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We are featuring the article, "Don't Treat Soil Like Dirt" from one of our past newsletters.

Don’t Treat Soil Like Dirt
By Thomas J. Akin
In our increasingly paved-over civilization, soil is a woefully under-appreciated asset. Just think what an amazing resource it is! Soil naturally filters all of our water. Soil enables us to grow all of our food, fiber, and flowers. Soil is home to millions
of life forms. And it was dropped here, free of charge, by the last glacier that came through, 12,000 years ago. Biological properties of soil Soil quality is interconnected by biological, physical, and chemical factors. All three can be improved by adding organic matter. The astute gardener knows  arthropods, insects, and animals (both invertebrate and vertebrate), bacteria and fungi. Increased biological activity improves soil quality, which, in turn, strengthens the root systems of plants. If you are using organic practices, 10-20% SOM (by weight) is ideal to maintain release of plant nutrients. Most of the soils of the Arnold Arboretum test between 10-20% SOM, primarily because grass clippings and leaves are left on the site and there is little or no tillage to oxidize soil organic matter. In my opinion, more SOM is better, both for the soil and the plants. However, research has shown, if herbicides are employed for weed control, levels higher than 4-8% SOM render them less effective.

Physical properties of soil Soil texture (determined by the percentages of sand, silt, and clay) is fairly immutable; unless another glacier passes by or the top 12 inches of soil is otherwise replaced, we will have to work with the soil we have. However, both soil structure (how the soil particles are glued together) and soil tilth (how tightly the particles are glued) can be modified. Microbial biomass and microbial exudates are the glues that coat, separate, and hold soil particles in place. Air movement (oxygen in particular) and water are essential for all biological processes. Good structure and tilth allow air to diffuse throughout the soil, water to infiltrate freely, and permit root systems to explore and mine the soil for nutrients to the fullest extent. Organic matter in soil is a dynamic mix of decaying plant material, the agents of decay, and humus. Worms, insects, arthropods, bacteria, and fungi first consume the least-resistant forms of soil carbon such as plant proteins, sugars, and fats. Resins, cellulose, and lignin, to name a few, are decay-resistant plant components; they are more chemically complex and require numerous modifications by microbes before decay is complete. As plant materials are consumed, decay by-products are themselves transformed. Carbon dioxide is generated and SOM evolves into its most chemically stable form, humus.

Humus
Humus consists of two decay-resistant organic acids, humic acid and fulvic acid.
Humus, like clay minerals, has large surface areas of negatively charged sites that attract and hold positively charged ions or cations. Cations such as potassium
(K+), calcium (Ca2+), magnesium (Mg2+), and ammonium (NH4+) are the most desirable. These are joined by strictly acidifying cations such as hydrogen (H+) and aluminum (Al3+). Other naturally occurring elements such as the micronutrients (copper, zinc, molybdenum, etc.), sodium (Na+, not a plant nutrient), and heavy metals such as lead (Pb2+), nickel (Ni2+), and cadmium (Cd2+) may also be attracted to the negatively charged sites. Because of this electrical relationship with cations, humus is a sink (or storage reservoir), that
readily absorbs plant nutrients.

CEC
A soil’s ability to attract and hold cations is called its Cation Exchange Capacity (CEC) and is largely dependent on the content of SOM and clay minerals. Of the two, it is argued that, especially in New England, SOM is more important because organic matter levels can be manipulated while the chemical reactivity of clay
minerals is relatively low.

Soil pH
Soil pH governs the solubility of most of the essential plant nutrients. If soil pH falls below 5.5, many essential elements are rendered insoluble. Soil pH is a measure of the concentration of hydrogen (H+) ions dissolved in the soil water solution. Buffer soil pH is a measure of the concentration of hydrogen (H+) ions absorbed onto the soil colloids (SOM and clays). This type of acidity is said to be held in reserve because it is temporarily sequestered on the colloids. If the active acidity is neutralized with calcium limestone, the hydrogen ions held in reserve on the colloids will be replaced by the calcium. The replaced hydrogen (H+) then will enter into the soil solution and will take part in the active acidity. The Base Saturation numbers presented in the soil testreport indicate the percentages  of potassium, magnesium, and calcium on the soil colloids; these numbers, along with the percentages of hydrogen, aluminum, and ammonium, constitute the CEC number. The  CEC and the Base Saturation levels are some of the most important numbers on soil test reports.

Amendments
Each soil amendment comes with a unique microbe population; the greater the biological diversity, the better the chances for improved soil. Bacteria and fungi are responsible for degrading carbonaceous materials. They require nitrogen to complete their lifecycles. The Carbon: Nitrogen (C:N) ratio is vital to choosing amendments. Nitrogen in the soil, usually in the form of nitrate (NO3), is incorporated into the microbial biomass as amino acids and proteins. Absorption of all available nitrogen by microbes is called “immobilization.” Nitrogen deficiency shows up as chlorosis of the older leaves and gradually moves up the plant. If the C:N ratio exceeds 30:1, as it does in pine sawdust (C:N ratio of ~300:1), microbes may not have enough nitrogen to degrade the available carbon. The bacteria and fungi are then not able to multiply. In manure (C:N ratio of ~10:1), nitrogen is plentiful and the microbial processes can proceed. When nitrogen is released from the microbial biomass back into the nitrate form, the nitrogen is once again available to plants. A C:N ratio of 30:1 or slightly higher will immobilize nitrogen to a slight degree and organic matter levels will increase. With the addition of organic amendments with C:N ratios of less than 30:1, soil organic matter levels may actually decrease when too much nitrogen is present. This may occur under high temperatures and adequate soil moisture, when microbiological activity is at its

Testing
UMass Soil Testing Lab compost analysis and soil testing is available to the general public as well as professionals. A compost analysis includes pH, C:N ratio, total N (nitrate and ammonium), total C, moisture content, and bulk density (tons/cubic yard). The bulk density number and C:N ratio indicate the additional nitrogen needed to supplement large amounts of organic amendments. Improving the soil is good stewardship, plain and simple. First test the soil, then follow the recommendations! Be vigilant concerning organic matter levels and soil pH. Let’s be good stewards of a precious resource, the living soil!

Thomas J. Akin is Conservation
Agronomist at the USDA Natural
Resources Conservation Service
Soil Conservation Service in
Amherst, MA, and former Assistant
Superintendent of Grounds
at the Arnold Arboretum in
Massachusetts.

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