EFFECTS OF PREPLANT PHOSPHORIC ACID TREATMENT ON P RETENTION OF TUFFS OF THE CANARY ISLANDS

Several types of volcanic tuff are found in the Canary Islands, where they are popular with growers as a cheap and abundant alternative for soilless culture. Textures and composition vary according to type, but all have in common a high, buffered pH and a P sorption capacity of agricultural importance. Four tuffs, two coarse black unweathered tuffs and two fine red weathered (alterated) tuffs, from four different localities, were selected to evaluate changes in P sorption after a preplant treatment with phosphoric acid to achieve pH 6. Granulometry, pH, and exchange capacity (CEC) were determined; neutralization curves showed that both red tuffs needed more acid to correct pH. Treated tuffs were fertigated during five weeks with a standard nutrient solution to determine short-term effects on the solution P content retention; rockwool slabs were used as control. Drainage was collected daily, and phosphorus content was determined. In red tuffs, there was a strong initial P release that gradually declined and was practically null at the end of the trial. The black tuffs did not change the P level of the solution during the first week, but afterwards they exhibited an increasing sorption inversely proportional to the decrease in the P level of the solution. Results showed a strong relationship between weathering (significantly more than particle size alone) and the amount of P sorbed from the nutrient solution. However, the present trial shows that preplant phosphoric acid treatment alone is not enough to sustain the necessary P level of the nutrient solution. For the black tuffs studied, the solution needs extra P (particularly for high demand crops), and for the red tuffs, part of the phosphoric acid should be replaced by nitric acid in order to avoid the strong initial release of P that unbalanced the nutrient solution composition.