Direct assessment of mineral phosphorus availability to tropical crops using ³²P labelled compounds

The availability to plants of phosphorus (P) derived from sparingly soluble iron and aluminium phosphates was directly assessed with ³²P labelled compounds in two glasshouse trials. In the first experiment, the comparative availability of all mineral phosphate (P) compounds to maize increased with time (14 to 42 days post emergence) and plant total P uptake, but P source did not affect the growth or total plant uptake of P. The comparative availability of the amorphous AlPO₄ (Al-P), crystalline AlPO₄ (variscite), amorphous FePO₄ (Fe-P), and crystalline FePO₄ (strengite) compared to KH₂PO₄ (=100) was 53.1, 3.4, 38.9, and 1.9%, respectively. In the second experiment, the availability of Fe-P, strengite, and KH₂PO₄ to several topical crop species was examined. There was no difference between maize, sorghum, mungbean, cowpea or soybean in their ability to utilise Fe-P or KH₂PO₄, although maize utilised strengite more than the other species. The major difference between these species in their ability to acquire P appears to be a difference in ability to locate soluble soil P rather than differences in their ability to access different pools of soil P. The advantages of using neutron irradiation to directly measure P absorption from mineral P compounds over traditional methodologies is discussed.