Ready to get into the zone? First, let's take a clear look at the difference between soil sampling by grid vs by zone. Grid management is built around soil test data. A field is divided into, say, 5-acre squares and a soil sample is taken from the centre of each.
Zone management incorporates yield data, imagery, possibly EC testing, etc., to construct productivity zones, which govern how to manage the field. Zones are a function of soil attributes plus environmental conditions that determine yield.
Grid sampling has been popular since GPS data became available. Now that good yield and elevation data is also available, as well as imagery, growers and their crop advisors are able to draw a more precise picture of a field’s characteristics and yield variability by layering additional data on top of the soil test results.
Clint Nester has had a front-row seat to the evolution of soil sampling over the years. As an agronomist with Nester Ag, which manages over 150,000 acres in Ohio, Michigan and Indiana, Nester says his business is now focussed entirely on zone sampling.
'NATURE IS NOT IN SQUARES'
“Our feeling is that nature is not in squares, not by any means,” says Nester. “We use yield zones to let the crop tell us where changes are occurring.” Nester’s zone size averages six to 10 acres at US$7.50 an acre, and is used for two years.
In an ideal world, each grower would have four to six years of calibrated yield data, which Nester uses to develop zones. Then, it is crucial to get the client into the field to make sure things match up, and explain any anomalies.
He also uses satellite imagery, describing it as “another tool in the toolbox, but we put more emphasis on soil type, yield data and elevation data.”
According to Nester, one of the key benefits to zone testing is to identify a field’s lime needs. “We pay close attention to the calcium/magnesium relationship in the soil,” he says. “If we can get this balance right it really helps our growers – they see better yields, better soil quality, better water infiltration and better nutrient utilization.”
ZONE SAMPLING EMERGES AS KEY
Roger DuMond, an agronomist who oversees soil sampling for Kova Fertilizer, Inc. in Greensburg, Indiana, agrees that accurate soil sampling translates into higher productivity and better yields.
Kova has been using Trimble’s Farm Works Software since the 1990s, when it began offering precision ag and GPS-related services. DuMond recalls the first year grid sampling for clients, taking three samples in a 10-acre plot. When GPS came along, they opted to move to smaller units for improved management, so moved to four samples per 10-acre field, or 2.5-acre grids.
“Within a few years we started using a ‘smart grid’ concept, laying other maps over that field to break it into other units. This was the evolution of zones.”
DuMond is somewhat of an expert in variability – and the folly in underestimating its importance. He works in an area known for its ‘glaciated’ soils, or soil that changes along lines formed by glaciers. Here, interpolation is not considered a good thing.
“Most mapping systems take data and interpolate the data in between,” he says. “But here, you shouldn’t. There ought to be zones in the field that you can keep uninterpolated, especially here when you have rapidly changing soil.”
NOT ALWAYS EITHER/OR
Ultimately, he says each strategy has its shortcomings. “Either you have a blind grid or with zone sampling, you have to remember that zones mean different things depending on what purpose you’re using it for. A management zone from EC tells one story and soil testing may tell a different story and take you in a different direction. Until you marry it together you don’t have a good sense.
He also notes that many growers opt to switch it up, using one sampling strategy for, say, applying lime and another one for VR seeding. “You only really know by bringing it back to the results, the yield data.”
The most important advice he has for growers, whether they are doing grid or zone sampling, is to monitor the results and tweak the system for improvements. “There’s a constant re-evaluation of what we are doing and measuring the results in the field,” he says. “You have to do that to justify your agronomic recommendations, and to show to the customer.”
Don't miss Part 3 in this series!