How to get the most of P fertilizer in alfalfa stands
Phosphorus (P) is relatively immobile in soil. Our previous research has shown that the majority of fertilizer P recovered in soil as extractable P remains in the top 5 cm layer, even after long-term annual applications of P to grass or alfalfa. This suggests that P fertilizer would be less effective when surface broadcast. For this reason, placement of P fertilizer is very critical; that is, it must be located where roots can intercept it. A field experiment was initiated in 1992 on the existing alfalfa stands on a P-deficient Black Chernozem (Typic Boroll) soil at Ponoka, Alberta, Canada to compare the effects of surface broadcasting versus subsurface banding (using a special disc drill) of annual and one-time initial applications of P on dry matter yield (DMY). The alfalfa was harvested for dry matter yield two times during growing seasons in 1992 to 1995. There was a marked increase in DMY from P applications in all the four years. Disc-banding at 15 cm spacing produced greater forage yield than surface-broadcasting, regardless whether P was applied annually (by 828 kg DMY/ha) or as single initial applications (by 565 kg DMY/ha). The P-use efficiency (averaged across P rates over a 4-year period) was greater with subsurface banding as compared to surface-broadcasting by 50 kg DMY/kg P/ha for annual applications and by 21 kg DMY/kg P/ha for single initial applications.
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