems are sometimes noted on silage corn when manure hasn't been applied for a while. It also can be deficient following topsoil removal from parts of fields as land is leveled for furrow irrigation. Sometimes, crops outgrow the problem as the soil warms up and organic sources become more available to plants. Zinc sulfate (about 35 percent zinc) applied to soils is one of the materials used to correct zinc deficiencies. If the deficiency is due to high pH, or if an orchard crop is zinc-deficient, a foliar application is commonly used. If a soil test before planting an orchard reveals low zinc levels, zinc sulfate should be soil-applied.

Boron deficiencies show up in alfalfa when growing on eroded knolls where topsoil and organic matter have been lost. Root crops seem to need higher soil boron levels than many other crops. Cole crops, apples, celery and spinach are also sensitive to low boron levels. The most common fertilizer used to correct a boron deficiency is sodium tetraborate (about 15 percent boron). Borax (about 11 percent boron), a compound containing sodium borate, also can be used to correct boron deficiencies. On sandy soils low in organic matter, boron may be needed on a routine basis.

Manganese deficiency, usually associated with soybeans and cereals on high pH soils and vegetables grown on muck soils, is corrected with the use of manganese sulfate (about 27 percent manganese). About 10 lbs. of water-soluble manganese per acre should satisfy plant needs for a number of years. Up to 25 lbs. per acre of manganese is recommended, if the fertilizer is broadcast on a very deficient soil. Natural, as well as synthetic, chelates (at about 5 to 10 percent manganese) usually are applied as a foliar spray.

Iron deficiency occurs when blueberries are grown on moderate to high pH soils. Iron deficiency also sometimes occurs on soybeans, wheat, sorghum, and peanuts growing on high pH soils. Iron (ferrous) sulfate or chelated iron are used to correct iron deficiency. Both manganese and iron deficiencies are frequently corrected by using foliar application of inorganic salts.

The CEC in soils is due to well humified ("very dead") organic matter and clay minerals. The total CEC in a soil is the sum of the CEC due to organic matter and CEC due to the clays. In fine-textured soils with medium to high CEC-type clays, much of the CEC may be due to clays. On the other hand, in sandy loams with little clay, or in some of the soils of the southeastern U.S. containing clays with low CEC, organic matter may account for the overwhelming fraction of the total CEC.

There are two practical ways to increase the ability of soils to hold nutrient cations, such as potas-sium, calcium, magnesium, and ammonium:

One of the benefits of liming acid soils is to increase soil CEC! Here's why: As the pH increases, so does the CEC of organic matter. As hydrogen (H+) on humus is neutralized by liming, the site where it was attached now has a negative charge and can hold onto Ca⁺⁺, Mg⁺⁺, K⁺, etc.

Estimating Organic Matter's Contribution to a Soil's CEC

The CEC of a soil is usually expressed in terms of the number of milliequivalents (me) of negative charge per 100 grams of soil. (The actual number of charges represented by one me is about 6 followed by 20 zeros.) A useful rule of thumb for estimating the CEC due to organic matter is as follows: for every pH unit above pH 4.5, there is 1 me of CEC in 100 gm of soil for every percent organic matter. (Don't forget that there will also be CEC due to clays.)

Example 1: pH = 5.0 and 3% SOM → (5.0 — 4.5) x 3 = 1.5 me/100g
Example 2: pH = 6.0 and 3% SOM → (6.0 — 4.5) x 3 = 4.5 me/100g
Example 3: pH = 7.0 and 3% SOM → (7.0 — 4.5) x 3 = 7.5 me/100g
Example 4: pH = 7.0 and 4% SOM → (7.0 — 4.5) x 4 = 10.0 me/100g

Many soil testing labs also will run CEC, if asked. However, there are a number of possible ways to do the test. Some labs determine what the CEC would be if the soil's pH was 7 or higher. They do this by adding the acidity that would be neutralized if the soil was limed to the current soil CEC. This is the CEC the soil would have at the higher pH, but is not the soil's current CEC. For this reason, some labs total the major cations actually held on the CEC (Ca⁺⁺ + K⁺ + Mg⁺⁺) and call it effective CEC. It is more useful to know the effective CEC the actual current CEC of the soil than CEC determined at a higher pH.

