EFFECT OF LONG-TERM APPLICATION OF PHOSPHORUS FERTILIZER ON BICARBONATE-EXTRACTABLE SOIL PHOSPHORUS IN A MYWYBILLA SOIL
C.W. Dowling
Incitec Fertilizers, PO Box 623, Toowoomba, Qld 4350

This poster reports on changes in soil bicarbonate-extractable phosphorus (P_B) following nine additions of four rates of fertilizer phosphorus (P) to a Mywybilla clay in a dryland sorghum/barley rotation on the central Darling Downs, Queensland.

MATERIALS AND METHODS

Soil samples (0-10 cm depth) for P_B analysis (2) were taken from each P rate that received 0 or 120 kg/ha N presowing in each crop. Sample collection was carried out in August 1993 after nine cropping cycles spanning a ten year period.

RESULTS AND DISCUSSION

After nine crops, P_B in the soil increased significantly (P<0.05) with P application rate (Table 1), the increases being greater for those treatments receiving no fertiliser N.

Table 2. Effect of four rates of P on the P_B (mg/kg) of the soil after nine separate fertiliser applications.

Annual fertiliser P (kg/ha)	Annual fertiliser N (kg/ha)
	0	120	Predicted
0	14.1	12.4	101
10	31.6	24.1	25
15	35.8	33.4	33
20	56.9	40.6	41

¹ Mean soil P_B value found prior to the first crop (range 7-13 mg/kg).

The rate of increase in P_B was found to closely fit the soil P prediction equation described by Best et al. (1), P_B = P_B⁰ + 0.3 [∑ 4 /(4+(n-1))]R where P_B⁰ is the initial soil P_B concentration, n is the number of years of application and R is P fertiliser application rate. Where no nitrogen was applied, the prediction of residual P was inferior (R²=0.86) to that when N was applied (R²=0.99). Given the close correlation of the actual and predicted results from this experiment, with nitrogen adequate conditions, the equation may be applicable to cereal crop rotations on a range of clay soils.

REFERENCES

1. Best, E.K., Strong, W.M., Cooper, J.E., Glasby, J.M. and Pumfrey, A. 1991. Queensland Wheat Research Institute Biennial Report 1986-1988. pp. 91-93.
2. Standley, J. 1993. Analytical Note No. 4, QWRI, Toowoomba, Qld. 15 pp.