Got my soil test back results back today, and I thought I would take a few moments to share a little bit of what I have learned in the past couple months. The recommendations I’m using are from Steve Solomon’s book, The Intelligent Gardener.
CEC
Before you can even look at a soil test, you have to understand cation exchange capacity. Some of you may remember from high school chemistry that elements are made up of ions that carry with them a charge. Positively charged ions are called cations (pronounced cat-ion) and negatively changed ions are called anions. The major cations in soil chemistry are calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), and hydrogen(H). These cations are attracted to negatively charged soil particles (usually clay) similar to the way a balloon sticks to a sweater. These cations can then be exchanged by the plant for other cations, usually the plant exchanges H from H2O for whichever cation it is in need of. The total number of cation exchange sites is the cation exchange capacity in milliequivalents per gram (meq). One meq of one cation will push out one meq of another cation.
Another thing to note is some of the units are given in pounds per acre and some are given in ppm. An acre is assumed to weigh 2000000 pounds, therefore 1 pound per acre is 2 ppm. In the discussion below, I have converted everything to ppm as I think it makes more sense than pounds per acre.
So let’s take a look at my soil test results. I added a column to the right so we could compare the previous testing cycle to the current one.
Total Exchange Capacity
When you get you soil test results back, you will get a total exchange capacity (TEC) reading back. This is the total number of exchange sites for the five major cations in soil chemistry, Ca, Mg, K, Na, and H. This is how much nutrients the soil can hold on to, any excess cations will push off other cations from the exchange site and wash away into the subsoil (or in our case runoff). Since my soil has a pH above 7.0 and has a high Ca content, it would be classified as a calcareous soil. This type of soil requires a different kind of soil test, AA8.2, to calculate the correct Ca value. Using the AA8.2 Ca value, I recalculated my TEC accordingly (not shown on the comparison). My previous TEC was 23.43 and it went up slightly to 26.7 this time. However, when I use the AA8.2 Ca values, my new TEC becomes a more believable 14.7.
pH
The next number you will look at on your soil test is the pH. If you recall, pH stands for potential hydrogen and it is directly related to the number of H cations attached to the exchange sites. A soil with a pH greater than 7.0 will not have any H cations, while a soil with a pH below 7.0 will begin filling up these exchange sites with H. My pH went up slightly from 7.2 to 7.3 despite my addition of sphagnum peat moss and fermented fruit juices. The high pH does make some of the micronutrients less available, but otherwise is not detrimental to the soil. No correction is desired.
Organic Matter
The organic matter component of the soil comes from compost, castings, or general breakdown of plant materials. This mix was made with 33% compost and is very full of worms, so it is not surprising that my organic mater is high. It rose slightly from 46.10% to 47.13%. Organic matter generally contributes to the N production of a soil as organic matter breaks down it gets converted into nitrates. With such a high organic matter component, no additional nitrogen is required.
Sulfur
Sulfur is an anion used in plant nutrition. Elemental sulfur can also be used to lower the pH of soils with a pH above 8.0. The S on my test dropped significantly from 216 ppm to 166 ppm. Many of the micro nutrients I will be adding, as well as the gypsum are sulfates that contain S. With a TEC greater than 10, I am targeting 35 ppm S, so no additional sulfur is required.
Phosphorous
Phosphorous is another anion used in plant nutrition. Phosphorous in the soil is usually quickly tied up with calcium and iron making it unavailable to the plant. The soil test measures phosphate (P2O5), to get the elemental P value, multiply that value by 0.44. My phosphate rose from 624 to 1003, perhaps due the microbes in the Mammoth P. Since my target level is P = K, no additional phosphorous is required.
Calcium
Calcium is usually the most abundant cation in a soil. It is used in the plant for cell walls as well as other metabolic functions. My calcium actually increased over last time from about 3640 ppm to 4108 ppm, likely more calcium carbonate was freed up with organic acids exuded from the plant roots and microbes. Since I have a high calcium content and my pH is above 7.0, I will be using the calcium value from the AA8.2 test as previously mentioned, that brings my Ca value down to 1806 ppm. I didn’t get the AA8.2 test last time. My target is 85% of the TEC, so I am short about 693 ppm of Ca. To get to this level of calcium would cause me to go over the recommended maximum amount of gypsum, so I will just add the maximum recommended amount of gypsum. 500 grams of gypsum is required.
Magnesium
Magnesium is a cation required for metabolic processes, particularly photosynthesis. Mg is a large molecule and excess Mg in the soil has a tendency to tighten it up and not allow for enough oxygen in the roots. My magnesium went up from 335 ppm to 450 ppm. My target value for magnesium is 5% of the total TEC, so I have an excess. Because adding quite a bit of Ca, it should help to push off some of that Mg from the exchange sites. Remember, one meq of Ca will push off one meq of Mg.
Potassium
Potassium is another cation used by the plant, particularly in the generative (flowering) phase. My K went down from 464 ppm to 409 ppm. The target value for K is non linear and done by a lookup table in Solomon’s book. According to the table for my TEC, 14.7 I need 190 ppm, so I am well over the target value. Again the Ca cations will push out K cations with the addition of gypsum.
Sodium
Sodium is a cation that is used very sparingly in plants. My sodium dropped slightly from from 111 ppm to 43 ppm. My target value is 0, though I plan to do some foliar applications of sea salt for “trace” minerals.
Boron
Boron is used in very small amounts, but is vital to calcium uptake. My Boron didn’t really change much from 1.35 to 1.51 ppm. My target value is 2 ppm, leaving me short about 0.5 ppm. 2.5 grams of borax is required.
Iron
Iron is generally abundant in soils. It should be in mine as well because I used basalt rock dust. According to my test Fe stayed pretty much the same going from 65 ppm. to 63 ppm. My target value is 75 ppm, leaving me short about 12 ppm. 20 grams of iron sulfate is required.
Manganese
Manganese becomes less available as pH levels increase. My current levels went up from 11 ppm to 26 ppm thanks to the additions of manganese sulfate. Since my target is 50 ppm, I still need another 24 ppm. Since the application limit is 5 ppm, 7.75 grams of manganese sulfate is required.
Copper
Copper is another nutrient that is used in small quantities. My current value rose from 1.51 ppm to 2.19 ppm. This is likely from the addition of TM-7. My target value is 5 ppm leaving me short about 3 ppm. Since the application limit is 2 ppm, 4 grams of copper sulfate is required.
Zinc
Zinc is the final micronutrient I will be trying to balance this round. My current value rose from 6.69 ppm to 13.44 ppm. My target value is 1/10th of P, so 19 ppm leaving me short about 6 ppm. Since there is a 5 ppm application limit, 7.25 grams of zinc sulfate are required.
Without going over the safe application limits, my soil prescription this cycle looks like this:
500 grams gypsum
2.5 grams borax
20 grams iron sulfate
7.5 grams manganese sulfate
4 grams copper sulfate
7.25 grams zinc sulfate
So suspect they should.
