Saturday, March 1, 2014

How Your Soil Test Results are Like a Margarita

Posted by: Tony Koski, Turfgrass Extension Specialist

A couple of weeks have passed since you dropped off that soil sample from your yard or garden. And then it arrives: The Test Results. The bewildering assortment of numbers in units like parts per million, and millimhos per centimeter is chilling and confusing. If you are lucky, the lab has provided some interpretation of these numbers and some recommendations for fertilization of your yard or garden. You could just ignore the raw numbers and only pay attention to the fertilizer recommendations given by the lab, but for those of you who are more curious about what those mysterious numbers mean… read on my dear friends.

Here’s what your lab report will look like:
What about pH? This is pronounced “pee-H” (not “ppppppphhhht” as I once overheard).  If you live in Colorado, it’s likely that number is going to be above 7.0 (neutral). We just don’t have acidic soils like in the Midwest or east coast. Our alkaline soils (generally 7.5-8.3) means that plants such as blueberries, azaleas and rhododendrons struggle (i.e. die). There’s not much you can do to change pH. Adding pine needles, coffee grounds or sulfur are often suggested by people, but they are not effective at lowering pH. There is so much “free lime” in our soils that it is impossible to change soil pH—at least in your lifetime. Instead, take that “free lime”, have a margarita and crank Jimmy Buffet.

Since you enjoy salt with your margarita, let’s talk about EC (electrical conductivity), which is a measurement of the salts in your soil. Our example results are low; this number will be higher if you’ve added fertilizer, fresh manure or compost in recent months. Plants really don’t like salt. Check out this great blog from Dr. Ajay Nair at Iowa State University. 
Salt damage on peppers.
Vegetables tend to be most sensitive and will grow best if this number is below 2.0 mmhos/cm. If you have high EC, then plan on leaching with clean water. If you suspect you are irrigating with salty water, then testing your water is necessary (and it comes complete with another set of confusing numbers!). If your soil has a high EC value, then be careful in the future about the soil amendments you add, which can increase this number. Turfgrass can tolerate EC levels of 3.0-5.0 mmhos/cm without problem; landscape plants differ greatly in their tolerance to salts.
Lime estimate (seriously, why are all soil test measurements like a margarita?) is a rough indicator of the relative amount of free lime present in your soil. As mentioned above, it’s probably going to be “high” or “very high.” Only soils with “low” or “very low” estimates are worthy of consideration for acidification. Want to do a cool science experiment? Take a sample of your dried soil, pour some household vinegar on it and watch it fizz! This is the acid in the vinegar reacting with the lime in your soil.

OM = organic matter. Organic matter is the percent of organic compounds (i.e. decomposed leaf matter, grass clippings, manure, compost, plant tissues and roots, etc.) present in your soil. The magic number for organic matter is about 5%, yet this can vary widely, depending on where you tested your soil and what you have done to modify it. The longer you grow plants in soil, the higher this number will be. A 25-30-year-old lawn may have as much as 4-5% organic matter.  New, urban landscape soils may only have 0.1-0.5% organic matter (if you’re lucky). Organic matter helps “loosen” the soils, provides food for microorganisms, increases water retention, stores nutrients like phosphorous and iron and is really just a good thing. If you have a low percentage, you can increase this over time by slowly adding organic matter, preferably using high-quality compost. If you mulch your landscape with wood chips, this will also slowly increase organic matter over time (another reason to love mulch).  

The next column is NO3-N (nitrate nitrogen). The number you see in your soil test is likely to be very different than what it was in your garden at the time of the sampling. That’s because nitrogen in soil changes rapidly from one form to another as soon as you pull the sample from the soil. This number can be very high if you sampled soon after amending with manure or fertilizing. It’s also important to note that nitrogen is the nutrient you will most likely need to add to improve plant growth, especially in turfgrass. To be honest, I pay very little attention to this number when I’m advising people on their soil test results.

Phosphorous (P) and potassium (K) are next. Phosphorous tends to be correlated with organic matter. If your OM is high, P levels will also be high. If your report comes back with low P levels, then I would challenge you to look closely at plant growth to determine if adding P is necessary.  You will never see a P deficiency on turf, so it’s totally fine that our lawn fertilizers are increasingly P-free. With vegetables, P deficiencies will result in yield reductions, so look for a number between 20-40 ppm.  One of the easiest ways to add P to a garden soil would be adding reasonable amounts of well-composted manure. Potassium levels in Colorado are usually off-the-charts, unless your soil is sandy. Adding K generally isn’t necessary in Colorado soils.

Iron chlorosis on maple.
Let’s talk about micronutrients (zinc, iron, manganese and copper).  The key point with this is “micro”, meaning necessary in very small amounts. As long as each is between 5-10 ppm, you’re in good shape. You probably hear a lot about our “iron deficient soils.”  It’s not that our soils have a lack of iron; it’s just that at our high pH iron tends to be in a form unavailable to plants. As pH increases,iron availability decreases; this is why we may see chlorosis (yellowing) on plants like maples and oaks. It’s also important to note that chlorosis can be caused by compacted, cold, wet, dry and salty soils.

The last thing to mention is soil texture. Soil texture is an approximation of the percent sand-silt-clay in your soil. It may also indicate water infiltration rate, but since the analysis doesn’t consider soil compaction, this number may not be accurate. You can’t change soil texture, unless you add different soil to your existing soil, which we strongly recommend you don’t do

Truly, everyone thinks their soil stinks. Even the folks in Iowa. It’s better to accept your fate, grow what you can and learn to deal with your soil’s limitations. It’s really not as bad as you think. I can only recall one or two soil test reports that made me gasp and cringe—and neither was from a homeowner’s yard. If you’re really struggling growing anything, then consider your cultural practices (watering, fertilization, tilling, mulching) or simply resort to raised-beds or containers. As Jimmy Buffet once said, “Breathe in, breath out…move on…”
For a plethora of excellent information, check out the Colorado Master Gardener Garden Notes—it’ll keep you busy until the weather warms. 


  1. Tony, thanks for the "margarita" breakdown of soil reports. It added a depth of understanding to my soil rest report (completed by CSU).

  2. Hi Karen, thanks for writing such a practical bit of advice on gardening. I love your metaphor using the margarita example.
    Good job.
    George Brinkmann

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  4. Well, I think I have a clue as to why my garden isn't growing. I've done just about everything I can to get something to sprout, but it's been to no avail. I just thought I didn't have a green thumb like my wife, but it could be the soil. I'm still not sure, but it doesn't hurt to test it out. I'll look around to find where I can test the pH and then go from there.

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