COWPEA VARIETAL DIFFERENCES IN UPTAKE OF PHOSPHORUS FROM GAFSA PHOSPHATE ROCK IN A LOW-P ULTISOL

A greenhouse experiment was conducted to evaluate varietal differences in the uptake and availability of P from Gafsa phosphate rock (PR) to five cowpea cultivars grown in a low-P Paleustult soil from Ghana, us ing the A value technique. The P-32 radioisotope used as a tracer was P-32-labelled triple superphosphate (TSP). Each cultivar received sole or a combined application of the two fertilizers (TSP, PR). From thes e treatments it was possible to estimate for each cultivar, A(R) + A(S ), A(S) and by difference A(R) (A(R) and A(S) stand for A values for p hosphate rock and soil, respectively). Using this approach we measured significant genotypic differences in P uptake from PR and A(R) values . The ranking of the cultivars in P uptake from PR was the same as for A(R), i.e. Asontem > Vallenga > Soronko > IT81D-1137 > Amantin. Simil arly, ranking for uptake from soil P was the same as A(S), i.e. Vallen ga, Soronko, IT81D-1137, Asontem and Amantin. Thus although PR could n ot be labelled directly, using the A value approach it was possible to distinguish between P availability from PR and soil to the plant. The recoveries of applied TSP ranged from 8.0% to 9.4% and those of PR fr om 2.3% to 3%. The other advantage of the A value method is that it ma de it possible for the different genotypes tested to be compared direc tly in units of a standard fertilizer, TSP in this case. Thus for Vall enga in this soil 3.01 kg of P in Gafsa PR was capable of supplying th e same amount of P that could be supplied by 1 kg P of TSP; whereas fo r Amantin a higher amount i.e. 3.5 kg P as Gafsa PR was needed. This i nformation is useful for adjusting application rates to be recommended for different P fertilizer sources in field trials so as to achieve s imilar effects.