As commodity prices continue to deteriorate and production costs — if not increasing, don’t seem to be going down — farmers must be alert to any opportunity to improve efficiency.

Site specific nutrient management offers one possibility, says Texas AgriLife Extension specialist Ronnie Schnell. Taking advantage of residual nitrogen, possibly deeper in the soil profile than is typically determined with usual soil sampling protocols, also provides an opportunity to save on fertilizer costs and to decrease potential for environmental damage.

Schnell, assistant professor in the Soil and Crop Science Department on the College Station campus, discussed precision agriculture (PA) at the recent annual conference of the Texas Plant Protection Association at Bryan.

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“PA technologies are quickly becoming commercialized and made available to growers,” he says, noting that growers can now collect substantial data within fields that allow them to manage crop inputs on a site-specific basis.

“Significant spatial variability of soil profile nitrate nitrogen may exist within fields, creating challenges for sampling and crediting for residual nitrogen,” he says. Recent research using electrical conductivity (EC) shows that soil texture may be used to create management zones and to indicate areas with residual nitrogen.”

Nutrient management may be accomplished on a single unit basis, on a grid, or by zone, Schnell says. Factors involved in creating a site-specific program include the soil EC, soil surveys, remote sensing, aerial maps, and yield maps.

MULTI-YEAR YIELD MAPS

“We have an opportunity to be more efficient with fertility across the field,” he says. Multi-year yield maps provide better information for nutrient application decisions. Yield maps over several years show farmers “where in the field consistently high yields or consistently low yields occur.

“We can merge maps from several years into a single map and create management zones. We can identify areas with 10 percent below average yield, areas with average yield, and areas that are 10 percent greater than average.” With that information, farmers can develop prescription fertilization programs for specific zones.

Sensor-based soil analyses using EC, soil salinity, water content, and bulk density also offer valuable data to use in creating site-specific application programs.

“Management zones may be based on soil type — clay content, for example,” he says. “EC-based zones are useful if EC values are derived by soil texture differences. We expect yield differences between soil types and with crop selection.”

Soil sampling also plays a role, but farmers need to dig a bit deeper than usual. “Sampling to a 4 foot depth makes a difference.” Many fields, depending on past crop choices and yield history, will have nitrogen reserves below the usual sampling zone.

CREDITING RESIDUAL NITROGEN

“We can obtain similar yields (compared to adding recommended nitrogen rates) by crediting residual nitrogen down to 2 feet,” Schnell says. Nitrogen is the key nutrient. Phosphorus is more mobile and less likely to stay within attainable range.

The key, he says, is information — knowing where soils within a field differ enough to alter yield potential, and knowing if residual nitrogen is available to offset fertilizer needed to meet crop yield goals.

“During 2014, profile nitrogen content (2 feet deep) was 1.5 times greater in zones with greater clay content compared to the zone with the lowest clay content. Management zones with greater clay content and residual nitrogen reduced optimum fertilizer rates for corn production by 40 pounds to 60 pounds per acre. Study results reveal potential for spatial management of profile nitrogen levels using EC-based zones.”