by Dr. Calvin Trostle, Extension Agronomy, TAMU Soil & Crop Sciences, Lubbock, (806) 723-8432, ctrostle@ag.tamu.edu
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.
Overall soil testing information from Texas A&M is found at http://soiltesting.tamu.edu/ Included is instructions on how to sample, what types of test you can choose to meet your needs (basic nutrients and pH vs. a complete analysis of nutrients, organic matter, salts, etc.). Prices are listed as well. Furthermore, discounts are available for a variety of tests when groups of eight or more samples are submitted at the same time for the same test.
Information is also available for testing of forages, plant tissues, water, and biosolids.
Here are eight additional 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 (see more in #5 below). 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.
The two primary philosophies of soil testing are generally “provide the nutrients needed for the current crop” vs. “build and maintain,” or let’s increase the background residual fertility. Each has its merits. The former is more likely the approach taken by public (university) soil test labs, which are expected to base soil test recommendations on years of field validation trials for different crops across a range of soil types. Private labs likely use a similar basis for gauging crop nutrient requirements (and very well may use the university data) but may be more inclined to recommend a higher level of fertilization for some nutrients to increase the background level of fertility. (This is most commonly associated with P and K; we do not “build” soil nitrogen, or N, which is relatively mobile in the soil when applied or converted to the nitrate form; any build-and-maintain approach for nitrogen generally involves the application of compost, manure, etc. where N release occurs over a couple of years).
In general, both philosophies should include a yield goal and consider existing residual soil fertility that is available to your next crop. You know that build-and-maintain may cost a little more, but if it reflects your goals, then this is acceptable additional expense. If you are not sure of your preferred soil test lab’s approach to their recommendations, ask. You have a right to know. The same applies if you have a crop consultant or fertilizer dealer making recommendations for your farm.
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.
Ask what soil test lab the consultant or fertilizer dealer uses. Even if you are not charged for the soil tests (part of the consultant’s fee or you are expected to purchase your fertilizer from that dealer), you should inquire who is conducting the test and understand what the recommendations might be (see #1). Also, since you did not submit the soil test reports, were they returned to you with recommendations based on your yield goals? What are your yield goals? If you have been working with a consultant for many years, they may already know your target yield goals for individual fields based on experience. But you should ask. If you have made other changes in your production practices that may influence yield goals hence recommendations you need to share that information.
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 is would provide more information to better pinpoint fertilizer recommendations. We do recommend, however, greater attention to soil nitrate-N below 6”. Thus, the Texas A&M Soil Test Lab now provides a “Profile N” soil test form (see http://soiltesting.tamu.edu/files/profilesoil.pdf). 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. On the form (Fig. 1) you mark the depth of the subsoil sample as 6-12”, 6-18”, or 6-24”. 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.
Fig. 1. Screenshot of the Texas A&M Soil profile test form. Note at the botton where the subsequent sampling depth of this second companion sample is marked. Testing choices include standard soil analysis (check box for Test 1) or soil profile nitrate-N only (check box for Test 11)
5) Do you use a soil test lab that is out of state? If so, how do we know their soil fertility recommendations are appropriate for your farm? Nebraska soil test and fertility recommendations may be appropriate for corn there, but what about for a Texas field? What if you send soil test samples for cotton to Nebraska where cotton is not grown? So how do they make recommendations for cotton? (Do they get data from decades of Texas soil fertility research for cotton and place in their database?) You have a right to know. States outside of Texas may even use a different soil test method for some nutrients that is not appropriate for some Texas soil types.
This is an even bigger question than what soil test philosophy an out-of-state lab uses. You need assurance that their recommendations are appropriate for Texas. So ask.
6) It is possible in some cases to compare soil test recommendations from other labs to what Texas A&M recommends. The essential key is the other lab must use the same standard soil extraction and analysis method that is the basis for Texas A&M AgriLife testing and recommendations. Texas A&M methods for all soil tests are posted at http://soiltesting.tamu.edu/webpages/swftlmethods1209.html
Texas A&M posts online numerous charts for N, P, and K that also reflect a yield goal. This is done for the major crops of cotton, corn, grain sorghum, wheat, winter canola, and a few forage crops. You may view these charts at http://soiltesting.tamu.edu/webpages/recommendations.html For these crops you will find the recommended fertilizer application for a target yield goal based on a soil test value (what chemistry measures) of the nutrient. Unfortunately, there is less information for other crops, so Texas A&M recommendations become uncoupled from yield goal (no range of yields are provided). So, for crops like sunflower, peanut, sesame and several forages this comparison cannot be done.
If you make this comparison, you can rightly assume the Texas A&M AgriLife recommendation is based on providing what your crop needs for the coming crop year. In addition to ensuring the two labs are using the same extraction and analysis method, you will need to know what soil test philosophy is used by the other lab.
7) Using soil test information when buying and selling farmland. If you are considering purchasing farmland, have you thought about asking for recent soil test reports? Or better, can you get permission from the seller to soil test the property? This is rarely if ever done, but why not? Information from soil testing could potentially demonstrate a swing of $50 per acre or more on the value of the land. If residual soil fertility is high and you find there is 60 lbs. of soil nitrate-N below 6” (that is probably worth at least $25 per acre), you can better justify your price. If you find that soil test P is ‘low’ (10-20 ppm P2O5 equivalent) and you have a build and maintain approach to your cropping, then you can calculate how much P fertilizer—a significant cost—it will take to reach your goals. This of course detracts from the value of the land to you.
If you are denied the opportunity to soil test a unit of land for sale, what does that say? If should decrease your interest in paying as much for the land. On the other hand, if you are selling land that you know has good soil nutrient status, invite prospective buyers to soil sample. You could do this yourself and provide the information though this is not independent.
8) How long should you keep soil test reports for each field? I recommend you treat them like previous years’ tax returns. Keep them a long time, even more than 25 years. A historical record of soil tests can show you what changes have occurred in your soil over time. Have you improved overall soil fertility (this would be for nutrients other than nitrogen)? Or has it degraded over time? Perhaps at some point you change how you tilled the soil or applied your P, then you can see if that is reflected in your soil tests.
And like #7 above, if you are considering purchasing a unit of land, ask if they have soil sample reports over past years. If you were buying a vehicle or tractor, you would like evidence the oil has been changed regularly. That reflects better care. Soil tests reports can do the same for buyers—and sellers.
Calvin Trostle
Professor and Extension Specialist
Lubbock, TX
803.746.6101
ctrostle@ag.tamu.edu