Background
Plants have evolved under specific environments, which in turn influence their needs as agricultural crops. For example, alfalfa originated in a semiarid region where soil pH was high; alfalfa requires a pH in the range of 6.5 to 6.8 or higher (see figure 18.1 for common soil pH levels). On the other hand, blueberries, which evolved under acidic conditions, require a low pH to provide needed iron (iron is more soluble at low pH). Other crops, such as peanuts, watermelons, and sweet potatoes, do best in moderately acid soils in the range of pH 5 to 6. Most other agricultural plants do best in the range of pH 6 to 7.5.

Several problems may cause poor growth of acid-sensitive plants in low pH soils. The following are three common ones:

aluminum and manganese are more soluble and can be toxic to plants;
lack of calcium, magnesium, potassium, phosphorus or molybdenum (especially needed for nitrogen fixation by legumes); and
a slowed decomposition of soil organic matter and decreased mineralization of nitrogen.

The problems caused by soil acidity are usually less severe, and the optimum pH is lower, if the soil is well supplied with organic matter. Organic matter helps to make aluminum less toxic and, of course, humus increases the soil's CEC. Soil pH will not change as rapidly in soils that are high in organic matter. Soil acidification is a natural process that is accelerated by acids produced in soil by most nitrogen fertilizers. Soil organic matter slows down acidification and buffers the soil's pH because it holds the acid hydrogen tightly. Therefore, more acid is needed to decrease the pH by a given amount when a lot of organic matter is present. Of course, the reverse is also true more lime is needed to raise the pH of high-organic matter soils by a given amount (see "Soil Acidity" box below).

Limestone application helps create a more hospitable soil for acid-sensitive plants in many ways, such as:

neutralizing acids;
adding calcium in large quantities (because limestone is calcium carbonate, CaCO₃);
adding magnesium in large quantities if dolomitic limestone is used (containing carbonates of both calcium and magnesium);
making molybdenum and phosphorus more available;
helping to maintain added phosphorus in an available form;
enhancing bacterial activity; and
making aluminum and manganese less soluble.

Almost all the acid in acidic soils is held in reserve on the solids, with an extremely small amount active in the soil water. If all that we needed to neutralize was the acid in the soil water, a few handfuls of lime per acre would be enough to do the job, even in a very acid soil. However, tons of lime per acre are needed to raise the pH. The explanation for this is that almost all of the acid that must be neutralized in soils is reserve acidity associated with either organic matter or aluminum.

Soil Acidity

Background

pH 7 is neutral.
Soil with pH levels above 7 are alkaline, those less than 7 are acidic.
The lower the pH, the more acidic is the soil.
Soils in humid regions tend to be acidic, those in semi-arid and arid regions tend to be around neutral or are alkaline.
Acidification is a natural process.
Most commercial nitrogen fertilizers are acid forming, but many manures are not.
Crops have different pH needs pro-bably related to nutrient availability or sus-ceptibility to aluminum toxicity at low pH.
Organic acids on humus and aluminum on the CEC account for most of the acid in soils.

Management

Use limestone to raise soil pH (if magnesium is also low, use a high magnesium or dolomitic lime).
Mix lime thoroughly into the plow layer.
Spread lime well in advance of sensitive crops, if at all possible.
If the lime requirement is high some labs say greater than 2 tons and others say greater than 4 tons consider splitting the application over two years.
Reducing soil pH (making soil more acid) for acid-loving crops is done best with elemental sulfur (S).

