Phosphorus Use Efficiency and Long-Term Trends in Soil Available P in Wheat Production Systems.

Efficient P use in crop production is important for both economic and

environmental reasons. Short term studies often show low fertilizer P

recovery, but efficiency of P use may be underestimated because the

value of the residual P in the soil is ignored. Fertilizer P use

efficiency was evaluated on an aridic haploboroll in a continuous wheat

(CW) and a three-year rotation of summerfallow-wheat-wheat (FWW) using

data from a 39-yr study initiated in 1967 at Swift Current, SK, Canada.

Each rotation received P only (P) or N plus P (NP) fertilizer. We

measured annual grain P removal and changes in Olsen bicarbonate

extractable P in the 0-15 cm soil depth. In 1993, subplots which

received no further P were established to evaluate the residual effect

of P fertilizer applied in the preceding 27 years. Where P was applied

each year, grain P removal averaged 54 to 78% of fertilizer P, with

values as high as 65 to 109% in 1994 to 2005, the period of lowest water

deficit. On average, the P-only treatments removed 13% less P in grain

than NP treatments. In the P-only systems, Olsen-P increased linearly

with time but, in the NP systems, Olsen-P reached a maximum after 20-22

yr and then stabilized. The cumulative P balance (fertilizer P minus P

removed in grain) accounted for 60% of the variability in Olsen-P over

the course of the experiment. The rate of Olsen-P increase was greater

in the FWW than the CW rotation possibly due to P mineralization during

the summerfallow year. Crop P removal from 1967-2005 where P was

withheld averaged 105 and 90% of fertilizer P for the NP and P only

systems, respectively. Residual P in prairie soils is retained in

plant-available forms and wheat crops may recover close to 100% of

applied P over time.