How to counteract the looming phosphorus deficiency



  • Phosphorus is essential for many critical plant functions, but it’s often under-applied and, in many cases, soil levels are being depleted
  • There are resources to help you monitor nutrient removal for all crops so you can better understand how much you need to apply
  • Phosphorus is typically applied with the seed or banded near the seed row, but row width and the type of crop are all factors to consider

Phosphorus is essential for many critical plant functions, but it’s often under-applied and, in many cases, soil levels are being depleted. So much so that well-known soil scientist Rigas Karamanos often refers to it as the forgotten macronutrient.

Karamanos has enjoyed a long career in both the private and public sectors and is the senior agronomist for Koch Agronomic Services based in Calgary. Simply put, Karamanos says crops in many fields across the country are using more phosphorus than what’s supplied in fertilizer applications. Gaps are accentuated in years with high yields.

Depleting soil P levels is like deficit spending. If it continues for too long, it can be very difficult and expensive to climb out of the hole.

While Ontario has some livestock-intensive areas where P levels are too high, Tom Bruulsema, phosphorus program director for the International Plant Nutrition Institute in Guelph, reports that it’s more common to find levels in decline on cropland growing corn, soybeans and wheat.

To monitor nutrient removal for all crops, the Institute has a handy mobile app for iPhone and iPad devices called the Crop Nutrient Removal Calculator.

Why is phosphorus deficiency a concern?

The application of nitrogen has increased dramatically over the years, while phosphorus usage has remained largely constant. More nitrogen can provide an immediate and obvious yield boost when other factors are not limiting. Higher levels of P, beyond the starter rates typically applied with the seed, may not provide much of a yield boost in the year of application.

However, over time the soil will become increasingly phosphorus deficient if crop removal rates consistently outpace what is being added. Studies show that depleted levels can decrease crop yield potential, and you can’t entirely compensate for this by a one-time fertilizer application.

What factors contribute to phosphorus deficiency?

In Manitoba, depletion is a trend that worries soil scientists. A recent factsheet on the topic was co-authored by John Heard (Manitoba Agriculture Food and Rural Development), Cindy Grant (Agriculture and Agri-Food Canada) and Don Flaten (University of Manitoba).

The factsheet notes many cropping plans now include more canola, soybeans and corn, which remove more phosphorus than cereal crops do.

Phosphorus for the short or long term?

Heard, Grant and Flaten address the sufficiency versus sustainability issue with analysis based on the Ontario Soil Fertility Handbook.

In a short-term sufficiency approach, the cost of phosphorus is paid for by the yield increase in the current crop year, and no economic value is given to the residual effect of fertilizer. This approach is most common with short-term land tenure, such as annual land rental. It may also be the approach when crop prices are low relative to P prices, or if the land has limitations to yield other than fertility.

With long-term sustainability, there’s recognition that the nutrient applied has residual benefit to subsequent crops. This approach is suited to long-term land tenure and high-yielding, high-value crops. Soil phosphorus levels tend to stabilize in the medium range, rather than dropping to low levels.

How is phosphorus applied?

Phosphorus is relatively immobile in the soil, and germinating plants have a high requirement for it early in the growing season. That’s why the nutrient is typically applied with the seed or banded near the seed row. Row width, the crop and other factors determine how much can safely be applied with or near the seed.

For Prairie farmers, Rigas Karamanos points to a calculator developed by South Dakota State University that may aid in seed row placement decisions.

Don Flaten says that for phosphorus not placed with the seed, banding is a better strategy than broadcast applications. Especially in the fall, phosphorus applied to the soil surface is susceptible to loss through runoff, and that can contribute to environmental issues downstream.

Flaten is concerned with what he calls “naked planters” that don’t allow for starter phosphorus with the seed or for banding. He points out that such planters can be retrofitted.

In Ontario, the practice of placing dry starter fertilizer in a band two inches below and two inches to the side of the corn seed has given way to a small amount of liquid “starter” or “pop up” fertilizer applied with the seed. Convenience and capacity are the drivers. As a result, less phosphorus is being applied at planting, and application is done primarily by broadcasting – not the optimal way to maximize the benefit of the nutrient.

Many farmers don’t apply any phosphorus when growing soybeans. While an increasing number of Ontario farmers use air seeders for wheat, which enables application with the seed, many still use box drills with no fertilizer capability.

“The trend in Ontario is towards large planters without dry fertilizer capability, more broadcasting, but less tillage. The result is more P concentrated in the top few inches of soil where it is vulnerable to runoff,” Bruulsema says.

The phosphorus challenge in Ontario is two-fold: maintain productive soil levels and reduce loss to water sources in the Great Lakes basin. Significant algae blooms in Lake Erie in 2011 and 2013 have put renewed emphasis on monitoring the amount of phosphorus that may be finding its way from farmland to waterways.

Takeaway messages

Phosphorus depletion is an issue we ignore at our peril. Yields will gradually erode as soil phosphorus becomes a limiting factor. As John Heard says, phosphorus fertilizer is a conversation, not a single answer. Your equipment, crop rotation, tillage practices and even land tenure are all part of the equation. “Depleting soil P levels is like deficit spending. If it continues for too long, it can be very difficult and expensive to climb out of the hole,” Bruulsema says.

From an AgriSuccess article (March/April 2016) by Kevin Hursh (@KevinHursh1) and Peter Gredig (@agwag).