Increasing the pH of acidic soils is usually accomplished by adding ground or crushed limestone. Three pieces of information are used to determine the amount of lime that's needed:

What is the soil pH? Knowing this and the needs of the crops you are growing tell whether lime is needed and what target pH you are shooting for. If the soil pH is much lower than the pH needs of the crop, you need to use lime. But the pH value doesn't tell you how much lime is needed.
What is the lime requirement needed to change the pH to the desired level? There are a number of different tests used by soil testing laboratories that estimate soil lime requirements. Most give the results in terms of tons/acre of agricultural grade limestone to reach the desired pH.
Is the limestone you use very different from the one assumed in the soil test report? The fineness and the amount of carbonate present govern the effectiveness of limestone how much it will raise the soil's pH. If the lime you will be using has an effective calcium carbonate equivalent that's very different from the one used as the base in the report, the amount applied may need to be adjusted upward (if the lime is very coarse or has a high level of impurities) or downward (if the lime is very fine and is high in magnesium and contains few impurities).

Soils with more clay and more organic matter need more lime to change their pH (see figure 18.2). Although organic matter buffers the soil against pH decreases, it also buffers against pH increases when you are trying to raise the pH with limestone. Most states recommend a soil pH around 6.8 only for the most sensitive crops, such as alfalfa, and about pH 6.2 to 6.5 for many of the clovers. As pointed out above, most of the commonly grown crops do well in the range of pH 6.0 to 7.5.

There are other liming materials in addition to limestone. One of the commonly used ones in some parts of the U.S. is wood ash. Ash from a modern air-tight wood burning stove may have a fairly high calcium carbonate content (80 percent or higher). However, ash that is mainly black indicating incompletely burned wood may have as little as 40 percent effective calcium carbonate equivalent. Lime-sludge from wastewater treatment plants and fly ash sources may be available in some locations. Normally, minor sources like the ones mentioned above are not locally available in sufficient quantities to put much of a dent in the lime needs of a region. Because they might carry unwanted contaminants to the farm, be sure that liming materials are thoroughly evaluated for metals and field-tested before you use any new byproduct liming sources.

"Overliming" Injury

Sometimes there are problems when soils are limed, especially if a very acidic soil has been quickly raised to high pH levels. Decreased crop growth because of "overliming" injury is usually associated with lowered availability of phosphorus, potassium, or boron, although zinc and copper deficiencies can be produced by liming acidic sandy soils. If there has been a long history of use of triazine herbicides, such as atrazine, liming may release these chemicals and kill sensitive crops.

Need to Lower the Soil's pH?

When growing plants that require low pH, you may want to add acidity to the soil. This is probably only economically possible for blueberries and is most easily done with elemental sulfur (S), which is converted into an acid by soil microorganisms over a few months. For the examples in figure 18.2, the amounts of S needed to drop the pH by one unit would be approximately 3/4 ton per acre for the silty clay loams, ½ ton per acre for the loams and silt loams, 600 lbs. per acre for the sandy loams, and 300 lbs. per acre for the sands. Sulfur should be applied the year before planting blueberries.
Alum (aluminum sulfate) may also be used to acidify soils. About six times more alum than elemental sulfur is needed to achieve the same pH change.

Special soil problems are found in arid and semi-arid regions, including soils that are high in salts, called saline soils, and those that have excessive sodium (Na+), called sodic soils. Sometimes these go together and the result is a saline-sodic soil. Saline soils usually have good soil tilth, but plants can't get the water they need because the high salt levels inhibit water-uptake. Sodic soils tend to have very poor physical structure because the high sodium levels cause clays to disperse, leading to the breaking apart of aggregates. As aggregates break down, these soils become difficult to work with and very unpleasant for plants. Wet sodic soils with an adequate amount of clay end up looking and behaving like chocolate pudding.

Saline soil. Electrical conductivity of a soil extract is greater than 4 ds/m, enough to harm sensitive crops.

Sodic soil. Sodium occupies more than 15 percent of the CEC. (Soil structure can significantly deteriorate in some soils at even lower levels of sodium.)

Saline and sodic soils are commonly found in the semi-arid and arid regions in the western U.S., with pockets of saline soils found near the coastline. Sometimes, the extra moisture accumulated during a fallow year in semi-arid regions causes field-seeps, which lead to the development of saline and sodic patches.

