Potentially mineralisable phosphorus as asource of plant available phosphorus. (75)

Stanlee Juma, H.A. Maida


Inorganic phosphorus availability for plant uptake is often very low in highly weathered acid soils, particularly those that contain large quantities of Al and Fe hydrous oxides. Conservation tillage use is increasing in tropical agriculture, necessitating consideration of crop residue as a potential source of plant-available P, and of the importance of the mobility of this P in soils from both agronomic and environmental standpoints. The extent to which the predictive value of Mehlich 3, which is used as an index of available P for soils in Malawi, is improved by including mineralisable P as a contributory fraction extracted by the Mehlich 3 method has not been investigated in detail using Malawi soils. The main objective of the present study, therefore, was to determine whether the efficacy of Mehlich 3 was improved when mineralisable P was taken into account. The soils selected for laboratory and greenhouse investigations varied from 9 mg kg<sup>-1</sup> to 50 mg kg<sup>-1</sup> in their Mehlich 3 extractable P status. The Mehlich 3 extractable phosphorus was strongly correlated with pH (r = -0.766; P< 0.01). The correlation between organic matter and organic P was also highly significant at the 0.1 per cent probability level. The phosphorus uptake was negatively correlated with organic matter (r =-0.621*), and positively related with total P (r = 0.644; P< 0.05), pH (r = 0.793; P< 0.01), inorganic P (r = 0.961; P< 0.001), Mehlich extractable- P (r = 0.870; P< 0.001) and mineralised P (r =0.868; P< 0.001). There was an increase in P uptake with an increase in the incubation periods. The Mehlich 3 extractable phosphorus was strongly correlated with total inorganic phosphorus (r = 0.881; P< 0.001), mineralised phosphorus (r = 0.815; P< 0.001), organic phosphorus (r = 0.993; P< 0.001), pH (r = -0.766; P< 0.01), and organic matter (r =?0.553; P< 0.05). The present findings confirm that the efficacy of the Mehlich 3 method may be improved if mineralisable soil P fraction is taken into account.


Inorganic phosphorus, mineralisable phosphorus, mehlich 3 method, soils

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