The response to phosphate of two annual pasture species. II.* The specific rate of uptake of phosphate, its distribution and use for growth

Abstract

Subterranean clover and Wimmera ryegrass were grown in pots at a range of levels of applied phosphate for up to 33 days after emergence. From the results of sequential sampling, the rates of uptake of phosphate, its distribution and its effect on the rate of assimilation were calculated.

The assumption involved in the widely used approximate method for calculating the average specific absorption rate (SAR) of nutrients by roots was found to be invalid. Hence instantaneous values of SAR were calculated from multiple regressions which allowed the simultaneous determination of the effects of phosphorus level and of time. For both species, SAR increased until about 2 weeks after emergence, and then declined. The effect of level of application on SAR differed between the species. For the ryegrass, SAR increased with phosphate level and reached a maximum value. For the clover, there was a level of application below which phosphate was almost ineffective. Above this level, the slope of the relationship between SAR and phosphate applied increased with increasing level of phosphate. It is suggested that this upward-curving relationship occurred because the proportion of added phosphate in the soil solution increases with increasing phosphate level. There was no evidence of a maximum value of SAR for the clover within the range of levels of phosphate used. At all levels of application, the SAR for the clover was lower than that for the grass.

Ryegrass transferred a greater proportion of the phosphate to the tops than did clover. The net photosynthetic effectiveness of the tops of both species increased with phosphate concentration. The ryegrass had higher values for photosynthetic effectiveness than the clover at high phosphate concentration. At low phosphate levels the ryegrass allocated a greater proportion of its growth to roots than did the clover.

*Part I, Aust. J. Agric. Res., 26: 137 (1975).