How much fertilizer does canola need?

Feb. 9, 2012 — Issue 5

March 17, 2016 - Issue 3

John Heard from MAFRD uses these 2-litre bottles to demonstrate how much of N, P, K and S it takes to produce a bushel of canola. N is in the 2 white bottles on the right. Phosphate is in the middle bottle. Potassium (K) is the pink fertilizer. S is the grey fertilizer.

Canola needs a certain amount of nitrogen, phosphorus, sulphur and potassium for each bushel of yield. By factoring the total need for a target yield, growers can subtract soil reserves and estimated organic-matter mineralization to determine fertilizer rates.

Nitrogen

Overall need: Past research and experience show that canola needs 3 to 3.5 pounds of available nitrogen (N) per bushel of seed yield. Based on this, a 50 bu./ac. crop must have 150 to 175 pounds per acre of available nitrogen. New thinking is that higher-yielding modern hybrids are more efficient at converting nitrogen to seed yield. If canola can achieve higher yields the crop may take up closer to 2.5 pounds of available N per bushel.

In that case, a target yield of 60 bu./ac. could be achieved with 150 lb./ac. of available nitrogen. Limited research supports this high-yield relationship to N uptake, but more research would be helpful to support this new equation.

Fertilizer rate recommendations are based on the total need (uptake) minus the following sources:

Soil reserves: Soil tests will indicate soil reserves. Carryover nitrogen can be difficult to predict. Often about half the soil test N is immobilized and not available to this year’s crop.
Organic matter (OM): Mineralization of soil OM will provide available nitrogen to the crop, but the amount varied widely based on soil type, soil OM levels and moisture. In general, OM can generate 6 to 30 pounds of available nitrogen per acre for each percentage point of OM. Rigas Karamanos, a soil scientist with decades of experience in western Canada, uses 14 lb./ac. per OM% and that of that amount, only 80% is available to the crop that year. Using Karamanos’s number, soil with 5% OM will supply 56 lb./ac. of N through the growing season (5 x 14 x 0.80 = 56). He adds that with more than average moisture, mineralization can increase by up to 25% and in drier conditions, it can be 25% lower
Other sources: Lightning strikes actually can add 5 to 10 pounds per acre. The energy from lightning breaks apart nitrogen (N2) molecules, allowing them to combine with oxygen. They dissolve in rain and form nitrate. Careful in banking too much on getting this amount.

Of the N canola takes up, around 60% is removed with the seed and 40% is returned to the field in the residue.

This graph shows how mineralized nitrogen increases with organic matter. Source: R.H. McKenzie and Alberta Agriculture and Forestry, 2012

This graph shows how mineralized nitrogen increases with organic matter. Source: R.H. McKenzie and Alberta Agriculture and Forestry, 2012

Phosphorus

Overall need: Canola needs about 1.25-1.5 pounds of phosphate (P2O5) per bushel of yield, which means a minimum 62.5 pounds per acre of available phosphate for a 50 bu./ac. crop.

Soil reserves: If soil tests show available P levels at less than 10 parts per million (20 lb./ac.), this is considered “low” and growers may want to use a rate higher than crop removal to build soil reserves. At a soil test of 10-20 ppm, rates that match removal are recommended.

Of the phosphate canola takes up, around 67% is removed with the seed and 33% is returned to the field in crop residue. For a 50 bu./ac. canola crop, the “removal” rate would be around 40 lb./ac. of phosphate (80 lb./ac. of MAP, for example). Of that, a safe seed-placed rate of 20 lb./ac. of phosphate will ensure that each canola seed has a phosphate prill or droplet close enough to provide the benefit of early pop up, which is especially important when seeding into cool soils. The rest can go with the nitrogen blend placed outside the seed row.

Organic matter: Each percentage point of OM provides up to 5 pounds per acre of phosphorus. This amount can be highly variable, however it is possible that OM of 4% can provide another 20 pounds of phosphorus through the season.

Sulphur

Overall need: Canola needs 0.55 to 0.8 pounds of sulphur (S) per bushel of yield, making it a high S-using crop. A 50 bu./ac. canola crop needs 30 to 40 lb./ac. of available S. Of the S canola takes up, around 55% is removed with the seed and 45% is returned to the field in the residue.

Fertilizer rate recommendations are based on the total need (uptake) minus the following other sources:

Soil reserves: Sulphur is often highly variable across a field — it might be 10 pounds per acre in one corner and 2,000 pounds in another. If soil is low in S, fertilize according to soil test recommendations. If soil is moderate or high in S, a small amount in the fertilizer blend — say 10 lb./ac. — may be required to offset the high amount of variability in a field.
Organic matter: Expect 2-3 pounds per acre of available sulphur for each percentage point. OM of 4% provides 8-12 pounds of sulphur.

Potassium

Overall need: Canola plants need 2.3-2.5 pounds of potassium (K2O) for every bushel of seed yield. A 50 bu./ac. crop needs 125 pounds per acre of available potassium. Of the K canola takes up, at least 75% is returned to the field in the residue.

For this reason and because our relatively “young” Prairie soils continue to produce plant-available K through break-down of soil particles, crops have rarely shown a response to K fertilizer. But if soil tests show less than 300 pounds per acre (150 ppm), canola may benefit from a potassium application.

Moisture

Moisture is a major limiting factor for canola yield potential and must be included when setting fertilizer rates and budgets. Canola needs an inch of available moisture through the growing season for every 4 bushels per acre of yield potential. A 50-bushel crop will need roughly 12 inches of moisture.

Click your province for a link to soil moisture information:  Alberta Saskatchewan Manitoba

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