CAUSES OF POOR GROWTH OF PLANTS ON ACID SOILS AND BENEFICIAL EFFECTS OF LIMING

Abstract

Two greenhouse expts. were conducted to determine the influence of the exchangeable Ca supply, soil acidity, Mn, Al, and Fe as factors affecting the growth of alfalfa on Mardin silt loam having a pH of 4.75 and containing 600 lbs. of exchangeable Ca/acre. When the exchangeable Ca content was reduced to 150 lbs./acre by diluting the soil with sand, the yield of alfalfa increased as much as 500%. The growth of alfalfa on a peat-sand culture at pH 4.8 containing 500 lbs. of exchangeable Ca was as good as that on the Mardin soil limed to pH 6.5 and containing 3800 lbs. of exchangeable Ca/acre. The poor growth of alfalfa on the unlimed soil was, therefore, not due to the low exchangeable Ca supply in the soil or to the low Ca content of the plants. Appln. of gypsum to the soil at varying levels of exchangeable Ca and at different pH values not only failed to increase the yield of alfalfa but caused a reduction in yield when the soil was high in exchangeable Mn. The Mn and Al concns. in the soil soln. were greatly increased by the appln. of gypsum to the soil. Symptoms of Mn toxicity appeared on the plants when the Ca-Mn ratio in the plants was less than 75. Mn toxicity was observed on all plants growing on the unlimed soil. The toxicity was reduced when the soil was diluted with sand and was increased when gypsum was applied to the soil. Al and Fe added to the soil as the hydrous oxides had little influence on plant growth, but when added as the sulfates, the yield of alfalfa was reduced. The amt. of readily soluble Al and Mn, as detd. by rapid micro-chemical soil tests, and the concn. of these 2 elements in the soil soln. were decreased by either liming the soil or diluting the soil with quartz sand. The beneficial effect of liming or diluting the soil with sand may be attributed to the decrease in concn. of Al and Mn in the soil soln.