Extension Agent: Nitrogen management in crops

While the farm commodity prices have certainly increased recently allowing for increased profits, dramatically higher prices for production inputs, such as nitrogen fertilizer, have sharply increased production costs of crops. This, coupled with the ongoing drought, has made a big impact on area farmers' decision to fertilize for 2012 crops.

At this time, 95 percent of the cropland in Victoria County is not fertilized. Typically, 70 to 95 percent has already been fertilized by this time. With most of the area only getting about 3/4 inch of rain last week, the decision to fertilize has become even more complex.

Should I put out my full rate of fertilizer or half rate?

Normal fertilizer rates with poor or less-than-desired yields, like last year and anticipated again this year because of the drought, will mean a potential accumulation of nitrogen in the soils. Even in good years, nitrogen fertilizer rates on cropland have been accumulating on some sites.

For the past several years, increasing concerns regarding nutrient contamination of surface and groundwater resources have refocused attention on nitrogen management in crops.

Field studies conducted by Dennis Coker and Mark McFarland, Texas AgriLife Extension Service Agronomists, were conducted at 26 sites throughout the Upper Coastal Bend (including Victoria County for the past three years) and Central Texas Blacklands from 2008-2011 to determine the plant availability and effects on corn grain yield of residual soil nitrate to a 24-inch depth of soil.

To evaluate this, treatments included standard nitrogen fertilizer rates (full rate of nitrogen based on the producer's crop yield goal) and reduced rates where carryover nitrogen measured by soil testing to depths of 6, 12 and 24 inches were credited to the fertility rate, thus applying less nitrogen. Residual soil nitrogen to a depth of 24 inches ranged from 16 to 54 pounds per acre in 2008, 50 to 108 pounds per acre in 2009, 25 to 85 pounds per acre in 2010 and 21 to 76 pounds per acre in 2011 across all 26 study locations.

The end result: Corn grain yields and bushel weights were not affected at 25 of the 26 sites when carryover nitrogen was credited to 24 inches, indicating efficient recovery of profile soil nitrogen.

Similar field studies were conducted at 16 sites throughout the Upper Coastal Bend and Central Texas Blacklands from 2008-11 to evaluate dryland grain sorghum yield response to carryover soil nitrate to a 24-inch soil depth. Treatments again included standard nitrogen fertilizer rates based on yield goal and reduced rates where carryover nitrogen measured by soil testing to depths of 6, 12 and 24 inches were credited. Carryover soil nitrogen to a depth of 24 inches ranged from 14 to 44 pounds per acre in 2008, 47 to 104 pounds per acre in 2009, 29 to 159 pounds per acre in 2010 and 29 to 33 pounds per acre in 2011 across all 16 study locations.

The end result: Grain sorghum yields on all 16 locations were not reduced when carryover nitrogen measured to 12 inches was credited. Further, grain yields at 14 of the 16 locations were not reduced when carryover nitrogen was credited to a soil depth of 24 inches. These results demonstrate that grain sorghum also is very effective at recovering residual soil nitrogen measured by soil testing to a depth below the traditional sampling depth of 6 inches.

Previous research on cotton conducted from 1998 through 2004 showed that only 13 of 55 locations, 24 percent, responded to the application of supplemental nitrogen fertilizer.

Soil test data indicated that 33 locations, 60 percent, had 100 pounds per acre or more of residual soil nitrogen. Maximum yields on these locations ranged from about 1 to 2 1/2

At current prices, adoption of deep sampling in corn, grain sorghum and cotton production systems could reduce input costs by an estimated average of $36.33/acre, range of $9 to $109 per acre.

With recent increases in nitrogen prices, it is essential that producers get the most out of every pound of nitrogen applied. Deep soil sampling offers one opportunity to manage fertilizer inputs more closely to optimize production economics. This is especially true since the past several years of drought may have resulted in increased soil nutrient accumulation because of insufficient moisture resulting in less-than-desirable growth, yields and use of nutrients.

If there was ever a year to utilize deep soil sampling to gauge fertilizer recommendations, this is it.

Joe Janak is a Victoria County extension agent.