Tuesday, April 23, 2013

The Soil Doctor Writes a Prescription


Since my last post spring has arrived in St. Louis, bringing with it sufficient warmth to open the flowers on our serviceberries and magnolia, shown in the above photo. We have had a wetter and cooler than normal spring season. Last week I measured a total of 4.0 inches (about 10 cm) of rain at our location. I had prepared the bed for the onion and leek seedlings before the rain began but I haven’t planted it as I have been waiting for the saturated soil to drain. While I was waiting, I weeded and mulched the herb bed, just in time for today’s rain. I don’t think we will get nearly as much rain this week as we did last week so I will be able to plant the onions within the next couple of days. Then I must get busy on getting the rest of the cool season crops planted before summer begins and it is time to plant the warm season crops.

A couple of posts ago I discussed Steve Solomon’s new book The Intelligent Gardener: Growing Nutrient-Dense Food. The book describes the re-mineralization process he has outlined for us home gardeners so that we may grow the most nutritious food possible in our gardens. I had a suspicion that our garden soil may not be as well mineralized as it could be and decided I’d like to test his program this year. So I collected 15 plugs of soil using a tubular soil sampler (it could be done with a stainless steel trowel as well but I sprung for the soil sampler because I expect to collect soil samples each spring from now on). Each plug was taken from the approximate middle of one of our vegetable garden beds; there are 15 of them in all, each 100 square feet in area. I mixed the fifteen plugs together as described in the book and labeled that sample Garden. Since I’d been using a version of the Complete Organic Fertilizer (COF) in Solomon’s earlier book, described in this post, on these garden beds for varying periods of time, I was curious as to what effect it may have had. To understand this, I took another 15 plugs of soil from the lawn in between the two largest groups of beds, mixed those plugs together, and labeled that sample Yard. I then packaged and sent both soil samples to Logan Labs, LLC, using their downloadable instructions and form. About a week later the soil report shown below landed in my email box.


From a quick scan of the results I learned some interesting things about our soil. If you look at the pH (level of acidity: 0 to 7 is acidic, 7 to 14 is basic) of both samples, you will see that the soil in the yard is more acidic (lower number) than the soil in the garden and that both values are on the acid end of the pH scale. (Fun but ignorable fact: because the pH scale is logarithmic, the yard soil is actually more than 10 times as acidic as the garden soil.) This makes sense because St. Louis gets about 40 inches of rain a year, enough to leach a significant amount of calcium out of the soil and thus raise its acidity. The garden soil has a less-acid pH than the yard soil because of the garden lime in the original COF formula that I’ve been using. Just about every gardener around here uses garden lime to raise soil pH and calcium to levels conducive to good vegetable growing. The pH value of 6.4 in the garden sample is about what most veggies want.

The organic matter level is nearly the same in the yard and garden samples, running around 4%. Solomon says that north of the Mason-Dixon line, levels of 7% are not difficult to achieve. South of the line, 4% is about as good as you can expect because long, hot summers burn up the organic matter in the soil. St. Louis is close to the line, perhaps just a bit south, so 4 to 5% organic matter is probably about the best I can expect. Considering that I have been adding only the minimum amount of compost according to Jeavons’ book and that last summer was the hottest, longest summer I have experienced here, I’m quite pleased with 4% organic matter. I may look into adding a bit more compost to each bed in the future to see if it might raise the organic matter a little higher, if I have enough extra compost for this.

The total exchange capacity has a lot to say to anyone who takes the time to study chapter 5 of Solomon’s book carefully. To very briefly summarize Solomon’s argument, the various minerals that plants need can be held onto (a chemist would say adsorbed to) either or both of the organic matter or clay fractions of soil. The more of each you have, the more minerals you can potentially store in your soil, available on demand to your plants. Clay has a higher holding capacity than organic matter for many of the minerals plants need in higher quantities but only organic matter can hold onto phosphorus and sulfur.

Our soil is a silt loam soil derived from wind-blown glacial deposits. Its clay content is rather low. Since summers are long and hot, I can’t hold a lot of organic matter in the soil. Thus my soil starts out with a major disadvantage: the soil mineral pantry will always be on the small side, sort of like the pantry in our 85 year old house is too small to hold everything Mike and I would like to keep in it. That’s what the total exchange capacity of around 7 means. Those of you with more clayey soils have an advantage over me because your soil’s pantry is potentially larger even if we have the same percentage of organic matter in our soils. What this means to me is that re-stocking my garden soil’s pantry by side-dressing with extra fertilizer in midseason could boost my yields to some degree. I will consider that possibility as the garden season progresses. Certainly with that small exchange capacity I need to be attentive to re-mineralizing every year, especially for crops that need high nutrition to grow well.

In order to understand what the rest of the results on the soil report mean and how to develop a soil prescription from them, you’ll need to read chapters 6 and 7 of Solomon’s book. He has developed a worksheet to help with that process. You can find the various versions of it in Appendix C, or you can download a pdf file of the latest version of the worksheets at this site. Since I have an acid soil, I used the acid soil worksheet in the downloadable version.

Please forgive me for not subjecting you to the level of detail Solomon does in the example in his book; I’d really like for you to read the book yourself rather than attempt to summarize it for you. What I’m going to do here is discuss the pattern of mineral deficiencies and excesses for my soil and the COF prescription that I’ve designed to address those patterns.


Above is side 1 of the filled-out worksheet, in which I used the information from the soil report to determine the pattern of deficiencies and excesses for my soil. Note that sulfur and phosphorus are both deficient. The phosphorus deficiency is to be expected since I do not use N-P-K  fertilizer (the P represents phosphorus) on my garden and the organic matter level is not high. Plants need and use a lot of phosphorus so many soils are deficient in it, thus the widespread use of N-P-K fertilizers. My soil also has a considerable sulfur deficiency, as does the example soil in the book. As for the rest of the minerals, magnesium, potassium (the K in N-P-K), iron, and manganese are all present in excess in my soil, while the rest of the minerals are deficient.


