Abstract
A glasshouse pot-experiment was conducted to investigate the effect of phosphorus fertilization on growth and P uptake of soybean in three Ghanaian soils (Kumasi, Akroso and Ofin, occurring on a common catena) and to estimate critical P levels in soybean plant tissue and soils. Soybean cultivar Jupiter, was grown at six different P rates, 0.0, 7.5, 15, 30, 60 and 120 mg P kg-1 soil, in each of the three soils and harvested at the Dowering stage (45 days after planting); the whole plant tops were analysed for P content. Samples of incubated soils similarly treated with P were analysed for available P by Bray P1 extractant. Plant height, dry matter (DM) yield and percentage P content in plant tops increased with P fertilization up to the application rate of 30 mg P kg-1 of dry soil. Phosphorus utilization efficiency (PUE) decreased from a mean value of 0.56 to 0.40 g DM mg-1 P in plant tops at zero P and 30 mg P kg-1 soil application rates, respectively. Optimum DM yield, 93% of maximum, was attained at a P application of 30 mg P kg-1 soil in all three soils, and at tissue P contents (internal critical P requirement) of 0.22% in Ofin and 0.27% in each of Kumasi and Akroso and at mean Bray Pl levels (external critical P requirement) of 6.3 mg P kg-1 soil in Kumasi and 8.5 mg Pkg-1 soil in each of Akroso and Ofin. For the Kumasi-Akroso-Ofin soil association, a plant tissue-P content of 0.27% and Bray Pl value of 8.5 mg P kg-1 could be used as general working values for diagnosing P sufficiency in soybean
