FIRST 12 YEARS OF A LONG-TERM CROP ROTATION STUDY IN SOUTH WESTERN SASKATCHEWAN — BICARBONATE-P DISTRIBUTION IN SOIL AND P UPTAKE BY THE PLANT

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Abstract
The first 12 yr (1967–1978) of an ongoing crop rotation study being conducted on a Brown Chernozemic loam soil at Swift Current, Saskatchewan, has already been assessed for yield, economics and soil N changes. In this paper the effect of crop rotation, N and P fertilizer and environmental factors on NaHCO3-extractable in-organic P (bicarbonate-P) and plant P were assessed. The soil had a high level of bicarbonate-P at the initiation of the experiment. Withholding P fertilizer for 12 yr did not reduce the level of bicarbonate-P but, frequent P application, using generally recommended rates of P, increased the bicarbonate-P in the top 15 cm of soil; the bicarbonate-P levels below 15-cm depth were unchanged. Bicarbonate-P changes during fallowing were significantly related to changes occurring in cropped treatments (r = 0.62** to 0.85**). There was no evidence of net P mineralization. Between spring and fall 66–88% of the variability in bicarbonate-P occurring in the top 15 cm of soil was related to soil moisture, air temperature, and their interaction. Invariably, bicarbonate-P increased between late fall and spring thaw. P uptake by plants was linearly related to time through to shot blade stage; the rate slowed thereafter, but uptake continued to maturity. The amount of P uptake was directly related to dry matter yield and positively influenced by N fertilizer. It was concluded that bicarbonate-P might not be the best indication of P availability and that a more detailed study of the cycling of P in these rotations would be invaluable. Key words: Bicarbonate-P, uptake, temperature effects, moisture effects