Written by: Chris Brown, OMAFRA

Most people will have a good idea of their bank account balance. Imagine not knowing when making a purchase if there was enough to cover the acquisition. Or, if there are sufficient resources, knowing whether to build equity by investing more.

Soil fertility is a “bank account” of nutrients. It seems odd that consistently in surveys there are only about one-third of crop producers that take regular soil samples and even fewer that take manure samples. So, here is the question; If you do not know the fertility status of the soil, how do you know if you’re getting a return of investment from dollars spent on fertilizer?

A recent example of a dairy operation with a corn-alfalfa rotation discovered that after years of not testing the soil or the manure, the soil fertility levels were dangerously low with less than 10 ppm for soil phosphorus (P) and less than 80 ppm for potassium (K. The operator thought that returning the manure nutrients ahead of corn would be sufficient. They were surprised to learn that 3 years of alfalfa on a 3-cut schedule can reduce the K soil test by + 40 ppm (P soil test + 6 ppm. Manure nutrients alone cannot replace that amount.

The opposite also occurs, where fertility levels are above the range that will show an economic response to additional nutrients. In those cases, there is seldom a return to investment of additional fertilizer (beyond seed-placed starter. In addition, P soil test levels above 35 ppm increase risk for loss through runoff or through tile water. And yet, in addition to manure there is often a generous addition of fertilizer just to ensure nutrients removed by the crop are replaced.

Questions to consider in balancing the fertilizer account include:

1. Where are current fertility levels in the field?
2. How much P and K are removed from a “normal” planned crop rotation?
3. Where during the rotation are nutrients generally applied?
4. If nutrients come from organic amendment sources is there a nutrient analysis?
5. Are fertilizer levels compensated to account for organic amendment sources?

The tools in the new agrisuite software (found at: www.ontario.ca/agrisuite) can help provide some answers to the above questions.

Consider that a high yielding corn-soybean-wheat rotation will remove up to 190 lbs of phosphorus and between 150 to 300 lbs of postash (depending on whether the wheat straw is left in field or removed). That represents an approximate change in soil test of 5 ppm P (sodium bicarb) and 15 ppm potassium. High crop yields can result in a relatively rapid change in soil fertility levels. Where and when nutrients are applied in the rotation and the use of organic amendments can maintain or build the soil nutrient bank account, but without regular testing do you know your account balance?

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