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Soil Science Basics

If you aspire to have a SWARD, which is a lawn that is the source of envy from all, it requires that you run a soil test, to “fix” imperfections. Have you run a soil test yet? In general, we run a soil test to determine three critical factors:

  1. Soil pH: determine if the soil is acidic (“sour”), neutral, or basic (“sweet”)

  2. Phosphorous levels (“P”)

  3. Potassium levels (“K”)

If you follow our “horticulturally-sound” advice, you’ll soon be the “big kahuna of turf”, at least in your cul-de-sac or neighborhood. We know how quickly your sward responds to proper management, especially if you adhere to the proper timing of products, all season long. THE Turf Plan® is predicated upon accurate application of fertilizer and/or pesticides. Once you’re sure how much actual turf you have (your “sward” size) your next step is to get a soil test. Read on.

You should test your soil every 3 to 5 years, especially if your sward is not up to standard, or if you’ve been following our advice for several seasons, but haven’t achieved satisfactory results. Additionally, if you have “problem area” it’s a good idea to run a separate soil test for that general area. If you aspire to be the neighborhood lawn stud, soil testing will only get you there faster.

How to Take the Soil Sample

  1. Decide if you need one general sample, or whether or not you’ve got some serious problem areas that you’d like to do some “detecting” work. If so, segregate your problems areas

  2. Get a garden trowel or digging tool and a large PAPER grocery sack

  3. Plan on getting at least 4 to 6 “cubes” of soil from random areas, about 2″ to 3″ wide, and 2″ to 3″ deep.

  4. Before digging the soil cube pull all the grass out and clear away the organic debris (“duff” and/or thatch). Don’t collect any dog poop because it will skew results!

  5. Take a cube of soil from 4 to 6 different spots in your yard, placing them into the grocery sack as you collect them. Our interest lies in the actual rooting zone for turf. Backfill the divot with some bagged topsoil, rather than clay.

  6. If you have problem areas, repeat the process for the bad spots, but keep that sample separate! Be sure to label the bags “good” and “bad” lest you mix them up!

  7. Take the bag(s) into the garage. Leave it open to air for a few days to allow the soil to dry, until it’s very easy to crumble into small clods and homogenize (mix). When the soil is in a crumbly state, carefully mash it all up with either a baseball bat or a piece of wood. Mix the sample evenly. This is important. Remove stones, sticks, debris, thatch, etc.

  8. The soil lab only requires at least one cup of nicely granulated soil. Use any left-over soil to fill some of your divots.

  9. Pick up a soil sample box from our warehouse and a the proper form that is required by Mizzou’s soil testing lab. We’ll tell you how to label the box and the sample. You’ll mail it on your own to the. You need to send a check for $10 for each sample you submit.

  10. Results should be back to you within 10 days. The lab recommendations will be on the bottom of the test results. Bring those results in to the warehouse and we’ll help you interpret the results and make the necessary corrections. We sell all of the products you need to fix your soil.

  11. We recommend that you take your soil test between the months of April and June. That way, you’ll be sure to have your results in plenty of time to take corrective actions in September, which is the critical month for turf seeding and renovation.

  12. The absolute best way to amend soil problems is to roto-till the required materials into your topsoil but that’s not practical for established lawns. Thus, we recommend that you aerate your lawn very, very heavily in September, and then apply the required limestone, P and/or K, based upon your test results. Now when we say “heavily” we mean 4 passes at a minimum! You need to poke your sward with hundreds of thousands of holes because these nutrients are not very water soluble. Applying the nutrients into a swiss cheese lawn places the nutrients directly into the rooting zone. Don’t even think about applying lime, P and K atop an un-aerated lawn! surface! You’re just wasting your time and possibly polluting the water that runs off your property (especially with phosphorous).

How to Adjust your soil pH Level:

Most soils in St. Louis county are acidic, as you’ve already heard. A soil test value will take into account the pH and it will provide an amount of calcium required to bring the pH into the optimum range. Optimum pH for turf is 6.2 to 6.7. It is NOT 7.0, which is “neutral” on the pH scale. If your soil is acidic, adding lime will slowly bring the value into the optimum range. The test result might state: apply 40 lbs of lime per thousand square feet. Some soils are so acidic, 5.2 to 5.5, that it requires at least 150 lbs of lime per thousand! No kidding, that’s three full 50 lb bags, over a 20’ x 50’ patch of grass! We only want you to apply 50 lbs per thousand per year. Thus it will take you three consecutive seasons, applying a big bag of lime, for every thousand sq ft of turf. You will need to aerate every fall, before applying the lime. Calcium carbonate, the neutralizing element in lime, is not very mobile in the soil. The coring helps move nutrients deeper into the rooting profile, faster.

