Abstract
The minimum P concentration in diagnostic plant parts associated with optimum plant growth is called 'internal P requirement' or 'critical P concentration'. A pot culture experiment, using P-deficient soil of Missa series (Typic Ustochrept), was carried out to determine the internal P requirement of black gram or 'mash' [Vigna mungo (L.) Hepper]. Phosphorus fertilization enhanced crop growth; the maximum increase in grain yield was 373% over the control. Near-maximum grain yield of black gram was associated with 28 mg fertilizer P kg·1 soil. Critical P concentration was 0.37% in whole shoots (<30 cm tall) and 0.28% in most recently matured leaves. Phosphorus content in mature seeds of black gram was a good indicator of the P fertility of the soil; the critical P concentration in mature seeds was 0.35%. Phosphorus extracted by three soil tests (NaHC03, AB-DTPA, and Mehlich 3) correlated well with each other and the P contents of diagnostic plant parts. Critical soil test P levels for pot-grown black gram were 8 mg P kg·1 soil (NaHC03); 6.5 mg P kg·1 soil (AB-DTPA); and 23 mg P dm·3 soil (Mehlich 3). As the 'universal' soil tests (AB-DTPA and Mehlich 3) are inexpensive, easier, and less time-consuming than the NaHC03 test now used routinely in many countries, they could be used for rapid site and crop-specific fertilizer recommendations.
