Effect of banded fertilizers on soil solution composition and short-term root-growth .1. Ammonium-sulfate, ammonium-nitrate, potassium-nitrate and calcium nitrate

Abstract

To simulate the effects of banded fertilizer on soil solution composition, layers of KNO3, NH4NO3, Ca(NO3)2.4H2O or (NH4)2SO4 were placed in contact with soil columns and allowed to equilibrate for 5 days at a moisture content equivalent to 10 kPa matric suction. Five soils were used in the columns: 0-10 cm samples from a Kurosol, a Ferrosol, a Vertosol and a Kandosol, and a 50-60 cm sample from the Kandosol. After the contact period, soil sections were recovered at successive 5 mm increments from the fertilizer layer, the last section being 45-50 mm from the layer. Soybean seedlings (Glycine max (L.) Merr. cv. Forrest) were grown for 48 h in each section and relative root elongation (RRE) was determined. Soil solution was then extracted from each section and analysed. Nitrate salts moved 50 mm in all soils (measured as a change in the electrical conductivity of the soil solution), but the movement of the sulfate salt was dependent on soil type and was least in the Ferrosol. As distance from the fertilizer band decreased, concentrations of cations in the soil solution increased, often reaching a plateau value. In all soils, solution Ca concentrations near the (NH4)2SO4 fertilizer band were lower than those further away, indicating precipitation of CaSO4.2H2O had occurred. This effect was particularly evident in the Vertosol and Ferrosol. The relationship between electrical conductivity of the soil solution (ECss) and Ca activity ratio (CAR) was dependent on soil type and fertilizer source. For the sulfate source, all soils exhibited a common curvilinear relationship in which CAR decreased with increasing ECss. However, for the nitrate salts, the Ferrosol and Vertosol had a higher CAR for a given ECss than the other soils because of the higher Ca2+ activities in their soil solutions. Relative root elongation decreased curvilinearly with increasing ECss, and a common curve was obtained for all soils and fertilizer sources. Although CAR decreased with increasing ECss, there was no common curve relating RRE to CAR for all soils and fertilizer sources. Osmotic effects, and not a salt-induced Ca deficiency, were therefore considered responsible for the decrease in RRE as distance from the fertilizer source decreased. A 10% reduction in RRE occurred at an ECss of 4.1 mS cm-1.