Loss of Lime by Leaching and Cropping

Fortunately, scientists are discovering that the rate of loss of lime by leaching is much less than was formerly believed to be the case; some of us used to assume that the annual loss was 500 to 1,000 pounds an acre. Recent data obtained by R. S. Stauffer in Illinois and Victor J. Kilmer, Orville E. Hays, and Robert J. Muckenhirn in Wisconsin show that when silt loams in their area are kept in crops, very little soil water percolates beyond a depth of 4 feet and that very little leaching of lime beyond the root zone can therefore occur. Their data were obtained with lysimeters that allowed natural surface drainage.

In some of the older lysimeter experiments, surface drainage was not provided, and all the rainfall was forced to enter the soil. Under such conditions, the amount of drainage water causing loss of lime may be multiplied several times, because it is that excess of water beyond a certain amount that the soil can hold or is used by crops that eventually drains out and causes leaching. With an annual rainfall of about 30 to 35 inches, the excess or percolation under cropping of the better soils when surface runoff is allowed may amount to only 2 or 3 inches annually; without runoff, it may amount to 8 to 10 inches.

That lime is not lost by leaching as rapidly as we once believed is substantiated by a consideration of the amount found present in many virgin soils. In eastern Wisconsin, for example, there exists a red clay loam soil which either contains lime carbonate throughout the soil profile, or is, at the most, only slightly acid in the surface layer. Geologists estimate that the parent material of this soil (lacustrine clay with some calcium carbonate) was laid down by glacial action about 25,000 years ago. If the annual loss of lime over that period had averaged 500 pounds an acre, the total loss for the whole time would amount to about 3 feet of powdered limestone. Obviously, nothing like that has taken place, otherwise the soil to a considerable depth would now be decidedly acid. Undoubtedly the native vegetation, a mixture of hardwoods and conifers, markedly retarded the net loss of lime by providing annual returns from the deeper layers to the surface in the form of deposits of tree leaves. I think that under these conditions the annual loss of lime carbonate by leaching was not more than a hundred pounds to the acre.

Investigations by J. W. White and F. J. Holben, carried on in connection with the old soil fertility test plots on Hagerstown clay loam at the Pennsylvania Agricultural Experiment Station, give information bearing directly on the leaching of lime under cropping. From a plot which had received lime (CaO) at the rate of 35,200 pounds an acre in 40 years, they calculated from their soil analyses that 4,353 pounds of CaO were leached out of the surface soil layer (0-7 inches) during the period. On an annual basis, that amounts to 109 pounds of CaO, or 195 pounds of CaCO,,. Because the loss occurred in a soil that was kept very heavily limed (maintained at a pH well above 7), it is higher than would be the case under most farming conditions (soil at pH 6.5 or less) in a similar climate. Moreover, some of the lime that was removed from the surface Soil layer was retained by the soil below and so was not lost for soil improvement and crop use.

For the northern States of the humid region, an average annual removal by leaching and cropping of 200 pounds of calcium and magnesium carbonates an acre would seem to be a conservative estimate. In farming, as usually carried on, much of the lime removed by cropping can be returned and should be returned in the form of manure and crop residues. If that is done, the annual net removal would be less than 200 pounds in many cases. Even with an annual net removal of 200 pounds an acre, an application of a ton of lime to the acre will resupply what is removed in 10 years.

As one goes from north to south in the humid region of this country, the rainfall, temperature, and character of soil in many places become much more conducive to loss by leaching. Possibly the average net loss by cropping and leaching in the Southern States may be 25 to 50 percent higher than that estimated for the Northern States. A ton of lime an acre should, on the average, put back at least what is lost in the Southern States in a period of 7 or 8 years. It may be considerably longer in certain cases, depending on type of soil and cropping system.