BRANDON – Public concern about water quality has focused attention on nutrients running off agricultural land into rivers and streams. Phosphorus runoff is often cited as a concern and phosphorus concentrations have continued to rise in nearly all water bodies across the Prairies.
Excessive nutrients in water can cause excess algae growth, oxygen depletion that kills fish and toxins that can affect wildlife and livestock that drink the water.
But the problem may not be caused solely by the amount of phosphorus applied to agricultural land. It may be more a question of balance between nutrient application and nutrient withdrawal and how available the phosphorus is to the plant, according to Don Flaten.
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The balance between phosphorus and nitrogen is also key.
During a recent seminar in Brandon, the University of Manitoba soil scientist explained that phosphorus loading into surface water is generally highest in April and May, when most rain and snow melt flows into drainage systems and then into streams, rivers and lakes.
“There are two things the public and government are looking at,” said Flaten. “There’s nutrient management on the farm itself and then there’s management of water leaving the farm.”
He said livestock producers face a challenge because of the high phosphorus content of manure, but crop producers who use commercial fertilizer also face a challenge because drainage systems are engineered to remove excess water from the fields, which ends up in rivers and lakes.
“We’re all in this together. Everybody involved in agriculture has a share in this problem, not only hog producers and feedlots.”
Flaten showed that documented phosphorus levels in surface water are always highest in years of high runoff, which is no surprise.
From 1980 to 1990, phosphorus concentrations in the Red River were measured where the river passes through Selkirk, Man. The long-term trend saw an increase in phosphorus. The check site had 0.28 milligrams of phosphorus per litre in 1980. That measurement rose to 0.45 mg per L in 1990.
The problem is that most manure applied to farm fields has a nitrogen to phosphorus ratio of about three to one. Most crops require a ratio of about four to one.
Flaten said this is how manure applications designed to meet a crop’s nitrogen requirements result in too much phosphorus in the soil.
He said in a typical barley field with an 80 bushel per acre target, the recommended nitrogen rate would be 78 pounds per acre. Using liquid hog manure, the application rate would be 3,305 gallons per acre. That volume of slurry would also put down 70 lb. per acre of phosphorus, expressed as P2O5 in soil test reports. This is more than the crop will need, but it’s in the slurry and can’t be taken out or reduced.
If this barley crop hits the 80 bu. target, it will have removed all of the 78 lb. of nitrogen put down by the slurry, but only about 35 pounds of phosphorus. The remaining 35 lb. of phosphorus stays in the soil. As more hog manure is applied in subsequent years, the phosphorus level continues to rise.
Using grass as an another example, Flaten said three tonnes per acre would be a reasonable target yield for a field fertilized with liquid hog slurry. The recommended nitrogen rate would be 150 lb. per acre, which would require 9,415 gallons of slurry per acre. This volume of slurry would also put down 199 lb. of phosphorus per acre. The target grass yield would remove only 30 lb. per acre of that phosphorus, leaving a surplus of 170 lb. per acre.
“For long-term sustainable balance, the nutrient application rate must equal the nutrient removal rate,” said Flaten. “Soils do not have an infinite capacity to retain or release nutrients.”
The question of long-term phosphorus buildup is important to future livestock expansion and is coming under public scrutiny across Canada. However, it’s not yet a crisis in the three prairie provinces, at least not if buildup is judged by conventional agronomic soil test results.
Statistics from 2002 indicate that 59 percent of Alberta fields have phosphorus levels that are medium or lower than medium. In Saskatchewan, 85 percent of the soil tests show medium or lower phosphorus levels. In Manitoba, 73 percent of soil tests show medium or lower.
But that fails to tell the whole story, said Flaten.
“There is a problem with using P balance by itself for assessing the risk of P transfer into the environment. P balance alone ignores the availability of the nutrient. P in a simple soil test does not reveal the soil’s capacity to retain P.”
Flaten said a more accurate method called degree of phosphorus saturation, or DPS, measures the concentration of phosphorus in the soil relative to the soil’s capacity to retain that phosphorus.
But DPS is new technology and has not yet been calibrated for Canadian prairie soils, landscapes, climate and farming practices.
Nor have the maximum acceptable nutrient accumulation thresholds been determined for these soils.
To further complicate the issue, the risk of phosphorus transfer into the environment depends heavily on factors that reach beyond the realm of soil chemistry.
“Not all land is equally at risk of contributing nutrients to surface and ground water,” said Flaten.
Soil erosion is the most visually obvious method of transferring soil nutrients to water sources. In the United States, soil scientists estimate 60 to 90 percent of the phosphorus loss on cultivated land is attributed to eroded soil particles that retain phosphorus.
During erosion, a soil that is good at holding onto phosphorus contributes the most to phosphorus pollution.
Erosion is considered to be the long-term cause of phosphorus in aquatic systems, which is why control measures in the U.S. focus on reduced tillage and permanent vegetative cover in riparian areas.
Leaching below the root zone is another way phosphorus is transferred to water. This can be significant in situations such as potato fields, where tile drainage is installed and channeled into the local drainage network.
Leaching is also a problem where the soil develops deep cracks, in years when the soil lacks oxygen due to excess water, where the ground water table is close to the surface and when manure is applied.
Runoff carries water-soluble phosphorus leached from dead vegetation or from the soil itself, especially during snow melt, but does not necessarily carry soil particles, said Flaten.
Runoff carries phosphorus into drainage systems even where a growing crop, crop residue or permanent cover protects the soil surface.
“Runoff may account for most P losses from cultivated land in Manitoba and Alberta,” said Flaten. “It is not readily controlled by erosion management because it is not erosion.”
Flaten said in some jurisdictions, such as Ontario and Minnesota, restrictions on phosphorus applications to agricultural land is the law. A farmer must apply for a permit to apply manure to cropland. The formula used by regulators considers a combination of soil test phosphorus data and transport phosphorus information.
“Site specific phosphorus soil tests and an understanding of the phosphorus transfer process have not yet been developed for our prairie soils. But I think it’s safe to say that phosphorus regulations are coming.”
“There’s no doubt some of our agricultural practices are contributing more nutrients to the water, but you cannot trace all our water quality problems to agriculture. You know, we all understand there are some things we can do better,” said Flaten.
“The key will be to make absolutely sure that no nutrient enriched water leaves our farm and finds its way into the waterways. That will be the bottom line.”
