Phosphate diffusion in soil and uptake by plants

Summary

The self diffusion of P³² has been determined by transient-state methods in four different soils over a wide range of P³¹ additions. All soils were moistened to a water content approximating field capacity and packed into the diffusion cells to a bulk density such that during the experimental period all pores were filled with liquid.

During any single experiment, the apparent diffusion coefficient (D _e ) showed a strong dependence on level of P³¹ addition for all soils. However, the absolute values were influenced by the period of moist storage prior to assembly into the diffusion cells and thus are of limited value unless they can be related definitely to the conditions existing in the soil during the diffusion period.

The concentration of phosphate in the soil solution also varied with the level of P³¹ addition made to the soil but no uniform relationship could be found betweenD _e and phosphate concentration for the four soils.

For an inert medium, such as glass beads, in which adsorption of phosphate onto the solid phase does not occur, there was very little change inD _e with change in solution concentration from 1 to 200µgP perml. In soils, adsorption occurs to varying extents, and it is suggested that the diffusion coefficient (D _p ) in an inert medium can be related to diffusion in soil by considering the proportion of the available pool which is present in the solution phase.

$$i.e. D_p .\frac{{solution - phase P}}{{total P}}$$

This relation was developed from extensive data on one soil, but appears to be valid for the other soils tested. The apparent diffusion coefficientD _e is shown to be related to both the capacity and intensity factors often used to characterise soil phosphate availability.