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Soil Testing in a New Era of Nutrient Management

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Soil Testing in a New Era of Nutrient Management

Precision agriculture means many things to many people. When it comes to nutrient management, it means a deeper understanding of soil fertility.

The low-cost availability of the global positioning system (GPS) launched a new era in agriculture.

“It was a game changer,” says Curt Woolfolk, senior agronomist for Western North America at The Mosaic Company. “That’s what really launched precision agriculture. It gave us the ability to mark areas of interest within a field, and then return to those areas at a later point in the growing season or subsequent years. One of the many advantages that this technology brought was related to soil sampling. Retailers and growers could now dissect fields based on GPS coordinates and soil sample at a much smaller resolution than ever before. The soil test values from those small management areas within the field could then be used to drive variable-rate fertilizer applications. GPS also allowed harvesting equipment to track low- and high-production areas within a field.”

With this powerful tool in hand, it becomes a question of how many points to sample. And how often? Woolfolk says 2.-acre square grids and zone sampling have become the industry norm, and as to frequency, he has a short answer: Consider doing it more often.

“We emphasize that it’s time to increase the frequency of soil testing, if you haven’t already,” Woolfolk says. “If you are doing it every four years, consider every two years. Sampling once a year is ideal to properly track your soil nutrient budget, but the economics typically drive this decision. You don’t need to do intensive soil sampling every year, but closely track crop removal or do some form of soil testing every year.”

In the past, taking soil samples meant gathering soil cores from 8–10 points across a field, mixing them together in a plastic bucket, filling a soil sample bag with a bit of the mix, and sending it off to the lab. With grid sampling, the retailer or grower treats each grid like a whole field, gathering 8–10 samples within that 2.5-acre square, mixing them and drawing a single sample.

“We do grid sampling in 2½-acre grids so that the field is split into areas,” says Glen Franzluebbers, director of professional ag services for Central Valley Ag (CVA) Cooperative, headquartered in York, Nebraska. “That way we know what is needed for fertility in every part of the field.“

“Another option is zone sampling. That’s where we take areas of the field, based on imagery, yields or soil types, and divide a field into zones, and then take soil tests in those zones. Grid samples fit better on soils and fields that are really variable from one end to the other. Zone sampling is better for more uniform fields, so we can reduce the number of different samples and still get a good result.”

CVA added precision agriculture to its offerings 18 years ago. Each year, more and more farmers undertake this suite of management tools in order to become more efficient and bring in more bushels and more profits to their operations. Soil testing is the foundation for understanding the variation in yield potential across a field.

“We do a lot of yield mapping, so instead of using a blanket yield goal across the field — instead of fertilizing the whole field for 200 bushels an acre — we can start looking at those yield maps and historic yields, and almost create a yield potential map,” says Franzluebbers. “Then we can variably apply the nutrients, according to what the real potential is in each area of the field. If there’s a poor-yielding area, where it might only make 150 bushels, then we should fertilize for 150 bushels.

“We can push other areas where there is a history of higher yields, where it has gone up to 200 or 250 bushels; we can fertilize for that,” he adds. “By treating these areas differently, you can maximize the yield across the entire field.”

Franzluebbers notes that CVA makes variable-rate nutrient applications and variable-rate seeding work with either the grid sampling or zone sampling.

“We sample anytime after harvest, all the way up to the spring, before planting,” he reports. “With grids or zones, or when we are doing composites or nitrate tests, we first focus on the nitrogen, phosphorus and potash, the main nutrients. Once we get those to sufficient levels, then we start looking more to sulfur, zinc, boron and some of the other micronutrients. It is key to have those macronutrients where they need to be before we start working on the micronutrients. For CVA, pH and lime application is also a big part of our program.”

These precision measurements of the soil nutrients have allowed farmers to become more and more efficient. The next threshold in efficiency and increased yield is paying attention to micronutrients, according to Woolfolk.

“If you have only been looking at primary nutrients — nitrogen, phosphorus and potassium — at this point, maybe even sulfur, as well as pH, then now is a great time to begin looking at micronutrients within the soil and understanding different crops’ micronutrient needs, to take your yield to the next level,” Woolfolk says.

Though these experts encourage increasing the frequency of testing, this does not necessarily mean doing a full grid sample every year. Sixty-four samples for a quarter section of farmland every year can be costly, Woolfolk acknowledges.

“As for testing frequency, intensive sampling every two years is a sound strategy. If on a 4-year soil sampling program, pick 3–5 points within the field to subsample every year. The point is to keep an eye on these soil nutrient levels through various cropping cycles” Woolfolk says. “If the needle has moved a lot, then you may have a weather event or crop removal situation that could warrant further investigation.”

Timing of soil sampling is typically driven by what crop is growing.

“Pulling soil cores in a mature crop can be challenging, so most people pull samples immediately following harvest or several weeks prior to planting,” states Woolfolk.“ The goal is to allow enough time for soil test interpretation, decision-making and fertilizer application prior to planting. Pull your soil samples at the same time every year. Soil test values can vary during different seasons. Factors such as soil type, microbial activity, temperature and soil moisture all play a role in nutrient concentrations and location.

Spring can also work well for soil testing in more southerly zones, but northern climates tend to have more compressed springs, which can limit soil sampling opportunities. Woolfolk noted that efficiencies with shipping and soil test laboratories over the past 10 years have significantly shortened the time for a crop advisor to get a soil test report and fertilizer spread on a grower’s field.

“Farmers make a sizable investment in fertilizer for each field, so it makes good business sense to monitor the levels of soil nutrients on a frequent basis” says Woolfolk.

“You don’t want to overapply, with all the different things going on in the industry, with nitrogen levels and phosphorus levels and environmental concerns,” Franzluebbers explains.

“But we also don’t want to underapply and hurt our yields. Yield goals are increasing, populations are increasing, so we do need to adjust our fertility plans according to what our new goals are, but we also have to keep in mind environmental impact, and also profitability for our growers. Soil testing is the key to understanding how much fertility we need to add — just enough and not too much — to reach our goals.”