EFFECT OF BANDED FERTILIZERS ON SOIL SOLUTION COMPOSITION AND SHORT-TERM ROOT-GROWTH .3. MONOCALCIUM PHOSPHATE WITH AND WITHOUT GYPSUM

A layer of Ca(H2PO4)(2).H2O (MCP) or MCP plus CaSO4.2H(2)O was spread over duplicate columns of six soils to simulate the effects of banded MCP or superphosphate (MCP plus CaSO4.2H(2)O) on soil solution composi tion. A separate column was set up without fertilizer addition for eac h soil to act as a control (background) treatment. The soils used were 0-10 cm samples from two Kurosols, a Ferrosol, a Vertosol, a Kandosol , and a 50-60 cm sample from the Kandosol. Prior to fertilizer additio n, the columns were wet up to the water content at a matric suction of 10 kPa. Following 5 days of fertilizer-soil contact, soil sections we re recovered at 5 mm increments from the fertilizer layer to a distanc e of 50 mm. Soybean (Glycine maa: (L.) Merr.) seedlings were grown for 48 h in each section and relative root elongation (RRE) was determine d. Soil solution was then extracted from each section and analysed. Th e distance of phosphorus (P) movement from both MCP and MCP plus CaSO4 .2H(2)O was better correlated with P buffer capacity determined at a s olution P concentration of 3.2 mu M than at 320 mu M. This suggests th at the precipitation reactions which occur at the fertilizer site when MCP dissolves are independent, of the soil, and it is only in soil se ctions further removed from the fertilizer source (i.e. with lower soi l solution P concentrations) that the P sorption properties of the soi l become important in determining the extent of P movement. The amount of inorganic P (P-i) in the soil solution was summed over all soil se ctions for each fertilizer source, and was correlated with citrate-dit hionite extractable Fe and Al using step-up regression techniques. Cit rate-dithionite extractable Fe was highly correlated with P-i (r = -0. 937, P < 0.001), and the addition of citrate-dithionite extractable Al did not significantly (P = 0-05) increase the variation accounted for . RRE decreased in proximity to the fertilizer. When RRE was plotted a gainst the electrical conductivity of the soil solution, all data poin ts fell below the regression line previously obtained for various salt s (Moody et al. Aust. J. Soil Res. 1995, 33, 673-87), indicating that the reduction in RRE was not due solely to osmotic effects. Multiple r egression analysis indicated that a combination of the activities of A l3+ (a(Al)) and Mn2+ (a(Mn)) explained 83% of the variation in RRE whe n both fertilizer sources were considered in all soils except the Kuro sols. There was evidence of organic complexing of soil solution Al in the two Kurosols and so an accurate estimate of Al3+ activity could no t be made. For the soils other than the Kurosols, separate regressions of RRE against ant and a(Mn) indicated a 10% reduction in RRE set act ivities of 1.9 and 70 mu M, respectively. Based on these activities, b anding of MCP and MCP plus CaSO4.2H(2)O caused Al toxicity in all soil s, and Mn toxicity in all soils except one of the Kurosols. Manganese toxicity occurred further from the fertilizer band than Al toxicity in the Ferrosol and the Kandosol. The dual occurrence of Al and Mn toxic ities indicates that both factors need to be considered simultaneously when determining the effects of banded fertilizer on RRE.