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The Power of Soil Testing

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The Power of Soil Testing

Today’s high crop yields are demanding more from our soils than ever before. Modern genetics and improved crop management through practices like 4R are driving better nutrient use efficiency and returns. However, with big yields comes bigger crop removals, and basing 2021 crop needs on historical averages may not be enough. Soil test surveys in Eastern Canada have shown soil test P & K levels have moved declined particularly for potash. ( Soil test surveys in Western Canada reveal significant declines in soil test P levels in many areas

The impact high yields have on succeeding crops will depend on the current state of soil test values, and the variability in test values across a field.  High field averages don’t reveal problem areas with an imbalance or below critical level; limiting yield. Felix Weber, Ag Business & Crop, puts it this way “Fertilizing without consistent soil testing is like driving in the dark without your headlights on. Testing allows you to prioritize where best to allocate your fertilizer dollars”.

Sufficiency vs Build-Maintain. There are different approaches to making fertilizer recommendations. Historically, many provincial and university recommendations used the sufficiency approach, supplying just enough P & K that maximize profitability in the year of application. This approach focuses on applying just enough fertilizer to maximize profit in the year of application, without regard to future soil test values. The sufficiency approach is best suited to rental land and those years where short-term cash flow is a concern. There is a growing body of evidence that supports a Build-maintain approach, also called target and maintenance. Build-maintain takes a longer-term approach that targets a critical(ideal) soil test value that meets crop needs and fertilizing to build up or drawn down to this level. Ontario trial data is indicating that the highest economic yields in a corn-soy-wheat rotation in all three crops are achieved with a build and maintain approach. Even where extra fertilizer was applied to lower testing fields, it still does not match the yield obtained from build soils ( Research is still defining what that ‘critical level’ is for different crops. In Western Canada, the critical level of P is 15 ppm, below which P efficiency is decreased. What’s interesting about the build-maintain approach is that it can be achieved using the 4R approach to sustainable fertility over the whole cropping cycle. To get there requires having reliable site-specific soil tests, yield, and other data to properly address in-field variability and drive profitability across the whole field.

Agronomist Perspective

For Stephane Gagnon, Agronomic Data Manager, synAgri, Quebec soil testing is the cheapest input on the farm and offers the highest return on investment. Stephane sees the majority of growers requesting bulk sampling, but this is shifting to GPS-tracked sampling and to a lesser extent grid sampling. For Stephane, growers are seeing immediate yield responses from these methods by being able to track changes in soil test values, and better plan fertilizer purchases. The agronomist can also then help the grower to address problem areas like low pH which is common in many areas of Quebec. Synagri’s soil test laboratory, is also seeing declining soil tests, with 75% of samples testing low in potash, and 50% needing lime Stephan uses the example of using GPS sampling to correct soil pH with a variable lime application; by only applying a single flat rate of lime to a field, each third receives either not enough, too much or just right. Stephan echo’s that they see the highest economic returns from a build and maintain approach; it helps growers focus on long-term soil fertility. Stephan explains it this way, “in conditions of low fertility the probability of roots finding enough of a nutrient is low and the best fertility strategy is to focus on building the soil to optimum fertility level and there-after feed the soil to feed the crop and maintain these test levels; the 4R approach fits this perfectly.” Stephan and Synagri were early adopters of the Wintex soil sampler, “yes, to do more sampling, but mainly because of the consistency in sampling depth, that provides us with the greatest assurance of the quality of the soil sample. Without quality samples, we have nothing.”

Marilyn Kot, Green Acres Tech, Saskatchewan takes a different approach to soil sampling that fits the larger acreage that her clients operate. “For me, GPS marked benchmark sampling allows me to accurately track changes in soil test values, leading to better fertility planning and crop decisions. The most difficult fields to make fertility recommendation is in the first year when we don’t have a field soil test history.”

Soil test results are only as good as how they are collected and handled before shipping to the lab. Marilyn is fastidious about the accuracy and detail of proper soil sampling.  She feels it is an area there is not enough attention being paid to its importance. Since soil testing labs may only use a teaspoon from a submitted soil sample bag, how that sample has been collected determines how accurate the results will be. Soil sampling is the greatest source of error in soil test accuracy. To ensure reliable results, Marilyn begins with collecting between 15 to 25 cores, thoroughly mixing those before subsampling for the smaller sample required by the lab. Marilyn was the first to acquire a Wintex 2000 soil sampler that allows her to accurately sample fields at depths up to 24 inches. The first year a field is sampled she prefers to collect a 0 to 6, 6 to12, & 12 to 24-inch depth sample that provides a profile of nutrients and soil characteristics. She also has acquired a Dualem mapper and RTK GPS that allows for mapping of soil texture, salinity, accurate elevations for topography, and water modeling. For Marilyn, it is the next step for clients towards better understanding and managing the variability in fields and to manage crops for that potential long term.

Felix: “Fundamentally, accurate GPS sampling is the starting point for consistent results. Better data provides greater confidence in test values, helps track changes and trends in soil test values for all nutrients over time, use fertilizer $ efficiently; and not leave yield or income on the table.” Contact:

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