Interpreting Your Soil Test Report
I remember it just like it happened yesterday — though more years have passed than I care to admit. It was the day I carried an apple leaf sample into our local extension office to diagnose a problem of discolored leaves. As I presented the sample to the extension agent, I began to describe the curled brown edges of the leaves. Looking back now, I realize that the extension agent must have had a lot of patience because I’m sure she could see the sick leaves as well as I could! She looked up from her microscope and let out a long, drawn-out “Hmmm.” I began to explain that I had been very vigilant, spraying every ten days, even after rains. I had even brought in the container of multipurpose fruit spray that I had been using for her to examine, thinking the only possible reason for the yellowing leaves had to be that the spray wasn’t doing its job.
“Well . . . ” she responded, “when was the last time you did a soil test?” I avoided the question and responded, “ I feed my apple trees twice a year, in the fall and spring with a complete fertilizer, and I don’t think fertilizer is the problem.” She must have had an anger management class somewhere in her past because she just smiled and said, “Many times the symptoms of trouble in plants are caused by nutrient deficiencies. Quite possibly, this is the most-overlooked of all causes. There is an interrelationship between a nutrient’s availability and the soil pH, air and soil temperature, available water, and amount of soil organic matter. A soil test is the best starting point in diagnosing the problem you’re having with your apple trees.” She reached into the cabinet behind her desk and pulled out a soil sampling kit and handed it to me. She proceeded to go over the instructions included in the soil test kit on how to take a proper soil sample, how to prepare the samples for testing, and how to send the samples to the soil lab at Virginia Tech.
Well, I went home and collected some soil from around the apple trees. The instructions were pretty easy to follow. About 15 days after I mailed the sample, an email appeared in my inbox with the soil test results. Well, it might as well been written in Greek or Chinese. It was a bunch of numbers. I began scratching my head, and for a moment considered simply fertilizing again, but I couldn’t help noticing that the report made no mention of adding fertilizer. So I figured I needed some help. I printed out a copy of the soil test results and headed to the extension office.
The extension agent took a quick look at my test results, drew some red circles and a line and asked if I had followed the recommendation. Naturally, I said, “What’s lime got to do with the my apple leaf problem?” At that point I thought I detected a slight roll of the eyes, but the agent proceeded to translate the report for me.
As the agent patiently explained, a soil test provides an estimate of the level of available nutrients in the soil. The soil test measures the available levels of the following nutrients: phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), zinc (Zn), manganese (Mn), copper (Cu) and boron (B). The units of measure are given in either pounds per acre, or parts per million.
Okay, I’m thinking, what’s the big deal, but before I could get a word in, she began to talk about some German dude named Liebig from back in the 1800s and his “Law of the Minimum” which states, “Plants are generally limited by only one single physical factor that is in the shortest supply relative to demand.” The extension agent went on to explain that this principle simply means that a plant’s health and growth could be adversely affected if only one nutrient is deficient. What’s that old saying? A chain is only as strong as its weakest link. So that’s why it’s important to pay attention to all the nutrient levels on the report.
Turns out there are three Primary Macronutrients: Nitrogen (N), Phosphorus (P) and Potassium (K).
Nitrogen, more than any element, promotes rapid growth and dark green color. In general, plants require more nitrogen than any other mineral element as an essential part of protein and chlorophyll.
Although nitrogen is the most important of all nutrients, you will not find a measurement of this element in your soil test results from Virginia Tech’s Soil Test Lab — and that’s the case for most soil testing labs. Why? Nitrogen is the most mobile nutrient in the soil, and as a result, soil tests for nitrogen are not reliable for predicting nitrogen fertilizer needs in many situations. Nitrogen fertilizer recommendations are based on the kinds of plants/crops being grown. It is assumed that there is no nitrogen in the soil, that the plants used most of the nitrogen in the previous year, and that what was left over was leached out by rainfall or evaporated as ammonia. However, if the soil has a high percentage of organic material, there may indeed be nitrogen in the soil, depending on the type of organic matter. However, the assumption of the lab is that nitrogen needs to be replaced each growing season. The nitrogen recommendation is made specifically for the plants or crops you plan to grow.
Shown below is a recent soil test performed on a rose garden; note the nitrogen recommendation (Note 225).
If you look at the “Lab Test Results” section, you will see that all the nutrients, except nitrogen, are listed from left to right, and beneath each nutrient is the “Result” — simply the amount of that nutrient found in your soil. The first four nutrients measured are phosphorus (P), potassium (K), followed by two secondary nutrients: calcium (Ca) and magnesium (Mg) The report indicates the available amount of these four nutrients in pounds per acre. The micronutrients zinc (Zn), manganese (Mn), copper (Cu), iron (Fe), and boron (B) are reported in parts per million.
