Effect of soil solution phosphorus in Volcanic Ash soils on the response of tomato (Lycopersicum esculentum) to added phosphate

Abstract

Sorption isotherms of six Volcanic Ash soils were used to determine ‘Optimum P’, i.e. the amount of added P required to produce a soil solution concentration of 0.2 µg P per ml. In a pot test, application of these amounts of P to the soils led to maximum or near maximum growth of tomato plants, (Lycopersicum esculentum), harvested at 35 days. After harvest the soil solution concentration of P in all the soils had fallen below 0.2 µg per ml whatever the rate of P added initially. The amount of P required to restore the soil solution to 0.2 µg P per ml was linearly related to the fertilizer P originally applied to the soil; the higher the P applied originally the lower was the future P requirement. The regression coefficients suggested that the applied P would be twice as effective at reducing subsequent P requirement in a Recent Volcanic Ash soil than in the other more developed soils. Measurement of extractable Al (NH4OAc at pH 4.2) showed that lack of growth at low levels of added P on a high P fixing soil could be caused by insufficient P due to high levels of Al, although with a soil of pH 6 this would not have been expected. re]19760217