13 February 2009

4 minute read

North America

High concentrations of soil phosphorus increase the risk of phosphorus runoff loss from agricultural land and lead to problems with surface water quality, such as excess algae growth. However, there is little or no information about the effect of applying different sources or types of manure phosphorus on the risk of phosphorus loss after manure interacts with soil, especially for prairie soils.

Therefore, a study was conducted by Darshani Kumaragamage and Don Flaten, soil scientists at the University of Manitoba, with the objective to quantify and compare phosphorus losses from soils treated with four sources of liquid swine manure and four sources of solid cattle manure, and to relate phosphorus runoff and leaching losses to manure phosphorus forms and soil test phosphorus measurements.

Key Findings, Implications and Precautions

1. Average amounts of phosphorus, in total, were greater in liquid swine manure than in solid cattle manure, when expressed on a dry weight basis. However on a fresh weight basis (as applied) solid cattle manure and liquid swine manure had similar total phosphorus contents. Therefore, to apply the same rate of total phosphorus, similar rates of manure need to be applied with both liquid swine and solid cattle manure.
   
   
2. The proportion of easily extractable or "labile" phosphorus was greater in liquid swine manure than in solid cattle manure. Thus, when manures are applied at the same rate of total phosphorus per acre, a higher rate of labile phosphorus is applied with liquid swine manure than with solid cattle manure.
   
   
3. When nutrients were broadcast and incorporated, runoff concentrations and total losses of soluble reactive phosphorus were greatest from synthetic fertilizer, followed by liquid swine manure and solid cattle manure (i.e., following the same pattern as for the rate of labile phosphorus applied). However, it is important to keep in mind that although this type of laboratory study is useful for comparing the relative availability of phosphorus to runoff from various nutrient sources, it represents a "worst case" scenario, with sloped, unplanted, bare soil subjected to unnaturally high rates of runoff for lengthy periods, as well as phosphorus application rates that are much greater than those that are typically applied on Manitoba farms. Fortunately, typical agronomic practices and weather conditions minimize the risk of loss of phosphorus from the most labile nutrient sources in real field situations. For example, average rates of synthetic fertilizer on Manitoba cropland is approximately one tenth of that used in the study; furthermore, synthetic fertilizer is usually banded below the soil surface, in the seed row, in spring, after our main runoff event (snowmelt) has passed. Much of our liquid swine manure is injected or incorporated; a beneficial management practice that is easily used with liquid forms of manure and that is well-suited to address the risk of runoff phosphorus loss from this manure source. Lastly, although solid cattle manure is practically impossible to inject and not easy to fully bury by incorporation, this type of manure has very low phosphorus availability to runoff.
   
   
4. Leaching losses of soluble reactive phosphorus in sandy soils followed a pattern that was similar to that for runoff losses: greater for soils treated with synthetic fertilizer and liquid swine manure than for cattle manure treated soils. Therefore, to reduce the long term risk of phosphorus leaching into groundwater on these soils, long term rates of phosphorus application should not exceed rates of phosphorus removal by crops, especially for synthetic fertilizer or liquid swine manure.
   
   
5. The sodium bicarbonate extractable phosphorus in manure as well as Olsen phosphorus content in soils amended by manure or fertilizer are good predictors of the runoff loss of phosphorus where runoff is not likely to occur immediately after applying manure. Therefore, these tests appear to be well-suited to situations where manure is applied in spring, summer or early fall, well ahead of snowmelt, the major runoff event in the prairies. Also, Manitoba's current soil phosphorus test for regulatory enforcement, the Olsen test, appears to be a reliable tool for assessing the risk of runoff phosphorus losses, regardless of whether phosphorus is applied in the form of synthetic fertilizer, liquid swine manure or solid cattle manure.
   
   
6. Given its greater chemical and environmental availability, phosphorus from liquid swine manure may be more agronomically available than phosphorus from solid cattle manure. However, the agronomic availability of phosphorus to crops was not evaluated in this study.

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