EFFECTIVENESS OF DIFFERENT PHOSPHATIC FERTILIZERS MEASURED USING LABELED SUPERPHOSPHATE AND PHOSPHORUS TAKEN UP BY PLANTS

The agronomic effectiveness of three P fertilizers (diamonium phosphat e, rock phosphate and compost) was studied in a greenhouse experiment using wheat. A radioisotopic method, using triple superphosphate label led with P-32, was used to evaluate the P in dried tops that was deriv ed from i) the soil, ii) labelled superphosphate and iii) the fertiliz er being studied. The ratio between P uptake from each fertilizer and P uptake from the soil was used to compare the effectiveness of the di fferent fertilizers. P derived from diammonium phosphate was greater t han P derived from the soil, except in one soil. P derived from rock p hosphate was always lower than P derived from the soil. The effectiven ess of compost depended on soil type. Compost can produce two kind of effects: i) a direct P contribution and ii) an indirect effect improvi ng P uptake from the soil. The radioisotopic method can be used to stu dy the effectiveness of fertilizers even when there are no differences in yield.