Before embarking on a program to improve saline or sodic soil, it is important to find out what is causing the problem. Do you have a high water table that contains salty water? Is there a saline or alkali "seep" in a portion of the field? Is it a generalized problem over the entire field without a high water table? If a high water table is causing salts to migrate upward to the root zone, installation of tile drainage may be necessary. Water surfacing in saline and alkali seeps usually can be reduced by changing from an alternate year fallow to a more intense annual cropping regime.

There are a number of ways to deal with saline soils that don't have shallow salty groundwater. One is to keep the soil continually moist. For example, by using drip irrigation with low salt water plus a surface mulch, the salt content will not get as high as it would if allowed to concentrate when the soil dries. Another is to grow crops or varieties of crops that are more tolerant of soil salinity. Saline-tolerant plants include barley, bermuda grass, oak, rosemary, and willow. However, the only way to get rid of the salt is to add sufficient water to wash it below the root zone. The amount of water needed to do this is related to the salt content of the irrigation water, expressed as electrical conductivity (ECw) and the salt content desired in the drainage water (ECdw). The amount of water needed can be calculated using the following equation:

Salts are Present in All Soils

Salts of calcium, magnesium, potassium and other cations along with the common negatively charged anions chloride, nitrate, sulfate, and phosphate are found in all soils. However, in soils in subhumid and humid climates with from an inch or two to well over 7 inches of water percolating beneath the root zone every year salts don't usually accumulate to the levels where they can be harmful to plants. Even when high rates of fertilizers are used, salts usually only become a problem when you place large amounts in direct contact with seeds or growing plants. Salt problems frequently occur in greenhouse potting mixes because growers regularly irrigate their greenhouse plants with water containing fertilizers and may not add enough water to leach the accumulating salts out of the pot.

The amount of extra irrigation water needed to leach salts is also related to the sensitivity of the plants that you're growing. For example, sensitive crops like onions and strawberries may have twice the leaching requirement as moderately sensitive broccoli or tomatoes. Drip irrigation uses relatively low amounts of water, so lack of leaching may cause salt build-up even for moderately saline irrigation sources. This means that the leaching may need to occur during the growing season, but care is needed to prevent leaching of nitrate below the root zone.

For sodic soils, a calcium source is added usually gypsum (calcium sulfate). The calcium replaces sodium held by the cation-exchange capacity. The soil is then irrigated so that the sodium can be leached deep in the soil. Because the calcium in gypsum easily replaces the sodium on the CEC, the amount of gypsum needed can be estimated as follows for every milliequilivent of sodium that needs to be replaced to 1 foot, about 2 tons of agricultural grade gypsum is needed per acre. Adding gypsum to non-sodic soils doesn't help physical properties if the soil is properly limed, except for those soils containing easily dispersible clay that are also low in organic matter.

Sources
Hanson, B.R., S.R. Grattan, and A. Fulton. 1993. Agricultural Salinity and Drainage. Publication 3375, Div. of Agriculture and Natural Resources, University of California, Oakland, CA.

Magdoff, F.R. and R.J. Bartlett. 1985. Soil pH buffering revisited. Soil Science Society of America Journal 49:145148.

Peech, M. 1961. Lime Requirement vs. Soil pH Curves for Soils of New York State. Mimeographed. Cornell University Agronomy Department, Ithaca, NY.

Pettygrove, G.S., S.R. Grattan, T.K. Hartz, L.E. Jackson, T.R. Lockhart, K.F. Schulbach, and R. Smith. 1998. Production Guide: Nitrogen and Water Management for Coastal Cool-Season Vegetables. Publication 21581, Division of Agriculture and Natural Resources, University of California, Oakland, CA.

Rehm, G. 1994. Soil Cation Ratios for Crop Production. North Central Regional Extension Publication 533. University of Minnesota Extension Service, St. Paul, MN.

Tisdale, S.L., W.I. Nelson, J.D. Beaton, and J.L. Havlin. 1993. Soil Fertility and Fertilizers. Macmillan Publishing Co., New York, NY.