Seedbed: What have we learned about phosphorus?

Monday, October 5, 2009

Some insights from a recent expert forum on how phosphorus is behaving in our lakes and in our soil

by KEITH REID

I recently participated in a forum of experts on phosphorus impacts on surface water, and reducing phosphorus losses from agriculture. It was a chance to hear some new ideas about phosphorus management, as well as confirm some of what we already knew.  Here are a few of the concepts that stuck with me after the forum:

Zebra mussels have changed P cycling within lakes.
Zebra mussels filter sediment and organic material out of the water to feed and this has two major impacts. The first is that the water is much clearer, so water weeds and algae can grow to greater depths in near-shore areas. The second is that the mussels recycle phosphorus near the shore that would normally sink to the bottom in deeper water, and release it in forms that are available to aquatic plants. The net result is that water quality declines close to shore, while the deeper portions of the lake may actually be starved for phosphorus.

It is not clear what this means for phosphorus management, but it is clear that it will be more challenging because it appears to take less P to impair water quality when zebra mussels are present than when they are absent.

Phosphorus bound to sediment is not our only challenge.
Conventional wisdom is that phosphorus binds tightly to soil particles, so controlling soil erosion reduces phosphorus losses. While this is largely true, it ignores the small but significant fraction of phosphorus that leaves the field in the dissolved form.

There is some evidence that no till increases the amount of dissolved P leaving the field, but this appears to be primarily in situations with surface applied fertilizer or manure. The impact of this dissolved phosphorus on water quality will depend on the dominant form of P entering the lake. 

Some lakes on the Prairies receive very little sediment P, since the flat topography and dry climate do not generate much water erosion, so any increase in dissolved P can hurt water quality. Where the biggest phosphorus input into a lake is with sediment, reducing soil erosion is still the best way to improve water quality, even if there is a minor increase in dissolved P.

Seasonality of P losses.

Much of our understanding of sediment and phosphorus losses from the landscape has been developed in areas where most of the runoff is generated by rain storms. Models developed for these conditions fall apart when faced with five months of snow accumulation that all runs off over a two-week period in the spring.

If we are going to reduce phosphorus loading to the Great Lakes, we will need to consider the timing and method of fertilizer and manure application relative to the areas contributing runoff during spring thaw.

The Phosphorus Index works well, but it doesn't do everything.

The P index was developed to identify fields with the greatest risk of phosphorus losses to surface water, so that manure applications could be avoided on these fields or appropriate risk management put in place. 

In situations where there is a lot more manure than crop land, though, the eventual outcome will be that more and more fields become unsuitable for application as the soil P tests increase.

Long-term sustainability means that there must be a balance between the nutrients arriving on the farm as feed or fertilizer, and the nutrients leaving the farm as crops or livestock. This may mean increasing the land area of the farm, or exporting manure to other farms, where the livestock operation is dominated by purchased feed.

In extreme cases, the build-up of phosphorus in the soil may represent another form of depreciation, where the lifetime of a livestock operation is dictated by the ability of the soil to absorb nutrients as well as the condition of the buildings. Once the buildings have worn out, the farm will need to be converted to another use until the nutrient excess in the soil is used up. 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