Soil Reaction and Liming

K, Lawton and L. T. Kurtz.

A farmer in the Midwest who plans to grow alfalfa probably will have his soil tested because he knows that alfalfa grows poorly or not at all in strongly acid soils.

But when he selects ornamentals to landscape his home he might find the soil not acid enough to grow acid-loving plants like azaleas.

He knows generally that most of the common field crops grow well in slightly acid soils: When he limes to grow alfalfa he will have favorable soil reaction for corn and soybeans.

But many soils of humid areas naturally are too acid for the maximum production of many field crops. Many crops and plants grow best only if the soil reaction is suitable. Adjustment of soil acidity to the proper level often is important in good management.

Liming to reduce soil acidity is an extensive and routine practice in most of the eastern half of the United States. Soil acidity develops gradually in humid regions as the calcium and magnesium are slowly lost from the soil by leaching and is speeded by crop removals and by use of the soil.

Soils in the western Great Plains and in dry regions usually are neutral or slightly alkaline, rather than acid. Liming there is seldom necessary and often is harmful. The natural high-lime condition in some places contributes to nutritional disorder and poor production of some crops. Lime-induced chlorosis of fruit trees is an example.

Location is not a sure indication that a soil needs lime. Although soils of the Eastern States generally need liming, the degree of acidity varies, and scattered areas of naturally calcareous soils neutral to moderately alkaline occur from Florida to Iowa. They may be a result of calcareous soil material or perhaps are Shelly-spots associated with ancient lakes or marshes.

Many soils naturally are extremely acid, and only poor yields of field crops can be obtained on them even if fertility is adequate. They need moderate liming to correct soil acidity for maximum yields of corn and small grains.

Additional liming to a slightly acid or neutral range is necessary for maximum yields of other crops, particularly alfalfa and some other legumes.

Besides their value as crops, legumes are associated with nodule bacteria (rhizobia) and so can supply much of the nitrogen needed by following crops. Liming then influences the legume crops and also nitrogen fertility.

Before nitrogen fertilizers were readily available on the market, legumes were necessary in almost all systems of farming. Fixation through legumes was the one way to add nitrogen to the soil. Liming to encourage good growth of legumes used to receive more attention than now.

Availability of nitrogen from sources other than legumes should not detract very much from emphasis on the need for lime. Liming has been recommended for some sections on a fairly scientific basis for 50 years, but half of the acreage in these areas would still benefit from liming.

BENEFITS FROM LIMING result from more than mere reduction of soil acidity. Two major plant nutrients, calcium and magnesium, are supplied by liming materials. Soils that have too little calcium and magnesium for best plant growth often are highly acid. Liming may supply calcium and magnesium and correct the soil acidity at the same time.

Calcium and magnesium are removed in crops in large amounts, and those removals, along with leaching, contribute to the gradual development of acidity, a normal soil process in humid areas.

Finely ground limestone is the liming material used in many sections. Calcium carbonate is the active agent in limestone for reducing soil acidity. The calcium carbonate reacts with and neutralizes the soil acidity. The calcium then becomes part of the soil supply that can be utilized by plants.

Dolomitic limestone contains magnesium carbonate and calcium carbonate and therefore supplies magnesium, another nutrient. Even some of the high-calcium limestones contain significant percentages of magnesium.

Basic slag, a byproduct of the steel industry, is an important liming material in some regions, particularly in the Southeast. Calcium silicate is the active ingredient in the neutralization.

Marl and chalk are soft, impure forms of limestone and are sometimes liming materials.

Oyster shells, in which the neutralizing agent is calcium carbonate, also are used.

Hydrated lime and burned lime, which contain calcium hydroxide and calcium oxide, are effective but usually are more expensive and are not applied very extensively. They supply little magnesium.

OTHER BENEFITS, besides neutralizing soil acidity and supplying nutrients, may result from liming.

Regulation of soil acidity is a means of controlling some crop diseases. An example is potato scab, which is less severe in acid soils.

Liming influences the solubility of many compounds in the soil. Large amounts of iron, aluminum, and manganese may come into solution in a strongly acid soil and may be adsorbed on the surface of the soil particle in a form that plants can easily take up. Sometimes high levels of easily soluble manganese and aluminum are believed to be toxic to crops a condition that liming can correct.

Liming influences the form of phosphorus in the soil. Phosphates are believed to react in highly acid soils with the active iron and aluminum to form complex substances. Calcium is the dominant ion on the surface of the soil particle in properly limed soils, and the phosphates apparently can be utilized more readily. In alkaline soils, phosphates react with the surface of calcium carbonate particles and crops utilize them less readily.

1. Losses of calcium and magnesium vary with climate, soil permeability, soil acidity, fertility, and cropping practices. The figures here were obtained by R. S. Stauffer, of the University of Illinois, from uncropped lysimeters containing undisturbed soil profiles.

Myriad bacteria, fungi, and other kinds of micro-organisms abound in fertile soils. Some of the organisms are active in the decay of crop residues and manures in the soil. Decay processes release some of the nitrogen, phosphorus, and other mineral nutrients from these residues for subsequent crops. The activity of the micro-organisms and consequently the phosphorus and nitrogen fertility generally increase when an acid soil is limed.

The solubilities of iron, manganese, phosphorus, copper, zinc, and boron generally are less in alkaline soils than in slightly acid soils. But molybdenum is more soluble in alkaline soils than in acid soils, and liming may result in increased uptake of this micronutrient by crops. Some concern that liming may cause a deficiency of some nutrients or a toxic excess of molybdenum is probably warranted for a few soils.

Liming to adjust the soil to a slightly acid or near-neutral reaction will not induce deficiencies of phosphorus or the micronutrients in most soils and cropping situations. Underliming is far more common than overliming. If a deficiency of boron or phosphorus is induced by recommended applications of lime materials, the soils probably were already near the borderline and would soon have needed phosphate or borax fertilizers anyway.

Excessive liming should be avoided, of course. Overliming is less likely on silt or clay soils than on sandy soils. Medium-textured and heavy-textured soils of the middle States have a high capacity for lime. Sandy soils and highly weathered soils have low capacities and are more easily overlimed. Soil organic matter also has a high capacity for lime.

Frequency of liming varies with climate, soil, and cropping practices. Soils of heavy texture may require re-liming only once in 10 years. Applications on other soils should be made oftener. The need for reliming like the need for initial liming can best be determined by testing the soil.