But back to the results from the soil under my apple trees. I was getting a little dizzy looking at all those numbers, P =51 lb/A, K=160 lb/A, Ca=1088 lb/A, and Mg=152/A. It was beginning to look like a bunch of gibberish. So I lied, and said, “Okay, piece of cake.” I couldn’t wait to see what valuable piece of information would be revealed in the next line, which was labeled “Rating” and which contained a bunch of letters and plus and minus signs.
Well all of a sudden the light bulb came on when the extension agent began to explain the letters. Turns out we don’t need to understand the numbers, so long as we understand the letters, ranging from VH, H, H-, M+, M, M-, L+, L, to L-, which tell us just how bad or how good those results are. I was actually right when I guessed that VH=Very High levels, H=High levels, M=Medium levels, and and L=Low levels. So the number 51 under P might not mean much to me, but I could easily see from the H- rating that I had relatively high levels of phosphorus in my soil.
The extension agent explained that “The Ratings” are derived from the test numbers. In general, when a nutrient has a low rating, plants almost always respond well to fertilizer. When a nutrient is rated M for Medium, a moderate amount of fertilizer is typically recommended to maintain fertility. When a nutrient is rated High or Very High, plants usually don’t respond to fertilizer and no fertilizer recommendation is made.
As an example, the agent pulled out a calibration chart on phosphorus to show how my test results (numbers) were translated to a rating. The lab result for the level of phosphorus was 51 lbs per acre; therefore, it fell into the H- (Low High) rating.
Continuing on with the soil test results: the micronutrients zinc (Z), manganese (Mn), copper (Cu), iron (Fe) and boron (B) test results are reported in parts per million, but don’t worry too much about these numbers either. The rating is our main focus. The micronutrients are rated a little differently; here the test results are rated with a simple Suff. (sufficient) or Def. (deficient). Only those micronutrients rated Def. will require corrective action. The soil test lab will make a recommendation for the material and quantity if needed.
Now I was getting a little frustrated; all the test results were either rated in the high range or sufficient, with the possible exception of calcium (Ca) which tested in the Medium range. Maybe I needed to tweak the calcium level to get it into the high range? So I popped the question, “What’s a good source of calcium and why wasn’t there a recommendation made to add calcium?” I was told to hold that thought until we finished going over the soil test.
The next item we discussed was pH, and the result for this element was 5.2. Now my jaw dropped when I was told that one of the most important — if not the MOST important — soil factors that affects a plant’s growth and health is the pH level of the soil. The soil pH test measures how acidic or basic (alkaline) the soil is. The pH scale is a logarithmic scale and ranges from 0-14; a pH level of 7 is neutral. A pH less than 7 is acidic and a pH greater than 7 is basic. Since the pH scale is logarithmic, a soil with a pH of 5.7 is 10 times more acidic than a soil with a pH of 6.7, and a soil testing 4.7 is 100 times (10 X 10) more acidic than a soil testing 6.7. The same principle holds true for soils testing above 7.0 — each whole number is 10 times more alkaline or basic than the next whole number. This explanation may be confusing for those of us who are “math challenged,” but the main thing to remember is that when the pH needle moves one whole point, it’s not just one point, it’s 10 times that because the pH scale is logarithmic. (Plaster)
Why is pH so important? The degree of acidity or alkalinity of the soil is directly related to the availability and uptake of soil nutrients to plants. At pH extremes, some nutrients become partially or completely locked up in the soil and become unavailable to plants. In short, the pH factor is the keeper of the nutrient key. Adding amendments or fertilizer to soils with extreme pH levels will have little or no effect on plant growth. Correcting the pH level opens the nutrient door and allows the amendments and fertilizers to be used effectively.
As we can see in the chart above, the thicker the bar, the more available the nutrient. A pH above 6.2 will insure that all nutrients are available to my apple trees. But the soil around my apple trees had a pH of 5.2, which meant that my soil was more than 10 times too acidic. My apple trees were having a hard time pulling the needed nutrients out of the soil. The Soil Lab at Virginia Tech will recommend adding lime to soils with a pH of less than 6.2 , since the pH of the soil for growing apples should be in the range of 6.2-6.5 Thus, the brown edges on my apple leaves could be the result of a nutrient deficiency, not because the soil lacks a particular nutrient(s), but because the nutrient or nutrients have become unavailable due to the low pH of the soil. Who would have thought acidic soil could be the problem?
