Seedbed: Does fertilizing to crop removal make sense?

Wednesday, December 19, 2007

Keep in mind that every field will eventually reach a point where it just doesn't make sense to keep adding more fertilizer with no possibility of an economic return

by KEITH REID

"I didn't get around to taking a soil test. We'll just replace what the crop is taking out. That should be close enough."

Have you ever used this excuse? And have you ever thought about what it might be costing you?

The number of fields fertilized to crop removal seems to be increasing. Some of this may be an unfortunate consequence of including crop removal as part of the nutrient management software, implying that the Ontario Ministry of Agriculture, Food and Rural Affairs now "approves of" crop removal for fertilizer recommendations, the same as our neighbours to the south. (I'll discuss the difference between what's allowed, and what makes the most economic sense a bit later.)

Sacrificing yield by using crop removal.
Crop removal is the amount of nutrients taken up by a crop of a given yield. On soils with low fertility levels, the rate of phosphorus or potassium which will generate the greatestdollar return is considerably higher than the amount of nutrient removed in the harvested portion of the crop. It may help to think of the soil and the crop as sharing the applied nutrients. In "hungry" soils there isn't as much left over for the crop. Once the soil fertility is built up, the soil will give back more readily to meet the crop needs.

Replacing only the nutrients removed by a yield that is already lower than it should be because of nutrient deficiencies will only worsen the loss. This means that you are providing even less than what should have been applied.

For nitrogen, the picture is even clearer, since we cannot easily build the supply of available nitrogen in our soils from year to year. The plant needs to have enough nitrogen to grow a large plant to support a high yield, so the nitrogen supplied to the plant must always be greater than the amount removed by the crop. Adjustments are made for the amount already provided by the breakdown of soil organic matter or added organic materials.

Over-spending by using crop removal.
Once the fertility of the soil is built up, the crop can get all the nutrients it needs without requiring extra fertilizer. This means that applying fertilizer would increase the cost of crop production without an accompanying increase in income, so net profit is reduced.

There will always be arguments about how much fertility is enough, and some systems promote applying "maintenance" fertilizer to ensure that the soil stays in the fertile range. Despite these disagreements about the details, it is clear that every field will eventually reach a point where it just doesn't make sense to keep adding more fertilizer with no possibility of an economic return.

Keeping a bad situation from getting worse.
In extreme cases, the level of nutrients in the soil has increased to the point where adding more will increase the risk of environmental problems. This is where the nutrient management regulations come into play to protect surface and ground water quality. These regulations focus on nitrogen and phosphorus, since those are the nutrients most likely to impair water quality.

Nitrogen applications on sensitive soils should not exceed crop removal by more than 15 pounds per acre to minimize the risk of nitrate leaching. Phosphate applications, where not required for optimum crop yields, are normally limited to no more than
70 pounds per acre (allowing a further build-up of soil P of about two parts per million per year).

Fields with a high risk of runoff to surface water might be limited to no more than crop removal, or no application at all adjacent to a watercourse.

Don't guess - soil test!
If we knew which category each field fell into (infertile, adequate fertility, excessive fertility), we could probably come pretty close in guessing the optimum fertilizer rates.
Unfortunately, we can't tell how fertile a soil is by just looking at it.

A representative soil test of the field is really the only tool we have to tell us where the fertility of the soil is now, and what is the best amount of fertilizer (or manure) to apply. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture and Food and Rural Affairs based in Stratford. keith.reid@ontario.ca