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Nutrient removal as a soil fertility-planning tool

By Lyndon Hicks, PAg. Regional Crops Specialist

October 2017

Fall soil testing is a great tool to give producers a snap shot of the nutrient levels remaining in their soil, but it is not the only tool. We can also consider using the nutrient replacement method, which involves replacing the nutrients removed by the crop. To obtain consistent high yields while maintaining sustainability in our agricultural systems, we should be using grain to replace lost nutrients. In other words, we need to balance the inputs with the outputs. 

How do we do this you may ask? Knowing which nutrients and how much each crop removes with the grain is the first step. Each crop differs in the nutrient content of the grain, and therefore differs for nutrients removed with the grain. Canadian Fertilizer Institute has a publication called “Nutrient Uptake and Removal by Field Crops” and it gives an estimate of the nutrient uptake and removal rates based on typical nutrient concentrations found in crops grown in Western Canada under good growing conditions. Table 1 is a condensed version of the table that you can use as a guide. 

High protein crops such as peas and canola need a lot of nitrogen. These crops use approximately three pounds (1.36 kg) of nitrogen for every bushel of grain produced. However, it is not all removed in the grain. For example, for every bushel of canola removed, 2 lb. (0.91 kg) of nitrogen is taken with it. In other words, a 50 bushel per acre canola crop uses 150 lb. (68.04 kg) of nitrogen, and 100 lb. (45.36 kg) of the nitrogen is removed with the grain. Using the replacement strategy, which targets a similar crop the following year, the 100 lb. (45.36 kg) of nitrogen that was removed should be replaced. Replacement strategy allows us to replace what we remove. 

The obvious method of replacing nitrogen is through fertilizer application. However, using pulse crops is also a way to manage nitrogen on that field. Pulse crops can fix nitrogen from the air and do not need additional fertilizer nitrogen to produce high yielding crops. In fact, seeding peas on fields where the most nitrogen was removed may be a good fertility management strategy and may save on fertilizer costs. 

It is not only nitrogen that we can use the replacement strategy with, but for other nutrients as well. Phosphorous is one nutrient that the replacement strategy works well with since it doesn’t move much in the soil. Oilseeds and grain legume crops take up and remove more phosphorous than cereals. With canola, every bushel of grain removes 1 lb. (0.45 kg) of phosphorous. With 50-bushel canola this year it means 50 lb. (22.68 kg) of phosphorous has been removed. Peas and cereals remove less phosphorous at approximately 0.7 lb. and 0.6 lb. (approximately 0.32 kg and 0.27 kg) respectively for every bushel produced. Oats and barley remove even less phosphorous than wheat. 

With phosphorous, the only replacement strategies are through fertilizer or manure applications. There are no quick fixes like using pulse crops for nitrogen. Therefore, the replacement strategy works well in combination with soil tests and crop needs.

We may also consider other nutrients, such as potassium, sulphur and possibly some micronutrients. Soil tests are good to use in combination with crop removal rates to ensure we are not depleting our soil resources. 

Using soil tests in combination with crop removal rates will help with not only calculating fertilizer needs next year but also crop choice. A field where 50 bushels of canola was taken off means there will be little nitrogen or phosphorus left. Crop choice in this field should be one that is less reliant on phosphorus (e.g., cereals and pulses) and one that may be able to fix their own nitrogen (pulses). Knowing the crop removal rates as well as the needs of the crop (crop uptake rates) is important for choosing crops and fertility programs for next year.

In summary, when you are planning next year’s fertility program, take into consideration the yields obtained this year and use the nutrient replacement strategy along with soil test results to most accurately predict needs for next year. Balancing the inputs with the outputs is a good step to sustainability and is one of many tools you can use.

For more information:

Table 1.  Rates adapted from Nutrient Uptake and Removal by Field Crops published by Canadian Fertilizer Institute for Western Canada 1998.

Crop

Average Uptake per Bushel

Average Removal per Bushel

 

lbs/bu

lbs/bu

 

Nitrogen

P2O5

K2O

Sulphur

Nitrogen

P2O5

K2O

Sulphur

Spring wheat

2.1

0.8

1.8

0.2

1.5

0.6

0.4

0.1

Winter wheat

1.4

0.6

1.5

0.2

1.1

0.5

0.4

0.1

Barley

1.4

0.6

1.3

0.2

1

0.4

0.4

0.1

Oats

1.1

0.4

1.5

0.2

0.6

0.3

0.2

0

Rye

1.7

0.8

2.4

0.3

1.1

0.4

0.4

0.1

Corn

1.6

0.7

1.3

0.2

1

0.5

0.3

0.1

Canola

3.2

1.5

2.3

0.5

1.9

1

0.5

0.3

Flax

2.9

0.8

1.8

0.6

2.1

0.6

0.6

0.2

Peas

3.1

0.8

2.7

0.3

2.3

0.7

0.7

0.1

Lentils

3.1

0.8

2.6

0.3

2

0.6

1.1

0.2

Faba beans

5.7

2

5.1

0.3

3.4

1.2

1

0.1

Soybeans

5.2

0.9

3.4

0.4

3.8

0.8

1.4

0.1

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