7/2/2013 - By Dr. George Snyder

Dr. George H. Snyder, Milorganite Technical Consultant

SOIL REACTION (acidity or alkalinity)

The pH concept is used to numerically describe the acidity of aqueous systems, such as soils. The topic of pH has been widely discussed, so the basics will not be repeated herein. Most turfgrass managers know that pH values below 7 indicate acid soils, and values above 7 indicate alkaline soils. They also know that turfgrass textbooks routinely state that soil pH should be in the range of 6.0 to 6.8 for optimum turfgrass growth. However, the reason for stipulating this range, and the exceptions to it, are not always known.

ACID SOILS

Plants are largely unaffected by an aqueous environment more acidic than pH 6.0. In other words, it generally is not the acidity per se that has detrimental effects on plant growth. Instead, the problem comes with acid effects on some, but not on all soils. Metals are dissolved in acid soils. Some metals, such as aluminum, can be quite toxic to plant roots. Even iron and manganese can cause toxicities when present in sufficiently high concentrations. At soil pHs below about 5.5 such toxicities can occur. This can be a problem in many soils, particularly those with appreciable clay. In contrast, soils composed mostly of quartz sand, including the majority of Florida soils, certain coastal soils in the southeastern USA, and root zone media used to construct USGA greens, contain little aluminum and little to only moderate amounts of iron and manganese. Consequently, excellent turfgrass growth can occur in sand soils with pHs in the mid-four and five range, in spite of what textbooks say (Fig. 1). If plant nutrients such as calcium and magnesium are adequate, managers need not fret about pHs in the fives in sand soils.Calcium and magnesium may, however, be low in very acid soils. Liming with calcium carbonate (supplying calcium) or dolomite (supplying calcium and magnesium) supplies these nutrients in addition to increasing soil pH.

ALKALINE SOILS

High pH (i.e., substantially exceeding 7) is another matter. The same metals that are dissolved in acid soils are precipitated and largely unavailable to plants in soils with pH values exceeding about 7.5. Iron and manganese deficiencies are the most common in grasses grown at elevated soil pH, and many ornamental plants suffer these deficiencies as well. Susceptibility to these deficiencies, however, varies widely with species and cultivars. Susceptibility also can be significantly enhanced when growth is being pushed with high rates of nitrogen fertilization.

Chelates are chemicals that bond with metals and reduce their reactions with other elements. Some, but not all, chelates enhance micronutrient availability at elevated pH. EDTA, for example, which is one of the most commonly used chelates, usually provides little protection for the micronutrients at elevated pH. Calcium generally is abundant at elevated pH. EDTA has greater affinity for calcium than for iron or manganese. Consequently, an iron-EDTA applied to high-pH soil will release iron in exchange for calcium, leaving the iron unprotected from precipitation reactions. Milorganite® 6-2-0 is rich in organically-complexed iron which remains in a form available to plant roots at all expected soil pH levels. Plants known to be susceptible to iron deficiencies, such as ixoras, benefit from Milorganite fertilization (Fig. 2).

PHOSPHORUS AND pH

The availability of phosphorus also is reduced at elevated pH because of the formation of poorly-soluble calcium phosphates. At low pH, phosphorus forms insoluble precipitates with aluminum and iron when these elements are abundant. However, in sand soils, where iron and aluminum likely are not abundant, phosphorus solubility can be great at low pH. While this bodes well for plant availability, phosphorus leaching also is enhanced.

SOIL TESTING AND pH

Due to the effect of high pH on soil phosphorus solubility, certain soil-test procedures are preferred over others for phosphorus analyses. Many soil-test procedures use acidic solutions to extract plant nutrients as a method of estimating nutrient availability. When used for high-pH soils, the acid extraction procedures can dissolve phosphorus that is not plant available at the actual soil pH. The Olsen, or sodium carbonate, method has been developed for determining phosphorus availability in high pH soils.

