Bermuda Guide: Soil Tests

 Many people never have their soil tested and are satisfied with their lawns.  However, completing a soil test with a state Agricultural Cooperative Extension Agency can give you useful information about what's happening to your soil that you might never understand otherwise. Many Extension agencies recommend soil testing every 2 - 3 years and some lawn masters have it done twice a year, in spring and fall.

But even if you don't plan on a program of annual soil testing, I think its valuable to have a soil test done at least once, to understand where your soil is starting and what is typical for your situation.

Now, if you are facing some intractable problems, the soil test can be a key to solving them. 

A routine soil test package reports a large number of variables that can seem complicated and unnecessary.  It includes analysis of the plant-available levels of P, K, Ca, Mg, Zn, Mn, Cu, Fe, B, plus soil pH and estimated CEC, along with fertilizer and lime recommendations for the specified crop. Soluble salts and organic matter tests are also available.

However, for someone new to soil testing, there are two main questions that a soil test can answer that you can't really learn anywhere else. A soil report will tell you... 

1. Your soil's pH or acidity
2. The soil's macronutrient balance, particularly phosphorous (P) and potassium (K)

Once you know those two data points, you can begin to look at other things, like micronutrients and CEC levels

Soil samples are analyzed and computer recommendations generated usually within three working days of receipt. Completed soil test reports are emailed directly to you, or the email will give you a link to the results of your test.

The soil test results will tell us a lot about how to proceed.  Watch this video and several others on interpreting Ag extension soil tests  We will be looking at two important results. 

Soil Acidity or pH

The first major number is pH or acidity of the soil.  If this number is outside the 6.0-7.0 range, it will benefit you to begin to adjust it through applications of lime or garden sulfur.  Numbers outside that range mean that the grass can't access all the nutrients you are laying down with your fertilizer.  

Acidity or pH changes slowly.  You may want to plan several buffering applications throughout the next several years to achieve your final pH goal.

From Clemson Univ.:

Bermudagrass will grow best at a pH of 6 to 6.5. If a soil test shows a higher pH, sulfur can be applied to lower it. Apply 5 lbs of pelletized sulfur per 1000 square feet of turf. Apply sulfur only when the air temperatures are below 75 °F. In 3 months, recheck the soil pH to see what change was made. It may take several years for a large pH change to occur. Soils in the Upstate are typically acidic and usually do not need sulfur applications, but they likely may benefit from lime applications.  

Buffering applications that adjust pH can be made in the Spring (Feb-March) and again in the Fall (Sept-October).

 N-P-K - The Phosphorous problem

A second important number in the soil report is the level of Phosphorous in your soil.  Many soils are high in phosphorous and you can buy lawn fertilizers with or without it.  You just need to know which one.  If phosphorous is high, it may be detrimental to continue to add it to the soil.

Nitrogen is typically variable within the soil, as growing plants use it up and fertilizer applications replenish it.  

Similarly, the soil report will tell you the current state of Potassium that you can add to the soil with a lawn fertilizer.  Potassium may be added to enhance winter hardiness, if the soil test indicates it is near the bottom of its range.

Cation Exchange Capacity

A third interesting number is CEC or cation exchange capacity.  This is a basic measure of the soils ability to supply the nutrients (ions) to your grass.  Nutrient uptake by plants is dependent on CEC and a value above 10 is preferred.  Biochar and humic acid improve the CEC of your soil.  Knowing this number may tell you why your lawn is struggling and how heavy you may need to apply HumiChar or organic fertilizer.

The rest of the numbers are for micronutrients and they may be important if they are particularly low, and need to be directly supplemented. 

A calcium analysis of your garden's soil or an assay of calcium uptake in your plants is the best way to gauge your garden's gypsum needs. Most Cooperative Extension Service soil tests include calcium if you request it. Either very low (below about 5.5) or very high (above 8.2) pH values are also indicative of a need for calcium.

Gypsum application rate:  Apply 10 pounds per 1,000 square feet of garden area up to three times each growing season to correct calcium deficiencies. Over-application of gypsum is generally not a problem because it has low solubility and the increase in salinity is low. 

 

Reading a Sample Soil Report

Below is a sample soil test result from my own lawn:

So let's begin by asking ourselves the three big questions.

1.  What is our pH?   In this case, the pH is 6.6.  For most analysis, pH should be between 6.0 and 7.0.  And our pH sits comfortably in the middle.    Certainly, it is not at an extreme, and does not require adjustment in any way.  

This is good news for one of our major questions.   No lime or sulfur needs to be applied.

 

2.1  What is the state of Phosphorous?  Our phosphorous is 70.  Looking at the optimal range of 40 - 70, the lawn is sitting at the upper end of the ideal range.  Any more phosphorous will kick us into the excessive range.

Bottom line:  we don't need to include phosphorous, the middle number in our lawn fertilizer.  Our target should be 16-0-8 or 27-0-2.  (Having said that, P in this analysis isn't excessive so minor applications of phosphorous won't be overly detrimental.  We have flexibility.)

2.2  What about Potassium?   Our potassium is 127, and the optimal range is 200 - 310.  This means our lawn fertilizer could benefit from having some potassium included in the formula.  The deficiency isn't terrible, but some potassium should be included in the mix.

Once again, the 16-0-8 or 27-0-2 is looking good.

 

3.  What does CEC indicate?  Our CEC tested at 6.7  A general rule says that CEC should exceed 10.0 for best results of applying fertilizer to the lawn.  This suggests that the soil is possibly low in organic material and could benefit from an application of humic acid or biochar or an organic fertilizer.

For the micronutrients, the advice always is caution.  Often, these micros will increase when organic material increases, so the best solution is organic fertilizer, humic acid and biochar.  The problem with specific fertilizer corrections is the danger of over-application, creating toxicity instead of improving soil fertility. Magnesium and Boron are low in this soil report but it isn't worth making any adjustments at this point, other than the addition of organics, unless a specific problem is identified. 

The Bottom Line

My major take-away from this soil report is that I am pleased that everything is so normal.  I don't have any significant problems that need to be corrected.  I know which lawn fertilizer to use, and I understand that organics have a role to play in future soil development.

Was it worth it?  For the little effort that it took, I am completely satisfied with the result.  I knew that my yard was overlaid on a pile of construction debris, and I was worried that there were massive irregularities that I wasn't aware of.  This test put those misgivings to rest and gave me useful guidelines for the future.