19.4: Managing High-P Soils

High-P soils occur because of a history of either excessive applications of P fertilizers or, more commonly, application of lots of manure. This is a problem on livestock farms with limited land and where a medium-to-high percentage of feed is imported. The nutrients imported in feeds may greatly exceed the nutrients exported in animal products. In addition, where manures or composts are used at recommended rates for providing sufficient N to crops, more P than needed usually is added. It’s probably a good idea to reduce the potential for P loss from all high-P soils. However, it is especially important to reduce the risk of environmental harm from those high-P soils that are also likely to produce significant runoff (because of slope, fine texture, poor structure or poor drainage). Therefore, the environmental context should be considered. If the farm is near a critical water resource that is impacted by field runoff or tile drainage, aggressive measures are needed to reduce the impact. Conversely, small vegetable operations or urban farms on flat ground where fields are surrounded by grass berms or alleyways pose much lower risk, and high soil-P levels are generally more acceptable.

There are a number of practices that should be followed with high-P soils.

• First, deal with the “front end” and reduce animal P intake to the lowest levels needed. Not that long ago a survey found that the average dairy herd in the United States was fed about 25% more P than recommended by the standard authority (the National Research Council, or NRC). Using so much extra can cost dairy farmers thousands of dollars to feed a 100-cow herd supplemental P that the animals don’t need and that ends up as a potential pollutant.

• Second, reduce or eliminate applications of extra P. For a livestock farm, this may mean obtaining the use of more land to grow crops, to spread manure over a larger land area, or to swap fields with nearby farms that don’t have high-P problems. For a crop farm, this may mean using legume cover crops and forages in rotations to supply N without adding P. The cover crops and forage rotation crops are also helpful to build up and maintain good organic matter levels in the absence of importing manures or composts, or other organic material from off the farm. The lack of imported organic sources of nutrients (to try to reduce P imports) means that a crop farmer will need to be more creative using crop residues, rotations and cover crops to maintain good organic matter levels. Also, don’t use a high-P source to meet N demands. Compost has many benefits, but if used to provide N fertility, it will build up P over the long term.
• Third, reduce runoff and erosion to minimal levels. P is usually a problem only if it gets into surface waters. Anything that helps water infiltration or impedes water and sediments from leaving the field—reduced tillage, strip cropping along the contour, cover crops, grassed waterways, riparian buffer strips, etc.—decreases problems caused by high-P soils. (Note: Significant P losses in tile drainage water have been observed, especially from fields where large amounts of liquid manure are applied.)
• Fourth, continue to monitor soil P levels. Soil-test P will slowly decrease over the years once P imports, in the form of fertilizers, organic amendments or feeds, are reduced or eliminated. Soils should be tested every two to three years for other reasons, anyway. So just remember to keep track of soil-test P to confirm that levels are decreasing. Phosphorus accumulates especially rapidly in the surface of no-till soils that have received large applications of manure or fertilizer over the years. One management option in these cases is a one-time tillage of the soil to incorporate the high-P soil layer. If this is done, use practices that don’t result in building up surface soil P once again, such as applying P as starter near the seed and injection (especially liquid manure) instead of broadcast applications.