Liming of Acid Soil: II—Effect on Plant Available Manganese and Iron

Treatments of 0, 1120, 2240, 4480, 8960, 17920 and 35840 kg lime/ha significantly reduced the water-soluble, CaCl₂—extractable and exchangeable manganese content of a soil of Qulin, Missouri, U.S.A. Maximum concentration of soil manganese occurred within pH_s range of 4 to 5, and it attained a very low level beyond pH_s6. At about p_Hs 4.3 manganese was the most sensitive to changes in pH. All the extractants were equal when the extracted manganese was correlated with plant manganese. In the untreated soil, manganese (23 ppm water-soluble) was in toxic concentration. Manganese toxicity symptoms were observed in cotton and the plant also contained 1959 ppm manganese, which was above the reported toxic concentration (1130 ppm) for cotton. The Fe: Mn ratios of 1.36, 1.4 and 0.8 in cotton, wheat and soybean respectively suggested the occurrence of manganese toxicity in these crops grown in the unlimed soil. Liming significantly reduced the plant manganese content of maize, cotton, wheat and soybean. Treatment of 4480 kg lime/ha raised the soil pH to 5.1 and reduced the toxic concentrations of soil and plant manganese to a safe level. Iron content of maize, cotton and wheat plants was not influenced by liming, but the variations in soybean were not consistent. Negative correlation existed between the plant manganese and iron.