B. Bar-Yosef,
P. Imas

Apart from its direct role in plant development, P nutrition has indirect effects on plant growth and yield by influencing the uptake rates of other nutrients and by modifying the ionic environment in the rhizosphere. The objective of this work was to study nutrient uptake and dry matter production by greenhouse tomatoes (cv. F-144), in relation to P concentrations in irrigation water (Cp) and substrate chemical properties.

Four P treatments (0, 10, 30 and 60 mg P/l in the irrigation water) were tested in combination with three growth substrates: inert rockwool, mildly P adsorbing sand and strongly P retaining yellow tuff.

Leachate pH was higher in the presence than in the absence of plants in tuff and sand; the reverse was the case in rockwool. As Cp was raised the plant's influence on the pH increased in rockwool and declined in sand and tuff. The presence of plants enhanced Ca concentration, and reduced K concentration in the leachate from tuff relative to tuff without plants. In rockwool and sand the plants effect was appreciably smaller. The mechanism behind the interaction beween plant, substrate and P supply is release of H⁺ and organic acids by the roots, followed by reaction of these exudates with the growth substrates and nutrients. The P content in leaves and total dry matter production rate were significantly affected by Cp. The leaf P concentration was maximal in plants grown in rockwool, followed by plants grown in tuff and sand. At leaf P concentrations below 0.3 and above 0.65 g P/100 g, on day 109, a reduction in dry matter production rate, and a decline in leaf K concentration, respectively, were obtained. Calcium concentration in leaves was significantly lower in rockwool than in tuff and sand, in accordance with its concentration in the leachate of these substrates.