A Primer For Phosphorus

Lyle Cowell

Lyle Cowell

Nutrien

Senior Agronomist

Lyle Cowell is based in northeast Saskatchewan, where he has spent his career in agronomy with a focus on better soil management.  Lyle has always had the goal of connecting the three points of good research, agronomy extension and farm gate advice and application of soil fertility principles.

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Phosphorus is an essential nutrient for plant growth which must be accessed from soil or fertilizer.  Understanding the basic principles of phosphorus as a crop nutrient is critical to making best farm fertilizer 4R management choices.

A brief history of early phosphorus research

Dr. John Mitchell at the University of Saskatchewan recorded the first crop response to phosphorus in Western Canada in 1927. His groundbreaking research with radioactive-marked phosphorus fertilizer continued for nearly 20 years, with four main conclusions, which remain the pillars for phosphorus fertilizer management today:

• Western Canadian soils are naturally deficient in phosphorus
• Monoammonium phosphate (such as 11-52-0) is the best form of phosphorus fertilizer for most Canadian soils
• In addition to large yield responses, phosphorus is critical to ensure early maturity of crops in a short growing season
• Crops cannot use a large portion of phosphorus fertilizer applied each year, but can use residual phosphorus from fertilizer applied in previous years

Phosphorus fertilizer application became common practice on farms by the 1940s, and research continued to sort out best phosphorus fertilizer management. Yield responses in this early research averaged over five bushels per acre in wheat, with the largest responses in moist environments, clay soils, and on fallow.

More recent field research has measured smaller yield responses to phosphorus fertilizer. When phosphorus fertilizer is applied each year, a small pool of residual available phosphates can partly supply subsequent crops, and the apparent annual phosphorus response for these soils is reduced. As demonstrated by Dr. Mitchell’s research, phosphorus must be managed as a residual nutrient due to the low solubility of all phosphorus fertilizers. This small bank of soil phosphorus requires replacement each year to account for crop removal.

Why do plants need phosphorus?

Plants use high-energy phosphate bonds to store energy gained from photosynthesis for growth and development of plant cells. This energy is especially critical during rapid growth of seedlings, and for seed formation. When energy for growth or seed production is required, these bonds are broken, and the stored energy is released.

If phosphorus is deficient, crop development and maturity are delayed. This is particularly true during the early development of the seedling. For this reason, farmers often notice a ‘pop-up’ effect of vigorous seedling growth when phosphorus is applied. In most years delayed growth may not affect crop yield. However, delayed crop maturity in some years may devastate crop yield and quality due to fall frosts. If phosphorus deficiency is very severe the plant tissue will appear purple, especially on older leaves. Be careful with this symptom though, as normal genetics, cold damage, and herbicide injury can also cause a plant to be purple.

A final important benefit of adequate phosphorus nutrition is protection from plant disease. Plants that develop slowly provide a larger window for disease infection. Before application of phosphorus fertilizer became routine in Canada, cereal crops were often severely infected by root rot fungi. With the introduction of phosphorus fertilizers, cereal seedlings developed a strong root system quickly and were less susceptible to root rot.

How much phosphorus is required by crops?

Phosphorus is highly concentrated in seed with about 80% removed each year by harvest. According to the Prairie Nutrient Calculator most annual crops remove 25-50 pounds of P₂O₅ per acre by crop harvest each year. Keep these numbers in mind as a guideline for replacing phosphorus with fertilizer.

Phosphorus in soil

Plants’ mineral phosphate (PO₄) must dissolve in the soil for uptake into plant roots. Unfortunately, very little phosphorus in soil is in this readily available form. Most soil phosphorus is held in insoluble organic or mineral compounds. Mineral compounds include various calcium, aluminum, and iron phosphates. Organic phosphates can be part of soil organic matter, crop residues, or barnyard manures. When organic compounds decompose, phosphates are released. As plants remove dissolved phosphate from the soil water, more phosphate is released from these less soluble sources. In the reverse reaction, when soluble phosphates (such as fertilizers) are added to soil, they quickly react to form these less soluble compounds.

The solubility of mineral phosphates and the decay of organic compounds are strongly influenced by soil temperature – in cold soils, less soluble phosphate is available, and the phosphate moves to the roots very slowly. For this reason, it is important to have a good supply of soluble phosphorus fertilizer near the seedling roots in the cold soil in spring.

Forms and reactions of phosphorus fertilizer

Monoammonium phosphate (MAP) has proven to be the best granular form of phosphorus fertilizer for most of Canada. This fertilizer has a nutrient analysis near 11-52-0, depending on the manufacturing process. Smart Nutrition MAP + MST fertilizer (9-43-0-16S) combines MAP with micronized elemental sulfur. When added to the soil, phosphate in fertilizers reacts with soil minerals, which prevents significant movement of the phosphorus to plant roots. Over the course of a growing season, phosphorus moves only 3-5 mm from the fertilizer pellets. For this reason, only about 20-30% of phosphorus fertilizer is used in the year of application. Crops must rely largely on residual phosphorus from previous fertilizer applications. Annual fertilizer applications are an investment in current and future crop production.

Management of phosphorus fertilizer

Let’s review a few important points we know about phosphorus:

• Our soils are naturally deficient in phosphorus, with very large yield responses measured in early field research; with fertilizer application, we provide a small residual pool of available phosphorus for plant use
• Phosphates are not very soluble in soil water and move very little from the fertilizer pellet
• Phosphate movement and plant uptake is slowest in cold conditions
• Phosphates are most important for rapid growth in seedlings
• Crops use about 20-30% of the phosphorus fertilizer applied each year and rely on residual phosphorus fertilizer for the rest of their requirements
• All our annual crops will remove 25-35 pounds of P₂O₅ per acre each year

This combination of factors is the reason we have adopted a standard annual application of phosphorus fertilizer in or near the seedrow. This provides maximum phosphorus fertilizer efficiency to crop seedlings in cold prairie soils. Since phosphorus fertilizer is not very soluble in soil water, it is relatively safe to place near seed. As the crop root system grows and the soil warms, it is capable of using residual phosphates in the soil.