THE EFFECTS OF PLACING SMALL AMOUNTS OF PHOSPHATE FERTILIZER CLOSE TO THE SEED ON GROWTH AND NUTRIENT CONCENTRATIONS OF LETTUCE

Two glasshouse experiments are described in which the effects of applying starter phosphate fertilizer, 1 cm beneath the seeds, on early growth and nutrient concentrations of lettuce in well fertilized soil were determined. In Experiment 1, various rates of starter P, in the form of NH4H2PO4, were applied to soil containing a range of rates of incorporated triple superphosphate. Although there was little response of lettuce dry weight to the incorporated triple superphosphate, there was a large response (.apprx. 65% increase after 36 days) to the starter. N and P concentrations within the plants were increased by the starter treatments, whereas K concentration was reduced. The per cent P in the plants at 36 days from sowing could account for 60% of the variation in plant dry weight. In Experiment 2, the starter P was added as either the Ca, Na or K salt, with or without added (NH4)2SO4. Adding the starter P without NH4+ increased the P concentration of the plants by an average of 12% and the dry weight by an average of 39% at 30 days from sowing. The addition of NH4+ increased plant concentrations of P, Mg and N, but decreased plant K concentration. The effect of NH4+ on growth depended on the form of phosphate supplied as the starter. This variation in effect of NH4+ was attributed to the effects of other starter ions on the relative concentration of NH4+ in the soil solution.