How to Determine the Recommended Rate of Phosphorus Fertilizer

In the January 18 issue of Seednews, I have discussed developing nitrogen (N) fertilizer recommendations. In this article, I will discuss developing phosphorus (P) fertilizer recommendations.

For many prairie farmers, phosphorus is the second most limiting soil nutrient in grain and oilseed production. When it comes to fertilizer use, phosphorus is second only to nitrogen. Phosphorus is low or low in phosphorus in approximately 80% of prairie soils. A provincial research project in Alberta conducted with wheat, barley and canola found that 81% of wheat sites, 90% of barley sites and 72% of canola sites responded to the addition of phosphate fertilizer at 427 research sites.

Over the past 15 to 20 years, many prairie farmers have applied phosphorus fertilizers at rates below crop removal. This resulted in a slight drop in phosphorus from soil tests. The dramatic increase in the cost of phosphorus fertilizers could exacerbate this concern in 2022.

However, in manured fields, soil phosphorus levels have increased over the years due to repeated manure applications. Manured fields often have the advantage of not needing phosphorus fertilizer due to more than adequate soil phosphorus levels.

Soil phosphorus

Soil testing is by far the best way to determine which fields and crops need phosphorus fertilizer. A soil phosphorus test attempts to estimate the soil phosphorus available to plants, which is the part of soil phosphorus that can be taken up from the soil by plant roots.

Research in Alberta has shown that the Kelowna Modified Soil Test Method most accurately predicts crop phosphorus fertilizer requirements in a wide range of soils. Therefore, the modified Kelowna method is the recommended soil phosphorus test method in Alberta and Saskatchewan. The Kelowna method is the recommended soil phosphorus test method in British Columbia. In Manitoba, the Olsen phosphorus method (also called the sodium bicarbonate method) is the recommended method.

The Olsen method was developed for alkaline soils and is not intended and is not recommended for soils with a pH below 7.0. The Olsen extractor has a pH of 8.5 and therefore does not work accurately with acidic soils. Soil testing laboratories in eastern Canada and the United States often use the Bray soil testing method to make phosphorus fertilizer recommendations. The Bray method was developed for acidic soils and should not be used for soils with a pH greater than 7.0.

The Bray method has not been calibrated for western Canadian soils and therefore should not be used to make phosphorus fertilizer recommendations. The Bray method produces different analytical results compared to the modified Kelowna or Olsen methods. Farmers in Alberta and Saskatchewan should insist that the Modified Kelowna Soil Phosphorus Testing Method be used for their soils to obtain the best 4R guidelines.

The frequency of crop response to phosphorus fertilizers is strongly influenced by environmental conditions, particularly soil temperature and moisture. Therefore, soil tests cannot predict with 100% accuracy when crops will respond to the addition of phosphorus fertilizer.

For example, at research sites in Alberta, the observed response to phosphorus fertilizer, particularly with wheat, barley, and canola, tended to be higher under wetter, cooler spring conditions than under warmer spring conditions. Generally, farmers can expect greater crop response to phosphorus fertilizer in a year with wetter and/or cooler spring conditions than in a spring with conditions are warmer and drier.

Interpretation of phosphorus from soil tests

Across western Canada, crop response to phosphorus fertilizers has been correlated with the amount of soil phosphorus extracted from the zero to six inch sampling depth. Therefore, always sample zero to six inch depth separately from deeper depth samples to accurately determine phosphorus fertilizer requirements. This is also the case for potassium (K).

A soil testing lab will provide the level of soil phosphorus available to plants, usually in pounds per acre. Table 1 above provides a guide to soil phosphorus levels for the modified Kelowna method. Generally, grain and oilseed crops grown on soils with very low or low phosphorus levels have a high probability of response to phosphorus fertilizer.

Phosphate fertilizer recommendations for various crops are provided on provincial agricultural websites for British Columbia, Alberta, Saskatchewan and Manitoba. As an example, I will refer to Alberta Agriculture Agdex 542-3, Application of phosphate fertilizers in agricultural production. Tables 2 and 3 of Agdex 542-3 are provided for wheat. The phosphate fertilizer recommendations in Table 2 (below) are adjusted based on soil moisture content at planting. Table 3 gives an estimate of the probability of response to phosphate fertilizer.

To determine the recommended phosphorus level, simply look at the soil test level in Table 2 and match it to the soil zone on your farm. For example, from Table 2, if you are going to grow wheat in the black soil zone and your soil test level is 35 pounds of phosphorus per acre, the recommended phosphate rate would be 30, 35 or 40 pounds of P2O5 per acre, depending on whether seedbed moisture conditions are dry, moist, or wet, respectively. From Table 3 (below), a farmer can expect a 90% chance of at least a two bushel per acre yield increase and a 70% chance of a yield increase five bushels per acre. These probabilities are based on several years of field research. This information is also available for barley and canola on a probability basis. Phosphate recommendations are also provided for a number of other crops in the Agdex.

Phosphate fertilizer management

Phosphate recommendations are generally based on placement of phosphorus with or near the seed, as most field research has shown that placing phosphorus fertilizer with or near the seed is best for most crops. annuals. It is very important not to exceed the safely placed rate of phosphorus fertilizer for each crop grown. Table 4 (below) shows general safe levels of phosphorus placed in seed. Note that the recommended safety rates vary slightly between the three prairie provinces.

For cereal crops grown on soils that have average to low levels of available phosphorus, phosphate placed in the seed at the recommended rates is equal to or better than banding near the seed and far superior to broadcast and incorporation.

For canola grown on soils with very low to medium levels of available phosphorus, rates of up to 15 to 20 pounds per acre of P205 can be planted safely if seedbed utilization is 10% . For higher phosphorus rates, fertilizer should be either side or mid-row strip at seeding, or striped before seeding. When a field is high in plant available phosphorus, in-seed or banded fertilizer at rates of 15-20 pounds per acre of P205 can increase yield 30-50% of the time, depending on the soil area and environmental conditions.

Legumes absorb phosphorus from the soil fairly well and therefore do not respond as well as grains and oilseeds to phosphorus fertilizers. Research suggests that peas are most sensitive to phosphorus fertilizer when soil test phosphorus levels are below 30 pounds of phosphorus per acre using the modified Kelowna method. Beyond that, the probability of pea response to phosphorus fertilizer is low.

When soil phosphorus levels are high, an annual maintenance application of phosphate fertilizer could be considered to meet crop needs and replenish the soil phosphorus that is removed.

Canola has a relatively high demand for soil phosphorus. It is a non-mycorrhizal crop that uses different mechanisms to absorb phosphorus from the soil compared to other crops. This leaves the soil more phosphorus depleted compared to grain or legume crops. Crops that follow canola tend to be more sensitive to phosphorus fertilizers. Therefore, it is often wise to add an additional 10 to 15 pounds per acre of phosphate in addition to the phosphorus recommendation for grain or legume crops when following a canola crop with a good yield.

For optimum annual production, it is important to place an adequate amount of phosphorus fertilizer near the seed. Most annual crops absorb most of their phosphorus requirements during the first five to six weeks of growth after emergence. Placing phosphorus with or near the seed row with grain and oilseed crops was best for phosphorus fertilizer uptake. When soil test phosphorus is very low to low and when soil temperatures are cool, starter phosphorus in the seed row is often very beneficial for annual crops. However, it is important not to exceed the rate of fertilizer safely placed for each crop.

My next two articles will focus on developing fertilizer recommendations for potassium (K), sulfur (S), and other nutrients. Look for these topics in future issues of Seednews.

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