Understanding Soil TestingBy Darius Van d'Rhys (darius)
July 23, 2008
The most obvious answer is that our soil needs to be tested, so let’s look at that first. Then let’s use some simple terms to understand what the test results mean and what we can do to improve our soil for what we want to grow. Generally, your county extension service will have the name and address of a lab for testing.
For testing my soil, the local Extension Agent recommended taking samples from several spots in my garden. He said not to take the soil from the very top, but below the surface an inch or two, and then dig down to about 6” or so. Mix that soil in a bucket with samples taken at 4-5 other spots in the garden and let it dry. He provided 4 small cardboard soil sample boxes and said to fill them half full, then mail where directed. The reason for sampling several spots is that since soil is in continuous movement from gravity, moisture, root growth, shade, earthworms, microbes and more, the overall sample will be a better representation of my soil. Also he said samples taken in June will not be exactly like samples taken in, say, February or in September.
Another thing my Extension Agent told me is that I needed to map the kinds of crops I have, or want to grow, and label the samples accordingly. The recommendations of nutrients based on the soil samples in what will be my lawn will be different that where I want blueberries, and both of those will be different from where I want tomatoes.
When you receive your soil test results, most of the numbers will be in ppm (parts per million) and your agent should be able to interpret the results and recommendations. A short list of common average adequate levels is at the bottom of this article.
Now let’s take a look at soil nutrients. There are 2 basic nutrient element groups, anions and cations. We need to know what is an anion and what is a cation for a very good reason. Anions have a negative charge (-), which means they do not bind very well to the soil and can easily wash away. The cations, on the other hand, have a positive charge (+) and so they cling better to the soil particles. So if I know what clings to the soil and what does not, I already have a better understanding of why soil nutrients vary in replacement needs. I use a small trick to remember that anions have a negative charge... in my mind I remember the an from anion reminds me of anti, so the anions are anti-clinging.
The anion group contains nitrogen (N), phosphorus (P) and sulfur (S).
Nitrogen (N) is most important to plant growth and can be increased in the soil by adding compost, well-rotted manure or by growing a cover crop like legumes. The amount of organic matter in the soil directly impacts available nitrogen.
Phosphorus (P) is more complex to test and understand but most soils have enough phosphorus without any addition. Plant roots give off a small amount of a weak acid that breaks down phosphorus to make it available to the plant. Phosphorus levels in the soil can be increased by the addition of rock phosphates but they take time (a year or more) to break down and become available to plants.
Sulfur (S) is important for the formation of plant protein. Like nitrogen, it needs adequate organic matter in the soil and is released by bacterial action. Adding gypsum or elemental sulfur will raise soil levels.
The cation group contains potassium (K), magnesium (Mg) and calcium (Ca).
Potassium (K) is generally sufficient in most soils although many organic growers apply K Mag (potassium magnesium sulfate) to be on the safe side without soil testing. Manures contain high levels of potassium and make a good supplement.
Calcium (Ca) obviously doesn’t build strong bones in plants but it does build strong cell walls, and is necessary in seed formation. Calcium levels in the soil are directly related to pH.
Magnesium (MG) is also very necessary for plant growth. Lime and K Mag can help deficiencies. Magnesium, calcium, and potassium should be in balance for best growth, and that "balance" is about 15% magnesium, 75% calcium and 5% potassium. The other 5% is made up of misc. minor nutrients.
Average adequate levels for plant health and growth:
Nitrogen 100 lbs. per acre
Phosphorus 30-40 ppm
Sulfur 8-12 ppm
Potassium 150 ppm
Calcium 2,000 ppm
Magnesium 50-100 ppm
Soil in Hand, © Clayton Hansen, iStockPhotos #498015 used by permission
Soil and Flask, © Srdjan Srdjanov, iStockPhotos #3152207 used by permission
Sources: Scott Vlaun and Seeds of Change newsletters for the specific information on cations and anions, used by permission; Smyth County, VA Cooperative Extension Service, and many friends and online sources over the years.