, Volume 3, Issue 2, pp 111-125

The balance sheet approach to determination of phosphate maintenance requirements

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Abstract

A balance sheet model has been used to calculate the efficiency of phosphorus applied at stable levels of pasture production. This shows that P is used efficiently, with 70 to 90% of fertilizer and recycled P being utilized by plants. The remaining P not used by plants (10 to 30%) is lost due to chemical reactions or downward movement in the soil. Utilization efficiency declines as the maximum yield (in respect to P nutrition) is approached on any soil.

The fertilizer phosphate maintenance requirements of grasslands can be calculated on the basis of the efficiency of P utilization by pasture plants and the amount of P recycled in plant and animal residues at any stable P output, i.e. steady state. The amount of recycled P has been obtained by deducting from P uptake the amount of P removed in produce and lost by transfer under (a) dairy farming, and (b) cattle and sheep farming.

In this paper the phosphate maintenance requirements have been calculated for soils of low, medium and high production potential, each at 70 and 85% utilization at 90% of maximum production.