MICROBIAL ACTIVITY AND pH

pH affects other chemical and biochemical reactions in soils too, but the importance of these reactions for turfgrass management usually is not as great as the effect on nutrient availability. Microbial activity, particularly that of bacteria, generally is reduced in acid soils. Consequently nitrogen release from organic matter is reduced in low pH soils. However, nitrogen supply from this source (organic matter) is relatively small compared to normal rates of turfgrass nitrogen fertilization. The rate of thatch decomposition is reduced in acid soils as well, but thatch usually is controlled successfully only by mechanical cultivation means. On the other hand, certain diseases, such as ‘take all patch’ have been associated with high soil pH.

ADJUSTING SOIL pH

To increase the pH of acid soils, limestone (either calcitic or dolomitic) routinely is incorporated into the soil. However, for established turfgrass, these materials generally are surface applied. Consequently, pH may be increased at the soil surface, but is largely unaffected at rooting depth. Cultivation methods, such as core aerification and refilling, which vertically mix the soil will assist in distributing lime throughout the root zone.

Soil pH can be reduced by the application of sulfur or ammonium sulfate, but for very different reasons. Upon the application of sulfur, microorganisms create sulfuric acid (H2SO4) when they oxidize the sulfur (S) to the sulfate (SO42-) form, which reduces soil pH. Soil pH reduction by the use of ammonium sulfate {(NH4)2SO4} does not involve sulfur oxidation, since the sulfur already is in the sulfate form. Instead, the oxidation of the ammonium (NH4+) nitrogen to nitrate (NO3-) nitrogen that creates nitric acid (HNO3), which reduces soil pH. Likewise, applications of potassium sulfate (0-0-50; K2SO4) and magnesium sulfate (MgSO4) do not reduce soil pH because the sulfur is in the oxidized form (SO4-2). Applications of iron sulfate, manganese sulfate, and even aluminum sulfate can, on the other hand, decrease soil pH not because of sulfur reactions, but because iron, manganese, and aluminum absorb and inactivate hydroxyl ions (OH-), which are responsible for high pH. However, in most cases, the rates of iron sulfate and manganese sulfate used to correct nutrient deficiencies will not be great enough to significantly affect soil pH. The use of aluminum sulfate for pH correction is risky. If the pH is reduced too much, aluminum toxicities can result from the addition of aluminum to the soil. It generally is impractical to lower the pH of bulk samples of soils which contain shell and limestone fragments. These materials are composed of carbonates (calcium carbonate, magnesium carbonate), which neutralize most of the acidity in acidifying products. However, localized regions of reduced pH may be achieved temporarily, and micronutrient availability can be improved in these regions.

The Milorganite Log Book and Reference Guide, which can be obtained from Milorganite distributors, contains guidelines for the amount of limestone needed to raise soil pH and the amount of sulfur needed to lower pH. Limestone recommendations may be included in some soil test analyses (see next section).

TYPES OF pH ANALYSES

Soil pH is measured in a water slurry. Some laboratories also perform and report a “buffer pH”. The buffer pH is quite different from soil pH, and incorrect recommendations will result when the two are confused. The buffer pH is a tool for estimating the amount of limestone that is required to increase soil pH. For this measurement, acid soil is added to a high-pH buffer. The pH of the mixture is measured. The quantity of limestone required to achieve a target soil pH is estimated from the reduction in pH of the combination relative to the initial pH of the buffer alone. The pH of the combination generally is greater than 7. Upon seeing these results, i.e., pH values exceeding 7.0, advisors have made recommendations for sulfur additions to reduce soil pH. Of course, the actual soil pH already indicated that the soil was acid, which was the rational for performing the buffer pH test. The recommended sulfur additions would only exasperate the low-pH situation! Certain laboratories also perform a “salt pH”, which is designed to predict the pH of a soil after fertilizer has been applied. This test was designed for agricultural soils depleated of salts during the winter thaw, and generally is inappropriate for turfgrass.

SUMMARY

In summary, soil pH is arguably the most useful soil analyses that will be performed in a routine chemical soil test. It provides an insight into nutrient availability and is useful for predicting and correcting nutrient deficiencies. However, the ramifications of soil pH are subject to misinterpretation. The turf manager must understand the properties of the soils they are dealing with, and not be confused by the type of pH analysis that was performed.