Okay, now I see that adding lime is essential. And it turns out one of the major elements in lime is calcium (in the form of CACO3), meaning that when I add lime to raise the pH of the soil, I will also be adding calcium. There is a lime product called Dolomite Lime ( Ca,Mg)CO3 that contains not only calcium but magnesium, and it may be used if the soil tests low for magnesium. (Brady)
The next block on the test result form is the Buffer Index. The Buffer Index is used by the lab to determine how much lime is needed to change the pH of the soil. Any time the soil pH is below 7.0, there will be a buffer number of 6.60 or lower, depending on both the pH and the type of soil (sandy or clay). The lower the buffer number, the more difficult it is to change the pH of the soil. In the case of soil around my apple trees, with a very low pH of 5.2 and a low buffer number of 5.89, a lot of lime will be needed. Fortunately, the folks in the lab do the calculations and make a recommendation on the correct amount needed. Because the lab does all the work for us, we can disregard this measurement.
The next number is Est.-CEC (Estimated Cation Exchange Capacity). This is a measurement of how well the soil holds water and nutrients or how “sticky” the soil is to some nutrients. A more exact scientific definition is the measurement of the total exchangeable cations (positively charged ions) a soil can adsorb. Nutrient cations in the soil include calcium, magnesium, potassium and hydrogen. In general, the greater the amount of clay and organic matter in the soil, the higher the number. Clay particles are smaller than sand particles, therefore, clay has more surface area to attract positively-charged ions.
Okay, what does this mean? Simply, think of a fuel tank; the larger the fuel tank (higher CEC number), the greater the soil’s capacity to hang on to nutrients. As plants remove nutrients (cations) from the soil , the fuel tank replenishes the soil with nutrients which are then available to the plants.
Sandy soils (larger particles=less surface area) generally have a low CEC number (smaller fuel tank) which means that nutrients (fertilizer) need to be added more often. Clay soils (small particles= greater surface area) have a high CEC number (larger fuel tank) which means that nutrients need to be added less often. The pH of sandy soils also needs to be adjusted more frequently because water passes through sandy soils at a rate greater than through clay soils which have a higher water-holding capacity. As water passes through the soil, basic nutrients such as calcium and magnesium are flushed or leached out. They are replaced by acidic elements such an aluminum and iron, and over time, the pH of the soil becomes more acidic (lower pH). Also, the application of certain fertilizers such as ammonium or urea speeds up the rate at which acidity develops.
As a general rule, the lower the CEC number, the more fickle 0r volatile the pH of the soil, requiring more frequent applications of lime and more rigid pH monitoring. In general, the pH in sandy soils with a low CEC value are less stable and change at a faster rate than clay soils. The higher the CEC number the more stable the pH, adding organic matter to the soil generally increases the CEC value of a soil.
The next block on the soil test form; “The Percent Acidity” is a ratio derived from the Buffer Index. The higher the percentage, the higher the amount of reserve acidity in the soil. This number will rise and fall in relationship to the of the soil’s pH. In general, the home gardener can just ignore this measure, since it is used by the lab to calculate the amount of lime required to adjust the pH to the desired level. The term “N/A” will be entered into the block if the pH is 7.0 or above.
The “Percent Base Saturation” is the percentage of cation sites filled with exchange bases (Ca, Mg and K). Huh?? Unless you are a soil scientist, this number can be disregarded! But if you’re curious enough to want a detailed explanation, I can safely say that it is an expression of the soil’s “potential fertility.” (Plaster).
And if you want to know where the Percent Base Saturation number came from, take a look at the rose garden test results again. Calcium occupies 78.5% of the cation sites, followed by Mg with an occupation rate of 12.1 %, followed by K with a percentage of 1.7%. Doing the math, you add these numbers up: 78.5%+12.1%+1.75%=92.2%, which just happens to equal the Percent Base Saturation rate of 92.4% on the rose garden test results.
Since base saturation is simply an indirect expression of soil pH, base saturation is generally not used for making fertilizer or lime recommendations. In acidic soils, base saturation and soil pH change simultaneously. Liming increases base saturation and soil pH. Sulfur application will reduce base saturation and pH. Base saturation is not a useful concept in alkaline soils, where base saturation equals or exceeds 100 percent.
The “Percent Ca, Mag and K Saturation” refers to the relative number of CEC sites that are occupied by that particular nutrient and is a way of determining any gross nutrient imbalance.
At one time, many labs provided recommendations to achieve very specific ideal potassium (K), calcium (Ca), and magnesium (Mg) saturation ratios or percentages. This approach was never supported by data. Research at University of Minnesota and Iowa State University suggests that an ideal ratio or percentage does not exist. Even if the ratio or percentage is considerate optimum, a nutrient deficiency may still exist. A sufficient supply of nutrients in the root zone is the most important consideration in making fertilizer recommendations.
