Residual Effects of Calcium Silicate on the Movement and Availability of Nutrients in Tropical Soils

The residual effects of calcium silicate on plant uptake and movement of nutrients in a Gibbsihumox were studied in a series of three experiments. First, kikuyu grass and desmodium were grown in the field to determine the magnitude of response to residual Si applied several years earlier at various P and pH levels; second, the partial recovery of applied Si during five years of cropping was determined at 3 pH levels; and third, uptake of residual Si by rice grown on soil collected from the field experiment at the end of five years was studied in a growth chamber. A study of soil A1 extraction methods in relation to plant A1 uptake and yield was also conducted on some Hawaiian soils. The combined yields of seven harvests of kikuyu grass and desmodium increased significantly with increasing residual P levels, but was not significantly affected by residual Si or soil pH. The relative yield differential between the three Si treatments decreased sharply with time and at the end of 56 months yield from 1660 Si was only 2.5% higher than that without Si, whereas yield from 830 Si was less than the yield without Si. The decline with time in relative yield response to P applications, on the other hand, was small indicating a continued efficiency of P applied 56 months earlier in increasing yields on a Gibbsihumox. However, it should be pointed out that two supplemental additions of P were made 21 and 27 months after the initial treatments. Although residual Si produced highly significant increases in water-extractable soil Si, the levels at the end of the experiment were low which suggests that supplemental amounts of calcium silicate may be required to maintain yield response to Si. Modified Truog-extractable P was significantly higher at 1660 Si than at zero Si five years after Si applications. Phosphorus requirements of a Gibbsihumox were decreased significantly by residual Si and P in samples collected after 5 years of cropping. However, residual P was about 7 times more effective than residual Si in reducing P requirements. Multiple regression analysis indicated that in addition to the initially applied treatments, soil P and A1 and plant P, K, Mg, and possibly also Mn, Zn, Al, and Ca were important to plant growth in both species. Plant uptake by the sugarcane plant and ratoon crops, corn, and seven harvests of kikuyu grass accounted for 12 to 21 percent of the applied Si while exhaustive extraction of profile samples taken at the end of five years with 0.1N acetic acid, adjusted to pH 3*5 and containing 50 ppm P, recovered 1^ to 28 percent of the applied Si. There was no evidence that applied Si moved below 30 cm. Total recovery of applied Si ranged from 28 to k j percent which means that 57 to 72 percent of the applied Si remained in the soil in some form not readily displaced by phosphate solution. Rice plants extracted proportionately more native Si than did phosphate solution which resulted in comparable amounts of added Si being recovered by the two methods. Silicon uptake by rice accounted for 8 to 30 percent of applied Si and exhaustive phosphate extraction recovered 13 to 23 percent of applied Si. The amount of A1 extracted by various solutions decreased in the order of IN ammonium acetate + 0.2N barium chloride, pH *»-.8>lN ammonium acetate, pH ^ . 8 ^ 1 N barium chloride >. IN potassium chloride^ water ^.O.OIM calcium chloride. Aluminum extracted from five soils was in the order1 Akaka (Typic Hydrandept)> Halii (Typic Gibbsihumox)> Wahiawa (Tropeptic Eutrustox)> Lualualei (Typic Chromustert) = Kawaihae (Ustollic Camborthid) which was related to the degree of weathering and the amount of rainfall affecting the soil. Soil A1 extracted with unbuffered solutions, especially IN potassium chloride, was more closely related to plant A1 whereas A1 extracted with buffered solutions, especially IN ammonium acetate, pH ^.8 , was more closely related to plant yield than that extracted with other methods. However, R values for the soil Al-yield relationships were generally lower than those for soil Al-plant A1 relationships suggesting that while soil A1 has a strong influence on plant Al, it has considerably less effect on yield