Thursday, March 1, 2012

Should Lime and Phosphorus be applied at the Same Time?

Lloyd Murdock, Extension Soils Management, University of Kentucky

No matter how phosphorus (P) is applied or when it is applied it will be tied up with something. A soil pH of 6.5 is the very best one can do to reduce this fixation of P. If fertilizer P is applied below a pH of 6.5, especially below 6.0, more of it will be tied up with aluminium and iron in the soil. So not applying lime would be bad also. If the pH is above 7 then more of the P will be tied up by calcium in the form of insoluble calcium phosphates (rock phosphate). So you cannot win either way. If you have a low pH, say in the high 5’s, and you apply lime and you add P also. You would have to get the soil pH where the lime is placed above 7 for much of this additional fixation to occur and it is going to be difficult for this to happen since you are starting at a low pH and lime is a rock and is only slowly soluble.

Bottom line is do not worry about P fixation and keep the soil pH about 6.5. Under the best of circumstances, you are only going to get about 30% of the P you apply in the crop. Very small differences in P fixation would happen with or without the lime.

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