Seed-placed phosphorus can give plants an extra boost early in the season, but it’s important to think about your entire rotation
Seed-placed fertilizers give plants that extra boost early in the season and are especially useful to provide the phosphorus that plants need right from the get go. But with the introduction of new crops like soybeans into Prairie rotations, seed-placing phosphorus has become more complex.
Soybeans and canola are both highly sensitive to seed-placed fertilizers. Without the seed-placed fertilizers, it can be difficult to meet the nitrogen and phosphorus needs of these crops early in the growing season.
Although phosphorus placed with the seed isn’t toxic, the nitrogen that comes with it is toxic to the emerging seedling. So, especially if canola or soybeans are grown continuously or are grown in rotation with each other, soil reserves of phosphorus can be depleted over time. The need to significantly reduce or eliminate seed placed fertilizers to avoid seedling damage can deplete phosphorus reserves and reduce yield potential over time.
The role of phosphorus
Phosphorus and nitrogen react differently in the soil. Surplus phosphorus is not likely to leave the soil in large amounts because it is not very mobile, which means that unless there are erosion losses or it moves dissolved in surface water it stays put where it’s placed for the most part. Although the amount of phosphorus that can be lost by erosion or run-off is usually minimal and doesn’t have a huge impact agronomically, it can have detrimental environmental effects.
Phosphorus is essential to the energy functions of plants and is needed right from the beginning to establish plant growth. Because it’s not mobile in the soil there has to be an adequate supply of phosphorus close enough to the seed for the emerging roots to easily access. In order to meet the starter phosphorus needs of sensitive crops such as canola and soybean, using the commonly available monoammonium phosphate or ammonium polyphosphate fertilizers, too much nitrogen would need to be applied, which would cause significant seedling injury and reduce plant stands.
Traditionally when most western Canadian farmers grew longer rotations that included cereals and other crops the uptake and removal rates of nutrients like nitrogen and phosphorus remained fairly balanced across the prairie landscape. In these rotations more fertilizer could be added to the cereal crops that would then remain available to subsequent crops like canola, where there wasn’t the same opportunity to use seed-placed fertilizer at the higher rates required.
But as more and more farmers switch to continuous canola and/or canola and soybeans in tight rotations the balance of nutrients, particularly phosphorus is not being maintained because rates of seed placed fertilizer have been reduced to prevent seedling damage and soil phosphorus has been drawn down to make up the deficit. When that soil phosphorus isn’t replaced at the same rate of removal a phosphorus deficit occurs.
Increasingly soil scientists and agronomists are looking at the need to fertilize to meet the complete needs of a diverse rotation rather than just trying to meet the needs of a single year’s crop. Balancing out the nutrients in this way ensures all crops in the rotation, over multiple years, get what they need and precious soil reserves, especially of phosphorus, are not seriously depleted over time.
Cynthia Grant, a soil management and fertility scientist at Agriculture and Agri-Food Canada’s Brandon Research Facility has studied phosphorus depletion in soils over many years. When there is an inadequate supply of phosphorus being added as starter fertilizer (as is the case with sensitive crops such as canola and soybeans) residual phosphorus in the soils will be depleted as the crops use it to meet their needs. Over time, says Grant, if this process is repeated the phosphorus buffer in the soil can become so depleted that crop yields suffer. Research by the University of Minnesota has shown that soils which become too depleted of phosphorus will not attain full productivity, even when attempts are made to replenish phosphorus levels using starter phosphorus.
Maintaining soil phosphorus at reasonable levels through the entire rotation is crucial to balance out the nutrient needs of different crops and maintain adequate fertility over time. Ideally, the rotation should include a mix of crops like canola and cereals. In years where cereals are grown there is an opportunity to increase the amount of fertilizer applied and help make up for any phosphorus deficit that is created when little or no seed-placed fertilizer can be used on sensitive crops like canola.
Studies have confirmed that excess phosphorus applied on cereal crops to target optimum yield does remain available in the soil for subsequent crops. Phosphorus does become less available over time as it’s tied up by calcium, magnesium or aluminium in the soil and becomes less soluble. Only around 30 per cent of phosphorus is used in the year of application. The remainder, as long as it’s not lost from the landscape in run-off or via erosion, will be converted back into a soluble form and recovered by the growing plants in subsequent seasons.
A number of long-term studies have shown that 80 to 100 per cent of applied phosphorus can be recovered over time. As long as farmers soil sample regularly to monitor the level of residual phosphorus in the soil they can develop a strategy to replace phosphorus through the rotation and balance what they apply with what the crops remove over multiple years to maintain a balanced fertility program.
“Soil test phosphorus allows you to see a change in phosphorus over time by monitoring your phosphorus level to see if it’s going up or down,” said Grant. “You want to maintain reasonable phosphorus soil levels for optimum yields and to give a phosphorus buffer for crops like canola and soybeans in the rotation.”
There will often still be a need for starter phosphorus early in the growing season, in which case side banding is recommended. Manure provides a good source of phosphorus that is released slowly. The ratio of nitrogen to phosphorus in manure is 1:1 or below and plant uptake is generally 2:1 or 2.5:1. Manure applied at a rate to meet the nitrogen requirements of the crop will provide enough P in soil reserves for several years.
“We need to maintain our soil phosphorus at reasonable levels,” says Grant. “Phosphorus deficits can reduce potential crop yields, particularly if you are reducing your seed-placed phosphorus to lower levels for sensitive crops and you don’t have any buffer of phosphorus in your soil to maintain the fertility over time. If you are putting on lower levels and the plant can’t find the phosphorus it needs in the soil you are going to drop your yields.” †