Phosphorus is one of the macronutrients a plant requires to grow successfully, yet the phosphorus recommendations for a particular crop can vary, and a number of factors influence the amount and availability of phosphorus in the soil.
The provincial guidelines for recommended fertilizer rates are based on local research completed within each province.
In some cases, such as with field peas, the phosphorus recommendations are markedly different. In Alberta, Saskatchewan, and Manitoba, the safe amount of seed-placed phosphate is set at 30 lbs./ac., 15 lb./ac., and 20 lb./ac., respectively.
The trials which led to these recommendations likely focused on a narrow seedbed utilization, which is found by dividing the width of the spread of the seed and fertilizer by the row spacing, to mimic the practices of many farmers today; a seedbed utilization of around 10 per cent, for instance, would see seed applied with a one inch spread in nine inch row spacing. A lower seedbed utilization means a greater concentration of seed and fertilizer, leading to lowered recommendations for the amount of fertilizer that can be safely placed with the seed.
According to Dr. Rigas Karamanos, manager of Agronomics Solutions with Viterra, the research generated in each province probably reflects different ecological conditions. “Some of the research done in Saskatchewan,” Karamanos points out, “was done in areas on the drier side.” However, research trials in Alberta, some of which Karamanos was involved in, saw no problem with placing up to 30 lb./ac. of phosphate with the seed. “If you put 30 pounds [down] in Saskatchewan,” Karamanos says, “you’re most likely going to kill the crop.”
Under wet conditions, it is possible to place slightly more phosphate with the seed due to reduced availability and mobility of phosphorus. Even so, the outcome of the provincial research is based on both the experimental and the agro-ecological conditions. There is always an element of conservatism when setting guidelines, so the lowest rate deemed ‘safe’ of all the provincial trials is going to be the official recommendation.
Dr. Jeff Schoenau, a professor in the Department of Soil Science at the University of Saskatchewan and Saskatchewan Ministry of Agriculture Chair in Soil Nutrient Management, notes that many factors influence a phosphorus recommendation. Both the plant itself and the soil in which it is grown play important roles. A crop’s phosphorus demand, for instance, “is a function of its physiology,” according to Schoenau. High phosphorus users include high yielding cultivars of cereals and oilseeds, particularly canola, as well as forage legumes and C4 plants (like corn) with a high metabolism and growth rate.
Soil and phosphorus rates
On the soil side, factors to consider include everything from soil microbiology to the pH of the soil solution. For phosphorus, “solubility and availability are pretty much synonymous,” says Karamanos, and when looking at phosphorus solubility, “pH is the primary factor.” At a low or acidic pH, phosphorus reacts with aluminum or iron and precipitates out of the soil solution; at a high or alkaline pH, it reacts with calcium or magnesium.
Once phosphorus has precipitated out of solution, it is no longer in the plant available form of orthophosphate. Therefore, phosphorus availability is said to be maximized at a neutral pH, with an optimum range of pH 6.5 to pH 7.5.
While acidic soils are uncommon on the Canadian Prairies, alkaline or calcareous soils (basic soils with a high lime content) are not. Some crops such as legumes can be beneficial in improving phosphorus availability in these soils because they can acidify the rhizosphere, the area of the soil near the plant root.
Phosphorus, being a relatively immobile nutrient in the soil and moving anywhere from a few millimetres to a few centimetres in the growing season, needs to be near a plant’s roots to be effective. Soil texture influences the mobility of phosphorus, with orthosphosphate moving more readily in light textured or sandy soils. Consequently, additional starter phosphorus can be important in heavy textured soils, namely clays, as well as in cold or wet soils.
Phosphorus and microorganisms
The activity of soil microorganisms is reduced in cold soils, which can slow the cycling of organic phosphorus from these organisms into the soil. Soils rich in organic matter see more soil microorganisms, but their presence, particularly that of arbuscular mycorrhiza (AM) fungi, can be encouraged through particular farming practices.
According to Schoenau, “the formation of beneficial relationships like AM fungi on crop roots is beneficial in phosphorus availability.” This relationship can be encouraged through good crop rotations as well as through the reduction or elimination of tillage to preserve the AM fungal network in the soil.
Long-term land management, therefore, influences phosphorus availability. Anything that maintains soil moisture, from high cut stubble to catch snow in the winter to leaving the crop residue on the land (as opposed to bailing or burning), will help because moisture is important for phosphorus to move by diffusion and ensure available phosphorus. It is also possible to build up phosphorus in the soil. Soil conservation practices like zero till build organic matter over time and therefore build the phosphorus supplying power of that soil.
A soil which has seen repeated applications of phosphorus fertilizer in the past can, in a year where the price is high, have a reduced requirement of phosphorus fertilizer necessary to maximize yield. Such nutrient mining, Karamanos cautions, cannot be maintained indefinitely.
“People who have been fertilizing with phosphorus probably have a very good labile pool,” which consists of phosphorous adsorbed onto minerals and soil particles and the available phosphorus maintained in the soil solution which is replenished throughout the year by phosphorus in the solid phase, “and can afford for three or four or five years to discount it.”
However, even when fertilizer is put down with the crop, a phosphorus deficit can be incurred because higher yielding crops require more phosphorus than is typically seed placed or side-banded.
Signs of a phosphorus deficiency are not as clear in plants as with other nutrients like nitrogen. “We call [it] a hidden hunger,” Schoenau says, noting that a lack of phosphorus leads to overall reduced crop growth, reduced head size, and reduced number of seeds. In some cases, there will be purpling or a bluish-green colour developing along the leaf margins; because phosphorus is mobile in the plant and can be reallocated to younger leaves, these symptoms would be found on the bottom parts of the plant, among the older leaves. However, the discolouration “is not something you’ll always see in a field, even with yield reductions.”
It is unlikely that any farmer is over-applying phosphorus when only using commercial fertilizer. With repeated applications of animal manures, however, it could be a different story. Manure has a low nitrogen to phosphorus ratio, meaning the relative amounts of each nutrient are about the same, which can be problematic as plants require more nitrogen than phosphorus.
“When we apply manure solely to satisfy the nitrogen requirement,” Schoenau points out, “[we] end up applying more phosphorus than what we can use over time.” While Prairie soils have a good ability to adsorb phosphorus, repeated applications can lead to saturated adsorption sites and the phosphorus will stay in the soil solution.
During the spring melt period or heavy rains, this phosphorus can be moved off the field and into nearby water bodies, causing problems with water quality.
While each field has a different management history and soil makeup and each crop a different phosphorus requirement, the general guidelines given by the various provinces are good rules of thumb. When Karamanos was asked if he agreed with the provincial recommendations, he noted that it is important to look at the research and cited an incident where a farmer exceeded the provincial recommendations, saying, “it didn’t work very well.” †