How to Use Soil Sampling to Improve Nutrient Levels and Crop Yield

Crop Nutrition

How to Use Soil Sampling to Improve Nutrient Levels and Crop Yield

eKonomics News Team

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Imagine trying to diagnose and correct the problems limiting your crop yields without accurate and relevant information. Jim Schwartz, Director of Practical Farm Research (PFR) and Agronomy at Beck’s Hybrids in Atlanta, Indiana, says it’s impossible to do without intensive soil sampling and analysis to understand nutrient levels and trends.

Farmers can use soil sampling to create a cost-effective plan for managing nutrients, especially those that offer the greatest Return on Investment (ROI) if managed properly like phosphorus and potassium. By sampling, farmers learn about the amount of nutrients available to their plants and how nutrient levels change over time. Armed with these results, farmers can follow three steps to make decisions about investing in fertilization, improving plant health, and increasing crop yields.

1. Test soil regularly and consistently

Nutrients in a large percentage of soils across North America are becoming depleted at an alarming rate, causing many areas to fall below the critical level for potassium, as well as phosphorus. Record and near-record yields from high-performance hybrids deplete soil nutrients at an increasing rate while farmers have reduced fertilization to control production costs.

To reverse these trends, farmers should consider whether they need to invest in fertilizer before their yields and profits are harmed. Soil sampling is the key to deciding how much phosphorus and potassium to apply, and where. Sampling identifies fields – and areas within fields – that have been depleted.

“You don’t have to test every year, and you don’t have to test every acre. But every year you have to test some of your acres. Otherwise, your fertilization is just a random guess,” says Dr. Brian Arnall, Associate Professor at Oklahoma State University who specializes in precision nutrient management.

Guessing is expensive because fertilizer is one of the largest production costs for growers. Corn growers, for example, can spend roughly a quarter of their budgets on fertilizer per year.

Not applying enough fertilizer or skipping applications can be expensive and limit returns on what inputs are spent. Avoid these issues with accurate soil data. Sample and analyze in each field every two, three or four years to match the crop rotation cycle. Samples may be needed annually with extreme variances in nutrient levels.

Defining critical nutrient levels

A critical level is the soil-test level above which response to added fertilizer would not be expected.  Above this point, the recommended amount of a nutrient to be applied is zero in sufficiency fertilization approaches or crop removal in build-maintenance fertilization approaches. Critical levels vary by nutrient and across geography.

2. Improve the quality of soil sampling with technology

Agronomists and certified crop advisers encourage farmers to sample soils at the same time of year and at the same depth to establish a baseline of consistent results that can be compared year over year. To establish phosphorus and potassium levels, for example, the sample depth is six to eight inches. The goal is to collect enough samples to create a map of each field. Historically, farmers have applied nutrients at the same rate across the field. However, with more intensive soil sampling strategies in combination with yield maps, farmers can apply fertilizer where the need it most, increasing their ROI.

Whatever sampling method is used, collect multiple cores near each grid point or within each zone. Check out this article on looking at the best resolution. Whatever sampling method is used, collect multiple cores near each grid point or within each zone. Dr. Arnall recommends 15 cores from each area. This method can add up to hundreds of cores from each field, but the higher volume of cores ensures adequate covering of variability within the sample area.

Field soil test variability can occur for several reasons. At the top of that list is historical applications of manure, which can sometimes prove hard to uniformly apply. Other reasons for variability can include varying yields and removal rates across a field. For these reasons, it is essential to soil sample to capture these areas of “suspected” significant varying levels.

3. Pick a nutrient strategy to maximize investment

Farmers may consider soil sampling and fertilization as avoidable expenses, especially when increased production costs and depressed commodity prices squeeze their profits. That is understandable, but farmers should choose one of the following nutrient management strategies based on their farm’s capabilities and business goals.

Sufficiency: Apply enough fertilizer to achieve a yield target this year. This strategy is based on the probability of getting a response to fertilizer in the current growing season and helps farmers maintain nutrient levels by monitoring soil levels and applying fertilizer according to current need. Growers who farm on land owned by someone else – roughly 40 percent of cropland in the U.S. is rented – can restore nutrients in order to meet soil level requirements in cash-rent agreements.

Build-maintenance: Apply fertilizer at lower rates this year and during each of the next several years as part of a program to slowly raise nutrients to recommended levels. This strategy considers crop nutrient removal and replacement to bank nutrients for use in future years and may give farmers more flexibility to adjust fertilization for current cash flow during lean economic periods and spend more on inputs when market conditions improve.

If soil sampling shows nutrient levels are low and yields are declining, it’s time to add fertilizer. What about when nutrient levels are low, but yields remain high? Farmers in that situation may be tempted to reduce or eliminate fertilization to improve profitability. That’s a short-term, budget-cutting solution that actually limits yield and profits, says Dr. Arnall, who wants farmers to at least apply test strips of potassium and phosphorus as a low-cost way to learn how crops will respond.

“You might get a corn yield of 200 or 220 bushels without applying potassium,” Arnall says. “However, you can increase that yield by 30 bushels an acre simply by applying potassium. You don’t know what you’re missing until you add some nutrient and see what happens.” Additionally, that high yield may impact the following year’s crop by removing greater nutrient amounts than a lower yield and increasing the amount one needs to replace.