Soil Sampling to Optimize the Return on Nutrient Investments

By Dale Cowan, CCA-ON, 4R NMS
Agronomy Strategy Manager and Senior Agronomist
AGRIS Co-operative

While soil sampling has existed for decades, it should still serve as a cornerstone of your cropping operation today.

We have seen the emergence and continued development of data management systems that incorporate artificial intelligence and algorithms to interpret captured data.

These systems should provide a descriptive, predictive and prescriptive capacity to support meaningful management decisions that extend beyond the obvious approach. 

Soil testing may not always be viewed as a high-end technology – especially when compared to today’s site-specific technologies and cloud-based data analytics. However, soil sampling results will be a main requirement as we move forward in utilizing farmer and machine-derived data to improve farm practices and profitability.

Soil test results contain all three elements of a high-tech service.

The results of a soil test certainly describe the soil conditions. Test results show whether a soil is acidic or basic, for example. A soil test lab can measure soil organic matter (SOM) content. Farmers and agronomists can use this data as a surrogate measure of soil health. Details on primary, secondary and micronutrients and cation exchange capacity (CEC) can all offer insights into the soil’s productive capacity.

Soil test results are predictive of crop performance based on the values reported. A low soil pH will limit crop growth of most broad-acre crops we grow. If essential nutrients test low, they will be yield-limiting. We can use pH, SOM and CEC values to select appropriate herbicides to minimize crop response and optimize weed control.

These results are prescriptive since we have calibration data to turn soil test values, such as phosphorus and potassium, into nutrient recommendations. We can use these recommendations to amend the soil in very precise ways. We can examine pH and buffer pH to determine the precise rate of limestone we must apply to adjust the soil pH. We can use SOM to assess nitrogen contribution and offer insights on how zinc, manganese, copper and boron will behave in the soil.

As we embrace sustainability, soil sampling is a cornerstone activity for 4R Nutrient Stewardship compliance. No substitute exists for soil testing. We have no other way to assess the soil’s productive capacity and determine what amendments, if any, are needed to optimize crop performance.

To maintain 4R Nutrient Stewardship compliance, we must use current soil samples no older than 4 years.

Soil test results, much like a balance sheet, indicate the health of the operation. Just like you would not make financial decisions today from a balance sheet from 2017, you should not make nutrient plans with soil test results of that vintage either.

For farmers who simply opt to apply nutrients at a crop removal rate each year, this approach is not without risk.

If soil nutrient rates are low and producers are not aware of these deficiencies, simply fertilizing to replenish crop removal rate will seldom optimize yields.

On the flip side, if soil nutrient rates are excessive, fertilizing at the crop removal rate means producers are increasing the size of their environmental footprints. In the process, farmers may not be optimizing harvested yield and most likely are not maximizing nutrient use efficiencies.

Often, growers base their crop removal rate calculations on last year’s crop. Our build, maintain and drawdown approach to make nutrient recommendations incorporate soil test levels and crop removals to arrive at an optimum rate of nutrient applications. 

Site-specific soil sampling and utilizing yield maps allow a much higher degree of precision nutrient management and 4R compliance. When tests are low we build up with rates higher than removal. When nutrients are at target levels we maintain by applying crop removal rates. When tests are high then we reduce application rates to be able to drawdown high nutrient levels without fear of losing yield. Our approach is designed to produce soil test levels that can meet the demands of the crop while avoiding deficiencies and excessive amounts of nutrients.

A single soil report can provide good information. A series of reports over the years is better.

We can look at multiple years’ worth of reports to find trends in soil fertility and allow for adjustments in amendments. If soil nutrient levels are increasing, then our applications of nutrients exceed what our crops are removing from the soil.

If nutrient levels are declining, the opposite is true. If the values remain constant, we have a balance in our cropping system.

In years when net returns to cropping are marginalized, we can prioritize our nutrient applications. In the fields with high fertility, we can reduce the amendment rates there in the short term. We can reallocate amendments to fields that will need the most help to maintain yields.

When we face years with marginal returns, yield maintenance is vital.

The higher our yields, the more yield we have available to sell at any price. 

The economic and agronomic risks of universally cutting back nutrients on all farms are unacceptably high.

Without soil tests, we won't know which fields are in most need of amendments.

With all the technology we have available today, including soil sampling, field maps, yield records and sorting fields by productive capacity, it is easy for us to custom-blend nutrient programs targeted to address specific field variability.

Producers who soil sample on a regular basis are better managing their production risks and optimizing profits. These farmers are also gaining competitive advantages with more efficient production systems.

The photo above is from an area of a field that was soil tested in the fall. The area had a pH of 5.3. The winter wheat in this area is performing poorly, as the crop grows best at a soil pH of 6.9 to 7.0. The farmer did apply lime but it did not have sufficient time to react and change the pH quickly enough. The soil test results predicted that the wheat would grow poorly well ahead of planting. Utilizing soil testing and proper interpretation in a rotation can avoid costly mistakes.

Carbon Farming

As we venture into carbon farming down the road it will be clear that we don’t trade carbon we trade data.