Above is side 2 of the acid soil worksheet, in which I work out target amounts of minerals to include in my customized COF prescription for 2013. It was easier to work out the quantities and materials for my COF versus the book’s example soil because I have an excess of potassium and because I need to add sulfur and calcium but not magnesium. Gypsum (calcium sulfate) is the perfect choice for this situation. Two other minerals I need to add, copper and zinc, are added as their sulfate salts (salts with sulfur included in the formula). What I did was determine how much of each of these minerals I needed and how much sulfur they brought with them, then adjusted the amount of gypsum to add to remedy the rest of the sulfur deficit. We are cautioned to avoid adding over a certain level of sulfur each year. Our sulfur deficit is less than that so I don’t have a problem there. I’m really glad to be adding gypsum because our soil has the classic symptom of a soil that needs it: it’s sticky when wet and compacts easily. Solomon had the same problem with his soil and found that adding gypsum greatly improved his soil’s texture. I’m eager to see if it does the same for ours!

The gypsum brings with it about half of the calcium our soil needs. Because our soil already has too much magnesium, I don’t want to use the dolomitic form of garden lime to add the rest; dolomitic lime brings in some magnesium along with the calcium. What I need to use is high-calcium lime, often referred to as high-cal lime. I determined how much of that to include to remedy the rest of the calcium deficit.

To add phosphorus, I chose to use rock phosphate because I have a nearly-full bag of this at home. The analysis on the bag suggests nothing else on the worksheet is coming in with it; I hope that is the case. If calcium comes along it won’t be a major problem. Once I use up the material on hand, I’ll probably switch to bonemeal for the phosphorus requirement since it does not bring in anything that is in excess in our soil.

This leaves two more minerals to balance, sodium and boron. Both of these are needed in only small quantities and toxic at levels not much higher than the needed levels. St. Louis gets enough rain that sodium chloride (table or sea salt) will leach out of the soil, which is probably why our soil has a small sodium deficit. I don’t find any information about sodium in the water quality report of our water supplier (it’s probably low because we draw water from the Missouri and Mississippi Rivers rather than a well field) and we still have a sodium deficit after irrigating heavily most of last summer, so I will add sea salt for the sodium requirement. It brings in some valuable trace minerals as well. For boron, my target to add is below the application limit, and I will use borax as recommended.

Solomon says that pounds per acre is about the same as grams per 100 square feet, the size of my vegetable beds. There are 454 grams in one pound, which I need to know because my postal scale reads in pounds. Some of the minerals are added in just a few teaspoonfuls for a 100 square foot bed; Solomon has those measurement equivalents on page 191 of his book. I’ll spare you the calculations that I made to convert pounds per acre to the amount used in the prescription for our soil.

Two other components besides those mentioned above will be part of the 2013 prescription. The first is an oilseed meal, cottonseed meal in my case (lots of cotton is grown in southern Missouri) since that is what I have been using in the earlier version of COF. Solomon suggests using about 3 quarts of this per 100 square feet. The meal feeds the soil microlife which in turn makes nitrogen (the N in N-P-K fertilizers) available to the plants as the microlife eat, poop, breed, die, and decay. The second component is kelp meal, used to provide a lot of trace minerals as well as a little nitrogen. Solomon suggests using 1 quart of that.

So here is the Soil Doctor’s prescription for 2013 for our soil, to dig into each 100 square feet of bed space before planting crops.
 - 3 quarts cottonseed meal
 - 1 quart kelp meal
 - 5 pounds rock phosphate
 - 1 pound gypsum
 - 10 ounces hi-cal lime
 - 1.4 ounces sea salt
 - 1.0 ounces (about 2 tablespoons) copper sulfate
 - 0.8 ounces (about 1 1/2 tablespoons) zinc sulfate
 - 0.2 ounces (about 1 teaspoon) borax

To prepare the 2013 prescription, I’ll put the cottonseed meal into a large tin that formerly held popcorn. Then I’ll mix all but the bottom four items into that. Solomon suggests dissolving the borax in a full watering can, then watering the entire surface of the bed with that water, in order to get a reasonably even distribution. I believe I can do that with the salt and copper and zinc sulfates as well. The others won’t dissolve sufficiently in water to play that trick.

After I remove any existing vegetation from one of the beds, I’ll sprinkle the mixed prescription as uniformly as possible over the entire surface of the bed, sprinkle 3 or 4 five-gallon bucketfuls of my compost similarly, and then dig them into the bed using a broadfork. Following this I’ll dissolve the last four items into a 2 gallon watering can and lightly water the bed surface with that solution. Then I’ll plant the bed.

If you play soil doctor and come up with your own prescription, you will need to locate the materials for it. Most decent garden centers carry most of the materials I needed for mine and most you are likely to need for yours. Borax is the same borax that you find in the grocery store and use for various cleaning purposes. What I had a hard time finding were hi-cal lime, sea salt, and copper sulfate. For copper sulfate, I searched the ‘net and found out it is used by aquarium hobbyists and can be obtained in 5 pound quantities. Even for my size of garden this will be many year’s worth. Fedco carries sea salt and hi-cal lime as well as most if not all of the other materials in Solomon's book if you are unable to obtain them locally.

Watch this space for details on the results I get. If any of you want to try this, I’ll be curious to learn of your results as well. I’ll explain more fully why I’m doing this and how I will determine if it’s worthwhile in one of the next posts I make. In the meantime, enjoy life!

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