The silent fertilizer loss on rented farm land

Thursday, May 2, 2013

With a three-year crop rotation of corn, beans and wheat, about $80 per acre of phosphorus and potassium is being removed in crops each year. In too many cases, this value is not being applied to rented land

by PAT LYNCH

There is a subtle loss occurring on rented land across Ontario. It is not as obvious as someone taking trees or soil from a rented farm, but it is as just as big.

If someone is renting land, they never think of stealing some trees from a bush that was also on the rented land. Nor would they steal a few hundred tons of topsoil. But the fertilizer loss from rented land is just as great. It could be as high as $40-$50 an acre. On 100 acres, this is $4,000-$5,000.

With a three-year crop rotation of corn, beans and wheat, there is about $80 per acre of phosphorus (P) and potassium (K) removed in crops each year. The value is higher if straw or forages are harvested. And there is a further loss of micronutrients. Too often, there is less than $80 per acre added.

The issue involves, first, use of fertilizer additives or products that claim that you need less fertilizer because their product makes nutrients more available or, second, mining nutrients from the soil. No matter how you look at it, about $80 is being removed per acre in fertilizer nutrients. In too many cases, this value is not being applied to rented land.

I have seen good growers looking after land and then losing it to another grower. That grower is willing to pay higher rent. This new renter then gets good yields by not applying as much P and K. You can grow a decent crop with $30 per acre of P and K. And you can do this for a short time. But this is done by mining nutrients from the soil.

It does not have to be this way. I once worked with growers who rented land from a European group who owned a lot of land in Canada. In their rental agreement was a stipulation that the soil must be tested every three years and growers had to maintain soil test levels.

But, due to some inaccuracies in soil testing, I would not want to rely only on soil tests. Soil testing is not as accurate as some believe. On a good day, a soil lab will have a 25 per cent variation in tested values. Thus, a standard sodium bicarbonate test of 15 for P could be as low as 13 or as high as 17. And a soil testing 40 could actually be 35 to 45.

Combine this with the fact that it takes 25 to 35 pounds per acre of fertilizer (P) to move the soil test one point. Thus, if you depleted soil by 100 pounds of available P, the soil test might move three points. But, because of the level of accuracy in soil testing, this may never show up on a soil test.
If I was renting land, I would want to see soil tests every three years but also see documentation of fertilizer applied and removed. I would want this verified by a Certified Crop Advisor.

And what if you own your own land? It really does not matter. If you mine your own soil, the only down side is lower yields. BF

Consulting agronomist Pat Lynch, CCA (ON), formerly worked with the Ontario agriculture ministry and with Cargill.