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Topographic Sampling

Saturday, February 1, 1997
filed under: Fertility

North Dakota State University soils specialist Dave Franzen has seen grid sampling/variable-rate application (VRA) of fertilizer grow by leaps and bounds in the sugarbeet and potato fields of the Red River Valley, where well over 100,000 of sugarbeet acres alone were grid sampled in anticipation of the 1996 crop year.

Grid sampling isn’t cheap, however, and not all those fields realized a positive net return. As a rule, the more nutrient variability there is throughout a given field, the greater payback one can expect from grid sampling/VRA. If fertility levels are already quite uniform across a field, the added cost may not be offset by improved crop yield and/or quality.

Recent investigations, however, suggest that an alternate approach called topographic mapping (i.e., measuring a fieldscape’s “ups and downs”) could nudge the odds further in the grower’s favor. Ongoing North Dakota research indicates that using a field’s topography to map variations in soil fertility is cheaper — and often more accurate — than using grids or proceeding simply on the basis of soil type.

A substantial reduction in the required number of soil samples is one major reason why topographic mapping can have an edge on grids. NDSU research at several locations has shown that using the topographic method required just one-fifth to one-eighth the number of soil samples required by the standard one-sample-per-acre grid method.

“At one site, 36 samples were required to map the field when we used a 220-foot grid to do mapping; but only five sample points were required when we based the sampling on field topography,” Fran-zen says. “In another field, 79 points were required using the grid method, but only 14 using the topography method.”

Even more important, the topographical method generated better-defined fertilility boundaries than the grid method — and provided equal or greater accuracy when it came to reflecting actual variations in a field’s fertility levels.

Franzen, who came to North Dakota from Illinois, says that topographic mapping has not appeared to be an accurate guide to nutrient levels in that Corn Belt state. That could be, he suggests, because the heavy fertilization of Illinois fields frequently masks the effects of topography.

The North Dakota situation appears to be different. “Many people thought that mobile nutrients such as nitrogen could not be mapped on the basis of topography,” Franzen admits, “that there would be no pattern, that you would take a sample one year and when you took another the next year it would be vastly different, since these nutrients move.” However, North Dakota research has discovered that even mobile nutrients fall into field patterns — related to topography — that are fairly stable from year to year.

With regard to sunflower specifically, research conducted the past couple years at the USDA-ARS Northern Great Plains Laboratory near Mandan, N.D., has shown a positive economic impact from the use of topographic mapping. In 1996, according to Franzen, the use of topographic mapping/variable-rate application of fertilizer on sunflower ground resulted in an increase in net return of $11.50 per acre when compared to the conventional soil samp-ling and fertilizer spreading methods. That $11.50 figure was nearly double the $6.00-per-acre increase resulting from grid sampling/variable-rate application versus conventional methods.

The differences in net return among the three methods were due to a combination of variations in yield, sampling expense and, to a lesser degree, cost of the fertilizer. After being adjusted for some hail damage, the 1996 Mandan sunflower yields came out as follows: topographic/VRA — 1,870 pounds per acre; grid/VRA — 1,851 pounds; conventional — 1,651 pounds. Per-acre sampling costs were placed at 75 cents for the conventional, $5.00 for the topographic and $10.00 for the grid sampling, with actual fertilizer costs being $24.36, $24.96 and $26.31, respectively.

The 1996 Mandan study used 11-52-0 and 46-0-0 fertilizers, with the respective nutrient management grids treated randomly via one of three methods: (1) uniform area using a composite sample; (2) variable area using a 4.5-acre grid; and (3) variable area using topographic mapping. Of the three fields in the study, two were planted to wheat and one to sunflower.

Topography nutrient level mapping utilized 14 sample-point locations across the three-field area. Despite the low number of samples, the correlation of nitrate-N, chloride, phosphate and sulfate-S was higher than it was with most of the grid soil sampling strategies. The correlation was especially good when topography was correlated with nitrate-N, Franzen reports. Topographic sampling also did a better job of defining nitrate-N level boundaries within the fields. (Continued)

Along with the economic benefit of topographic sampling/variable-rate appli-cation of fertilizer in the 1996 Mandan study, Franzen points to an environmental benefit as well: A better job of matching fertilizer treatments to actual needs means there’s less risk of nitrate leaching or runoff in the spring following application.

“The total amount of nitrate left in the soil at the end of the year was substantially less than where we had uniformly applied fertilizer,” he says. “The fertilizer expense was about the same; but the crop was able to utilize it better and produce a better yield. That translated into lower ending nitrate levels — which is good for the environment; and a higher net return — which is good for the producer.

“This is what precision fertilizer application promised from the begininning, and it is exciting to see our first-year results verify our predictions,” concludes the North Dakota soils specialist.

The topographic sampling research, which is a cooperative effort of the NDSU Extension Service and USDA’s Northern Great Plains Laboratory, will continue in 1997. Along with Franzen, other lead investigators are Dr. Ardell Halvorson, director of the Northern Great Plains Laboratory, and Vern Hofman, NDSU extension agricultural engineer.
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