CHAPTER 10

Purpose of Liming Soil An Enigma

MORE THAN A QUARTER of a century ago, 400 samples of 21 different soil types of Missouri tested for exchangeable calcium showed a close relationship of the amounts of this insoluble but available plant nutrient to the soil’s capacity to give good crops yields. That was also the time when limestone (calcium carbonate, and calcium magnesium carbonate) was applied extensively for the purpose of reducing soil acidity, or raising the numerical value of the pH i.e. the degree of acidity of the soil. The pH value as a soil test report was easily obtained by special laboratory equipment. But measuring the exchangeable calcium (or magnesium) was a tedious manipulation. However, the latter related the soil to potential crop production, while the pH was a technical expression, not so readily connected with the nutrition of the crop plants.

At that early date, the evaluation of the soil types with their arrangement in order of decreasing production of crops and livestock and their similar classification as an arrangement into six groups, A, B, C, D, E, F—by the soil surveyors—showed a close agreement with their decreasing amounts of exchangeable calcium by soil test as set forth in the accompanying table. The more productive soils supplied calcium in this exchangeable form of plant roots in quantities three times as great as did the poorer soils. The total quantities so active were large enough, and the differences between soils were great enough, for an accuracy of measurement which left no doubt about it.

Measurements of the clay-humus in the soil made possible the evaluation of the degree of saturation of that part of the soil from which the calcium (and magnesium) are exchanged. Thus, soil testing for calcium moved itself more closely toward indicating potential nutrition of plants, especially the legumes as producers of proteins. The term “pH” expresses the concentration of hydrogen as an active ion, separated out of any compound. Like the calcium and magnesium, it too is a positively-charged ion. Hence, any of these three can exchange for any other on the clay-humus colloid. Unfortunately, the hydrogen is not a plant nutrient coming from that source, while calcium and magnesium are. So when the hydrogen from cropping, weathering, leaching, etc. replaces the calcium and magnesium, we say the soil becomes “sour” and the crop fails. But the real cause of legume failures is the “going out” of the nutrients, calcium, magnesium, potassium, etc., the presence of which makes the soil non-acid, and not by the “coming in” of the hydrogen by which it is made sour or acid.

 

Soils Arranged in Order of Decreasing Productivity Shows a Closely Similar Order for Their Amounts of Exchangeable Calcium

Groups Soil Types Exchangeable Calcium, milligram equivalents per 100 gms. soil
A Marshall 14.80
Summit 12.64
Grundy 12.71
B Knox 11.32
Pettis 10.86
Crawford 10.68
Chariton 10.21
Eldon   8.99
Oswego   9.19
C Putnam   8.89
Bates   7.86
Memphis   6.22
Hagerstown   5.91
Lindley   8.64
Union   6.64
D Cherokee   5.57
Baxter   5.57
Gerald   3.82
Boone   7.59
E Lebanon   5.51
  Clarksville   4.29

Mo. Agr. Expt. Sta. Bul. 387., pg. 85., 1937. Report for year ending June 30, 1936.

When those two exchanges or soil phenomena occurred simultaneously with the legume failure the soil’s increased acidity was quickly recognized. We made the error of considering the soil acidity, the hydrogen concentration, the cause of crop failure when it was the fertility deficiency of calcium (and magnesium), or the failing plant nutrition, that was responsible. The validity of this conclusion was experimentally established by preparing a purely hydrogen-saturated clay and then separating lots of it as a series of increasing degrees of saturation by calcium. Half of these were prepared to give a series of increasing calcium saturation with reciprocally decreasing saturation by hydrogen or with decreasing degrees of acidity.

The other half of the lots was a series given barium to just replace the decreasing hydrogen or to approach the neutral condition for all of the lot. This gave increasing saturation by calcium and reciprocally decreasing saturation by the non-nutrient barium. These clays were added to sand in such amounts as to supply constant totals of exchangeable calcium per pot or sand-clay combination and were planted to soybeans. They resulted in literally duplicate series of crop growths according to the differing saturation by calcium that resulted, irrespective of the presence or absence of the acidity as shown in the accompanying illustration. Thus the enigma of the purpose of applying lime was solved long ago when such experiments told us that the benefit from liming the humid soil comes from its nourishment of the crop by calcium, and not from its reduction of the soil acidity by the carbonate accompanying the calcium in the limestone. Seemingly this truth is still much hidden in unopened reports.