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Where has the P gone?

By: Shannon Chant, M.Sc., PAg., Regional Crops Specialist,

November 2017

Phosphorus (P) is essential for plant growth, especially during early growth for cell division. P is mobile within the plant, so when P is absorbed in early growth stages, it can be used in other parts during seed formation. In the past number of years, the amount of P in soil tests has been declining. This happens at a higher rate than expected when yields are higher than anticipated. The P may not have been replaced in following years and this led to a decline in soil P levels. Saskatchewan soils tend to be low in plant-available P and soil test summaries have shown that over 80 per cent of fields would benefit from additional P. 

Phosphorus moves very little in the soil so it will not leach like nitrate or sulphate but it will not move to plant roots by soil water. Crop response to P fertilizer varies from one year to another because there are many factors that influence P availability and crop growth. In many cases, not all of the P fertilizer is available in the year of application but it does become available in future years. 

Phosphorus availability is dependent on a number of factors:

  • Soil texture: soils high in clay content fix more P than soils with less clay
  • Calcium carbonate content: more P is converted to less-available P forms in soils that contain more calcium carbonate
  • Soil temperature: low soil temperature will reduce P availability by slowing the movement from soil to the root and by reducing the conversion of organic matter to plant-available 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
  • Plant root type: plants with fibrous roots explore more soil volume in the top six inches than tap roots and are better able to recover P
  • Micro-organisms: some soil micro-organisms can make P unavailable while they break down organic matter. Other micro-organisms form associations with plant roots, which increases the surface area of roots and their access to P. Another type of micro-organisms discharge acids into the soil that can help make P available to roots. This third type is how JumpStart® works

There are calculations that can be done to determine approximately how much P has been used by the crop. These values can also be used to determine an approximate amount of nutrients required for the upcoming growing season using past yields from your farm or from the area for each crop. Fertilizer efficiency must be taken into account when doing the calculations. Efficiency will vary by fertilizer type.

The uptake of phosphate (P205) per bushel per crop is:

  • wheat: 0.7-09 pounds
  • barley: 0.7-0.8 pounds
  • canola: 1.3-1.6 pounds
  • flax: 0.8-0.9 pounds
  • pea: 0.8-0.9 pounds
  • lentil: 0.7-0.9 pounds

More information on phosphorus production is available in the fact sheet Phosphorus Fertilization in Crop Production.  You can also contact your local Regional Crops Specialist or agronomist.  

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