Agriculture can help to keep P out of the Great Lakes

Wednesday, April 3, 2013

Reducing P loading in the Great Lakes requires thinking 'outside of the box.' But there are ways producers can keep nutrients out of the water and also retain more on their farms

by PAT LYNCH

These days, the media are paying more attention to phosphorus (P) getting into the Great Lakes and the finger is being pointed at agriculture. And, indeed, we are responsible for some of it.

There was a Great Lakes study over 25 years ago which looked at phosphorus loading of the Great Lakes and one of the causes was milk house wash water going directly into tiles. Once dairy producers realized this, they developed alternatives. Wash water was diverted from tiles that fed directly into streams. This reduced P loading of the Great Lakes. Other actions taken by urban people also helped. The result was that P levels dropped significantly in the Great Lakes – to the point that there was concern lower P levels might hurt certain fish species.

Now, once again, there are worries that there is too much P in the Great Lakes. Some is being released from the bottom of the lakes due to warming of these bodies of water. But once more people are looking at agriculture to reduce our P additions to the Great Lakes.

Changing cropping practices in agriculture over the past 10 years have reduced some P loading. The movement to vertical tillage is also reducing the movement of P into tiles. One of the weaknesses of no-till is increased movement of P through tiles into water.

In the spring, about 50 per cent of water moves through tiles and the other 50 per cent moves across the surface of fields into streams and drainage ditches. If we can slow down this movement, we can reduce P loading. One of the ways to do this is by leaving trash on the ground. Ideally, 30 per cent cover will reduce erosion and thus P loading. The use of cover crops and reduced tillage will also help.

There is a suggestion that growers should go to banding all P fertilizer. You can do this with an air cart. But there are problems with this proposal. Many growers cannot justify the cost of an air cart. And we need too much P to realistically apply it all as a starter fertilizer in a band.

With a three-crop rotation of corn, beans and wheat, you will need about 150 pounds per acre of phosphorus as P2O5. Some of this must be broadcast. If you work this P into the soil, you can effectively keep it from getting into the water. This is how we handle manure. Incorporate to prevent movement across the top and close any channels to the tiles.

Mitigating P loading of the Great Lakes requires thinking "outside of the box." Ideally, we should be monitoring where the P is coming from. Most efforts to reduce P loading look at "non-point sources." The problem with this approach is that everyone is assumed to be adding the same amount of P.

In real life, there is an 80-20 rule. This states that 80 per cent of something is attributed to 20 per cent and it applies to food production, trouble with kids, and the like. I believe that 80 per cent of the P is coming from 20 per cent of the tiles. We should find these and address the problem. It may even mean that we must block offending tiles. This seems drastic but, if for some reason, you have one offending tile you have to address it.

And maybe it is a matter of diverting that tile. Maybe you can block it with a pressure relief valve. The valve is adjusted so that, if there is a danger of the tile blowing out, the valve releases the water. This water could be redirected back onto the field for recycling.
I know this sounds unrealistic, but I believe it can be done. If you told someone in the 1950s that you can plant soybeans without working the ground, they would have laughed at you.

We must take a realistic look at how we can keep P out of the water. If we do so, we will also be keeping other nutrients out of the water. And if you keep more on the farm you won't have to buy as much. BF

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