I recently encountered the new phenomenon (now a fad) of re-mineralization, while looking up some information (see http://growabundant.com/). That led to my purchasing Steve Solomon’s new book, The Intelligent Gardener: Growing Nutrient-Dense Food, which I am in the process of reading. The topic interested me because of my two gardening situations. I currently use raised beds filled with a soilless mix following Mel Bartholomew’s recipe: 1/3 peat moss, 1/3 vermiculite, 1/3 compost. Mel claims a scoop of compost is all you need to garden, but of course he insists that scoop of magical compost has to have all needed nutrients in it. Does it really? If it doesn’t that’s your failing, not his method. I have had mixed results with the soilless mix in my beds. Mechanically, it is great. It thaws early, drains well while retaining moisture, it has a light and fluffy texture that many plants love. But some plants (usually the heavy feeders) have not done well and that makes me wonder why. Are some essential nutrients missing? That is a complicated topic I have been thinking about for awhile and will be the subject of another post.
My beds are also located in a community garden where last year I had the opportunity to share half of an unclaimed plot for a garden. I used it for tomatoes, peppers and summer squash planted in-ground. The community garden was recently reclaimed from sodded fields whose history is vague to me. I doubt the field was ever tilled because of the large amount of ledge showing through in places. Maybe it was pasturage or hay field. At any rate, during the ice age it was under a mile thick layer of ice and the soil is some unknown mixture of silt, sand and gravel deposited by the glacier. So its fertility has always been in doubt in my mind, although fellow gardeners and myself have gotten decent crops from their gardens. I am curious to see how the soil tests out at and what could be done to improve yields for our gardeners using Solomon’s advice.
It all starts with a soil test.The photo above is a picture of a dried sample of the soil from the community garden that has been sent to Logan Labs in Ohio and to the UMass Extension service for testing. When dried, it crumbles easily and looks fairly sandy, but when we get our frequent heavy rains it compacts easily and becomes anaerobic. I have no idea of its actual soil type or composition. The worksheets Solomon uses assumes that the soil test used the Mehlich-3 extractant method, which is the method used by Logan Labs which he recommends. I also sent some of the same sample to the soil lab at UMass Extension, which uses a Modified Morgan method that includes micronutrients and is better suited to Northeast soils. The two tests will undoubtedly produce different numbers for nutrient concentrations and their target values will be different as well, so I am wondering how different the suggestions will be.
I called re-mineralization a fad because a lot of people have simplified it to just tossing a handful of rock dust around the garden. It has gotten so popular that many stores are encountering a large demand for rock dusts (I know, I just bought mine and got the last bag). Solomon's emphasis is much larger, however, trying to balance all the factors that make up the soil (pH, organic matter, minerals) so plants are given maximum opportunity to thrive and produce nutrient dense food. He talks about rebalancing the soil and that is the central theme of his book, not just mineralization. Using soil test results and the worksheets in the book, a prescription for supplements is developed to bring the soil into the right balance of pH, organic matter and ratios of minerals.
First step is to adjust the pH of the soil. The use of lime has been overemphasized in the past, and the recommended use of dolomitic lime can actually cause an excess of magnesium which tightens up the soil. Most vegetables prefer a slightly acidic soil, in the range 6.0 to 7.0. Solomon uses a target pH of 6.4. This target pH will actually be achieved when the four major cations (Ca, Mg, K, Na) are in the proper proportions. If you look again at the soil sample above, I am afraid that the white flecks you see are bits of lime from an over-eager distribution of lime early in the garden’s history. The last soil test in 2011 showed the soil to be alkaline with a pH of 7.6 with a very large Calcium content.
The next step is to address the amount of organic matter in the soil. Besides the usual mechanical improvement of the soil from adding organic matter and encouraging the growth of micro-organisms, the humus increases the ability of the soil to buffer not only cations (positively charged ions) but also anions (negative ions) so they are not leached away by rain and irrigation. There is no point in adding nutrients if the soil cannot retain them. The total ability of the soil to buffer/retain cations is called the Total Cation Exchange Capacity (TCEC) and is the key factor in rebalancing. Light (typically Southern) soils will have 2-4% organic matter and will have a TCEC less than 10. Heavier (typically Northern) soils will have 7-10% organic matter and a TCEC greater than 10. Solomon uses 7% organic matter as the target for northern acidic soils. He points out large additions of compost are not required once the soil reaches the desired level of organic matter and TCEC. The soil in the community garden in 2011 had a TCEC of 51.3 with 10.3% organic matter.
Next is balancing the major cations, Calcium (Ca), Magnesium (Mg), Potassium (K) and Sodium (Na). The target proportions are 68% of the ions buffered by TCEC should be Calcium ions, with 12% Magnesium ions. Soils having this 68:12 ratio will be loose and friable soils. Higher amounts of Magnesium will cause soils to tighten and clump. Potassium should occupy 4% of TCEC capacity, with Sodium at 2%. If a soil has this 68:12:4:2 cation ratio, its pH should be at the target 6.4. It may take several years to reach target ratios if you have an excess of one mineral, so the prescription may be more involved than just adding supplements. The community garden test from 2011 (Morgan method) had a TCEC of 51.3, of which 98% was saturated with Ca, leaving only 1.5% Mg and 0.6 % K, definitely not a balanced soil.
Next is to balance the anions, the negatively charged ions. Any clay in the soil will only buffer cations. You need sufficient humus in the soil to buffer your anions or they will be easily leached away by rain or irrigation water. The trick is to build up and retain the anions to adequate levels, starting with Phosphorus. Phosphorus is an essential element and low levels of P will reduce plant growth long before symptoms of deficiency evident themselves. The goal for Phosphorus levels is P=K, a level equal to the Potassium content of the soil. Our old 2011 soil test showed a P level of 4 ppm compared to a K level of 108, a very low level of Phosphorus and far from the prescribed balance.
The other key anions are Sulfur, Boron and Nitrogen. Sulfur is an essential element used in building many amino acids. If elemental Sulfur is added to the soil, microorganism will readily convert it to the sulfate anion. In balanced soils, the goal is S= 1/3 P, or one third of Phosphorus levels. Higher levels of Sulfur equal to 1/2 Mg can leach cations from the soil, which can be an advantage if you have an excess of a cation such as Calcium or Magnesium. The sulfates of most cations are water soluble and will then be washed from the soil by rain. For Boron in light soils, Solomon recommends levels about 1 ppm and in heavy soils, 2 ppm. Nitrogen is heavily used by plants and easily leached from the soil so its amount varies so widely that the standard soil test does not test for it. Usually it is added when planting and as needed during crop growth and is not a permanent fixture of soil fertility.
So my soil tests for the community garden have been sent off and it will be a week or two before I get results. Then I will use the worksheets to calculate recommended supplements. The problem I have, which is probably beyond my capabilities, is dealing with the excess Ca and high pH of the soil. The Ca saturation percentage on the 2011 soil test was 98%, leaving little room for the other cations. Adding Sulfur would seem like a possible solution for both the Ca excess and the pH, but the soil already has a very high S content, equal to 2x Mg. I have no clue why that is. Hopefully, the new soil test will be more encouraging and these anomalies will disappear, maybe just the result of a bad sample in 2011.