Farmers across Texas are familiar with standard soil testing procedures and many make use of soil tests to determine fertilizer applications for a wide range of crops and soil types. You have likely been encouraged to soil test annually and “Don’t Guess—Soil Test” to better pinpoint your soil fertility program. According to Dr. Calvin Trostle, Extension Agronomy, TAMU Soil & Crop Sciences there are considerations to help you capture more value from soil test results:
1) There are different philosophies of soil testing. Producers regularly comment to me that they sent the same sample to two different labs and received different recommendations. Why? There are several reasons why this could be. First, there are two components to soil testing and recommendations. On one hand there is the specific test method that is used. This includes how the nutrients are extracted from the soil and what method is used to analyze the nutrient. These may not be the same between two labs. On the other hand, an individual lab may have a different basis for what they recommend based on both the measured nutrient value and your goals.
2) Who soil samples your field and makes your recommendations? Particularly for large farms, producers may rely on a crop consultant or the fertilizer dealer themselves to conduct soil sampling on your different fields. Ensure they are taking representative samples for each sampling unit or field (at least one probe point per 4 acres, preferably 1 per 2 acres especially for smaller sampling areas). Also, if the individual who conducts soil sampling may not be familiar with different soil types or other production zones in your field (good areas, poor areas) that you observe, let them know so they can sample accordingly and not commingle soil samples from potentially different management zones.
3) Be alert for possible conflicts of interest. You know this, and it should go without saying. But if someone is doing your soil sampling for you, handles soil testing and recommendations, and you buy your fertilizer from them, this is a potential conflict of interest. Just so you know, regardless of the level of trust you may have. I have colleagues that strictly recommend you control the soil sampling process and sourcing of fertilizer recommendations that fully reflects your best interests. Then you shop for fertilizer based on cost, type, availability, and possible fertilizer application services.
4) There is a trend to increasing the depth of soil sampling. Initially, this was driven by recognition that there may be substantial amounts of the readily available form of soil nitrogen, or nitrate, below 6”—and sometimes lots of nitrate-N, even up to 100 lbs. of N per acre in rare cases—that is utilized by all crops. Some highly agricultural states now recommend standard soil samples to 24” deep for the basic, routine analysis. This includes Kansas and North Dakota. But you know that soil sampling to 24” is more difficult and will take more time. (Consultants and producers in the Texas High Plains tell me that they are generally readily able to sample to 18” without much difficulty.) But what is the value of better fine-tuning your nutrient needs?
Texas A&M AgriLife does not currently recommend deeper soil sampling for general soil nutrient analysis, though we acknowledge it would provide more information to better pinpoint fertilizer recommendations. We do recommend, however, greater attention to soil nitrate-N below 6”. To use this approach, you collect your standard soil sample (likely a 6” depth) which is analyzed for basic nutrients and any additional tests. A companion soil sample is collected at the same point beginning at 6” then deeper into the soil. This sample is analyzed inexpensively for nitrate-N only, and it is credited to your crop requirement. It represents a potential cost savings on fertilizer N, especially in wetter regions of Texas where nitrate could be lost out the bottom of the root zone from excessive rains. For more information on this or any other agricultural topic please contact the Hopkins County Extension Office at 903-885-3443 or email me at firstname.lastname@example.org.