The comparative effects of lime and gypsum upon plants grown on acid soils

In order to ascertain whether calcium deficiency is the primary cause of poor plant growth on acid soils, a greenhouse experiment was set up to study the comparative effects of lime, applied as calcium hydroxide, and gypsum on the growth and composition of barley, perennial ryegrass, and alfalfa. Other possible causes of poor plant growth in acid soils were examined in light of the results obtained in this study. The plants grown on limed soils absorbed much more calcium and gave much higher yields than those grown on gypsum-treated soils despite the higher concentration of calcium in soil solution in the gypsum-treated soils. For some unknown reason the plants were unable to absorb calcium from the acid soil even when supplied with an adequate amount of soluble calcium salt. There is good indication that manganese, which was absorbed readily by all plants, may have prevented the uptake of calcium. Indeed, an application of 200 pounds of Mn, as MnCO3, per 2,000,000 pounds of soil to an acid soil virtually prevented the growth of alfalfa. This amount of manganese is not much in excess of the amounts usually found in acid soils in exchangeable and water-soluble forms. Liming reduced the manganese content of all the three crops investigated and decreased the aluminum content of alfalfa. An application of 50 pounds of Fe, as FeSO4, per 2,000,000 pounds of soil appreciably reduced the yield of perennial ryegrass without affecting the iron content of the plants. The amounts of iron and phosphorus in the plants were relatively unaffected by any of the treatments. The results of this study would indicate that the poor growth of plants on acid soils is not necessarily due to the lack of an adequate supply of calcium, but that the response of crops to lime on acid soils is quite complex, involving several contributing factors such as toxicity of manganese, iron, and aluminum, the relative significance of which probably varies with different crops and soils.