Soil Testing and 4R Nutrient Stewardship

Each year, farmers aim to increase agricultural production and profitability while conserving our land resources for the next generation. These tandem goals drive sustainable soil fertility and crop nutrition decisions on cropland across the world.

In 2005, global fertilizer industry and environmental stakeholders began developing a standard theme to emphasize science-based stewardship in soil fertility and crop nutrition. The theme eventually became known as 4R Nutrient Stewardship, where each “R” referred to the “right” way to manage nutrients for crop production. The 4Rs are summarized as managing crop nutrition with the 1) Right Source, 2) Right Rate, 3) Right Time, and 4) Right Place.

To successfully implement 4R Nutrient Stewardship, you must start with a high-quality soil sample and an informative soil test. To begin, the fertilizer need and amount is determined through soil testing, which is based on regionally calibrated soil test levels for each crop. If you do not have a soil test, how do you know what the Right Rate is? Using crop removal rates or simply guessing without soil testing often leads to overapplication of fertilizer, cutting into profit.

A conventional whole-field composite soil sample (one soil sample per field) is certainly better than no soil sample. It gets you in the ballpark, but it does not detect variation in soil nutrient levels across the field. You might underapply fertilizer on high yielding parts and overapply fertilizer on low yielding parts. To get the Right Rate applied in the Right Place, precision soil sampling, either grid or zone, is the best way to determine the appropriate fertilizer rate and where to apply it in each field. Precision soil sampling is a proven tool to reduce over- and under-fertilization across fields, thus optimizing crop yield and profitability while reducing the potential risk of soil nutrient loss to the environment.

When you start soil sampling and making soil fertility plans for next year, keep 4R Nutrient Stewardship in mind. AGVISE Laboratories is a proud 4R Partner. To learn more about the 4Rs or become a 4R Partner, visit the 4R Nutrient Stewardship website.

Field Variability Screaming in Your Ear? Precision Soil Sampling is the Answer

Your land is variable. Each fall, you watch the combine yield monitor go up and down across the field. You know where crop yield will be the best in wet years and dry years. So, why do you still use a whole-field composite soil test to manage fertilizer inputs and ignore the obvious field variability affecting crop yield potential?

Precision soil sampling, using grids or zones, divides whole fields into smaller units for soil sampling and creates more accurate and useful soil test information. It tells you exactly where you need to apply more or less fertilizer within each field, unlocking untapped crop yield potential and fertilizer input savings. Grid soil sampling, which is the most detailed approach, typically breaks a field into 2.5- to 5.0-acre grid cells. The more adaptable approach is zone soil sampling, which divides the field into productivity zones that can be managed to their needs. A well-designed zone should represent the smallest practical management unit that still accurately represents the area (e.g. 20-40 acres). Zones are commonly created using data layers such as crop yield, satellite imagery, soil survey, topography, salinity, drainage, or a combination of several data layers.

Precision soil test data can reveal previously unknown production problems, which were otherwise masked in a whole-field composite soil sample. For example, more zone soil sampling has uncovered more and more low soil pH zones (below pH 6) in the long-term no-till areas of central South Dakota, southwest North Dakota, and north-central Montana. Previously, the whole-field composite soil sample had blended the low and high soil pH zones together and everything looked okay. But now, the zone soil samples are revealing where low soil pH is causing serious crop yield loss and where soil pH can be corrected with lime to improve crop yield. This is a good example of precision soil sampling revealing a long-hidden problem and showing us how to fix it.

If you break a field into smaller and smaller units (i.e. more zones), you will learn more and more about field variability. To illustrate the concept, we pulled soil test data from 23,000 zone sampled fields in 2020 and calculated the average soil test range (difference) between the high and low zones within each field. The summarized data is presented in the table.

Average soil test range within a field (high zone – low zone)
Number of zones per field Nitrate-N

lb/acre, 0-24 inch

Olsen P

ppm

K

ppm

pH Soil organic matter

%

3 27 9 88 0.57 1.10
4 38 14 108 0.76 1.52
5 45 17 137 0.89 1.73
6 55 21 164 1.12 1.68
7 61 23 184 1.25 1.59
8 65 24 183 1.26 2.04

As the number of zones increases in a field, the range in soil test values (high zone – low zone) also increases and highlights the true variability across the field. The trend is clear not just for soil nutrients like nitrogen, phosphorus, and potassium, but also for soil properties like pH and organic matter. This tells us that one whole-field “average,” was missing the highs and lows that occur naturally in many fields.

Precision soil sampling is the first step in understanding what is really happening in your fields. You can gain a clearer picture of what plant nutrient deficiencies might be occurring and where you can improve crop yield potential. The next step is creating variable-rate prescriptions for seed, fertilizer, lime, and even herbicides (consider soil pH and organic matter). These tools can help you improve crop yield, optimize crop inputs, and increase profitability within each field on your farm.