Articles
RESPONSE OF LETTUCE PLANTS GROWN ON DIFFERENT SUBSTRATES TO PHOSPHORUS FERTIGATION
The objective of this work was to study the response of greenhouse lettuce to P concentration in irrigation water (Cp) and to substrate chemical properties.
Four levels of P (0, 10, 25 and 60 mg P/l) were tested in three growth media: inert rockwool, mildly P absorbing sand and strongly P retaining yellow tuff.
The response in canopy fresh weight (CFW) to P concentration in irrigation solution (Cp) followed an optimum type relationship.
At any Cp, CFW decreased in the order tuff>sand>rockwool.
The Cp which gave maximum CFW in all substrates was 25 mg/l.
At Cp=25 mg/l, the P content in the canopy was ≈ 0.8%, coinciding with maximum CFW.
The flux of P uptake (Fp) vs. the average P concentration in the inflowing and outflowing substrate solutions (CpA), for the three substrates fitted the Michaelis Menten equation.
The threshold CpA corresponding to saturation in Fp was ≈ 25 mg/l.
Concentrations of P, K, Ca, Mg and pH of leachates with and without plants were differently affected by Cp in the three substrates.
A probable mechanism behind this interaction between plant, substrate and P supply involves the exudation of H+ and organic anions by the roots, followed by the reaction of these exudates with the substrates and added nutrients.