The effects of placing small amounts of phosphate fertilizer close to the seed on growth and nutrient concentrations of lettuce

Abstract

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 (Lactuca sativa L. cv. Avondefiance) 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 (about 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 ammonum 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 ammonium ions increased plant concentrations of P, Mg and N but decreased plant K concentration. The effect of the ammonium ions on growth depended on the form of phosphate supplied as the ‘starter’. This variation in effect of ammonium ions was attributed to the effects of other ‘starter’ ions on the relative concentration of ammonium in the soil solution.