The block labeled Organic Matter (percentage) is an optional measure for an additional $4.00. Now that I know just how organic matter can affect a number of soil fertility components, I think this information is well worth the $4.00.
For an additional fee of $2.00 the lab will proved another optional measurement, S. Salts (Soluble Salts). I did not request this measurement for my apple orchard soil test, but I did request it on the rose garden soil test. My reason was that the garden in question is a public rose garden, and over the years it has been fertilized very heavily. In addition, the rose garden is located adjacent to a sidewalk, and I had some concern about run-off from salts used to melt ice and snow on the sidewalk. The test results for S. Salts was 141 parts per million (ppm). Injury to plants may start to occur at soluble salt levels above 844 ppm , especially to seedlings and germinating seeds. Salt damage can be intensified during dry periods. Happily, the soluble salt levels (144 ppm) in the rose garden are well below the level that could cause injury.
If you have concerns about possible toxic elements such as lead or arsenic, you must engage a private laboratory since the soil lab at Virginia Tech does not offer this service.
There were two recommendations on my apple orchard soil test: to add boron and lime to correct the pH level. Because of the large amount of lime recommended, 3.5 tons per acre, I made 3 applications over a period of 9 months. It takes 2 to 3 years after application for lime to react completely with the soil; however, benefits from the addition of lime may occur within the first few months after application. How long the effects of lime last will depends on the kind of lime used, total soil acidity, amount of organic matter, type and amount of clay, and cropping and management systems used. Because of these multiple factors, a soil test is recommended every 3 to 4 years.
I leaned a lot during that office visit. Most importantly, I learned that a soil test is one of the best investments a gardener can make. The purpose of a soil test is to provide the gardener with information needed for making a wise investment in fertilizer and soil amendments. Ideally, soil samples should be taken a few months before any new landscaping is planted. Fall to early winter is the perfect time frame to test the soil and to add the recommend nutrients. This timing allows added nutrients to start working in the soil. As I mentioned, the general rule is that a soil test should be taken once every three to four years, but it should also be considered if there is abnormal growth or a change in plant color. And remember, soil samples should NOT be taken within 6 to 8 weeks of fertilizing or liming.
Looking back now, life would have been a lot simpler and my apple trees more productive if I had taken the soil test and followed the recommendations PRIOR to planting the apple trees. A soil test, if taken prior to planting, can be a tool to help you determine what kind of plants to plant. Why not embrace the existing soil condition? For example, if a soil test indicates a low pH, why not consider planting “acid loving (low pH)” plants such as blueberries, azaleas, rhododendrons, hydrangeas, or even a dogwood tree?
With your soil test report in hand, you can stop guessing and add necessary amendments in the right amounts to create a fertile environment for your lawn, your flower bed, your vegetable garden or even your apple orchard. Conducting a soil test is one of the best and least expensive ways of insuring that your landscape will flourish.
Thanks for joining us in The Garden Shed. We hope you drop by again next month.
“Nutrient Disorders In Fruit Trees,” Washington State University, Publication PNW 121W, http://whatcom.wsu.edu/ag/documents/treefruit/pnw0121e.pdf
“Essential Elements for Plant Growth-Law of the Minimum,” University of Wisconsin, http://soils.wisc.edu/facstaff/barak/soilscience326/lawofmin.htm
“Setting a New Trend,” Research Center for Chemical Risk, https://unit.aist.go.jp/riss/crm/crm_e/directors_address_2003e.htm
“Soil Test Interpretation Guide,” Publication EC1478, Oregon State University, https://catalog.extension.oregonstate.edu/ec1478
Soil Recommendations of Virginia Guidebook, Virginia Cooperative Extension, http://www.soiltest.vt.edu/PDF/recommendation-guidebook.pdf
“Soil Test Note #1, Explanation of Soil Tests, Virginia Cooperative Extension Publication 552-701, http://pubs.ext.vt.edu/452/452-701/452-701.html
“Fundamentals of Soil Cation Capacity (CEC),” Purdue University Cooperative Extension Service, Publication AY-238, https://www.extension.purdue.edu/extmedia/ay/ay-238.html
Plaster, Edward J., Soil Science & Management (2009), p. 266.
Brady, Nyle C., The Nature and Properties of Soil (14th ed.), pp. 387-393.
“Facts about Soil and Acidity,” Michigan State University, https://www.oakgov.com/msu/Documents/publications/e1566_soil_acidity.pdf