The Liming of Soils

by EMIL TRUOG

ABOUT three-fourths of the cultivated land in our humid regions needs lime. What would happen if farmers stopped applying it? Just this : Yields on lands where more than half of our agricultural crops are now produced would start to go down. For a few years we would not notice the decline very much. In a decade it would become quite pronounced. In 30 years or so the yields might be cut in half ; in 50 years the result would certainly be hunger and disaster. And not only would harvests drop; control of soil erosion would become increasingly difficult.

In recent years American farmers have become well aware of the need for liming: In 1944 they used nearly 25 million tons of agricultural lime, four times the amount applied just 8 years earlier and seven times more than in 1929. Calculations by C. E. Carter of the Department indicate that about 40 million tons of lime annually should be used for the next decade on our arable and pasture lands. That a usage approaching that figure may be attained in the near future seems quite probable.

Extensive educational work on liming by the State and Federal agencies, including hundreds of thousands of tests for acidity and numerous field demonstrations of results, has brought about a general and thorough realization among farmers of the widespread need for lime and its benefits. Because liming of acid soils has been shown to have a favorable effect as regards fertility on the physical, chemical, and biological properties of soils, farmers now appreciate, as never before, why liming of land is often referred to as the backbone of profitable and permanent agriculture in humid regions.

Occasionally the application of lime has reduced crop yields, and some farmers naturally doubted its value. Now we know why: For the most part, it turns out to be a lowered availability of boron and manganese. Sometimes the application of too much lime, especially to sandy soils that are low in active organic matter, makes the boron and manganese in the soil less available. In this country, James A. Naftel, of Alabama, was the first to demonstrate the relationship of overliming to a deficiency of available boron. His early investigations, made with a number of crops, notably cotton grown on Norfolk loamy sand, emphasize the great importance of carefully regulating the amount of lime added in accordance with needs as indicated by soil tests.

Boron is a nutrient element required in small amounts a ton of alfalfa hay may contain about an ounce by all crops for normal growth. The amount present in fertile soils in available form (that is, extractable with hot water) is usually not more than 1 to 5 pounds per acre plow layer. In some soils, particularly those that are low in organic matter and have been severely leached and exhaustively cropped, as is the case with many sandy soils of the South and Southeast, the content of available boron is so low that a slight reduction in availability, such as may be effected by liming, greatly reduces crop yields. The application of 25 to 50 pounds of borax to the acre remedies the condition, so that the lime needed to grow satisfactory crops of alfalfa, clover, and other crops may be added.

The relation of the availability of manganese to liming is much like that of boron. It has been known for 20 years or more that high pH (alkalinity) or heavy liming sometimes induces a deficiency of available manganese, but not until recent years was the frequency of the occurrence established by means of controlled experiments. Plants need but little manganese a ton of alfalfa hay usually contains about 1 pound but that amount is greater relatively than the requirement of boron.

The supply of available manganese in sands and loams that are low in organic matter and in certain feats that have little manganese-containing minerals is frequently too small for crop needs when their pH rises above 7 and more important when they are calcareous. The explanation for this is now quite clear. As long as the soil is acid (that is, the pH is below 7), a considerable part of the manganese tends to be in the divalent (reduced or manganous) form. In that form it acts like calcium or magnesium, particularly as an exchangeable cation, and is readily available because it is brought into solution as a bicarbonate through the action of the ever-present carbonic acid. As the pH rises to 7 and higher, there is a greater tendency toward the oxidation of divalent manganese to the tetravalent form by the oxygen dissolved in the soil solution, in accordance with the following reaction: 2Mn(OH)2+02= 2MnO2+2H2O. Tetravalent manganese in the form of manganese dioxide (MnO2) is insoluble in carbonic acid, and is, therefore, not readily available for crop use.

Because oxygen is required in the reaction for the formation of the highly insoluble manganese dioxide, we would expect that the excessive aeration of the soil would also favor the formation. That is the case. Thus, a combination of high pH and excessive aeration is the condition under which a lack of available manganese most frequently occurs. Because sands and looms that are low in organic matter and water-holding capacity are often over-aerated, it is these kinds of soils that are most subject to a lack of available manganese when the pH is high. Some peaty and very sandy soils are so low in total manganese and so severely leached that even when they are strongly acid they lack an adequate supply of this element in available form.

On the other hand, the heavier soils and those containing considerable organic matter hold much more water and do not easily become over-aerated; in fact, a lack of aeration often occurs in the heavier soils, especially during periods of heavy rainfall. Such soils are usually also better supplied with organic matter, which, on decomposition, releases its manganese in available form and favors the transformation of manganese minerals to the available type. A lack of available manganese at high pH, therefore, occurs much less frequently in the heavier soils than in the lighter ones. In either case the lack is usually remedied by an application of 25 to 100 pounds of manganese sulfate to the acre.

The discovery of the relationship between liming and the availability of boron and manganese and the remedy is one of the reasons for the recent rapid increase in the use of lime in some sections of the South and Southeast where the soils are generally very acid. This greater use of lime is a great boon there; it makes possible the production of good crops of the more valuable legumes, which, in turn, add badly needed nitrogen and organic matter to the soils.

It is of interest to note what Edmund Ruffin wrote about reduced yields that occasionally follow the application of lime (marl) in excessive amounts to acid soils: "There are many practices universally admitted to be beneficial yet there are none, which are not found sometimes useless, or hurtful, on account of some other attendant circumstance, which was not expected, and perhaps not discovered. Every application of calcareous earth to soil is a chemical operation on a great scale; decompositions and new combinations are produced, and in a manner generally conforming to the operators' expectations. But other and unknown agents may sometimes have a share in the process, and thus cause unlooked-for results. Such differences between practice and theory have sometimes occurred in my use of calcareous manures (as may be observed in some of the reported experiments) but they have neither been frequent, uniform, nor important."

Possibly the reduced yields of which Ruffin wrote more than a hundred years ago were caused by a lack of available boron or manganese, or both; prophetically, his words carry the inference that the cause of the reduced yields would some day be elucidated so that proper remedial measures could be taken. He found that sandiness and paucity of organic matter accentuate the overliming injury, that clover was less affected than other crops, and that its growth with attendant addition of organic matter would in time overcome the unfavorable condition for other crops. These observations are in accord with ours today. Possibly the reason that legumes that persist for more than one season, like red clover and alfalfa, are less affected by overliming than certain annuals is because by the second year their well-established root system are feeding to advantage for nutrients, such as manganese, below the limed layer. Also, by feeding throughout the growing season, they are able to take advantage of more favorable periods when there is abundant moisture.