Phosphorus Fertilization in Crop Production

Plant P

Phosphorus (P) is essential for plant growth and is required in large amounts by plants. P fertilizer is applied on about 85 per cent of Saskatchewan's cropland. Saskatchewan soils tend to be low in plant available P and soil test summaries indicate more than 80 per cent of fields would benefit from additional P.

Plants need P throughout their life cycle, especially during early growth stages for cell division and for the N-fixation process for pulses and legumes.

P is mobile in the plant, so it is absorbed during early growth and is later redirected for use in seed for­mation. Cereals, for example, rely heavily on the P taken up in the first four weeks of growth for crop establishment, tillering and final yield.

Plants absorb P as orthophosphate ions (H2PO4 and HPO4²-).

Table 1. - P uptake (lb. P₂O₅/A) by selected Saskatchewan crops

P Fertilization

Phosphorus is relatively immobile (moves very little) in the soil. Thus, it will not leach like nitrate and sulphate or be carried to plant roots by soil water. Furthermore, P from phosphate fertilizer will readily react with soil minerals making it less plant avail­able.

When granular phosphate fertilizer is added to moist soil, it quickly dissolves releasing orthophosphate ions to the soil solution. Most liquid P fertilizer is ammonium polyphosphate, which when added to soil reacts with water to form orthophosphate. Over time these ions react with calcium and other ions forming less plant available P compounds. No difference has been recorded in crop response to dry versus liquid fertilizer P formulations.

Manures are a source of inorganic and organic P. Organic P must be mineralized to orthophosphate to become plant available. Some manure, especially solid manures, have a low N:P ratio relative to crop requirements, so application of manure alone to match the crop N requirement can lead to P build up. Refer to "Managing Manure as a Fertilizer" Farm Facts.

Most crops recover only 10 to 30 per cent of the P in fertilizer the first year following application. Recovery varies widely depending on soil type and conditions, the crop grown and application method. However, Saskatchewan research has shown that the newly formed "less available P compounds" are more plant available than the "native soil P minerals". Thus, crops can continue to recover fertilizer P for several years after application and over the course of many years, utilization of fertilizer P will approach 100 per cent.

P availability is dependent on several factors.

Soil texture: soils high in clay content fix/absorb more P than those with less clay.

Calcium carbonate content: more P is converted to the 'less available calcium phosphate compounds' in soils containing more calcium carbonate.

Soil temperature: low soil temperature will reduce P availability by slowing the movement of P from the soil to the root and by reducing the mineralization of organic matter to plant available inorganic P.

Soil moisture: P is more plant available with 'good' soil moisture.

Soil pH: P is most plant available between pH 6.5 and 7.0. In soils with low pH (acidic) compounds of aluminum and iron may form which have low plant availability.

Plant root type: plants with fibrous roots explore more soil volume in the 0 to 6 inch depth than tap roots and thus are better able to recover P.

Microorganisms: Soil micro-organisms can immobilize P while breaking down organic matter, making it temporarily unavailable. However, once the organic matter decays the phosphorus is released (mineralized) making it available to the next crops. Other microorganisms form associations with plant roots which increases the surface area of roots and their access to P. Others discharge acids into the soil which can help to solubilize some P minerals.

Phosphorus solubilizing Inoculants

Several phosphorus solubilizing inoculants are registered for use on many crops. They are available in liquid, peat and granular forms.

Phosphorus solubilizing micro-organisms aid P utilization efficiency, but are not a substitute for a long term P fertilizer program, because they have no residual benefit. Fields should be monitored by soil testing to ensure that adequate background levels of P are available.

When the amount of P fertilizer that can safely be applied with the seed is less than the amount that is recommended by soil testing, seed inoculation can offer added flexibility.

Crop Response

Crop response to phosphate fertilizer varies from one year to the next because of the many factors which influence P availability and crop growth. Increased P uptake by the plant does not necessarily provide for an increase in yield. However, hundreds of Saskatchewan trials have shown, in the long term, an average wheat yield response of about five bu/acre to 20 lb actual seed placed phosphate fertilizer per acre. Trials have also shown the crop response may be as high as 10 bu/acre.

Granular monoammonium phosphate (MAP) (e.g. 12-51-0 or 11-55-0) is the most common P fertilizer used in Saskatchewan. The most common liquid P fertilizer is ammonium polyphosphate (APP) (e.g. 10-34-0). They contain nitrogen as well as P in the phosphate form.

Diammonium phosphate (DAP) (e.g. 18-46-0) is occasionally found in southern Saskatchewan. However, the P in DAP is less available than the P in MAP and APP on high pH soils. Most Saskatchewan soils have a high pH. Not as much DAP can be safely placed with the seed without causing germination or seedling injury.

Table 2. Banded phosphate fertilizer recommendations for zero to six inch samples collected from dryland fields with normal moisture conditions*

Application Methods

P fertilizer can be applied in several ways:

Seed placed

Seed placed fertilizer is the placement of fertilizer directly in the seed row with the seed. Fertilizer will be more or less in a band depending on the width of spread of fertilizer and seed.

For cereal crops, 50 lb/acre of actual phosphate P₂O₅, the maximum annual recommendation, can be safely seed placed. For other crops, see Table 3. Exceeding the safe rates can cause reduced germination and seedling injury. For more information on safe rates see the Ministry fact sheet Guidelines for safe rates of fertilizer applied with the seed.

Table 3. Maximum safe rates of seed placed monoammonium phosphate fertilizer (lb. actual P₂O₅/A)*

Banded near the seed

Banded near the seed refers to the placement of fertilizer in a band near but not with the seed, during the seeding operation. With side banding the fertilizer is placed one to two inches to the side and/or one to two inches below the seed. With below banding, the fertilizer band is one to two inches directly below the seed.

Whereas a limited amount of fertilizer can be safely seed placed, all (dry and liquid) recommended fertilizer can generally be safely banded near the seed provided at least one to two inches (depending on moisture conditions) of firm soil separates the seed and fertilizer.

Banded

Banded fertilizer is placed in narrow bands in the soil in fall or in spring prior to seeding. This method is not recommended as the only placement for P fertilizer because placing the P more than 1.5 inches from the seed can limit early supplies of P.

The availability of P is considerably enhanced when it is applied in a common band with Nitrogen (N), often referred to as a dual N-P band. Dual N-P banding all the fertilizer prior to seeding works best on warmer soils. On soils that are cooler at seeding or on soils that are quite P deficient, all or at least some of the P should be applied with or near the seed. If very high amounts of N are applied in a dual N-P band at or just prior to seeding, some delay of P uptake may occur.

Broadcasting and incorporating

Broadcasting and incorporating low rates of phosphate fertilizer is not as effective as placing phosphate with the seed or band­ing phosphate because of the likelihood of more rapid conversion to less available P compounds and insufficient contact with the small seedling root system. However, as application rates increase, the difference between application methods decreases.

Broadcasting and incorporating large one-time applications of phosphate fertilizer to build back-ground soil P levels has proven effective on Saskatchewan soils. A low rate of annual seed placed phosphate is often still needed for optimum yields.

Top dressing phosphate is acceptable for established forages. Dribble or coulter banding and nesting in established forages may increase P efficiency.

Mid-season foliar P applications for annual crops would be most suitable for a top up of P nutrition when applied in small amounts under conditions of soil P deficiency, rather than as a substitution for spring soil applied P fertilizer.

4R Phosphorus Management

The industry developed 4R nutrient stewardship plan; the Right Source, applied at the Right Rate, Right Time and Right Placement allows for phosphorus to be managed to optimize crop yield and quality while minimizing the risk of P movement into rivers and lakes.