Every five years, an inventory of soil test results from across North America is published by The Fertilizer Institute (formerly International Plant Nutrition Institute). The new 2020 inventory, to be released soon, provides insight into current soil nutrient levels but also includes data from past inventories, making it possible to see long-term trends and identify issues to which producers and their crop advisers should pay attention.
With these trends as a guide, producers can examine their historic and current soil test results to nail down some potential rate decisions for their 2021 crops. Using all available data and tools to the fullest will allow producers to get as close as possible to the right phosphorus (P) and potassium (K) application rates to achieve target yields — without wasting money.
The Fertilizer Institute’s soil test inventory includes millions of test results (about 7.5 million in 2015) solicited from both private and university labs, says Dr. Alan Blaylock, senior agronomist at Nutrien. “Some standardization of the data is always needed to make sure differences in soil-testing methods and so on are accounted for, and then the data is analyzed, and trends can be identified,” he notes. “But this is high-level ‘big picture’ data. It is not intended to provide local recommendations, but it’s a valuable guide to tell us what to pay attention to in terms of our own soil testing and nutrient management planning. Nutrient applications must be established with site-specific testing and local recommendations and conditions.”
Nutrients such as magnesium, sulphur and zinc have been added to the Fertilizer Institute inventory over time (zinc for example has been part of several of the most recent additions) so the long-term trend data for them is somewhat limited, but major nutrients P and K have the longest inventories and most data.
Blaylock says it’s expected that the 2020 inventory will reflect long-term trends identified in the 2015 inventory — there are some areas across North America, including the Canadian Prairies, with a majority of soil samples testing below optimum for both P and K. For more information on levels in your province, you can visit Nutrien’s eKonomics website (www.nutrien-ekonomics.com) and click on the ‘Geographic Data’ section.
Phosphorus in decline
Blaylock says the long-term decline in soil levels of P in the Prairies, where soil is naturally low in P, is an important trend to stay aware of, though there may be exceptions such as areas where large amounts of manure have been applied.
“Phosphorus is a nutrient crucial to maximizing yield in wheat and canola,” Blaylock says. “It enables proper tiller initiation and development in wheat and in the flowering process in canola, but it’s also equally critical for the pulse crops. It plays an important role in the nitrogen-fixing process. For all these crops and others, we have to make sure concentrations are high enough early in the growing season if we want to realize yield potential. Root systems are obviously smaller early in the season and any deficiencies will therefore be magnified when the plants are young.”
Big K uptake in forages
Past inventories indicate potassium is also in decline in some Prairie soils. While Blaylock says Prairie soils generally have naturally high levels of K and response to K in wheat and canola is not common, it could be a concern in higher-rainfall areas like Ontario and other eastern Canadian provinces. Over the long term, higher rainfall contributes to soil weathering and K loss.
There are also significant differences in potassium uptake between cereals and forages. A grain crop may remove 30 or 40 pounds of K per acre compared to a whopping 300 to 400 pounds for forage. Blaylock says the increase in soybean acreage in areas such as Manitoba has also likely reduced K levels. Yields also matter — for example, corn yields have been increasing by two bushels per acre per year, resulting in more K removal in harvested grain. One can get more information on crop nutrient removal using Nutrien eKonomics’ Nutrient Removal Calculator also found at www.nutrien-ekonomics.com.
Crop management practices are another factor. Much of the K absorbed by a crop remains in the straw and not the grain, so leaving crop debris in the field will allow it to eventually return to the soil. Blaylock says while plant roots extract P and K, they’re quite immobile and don’t move easily back down into the subsoil. So no-till/direct seeding, over the long term, can exacerbate the problem of subsoil nutrient depletion by stratifying immobile nutrients at the soil surface, but banding and subsurface applications that place these nutrients in the crop root zone help to reduce the problem.
“There is soil level itself and then we have to factor in everything else,” Blaylock says. “The crop being grown, the yield, what parts of the crop are removed, all this information must go into nutrient-management planning if you want it to be accurate.”
Pricing it out
Over the past few years, some producers haven’t had the financial leeway to add the nutrients they’ve needed to add back into their soil. However, Blaylock notes that this year, good commodity prices are expected and many farmers also made good profits last year. “Many growers are therefore in a position — at least if we get good moisture — to have better cash flow this year to replace some of what’s been mined from the soil nutrient bank.”
China is buying a significant amount of feed grain to return its pork production to normal, and Blaylock says this is pushing global prices up for feed grains as well as other grains and oilseeds. “But because every grower around the world therefore wants to take advantage of the higher demand and prices, fertilizer prices are up too right now because of that thirst for high yield.”
However, Blaylock says there is a good balance between fertilizer prices and the expected value of this year’s crop, so the economics are therefore still favourable to apply inputs at a level to attain high yields. Nitrogen and K costs, relative to the value of the crop, are still similar to long-term averages, and while P costs relative to crop value are a little higher, they are still not greatly different than long-term averages on a relative basis.
Test for the long term
To achieve accurate long-term nutrient management planning, Blaylock urges those farmers who aren’t doing regular soil testing to do so at the same time of year, with a solid grid or zone plan. Soil tests for immobile nutrients like P and K every second or third year may be adequate, but a consistent soil-testing plan is important. Monitoring these soil test results over time will also allow you to create your own soil test result inventory for your farm, monitor any changes occurring, and more accurately establish fertilizer needs.