How Does Harvest Impact Soil Nutrition? 

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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After harvest farmers are already making decisions for the following year’s crop. This includes decisions on fertilizer purchases and rates. Perhaps the hardest of the 4R principles to achieve is the ‘Right Rate’. We need to know what is needed for a given yield goal, what is likely available from the soil, and what is the balance of fertilizer application and removal. This of course has to be in context with applying rates of fertilizer that are economical to the farmer yet pose no risk to the local environment. One part of the puzzle is to know how much of each nutrient is required by the crop.  

Uptake or removal of nutrients?

It would be best to know exactly how much of each nutrient a crop needs to grow and create the seed that you will harvest. This is not as easy as one may think. As crops grow, they take up nutrients very rapidly when young and slower as they mature. Nutrients also move within the plant – some nutrients such as potassium and nitrogen are rapidly moved to new growth, while other nutrients such as sulfur or copper are not very mobile in the plant. As crops mature, leaves are often lost – most often older leaves, but in time most leaves will dry and drop off. As the crop becomes fully mature and dries down, nutrients may even be lost from the crop during rainfall as they are essentially leached from the plant cells. And then there are the root systems – these become almost impossible to measure! 

For this reason, we often look to crop nutrient removal through harvested grain as a benchmark to measure nutrient removal. It is easy to collect a grain sample and have the total nutrient content measured in a lab, and new lab methods can measure each nutrient much more accurately. This does not account for total nutrient uptake, but it provides a key measure to balance nutrient addition. Of course, this does not mean that removal must always be balanced by addition. You can start to look at your soil fertility as a bank with balances of addition and removal. If your soil test and your experience tell you that your field is not deficient in a nutrient, you do not need to add any fertilizer. Using grain nutrient removal becomes more valuable for deficient nutrients to help you plan long term fertilizer management. This can also help you understand that soil can become deficient in a nutrient over time. For example, your soil may not have been deficient in potassium when it was first farmed in 1930, so potash was never applied. However, 90 years later you have probably removed close to 1,000 pounds of the most available potassium in your soil and it may soon be time to start adding potassium to your soil bank. 

Where can I find accurate nutrient removal numbers? 

Be careful of the data that you use to estimate removal of nutrients. It is best to have values based on recent management practices, from local soils, with relatively new varieties, and from a source that has reviewed the data for accuracy. An example has been the transition of rapeseed to canola. Management has certainly changed in growing this important crop, and it is likely that the nutrient requirements of canola are different than rapeseed. Even if the numbers are from a local source, be sure the numbers relate to local crops and conditions. 

You should also remember that numbers are not exact. If we use 1.9 pounds of nitrogen removal per bushel of canola, this may be accurate but not precise – the range may be 1.6 to 2.3 pounds of nitrogen. Data based on a very large number of samples will improve precision, but we still need to remember the many variables in play from soil type to weather to crop health. In the world of crop nutrition, we are more often accurate than precise. 

There are two excellent resources to find nutrient removal values: 

• The eKonomics Nutrient Removal Calculator: This tool from the eKonomics team provides a broad base of crops with estimated rates of nutrient removal, and the values are reviewed to be accurate per crop. 
• The Prairie Nutrient Removal Calculator: This tool was created with the measurement of about 2,000 grain samples across Western Canada from 2020 to 2022, led by Fran Walley and Rich Ferrell at the University of Saskatchewan. It is also unique in measuring nutrient removal of some new crops and of micronutrients. Note that the calculator is based on the ‘75th percentile’ of data to help ensure that we do not underestimate removal. The calculator is simple and can be saved as a webpage to your phone, but below is a handy summary of numbers.  

A few notes on removal 

When we look at the data above, a few things likely stand out: 

• Certain crops remove A LOT of nutrients. For example, soybeans remove high rates of all nutrients. Canola removes far more sulfur than cereals. Pulse crops tend to remove a lot of potassium per bushel. 
• Crops like oats and flax are likely under-fertilized. A 40 bushel crop of flax will remove about 85 pounds of nitrogen per acre. A 140 bushel crop of oats will remove 100 pounds of nitrogen. Is this why these crops underperform in many rotations? 
• Micronutrients are essential but removal rates are small. A 60 bushel crop of wheat will remove about 0.02 pounds of copper. 
• Overall – explore the removal rates for your local crops and balance this with good soil samples. 

A good read

Want to learn more about nutrient removal and how harvest impacts soil nutrition? This open-access paper  The Copper Fertility of Saskatchewan Soils (Can J Soil Sci) is the classic research on which soil test recommendations are based on for Western Canada.