by Calvin Trostle, Extension Agronomy, Lubbock, (806) 723-8432, ctrostle@ag.tamu.edu
Jake Mowrer, Extension Soil Fertility & Water Specialist, College Station, (979) 845-5366, jake.mowrer@tamu.edu

Sub-soil Nitrate in Texas Soils

In the past 20 years Texas A&M AgriLife staff across Texas have become increasingly aware of the presence and amount of sub-soil nitrate nitrogen (N) in Texas soils. Below the standard recommended soil sampling depth of 0-6”, deeper soil sampling, usually 24” and sometimes 36” with subsequent analysis for nitrate-N only—which is the mobile form—has indicated in many cases substantial nitrate-N. This likely represents over-fertilization.

What should a producer do about this subsoil nitrate-N? Should it be credited to crop N requirement? If so, how much? Should it be fully credited 100% to crop N requirement?

Yes!

During numerous Extension programs we have conducted in different areas of Texas, when we ask farmers if, for example, they have 30 lbs. of nitrate-N at a depth of 6-24”, should they credit all of it to crop requirement, 10% of producers at most agree it should all be credited. About 1/3 will disagree, and the rest wait to see what we are going to tell them.

Former Texas A&M AgriLife Extension soil fertility specialist Dr. Mark McFarland used to bet producers the cost of a steel soil sampling probe ($75-100) that he was right to credit all subsoil N to crop requirement. How this was determined over decades is that when subsoil N was gauged, subsequent soil fertility research trials conducted with and without the allowance for subsoil nitrate-N showed that crop yield did not decrease if lower fertilizer N applications reflected accounting for subsoil nitrate.

Dr. McFarland’s efforts to increase adoption of this practice resulted in 104,000 acres of cotton and 129,000 acres of corn and grain sorghum combined utilizing this deeper soil test in the years 2012-2013. Participating farmers during these years saved an average of $23/ acre (cotton) and $31/ acre (corn and grain sorghum) by reducing fertilizer N inputs with zero detriment to yields. Across the state this resulted in a total of $12 million dollars increase in net returns and a 9.2 million pound reduction in applied N-fertilizer for those same years.

Some producers and consultants are understandably concerned about crediting all subsoil N to crop requirement. Yes, for all practical purposes you can do a full credit (perhaps that is our “wiggle room”), but if you are uncomfortable with this approach, at least give it major proportional credit. Perhaps you are only willing to credit 70% of the subsoil N. In the above example, then, that is still 21 lbs. of N, which reduces your fertilizer cost about $10-11/acre.

A common objection to full inclusion is that high rainfall can dilute subsoil nitrate-N and push it below the root zone. This is possible, but much of Texas doesn’t see this level of rainfall, and even when it occurs not all nitrate-N is moved below the root zone. Also consider that leaching will be more rapid in loams and sands than in clays. There are even times when producers may be so happy to have had a big rain so the possible loss of N is acceptable.

In addition to the substantial economic benefits, measuring for the presence of nitrate at depth in soils to reduce fertilizer inputs also reduces the likelihood of fertilizer escape into our state’s surface and ground water resources. These natural resources are something we can all be proud to assist in restoring and/or protecting.

How deep should soil nitrate-N be sampled?
For this discussion, soil sampling for nitrate-N should likely consider sampling as far as you conveniently can to a depth of 24”.  Most research sampling is often to 36”, or even 48”. However, this is impractical in many soils without special truck mounted hydraulic probes. AgriLife Extension specialists believe that in soils with sufficient clay content, we can rely on a 100% credit to 24”. In sandier soils, and below the depth of 2 feet in fine soils, we find the reliability of nitrate recovery is diminished. Though Texas A&M AgriLife has not considered expanding the depth of standard soil sampling depth for comprehensive soil tests, several neighboring states including Kansas State University are now recommending a standard 24” soil sample. In the meantime, Texas A&M AgriLife’s Soil Testing Lab (http://soiltesting.tamu.edu) has offered for the several years the…

Profile N Soil Test Form
This form (see below) was added by A&M’s soil testing lab several years ago, to emphasize the value of subsoil nitrate-N. Producers or consultants collect your standard 0-6” soil sample for full analysis (N, P, K, micronutrients and other macronutrients if desired, organic matter, salts, etc.), and a second paired sample is collected from the same probe points, and the profile sample form is marked for the depth of collection (second yellow arrow, 6-12”, 6-18”, or 6-24”), and this second sample is analyzed inexpensively for nitrate-N only. The A&M soil test lab will then combine the two soil sample analyses to determine available N and calculate the recommended level of fertilizer N to add based on crop and yield goal.

If you use another soil test lab that does not offer a similar profile N test, call them and ask how you can submit samples to obtain this analysis, and ask if they will credit all subsoil nitrate-N to crop requirement.

Examples of Common Subsoil & Excessive Subsoil N
As noted above Texas A&M AgriLife has increasingly been tracking subsoil N. The table below demonstrates average subsoil N levels from AgriLife Extension IPM agent Tommy Doederlein, Dawson Co. This is just one of many examples.

Table 1. Surface and subsoil nitrate-N levels in Dawson Co. Texas, 2011-2014 (date courtesy Tommy Doederlein). Subsoil nitrate-N levels are typical of many Texas soils when tested with deeper soil sampling.

The data shows that there is substantially more nitrate-N below 6”—four times the amount of N in dryland soils and eight times the amount of in pivot irrigated soils. Among the soil samples, the maximum soil nitrate-N level is reported each year. Incredibly, the amount of excess nitrate-N is unbelievably high in some soil samples. This is even more reason for producers to check subsoil nitrate-N for these levels indeed are subject to loss from the cropping system. These high subsoil nitrate-N levels represent major economic value but excessive amounts like 100 lbs. of subsoil nitrate-N/acre is more likely to move and be lost from the system. And there is no “banking” of soil N like you might find for P, K, and perhaps other nutrients.

For further information on soil testing in Texas consult the Texas A&M AgriLife Soil Testing Lab at http://soiltesting.tamu.edu.

Calvin Trostle
Professor and Extension Specialist
Lubbock, TX
803.746.6101
ctrostle@ag.tamu.edu