Your UK Soil Test Report: Use and Interpretation

My objective here is to help the reader understand what I look at, and why, when I get a soil test report to review. There is always a personal bias in things like this, but it is important for grain producers to understand why a soils extension specialist asks the questions they ask when reviewing one of these documents.

First, always check the top of the form (see next page) to make sure that the report received is for a sample you submitted. A number of situations have resolved themselves with that first check. Sometimes, mistakes happen. Second, re-acquaint yourself, mentally, with the area that was sampled. I have gotten off on the wrong foot by misaligning the sample with the area the sample came from. Verify the information pro-vided with the sample (primary crop, primary management, previous crop, previous management, etc.). Remember, the “Comments” section located in the report’s bottom half will contain several paragraphs specific to the ‘primary crop’. If you plant a different crop you will need to consult with your county extension agent to get appropriate information.

Moving down on the report, and starting on the left, you find the numerical lab results for ex-tractable phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and zinc (Zn), as well as the soil pH and buffer pH. Verify the units (UK reports extractable units in lb/acre, while some other labs report in ppm, causing the latter values to be half as large). As I look at these numbers, I do not place equal information value on each of them. In my experience, certain soil test report numbers are very valuable and some have little value.

Most valuable to me is soil pH. Soil pH impacts a host of soil health processes, nutrient (especially micronutrient) availability and the activity of certain herbicides. Knowing the pH range that your grain crop species need, you can quickly find a problem if your soil sample pH falls out-side, especially below (too acid), that range. The farther outside the range, the bigger the problem. The bigger the problem, the more lime (and time) will be needed. The need for more time to neutralize a higher level of soil acidity is another reason soil pH has great value. Buffer pH and soil pH are used to generate the lime rate recommendation on the UK soil test report.

UK uses the Mehlich III extraction procedure to estimate ‘plant available’ soil nutrient levels. The numbers are less important than the estimated “Level of Adequacy” shown by the graphed horizontal lines (>>>>>>>) to the right of the “Lab Results”. Remember, these numbers are mere estimates/indexes of plant available soil nutrition, not the absolute amount of a nutrient (e.g. lb P/acre) that the soil will deliver to the crop. This sample exhibits low, medium and high levels of available nutrient adequacy for K, P and Mg, respectively. This means that this soil test report ‘predicts’ that fertilizer dollars in-vested in K, P or Mg have good, minimal, or little ‘probability’ of positive economic return, respectively. We say ‘probability’ because seasonal weather makes nutrient adequacy predictions a bit of a crap shoot.

Further, the value of the numbers/estimated levels of adequacy is not equal for each of the different nutrient elements, depending upon what/how much we know. For Kentucky, I would say that we know most about extractable P, somewhat less about extractable K and Zn, even less about extractable Mg, and least about extractable Ca. The reasons for this are several, but most important is simply that we haven’t seen yield responses to nutrients like Mg and Ca – UK can’t provide an interpretation if the response patterns (critical soil test values for which crops, which soils, and for which environmental conditions) are not known/understood (mapped out).

This report contains numerical soil test results for boron (B), copper (Cu), manganese (Mn) and iron (Fe) just below the ‘Soil Test Report’ box, but above the ‘Comments’ section, because these ‘special’ tests were requested. We know a bit about, and can sometimes interpret, hot water extractable soil test B values. However, the Mehlich III Cu, Mn and Fe numbers have little to no value because we don’t know very much about crop response to these nutrient elements in Kentucky. And by the way, getting those same soil tests from another laboratory does not change the interpretive value of the numbers.

Last, I draw your attention to the box of “Calculated CEC Data” found on the right-hand side of the report box. The value for cation ex-change capacity (CEC) is calculated from Mehlich III extractable cations (K, Ca and Mg) and an estimate of exchangeable hydrogen (H) from the buffer pH. Then, the ‘saturation’ of the CEC by those same cations as a percentage (and summed together to give the percent ‘base’ saturation) is also calculated. I don’t find much value in calculated CEC values. Soil CEC is influenced primarily by the amount of clay (soil texture), the amount of soil organic matter, and the different mineral types found in the soil clay. Most Kentucky surface soils are silt loam to silty clay loam in texture, so differences in these calculated CEC values are driven by differences in soil organic matter (easy to measure) or clay mineralogy (difficult to measure). The different reasons for different test report CEC values means that their interpretive value is pretty limited without other information (texture/organic matter/mineralogy). In Kentucky, we have not found one situation where knowing the CEC has an interpretive consequence (causes modification of a nutrient recommendation). The same is true for the percent saturation numbers – you don’t need to know the percent K saturation – you only need to know that your soil test K value is associated with a medium-high level of adequacy for the K nutrition of your primary crop.

I hope this article helps the reader to under-stand the strengths and limitations to the soil test report – from UK or any other laboratory. Have a question? Don’t hesitate to email me (jgrove@uky.edu).