We’ve helped over 200 customers test their lawns, and it’s interesting that many of the soils in St. Charles County, around O’Fallon, are actually “sweet” (basic). And they’ve got a lot of magnesium (Mg), too. Soil pH is affected by the parent bedrock material. But, we’re not geologists, so we can’t give you much more background in this area. Regardless, sweet soils need to be acidified with elemental sulfur. As with the liming recommendations, you need to aerate heavily first, and then apply the sulfur. It’s a lot harder to give accurate sulfur recommendations, thus you’ll want to test your soil every year or two, until the pH is reduced to at least 6.7. If you’re interested in how sulfur works, it reacts with water, to form sulfuric acid, albeit quite slowly. Don’t worry, you won’t get acid burns or anything like that. If you need to acidify your soil, you must wait for cool weather in October, because too much sulfur can burn the crap out of the turf roots. Cooler, wetter weather is required.

Don’t waste your time or money applying “aluminum hydroxide” to lower the soil pH. Sulfur is far better, and less expensive. Plus, too much aluminum is toxic to plants! As with the liming recommendation, application of ground sulfur over two to three applications is more reasonable. And of course, core aeration is required in advance of the application.

Think of soil pH this way. When it’s optimum, between 6.2 and 6.7, turf roots have an easier time obtaining all those essential nutrients for plant growth. Micronutrient availability is completely dependent upon soil pH! When pH is either too high, or too low, those micronutrients are tied up. Everything is in balance. It’s a “holistic” approach to tending turf.

You now know how to take a good soil test, and “fix” the soil. Everything below is college level “Soils 101″. Read on, if you’ve got some time!


Most folks, especially those that have fertilized regularly, will get a soil test in which the result shows “high” or “very high” P levels. That’s good from the standpoint that P isn’t limited. It can be bad, though, because too much P forms “complexes” with calcium, manganese, iron and molybdenum, rending them unavailable to roots. Phosphorous is a very immobile element. That is, it stays put in the soil. Excessively high P can actually make nitrogen less efficient, too. That’s a problem. Applied onto the soil surface, it’ll take years to move into the top 2 to 3 inches of soil. Again, because P is so immobile, we recommend that you apply phosphorous fertilizer after many passes with a core aerator, to place the element into the entire root zone of turf. If your soil tests are high or very high in phosphorous, you should try to avoid applying fertilizers lacking this element (very low, or zero, for the second number in the analysis, such as “19-0-4″). We try to provide products with little, to no P.


Potassium, also called potash, is the element associated with “winterizer” fertilizers, or “stress conditioners”. It’s represented by the third number in the analyses of fertilizers. It generally tests out in the high category here in St. Lou. Some U.S. soils are K deficient, but St. Louis clays have a lot of it. K is more mobile than P, but not nearly as mobile as nitrogen. The stress conditioning aspects of high potassium come from the fact that inside the plant, potassium is the dominant positively charged cation. Potassium is involved in a lot of enzymatic activities and help maintain the charge balance of a plant. Again, if your soil test results indicate a need for more K, it’s best to apply it right after multiple aerator passes. Incidentally, fireplace ashes are mostly potassium. Rather than pitch your fireplace ashes, spread it thinly all over your lawn in the cool months of fall and winter, but be warned, the family mutt might track some soot back into your house, onto your nice, new and white carpet. If this happens, you’ll be sleeping beside the dog, dirty feet and all…

Fix the P, K, Ca and Mg!

Testing the soil fixes the P and K, along with the liming requirements. Lime is mostly calcium (Ca) but also magnesium (Mg).

In addition to these elements, there following 12 nutrients are essential for plant growth.

Macronutrients are required in large amounts. C, H and O are obtained from the air and soil, and are NEVER limiting!

C = Carbon, H = Hydrogen, O = Oxygen, P = Phosphorous, K = Potassium, N = Nitrogen, S = Sulfur, Ca = Calcium, Mg = Magnesium

Micronutrients are very low amounts. We don’t usually test for these.

I = Iodine, Fe = Iron, B = Boron, Cu = Copper, Zn = Zinc, Mo = Molybdenum, Cl = Chloride

(Note: this blog post was originally posted on Oct. 11, 2011)

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