Management effects on phosphorus transformation and implications for soil test recommendations
Peer Reviewed StatusNon-Peer Reviewed
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Crop rotations and fertilizer application on long-term rotation plots at Lethbridge and Breton have dramatically affected most soil phosphorus (P) fractions. At the Lethbridge site, soil cultivation has reduced organic P (Po) levels as a result of mineralization. This has caused a significant increase in resin extractable inorganic P (Pi; most biologically available), sodium bicarbonate extractable Pi (sorbed to soil surfaces), and sodium hydroxide extractable Pi (more strongly bound to Al and Fe compounds) levels. In non-fertilized treatments, continuous wheat (CW) resulted in greater P draw-down of all labile P fractions than in wheat-wheat-fallow (WWF) and wheat-fallow (WF) rotations. The addition of P fertilizer has significantly increased Resin-Pi, Bicarb-Pi and NaOH-Pi fractions. The addition of N fertilizer has resulted in increased Bicarb-Po and NaOH-Po levels in the CW, WF, and WWF rotations. At the Breton site, continuously cropped treatments, which had not received fertilizer, resulted in greater P draw-down of all P fractions except Residual-F. Addition of fertilizer had a significant effect on all P fractions (except NaOH-Po). The added Pin the fertilizer treatments positively affected the Pi fractions and the N in the fertilizer treatment positively affected the Po fractions. Bicarb-Po levels were found to be negatively· affected by soil pH. Finally, cropping without using phosphate fertilizer has resulted in a 30 to 41 % decline in Total-P in the Breton plots. A growth chamber study was conducted to compare four routine soil test P methods with plant uptake of P. Wheat and canola were each grown in eight soils from the Lethbridge and Breton plots with different pedogenic, crop rotation and fertilizer histories. Results of the study confirm that one calibration curve to predict fertilizer P requirements for a wide range of soils and crops is virtually impossible. Future soil tests will combine a chemical extractant with a computer model prediction of Po mineralization. Much more information is needed on root rhizosphere dynamics.
Part OfSoils and Crops Workshop
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