Ways to reduce dissolved P losses from your soil
Wednesday, June 1, 2011
For a start, applying phosphorus in the spring carries much less risk than applying it in late fall or winter
by KEITH REID
Anyone who has taken a soils course, whether at college, university or farm school, is familiar with the truism that "phosphorus sticks to soil particles, so if you control soil erosion you control phosphorus losses."
Unfortunately, while there is a lot of truth in this saying, life is too complex to be easily captured in a sound bite. There is more and more evidence that a significant part of the phosphorus leaving the landscape is not attached to soil particles, but is dissolved in the runoff water.
Dissolved phosphorus is all immediately available to algae, so the impact on surface water quality is greater and more immediate than from phosphorus bound to sediment (particulate P). To further complicate matters, controlling soil erosion may not reduce dissolved P losses at all, and may even increase them.
When should we worry about dissolved P losses? Every time there is runoff from the surface of the soil, it will carry a combination of dissolved and particulate phosphorus, but the proportion will vary with the topography, climate and past management of phosphorus.
In the Prairies, almost all of the runoff happens during the spring thaw and the flat topography means there is little water erosion, so most of the phosphorus is in the dissolved form. This is the region where there have been reports of no-till increasing phosphorus losses, since dissolved P can leach out of the crop residues and there was little particulate P in the first place.
Contrast this with a thundershower on a freshly planted field in southern Ontario, where the erosion can be significant. In this case, particulate P will dominate the losses from the field.
We should focus our attention on reducing dissolved P losses in areas where erosion has already been well controlled and where a significant part of the runoff is during spring thaw rather than from rain storms.
How can we reduce dissolved P losses? Building up a high level of available phosphorus at the soil surface will increase the concentration of dissolved P in runoff water. This could come from past applications of phosphorus building up the soil test values in the top soil, but more often it stems from fertilizer or manure that has been broadcast on the surface of the soil and left there.
The first step should be to ensure that P fertilizers or manure are either banded below the soil surface or incorporated soon after application. At the same time, ensure that phosphorus applications are matched to crop requirements, so soil P does not build up beyond what is needed for optimum crop growth.
In Ontario, the risk of runoff is greatest during spring thaw, so a much larger area can supply dissolved P to streams and lakes than during the growing season. Applying P in the spring will carry much less risk than applying P in the late fall and winter.
Managing the soil to encourage infiltration of rainfall will also reduce the amount of runoff and reduce the potential for dissolved P runoff. Avoid excessive tillage and maintain good soil structure and adequate residue cover to prevent surface sealing.
Does that mean we should work our no-till fields? NO! Tillage should be a last resort for reducing dissolved P, since it is compromising the benefits from controlling soil erosion.
This not only includes maintaining the long-term productivity of our soil, but keeping sediment out of our streams and lakes, where it can foul spawning beds for fish and silt up the harbours.
The exception to this may be the fields where erosion was never really a problem. Occasional tillage to bury a P-enriched layer on the surface may be of some benefit. Otherwise, it may mean adjusting the way we manage nutrients in our no-till fields to ensure they are placed below the soil surface. BF
Keith Reid is Manager (Eastern Canada), Soil Nutrient and GHG Management Agriculture and Agri-Food Canada, Guelph.