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Corn+Soybean Digest

Zone Vs. Grid Sampling Debate

There's been endless debate in recent years about which soil-sampling method is better — intensive grids (2.5 acres or less) or management zones. And the dispute continues, with advocates on both sides.

A recent Iowa State University study, conducted in producers' fields, may help clarify the controversy. It suggests that there is no single “best” soil sampling method for every field. But, in general, management zones appear to provide the better combination of cost and effectiveness for most situations.

The first question many folks might ask is: What are management zones?

“The criteria for establishing such zones can vary depending on the history and conditions of a particular field,” explains Antonio Mallarino, Iowa State soil scientist. “It starts with a soil survey (soil type) map as the foundation and can include yield maps, aerial photos, topography, soil electrical conductivity, previous cropping, fertilizer and manure application history and other factors.”

Soil electrical conductivity, or EC, is a measurement that correlates to soil properties affecting crop productivity, such as soil texture, cation exchange capacity, water-holding capacity, organic matter content and subsoil characteristics.

“Certain layers of information will be more useful in some fields or conditions than in others,” Mallarino points out. “However, both farmers and consultants will learn more about each field by using these various forms of information than by sampling arbitrary squares (grids) every four years.”

Mallarino says sampling in 2.5-acre grids generally does a better job of describing nutrient variability across a field than does sampling by management zones. That's because of the many more samples per given area. But the higher cost for those greater numbers of samples may override their value.

A “best” approach may be to do a one-time intensive grid sampling as the basis for determining nutrient status, then use management zones thereafter.

Murray Welden, Stillman Valley, IL, and his colleagues with Advanced Crop Care, Inc., generally sample in 2.5-acre grids. “That's what most of our customers want,” he says.

“The key factor is to take enough samples to represent a true picture of the field, whatever the system,” explains Welden. “If a field is rolling and variable, we may use grids smaller than 2.5 acres. If it's flat and black, we may go to 3.3- or 4.4-acre grids.”

Welden says sampling size needs to match the sophistication of the application equipment to be used. Not all applicators can handle small grids.

When Welden samples a farm for the first time, he likes to walk fields with growers to get background on each field and to learn the grower's goals.

“The management of some fields — cropping, tillage, past fertilizer practices and manuring — has become more of a factor than basic soil type in the current condition of some fields,” he points out. “We need to know that history to best determine the most effective soil sampling approach to use.”

Welden, like many specialists, believes the best soil sampling system on most farms is to start with intensive grids and then go to management zones in future sampling.

Dave Harms, an independent crop consultant from Bloomington, IL, recommends soil sampling by management zones, although he has experimented with grid sampling.

“In our research, we've taken individual fields and compared three different approaches to soil sampling — 2.5-acre grids, by soil type alone and by management zones — for cost and return,” Harms reports. “We've found that management zones are superior for return on fertilizer dollars. It allows each part of a field to be fertilized to optimum without going over on any other parts.”

Harms says one problem with sampling by grid is that a grid may straddle soil types. The limitation to management zones is that they need to be set up and monitored by a trained agronomist, he points out.

Mike Snyder, an independent crop consultant from Ashland, OH, believes management zones are the better way to go.

“The extra expense of intensive grid sampling and application seems to outweigh any cost savings on fertilizer,” he says. “I would rather use those dollars for P and K fertilizer or save my clients from spending the money in the first place.

“However, I do believe in grid sampling some farms with big fields as a way to establish management zones,” points out the Ohio consultant. “It's probably most important in finding those acres where there are extremes in pH.”

“Whether to use intensive grids or management zones is really more about a producer's yield goals and profit-per-acre goals,” says crop consultant Larry Arndt, owner of Agknowledge LTD, Spirit Lake, IA.

“If a producer is satisfied with an average crop, then larger management zones are ideal,” Arndt notes. “However, top producers want more than that. And they need the precision that comes from intensive grid sampling and other site-specific information. It may cost a few cents more per acre, but it produces more bushels. In our experience, when it's tied to prescription nutrient plans, it increases yields 5-10% across a field.”

Arndt believes intensive grid sampling is especially important when consulting on a farm for the first time. “We need to know the ‘base points’ and then build on that,” he says. “Four years later we may do something other than intensive grid sampling, depending on the management of a given field, such as the tillage and manure applications.”

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