“Don’t Treat Soil Like Dirt!” or “Is Your Soil Healthy?”

by Thomas J. Akin

I originally wrote this article for an ELA newsletter a little over 10 years ago; today I would call the article, “Is Your Soil Healthy?”  Let’s see how much I got right and how much the science has improved since then. Indented text shows my amendments to the original article that appeared in ELA’s print newsletter, The Ecological Landscaper, in 2004.

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.

Still true; soil is one of our most under-appreciated natural resources. For a great story on soil, check out a recent movie entitled Symphony of the Soil. You’ll never again think of soil as just dirt!  https://www.youtube.com/watch?v=F98Lo2Mef4c.

Soil quality is interconnected by biological, physical, and chemical factors. All three can be improved by adding organic matter.

Today, the buzz word is soil health, and the health biology in the soil determines to a large extent how well your landscape or garden is going to perform. You can learn more about soil health at the USDA Natural Resources Conservation Service website:  http://www.nrcs.usda.gov

The astute gardener knows that a healthy, biologically diverse soil promotes a bountiful harvest and lush landscape. Not long ago, manure from a neighboring farm was the source of a soil’s biological diversity. Now manure is more likely to come dehydrated in a plastic bag.

Today, compost is much more readily available on a commercial basis than it was 10 years ago. Innovative farmers and landscapers are making compost from yard waste, food waste, sea food waste, virtually anything that contains carbons. Applying compost to lawn areas is a great way to improve your landscape’s soil health on a grand scale. In late summer, top dress your lawns with a half inch to an inch of high quality compost (be sure to ask your vendor for their latest compost test results!) and watch the grass turn green with joy!

But, even from plastic bags, the addition of manure, leaf-based compost, bark mulch, or wood chips will increase your Soil Organic Matter (SOM) level, the amount of organic matter in the soil. Higher SOM levels attract a multitude of 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.

The increased biological activity also improves the soil’s structure, making the pore spaces in the soil much more resistant to collapse and compaction.

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. Howe3ver, research has shown, if herbicides are employed for weed control, levels higher than 4-8% SOM render them less effective.

I was probably being a little generous on the SOM percentages at the Arboretum, but if your SOM levels are between 5-10%, you’re doing a good job of creating a great home for all of those soil-dwelling life forms! One additional benefit of SOM that we know more about now is the increased water holding capacity of soils with high organic matter levels. Every additional 1% of organic matter in the soil can raise the water holding capacity by an additional 25,000 gallons of water per acre!

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.

All of this is still true today; two areas that I neglected to cover in 2004 are the issues of tillage and compaction. Tillage or any disturbance to the soil is bad for the soil biology; kill your rototiller and use mulch instead! Soil compaction is one of the biggest problems that goes unresolved. Walking paths that are not mulched become hard as pavement and almost as impervious, leading to runoff, erosion, and eventually gully formation. Once the pore spaces have been squished out of the soil, the biological activity is greatly reduced due to low oxygen.  Be wary of heavy equipment and lots of foot traffic. If there is a path that gets a lot of use, mulch it with wood chips! The soil and your trees will thank you.

Also, in place of “resins” referenced above, today I would use the word “biological exudates.” Almost 20 years ago a USDA soil biologist discovered a chemical compound in the soil which was named “glomalin” after the order of fungi “Glomales.” The following is the Wikipedia definition:  “Glomalin is a glycoprotein produced abundantly on hyphae and spores of arbuscularmycorrhizal (AM) fungi in soil and in roots. Glomalin was discovered in 1996 by Sara F. Wright, a scientist at the USDAAgricultural Research Service.” Glomalin is the glue that holds a lot of our soil together!

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 cations (positively charged ions). 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 (Al 3+). Other naturally occurring elements such as the micronutrients (copper, zinc, molybdenum, etc.), sodium (Na+, not a plant nutrients), 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.

All true still today; make that nutrient reservoir bigger by increasing your soil organic matter!

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.

Once again, the more organic matter, the higher your CEC will be and the healthier your soil will be.

Soil pH governs the solubility of most of the essential plant nutrients. If soil pH falls below 5.0, many basic cations (K, Ca, Mg) as well as phosphorus are rendered insoluble. If nutrients aren’t soluble, plant roots can’t absorb them.  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/magnesium limestone, the hydrogen ions held in reserve on the colloids will be replaced by the calcium. The replaced hydrogen (H+) then will enter in the soil solution and will take part in the active acidity.

The Base Saturation numbers presented in the soil test report 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.

Recent reviews of earlier research have since disproved the theory that plants grow better in soils where the cations are balanced to a particular ratio. The best guidance is to follow the soil test recommendations from Land Grant University or private accredited soil testing labs; if the nutrient levels are in the “optimum” range, your plants will thank you.

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 -3000:1), 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 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 (such as manures at 5:1), 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 highest levels.

Once again, I didn’t mention compost back then. Find a good compost vendor who makes an earthy smelling product with a C:N ratio of 20-30:1 and use it on your lawn, mulched beds, and mixed with potting soil. Remember, it’s all about the improving the biology!

Compost analysis and soil testing are available to the general public as well as professionals through the UMass Soil and Plant Tissue Testing Lab. 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. Similar soil testing services are provided by cooperative extensions in most states.

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!


About the Author

Thomas J. Akin is a former ELA Board member. He is Conservation Agronomist at the USDA Natural Resources Conservation Service in Amherst, MA, and former Assistant Superintendent of Grounds at Arnold Arboretum in Jamaica Plain, MA. Contact him at 413-253-4365 or Thomas.akin@ma.usda.gov.