Seed Bed: Seven simple ways to reduce P loss from your fields

Sunday, October 5, 2008

With phosphorus inputs into lakes a major issue, here are some tips to help you
play your part – and benefit economically as well

by KEITH REID

I've recently spent some time with the policy folks dealing with the Lake Simcoe Protection Plan, and reducing phosphorus inputs to the lake is a major issue. Since agriculture covers a major part of the watershed, agricultural practices are up for discussion.

There is a lot of debate about the exact effectiveness of various best management practices, and it is easy to get lost in the details. But there are some basic principles that we know will reduce losses of phosphorus from agricultural land. While the impetus for this list is environmental, with the increases in the cost of P fertilizer, some of these will be beneficial economically as well.

Here are seven simple things that you can do to reduce P loss from your fields.
1. Optimize crop response. Fields with high soil tests for phosphorus won't produce any more yield from adding more phosphorus as either fertilizer or manure. Target phosphorus additions to the fields which need it.

2. Don't use more than you need. Manure generally contains more phosphorus than the crop needs if you apply enough to meet the nitrogen needs of a crop. Consider targeting manure rates to P requirements instead, and balance the crop requirements with some nitrogen fertilizer. 

3. Place the P below the soil surface. Phosphorus binds tightly with the minerals in the soil. But if it is laying on the surface, there isn't much opportunity for this reaction to take place. Band or incorporate the fertilizer or manure, so that it doesn't run off in the next rainstorm.

4. Keep the soil in place. Because P binds so tightly to the soil, most of the P movement is with eroded soil. Anything that reduces soil erosion will reduce the amount of phosphorus moving off the field.

5. Keep the P out of areas that drain directly into a stream. The place where excess P causes problems is in lakes or rivers. The highest risk of P moving into these surface waters is from the areas immediately adjacent to the stream, and the areas where water flows off the field during snowmelt or heavy rain.
Use extra care with P management in these areas.

6. Don't spread when it's just going to run off. Most of our runoff occurs during spring thaw. Manure applied onto the surface of frozen or snow-covered ground is at high risk of being washed into neighbouring streams or lakes, carrying a high concentration of P with it.

7. Control point sources from the farm. Runoff from manure storage or tiles that carry milkhouse wash-water represent point sources of phosphorus. These should be controlled by proper storage or treatment. 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