Features Fertility and Nutrients
Updating crop nutrient management resources

Understanding particular nutrient requirements and removal rates of individual crops are key to sustainable cropping systems and fertility management. Over time, nutrient uptake demands have changed with advances in high-yielding crop varieties and new genetics. Researchers are in the process of providing a much-needed revision to the crop nutrient uptake and removal guidelines for crops grown in Western Canada, keeping this valuable resource relevant and useful.

“Over the years, the crop nutrient uptake and removal guidelines has been a very important document I use regularly in my research program and in work with agronomists and farmers,” says Fran Walley, professor at the College of Agriculture and Bioresources at the University of Saskatchewan. “At a workshop a couple of years ago there was considerable discussion regarding the widely varying values used to estimate nutrient uptake and removal in harvested crops. We discussed that the published information used by agronomists to estimate nutrient removal had not really been updated since 2001, and once we started looking at the data behind the guidelines, much of it can be traced back to 1992 or even earlier into the 80s. With the changes in cropping management systems since the early 90s and a switch to reduced tillage, along with new crop variety introductions with enhanced yield potential and improved genetics, we decided that revised guidelines reflecting the changes of nutrient uptake demands over time needed to be developed.”

Photo by Bruce Barker.

Walley and her collaborators Rich Farrell, John Heard and Lyle Cowell initiated a project in 2020 to determine and revise estimates of the nutrient uptake and removal of crops commonly grown in Western Canada. The goal was to collect 100 grain samples of each of 14 crops commonly grown in western Canada from commercial farm fields in Saskatchewan, Manitoba and Alberta. The number of samples collected in each province reflects the acreage of the various crops. The sample collection is being repeated in 2021, with 100 samples of each of the 14 crops being collected across the three provinces. In addition, 10 straw and pre-harvest seed samples are being collected in 2021 for each of the 14 crops for straw uptake estimates.

“The sample collection and preparation has been a major undertaking,” Walley notes. “Lyle Cowell with Nutrien has been co-ordinating the efforts working with agronomists to collect samples across Western Canada, along with John Heard [Manitoba Agriculture] for Manitoba. Working with farmers to collect samples from commercial farm fields and related field information about nutrient application rates and yields gives us a robust data set to start to understand crop nutrient uptake and removal rates. Similar to the existing guidelines, the analysis includes the main macronutrients of nitrogen, phosphorus, potassium and sulphur, but we are also adding a suite of micronutrients for analysis to the update.

“Once the guideline revisions are complete, the plans are to develop a user-friendly online and mobile app for determining nutrient uptake and removal estimates. Although there are currently great apps available, the data behind them may not be relevant or specific to Western Canada or to the cultivars being grown here.”

For agronomists and farmers, the crop nutrient uptake and removal guidelines and soil tests are two of the most important sources of information for making fertilizer decisions. The guidelines provide another set of data and more information to determine the nutrient status of the field and the soil in terms of its ability to supply nutrients for the next year’s crop. Soil tests are a really valuable tool for understanding the soil nutrient inventory, what nutrients are readily available in the soil or may become available during the growing season, and an estimate of total nutrients required to meet crop yield goals. Farmers can combine this information with their own knowledge about specific fields and crops in their cropping system to help identify fertility trends and better predict fertility requirements in the future.

“Combining the information from the two sources really helps to understand what is happening in the soil and what the fertilizer requirements are going forward,” Walley explains. “This provides a much stronger decision for making an investment into fertilizer inputs rather than relying on a soil test alone. With nutrient planning, it is increasingly important to think about the nutrients are still in the soil, what nutrients are exported off the field in the grain harvested, what the nutrient content of the straw is and what nutrients are going to be accessible to the crop next year. Crops are not able to extract all available plant nutrients from the soil, therefore for any given yield, the total nutrient supply in the soil, (soil plus added fertilizer) must be somewhat greater than the amount removed by the crop. Without accurate crop nutrient removal values, estimating fertilizer requirements to balance crop nutrient removal is not as accurate as needed.”

The guidelines also provide useful values for planning longer-term strategic soil fertility practices for maintaining or building nutrient levels in the soil. Strategies may be focused on nutrient sufficiency, which is basically applying only what is sufficient to meet crop demands, or to achieve environmental objectives. Or strategies could be implemented to build the soil nutrient reserve back up. Understanding the requirements of a crop and making sure the nutrients are replaced to reduce the risk of losing yield to insufficient levels is important. For example, high-yielding crops such as canola use a lot of sulphur, crops such as soybeans are known to have high removal rates of phosphorus and potassium, or micronutrients can be depleted through continuous cropping. Recognizing the particular nutrient requirements and removal rates of individual crops are key to sustainable cropping systems and fertility management.

“I am really looking forward to having the new revised guidelines completed,” says Walley. “These guidelines have been so valuable to me over the years, not only for understanding what nutrients a crop needs but also for answering other questions that come in. For example, I have had questions about estimating nutrient losses after the stubble in a field was burned, or the impact of baling the straw and removing it from the field and other questions. It is a handy piece of paper and in the near future a handy app to have available.

“This project has been a huge undertaking and has really taken a coordinated effort from a lot of people who are interested in revising that data to make it happen. The collaboration of agronomists and farmers from across Western Canada has really made the project possible. We look forward to completing the final data collection and analysis, and then be able to provide revised estimates of the nutrient uptake and removal of crops commonly grown in Western Canada. Ultimately in the end we will have really robust data specific to Western Canada that farmers and agronomists can use along with soil tests to help make decisions about fertilizer inputs. “