Seedbed: Are you 'mining' your soil or just making strategic withdrawals?

Thursday, April 2, 2009

Keeping track of the nutrients in your fields will help you decide if you need to add fertilizer or whether you can draw on the existing balance

by KEITH REID

"Can I get away without applying phosphate or potash this year?"

That has been a common question this year, with some justification given the spread between crop and fertilizer prices. The answer, as with most agronomic questions, is not a simple yes or no.

The first thing we need to be clear about is that this question is only valid in cases where there is already an adequate supply of nutrients in the soil. Soils that will respond profitably to added nutrients should receive those nutrients. This response may occur because the soil cannot provide enough to meet the total demands of the crop, or it may be that a higher concentration of nutrient to the seedling will increase early growth and crop yield (starter effect).

The common concern expressed about growing a crop with no fertilizer is that we are "mining" the soil. The implication is that productivity will decline if we don't add nutrients, and that we are setting up a situation where we will lose more money later than we are saving today. This fear is natural but, like any fear carried to extremes, it can get in the way of making rational decisions about nutrient use.

It may be useful to think about the supply of immobile nutrients in the soil (such as phosphorus and potassium) like a bank account. You make deposits each year in the form of fertilizer or manure, and withdrawals in the form of harvested crops.

Losses through erosion could be looked on as service fees. If the account balance falls below a minimum level, then you pay a financial penalty. In this case, you lose yield and profit.

One way to manage your "soil account" is to keep putting nutrients in each year. Another is to just keep making withdrawals until the check bounces (soil "mining").

A better way than either of these is to keep track of the balance in the account by soil testing. It is desirable to have some buffer above the minimum because we know that, if we get cool, wet weather, the release from the soil will be less than normal, but that there comes a point where a larger pool of P or K in the soil doesn't provide additional benefit.

If your soil test is built up to high levels (or with a RR rating, for Rare probability of Response), then we can be sure that skipping a year of fertilizer application will have no impact on yields. Many fields have enough residual soil fertility that they could be farmed for many years before a lack of nutrients affected crop production.

If you are in this situation, rather than mining the soil, you could be in the position of making a strategic withdrawal from the savings account in your soil. The dollars which would have been spent on fertilizer can then be used elsewhere on the farm where they can generate a greater return. BF

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