Organic Matter — The Gardener’s “Silver Bullet”
Some gardeners are just plain lucky; the soil in their gardens is dark and beautiful, and each year they produce a bountiful vegetable harvest. Now my luck is somewhat different. No matter how many times I have moved my garden, the spot I select turns out to be right on top of a mound of hard, red clay. I still cringe at the pain shooting though my knees as I attempted to push a spade into the clay of my current garden. I often think that the soil in my garden site is better suited for making ceramic pots than for growing vegetables. No matter how many gardeners, extension agents, or books I consulted on the subject of how to break up the cement structure of my garden soil, the addition of organic matter is always the solution that bubbles to the top.
So what’s the mystery behind the silver bullet — organic material — for turning your garden plot into a mean vegetable-producing machine?
Soil comprises four components: mineral matter, water, air, and organic matter. In Virginia, the typical organic matter level of ordinary, well-drained soil is from 0.5% to 2.5%. A soil with organic matter greater than 3% would be considered very high for a cultivated field. Due to relatively large amounts of organic materials being commonly added to our gardens, the organic matter in garden soils can be raised into the range of 5% to 10%.
Soil quality is the product of two factors: tilth and fertility. Tilth refers to the physical condition of the soil, and how well it allows for essential plant processes including seed germination, root growth, water infiltration, drainage, and root aeration. By that definition, the hard, compacted clay soil in my garden has a poor tilth. Fertility refers to the nutrients that are held in the soil and are available to plants.
Now how does organic matter play a role in soil quality? Organic matter serves as a reservoir of nutrients and water in the soil, improves soil tilth by helping to reduce compaction and surface crusting, and increases water infiltration into the soil. Organic matter is often referred to as the thread that holds the physical, chemical, and biological properties of soil together.
What is Organic Matter?
Organic matter is often thought of as the plant and animal residues we incorporate into the soil or those endless wheel barrels of leaves that I place around my vegetable plants in the spring to smother out weeds and hold in moisture. Well, that stuff is actually organic material not organic matter.
Organic matter is composed of plant and animal tissue that is in different stages of decomposition within the soil. To be exact, organic matter is organic material that has decomposed into humus. Humus (not to be confused with Hummus, that middle eastern chickpea dish) is organic material that has been converted by microorganisms to a resistant state of decomposition. Humus is simply organic material that has been completely decomposed into organic matter. Organic material is unstable in the soil, changing form and mass as it decomposes. As much as 90 percent of it disappears quickly because of decomposition; whereas, organic matter is stable in the soil and has decomposed until it is resistant to further decomposition. Usually only about 5 percent of organic matter is reduced to the mineral level yearly. That rate can increase if temperature, oxygen and moisture conditions become favorable for decomposition. It is the stable organic matter that is analyzed in soil tests.
The Importance of Organic Material and Matter
Organic matter and material provide a variety of benefits to soil, including improvement in the physical, chemical, and biological properties of the soil. Some of the main benefits are:
- Organic material placed on the surface: Mulch, leaves, straw, and grass clippings help reduce erosion by protecting the soil from rain run-off. Surface residues also provide a barrier to weeds and reduce water loss by reducing evaporation from the surface of the soil.
- Maintenance of adequate soil structure: Belowground residues are beneficial when it comes to soil structure. They directly influence soil structure by preventing soil compaction. During the breakdown of organic material by microorganisms, compounds are formed that are resistant to decomposition. Compounds such as gums, waxes, and resins, combined with the mycelia, mucus and slime produced by microorganisms, help bind together soil particles as granules or aggregates. A well-aggregated soil tills easily, is well-aerated, and has a high water-infiltration rate. Belowground residues supply energy to microbes and invertebrates. The decomposition of aboveground and belowground residues results in the release of proteins and improves the soil structure or aggregation. Better soil aggregation means improved permeability for water and roots.
- Organic matter helps maintain a stable soil pH: This helps keep the soil in the in the ideal pH range of 6.5-7.0 which is optimum for most vegetable gardens.
- Regular supply of plant nutrients and maintenance of soil fertility: Plants obtain essential nutrients (nitrogen, phosphorus, potassium) from decomposing organic matter. While the non-living part of organic matter (humus) does not directly provide nutrients, humus improves soil fertility by holding plant nutrients so they remain available for growing plants and do not leach from the soil.
- Coating of soil particles: The presence of adequate amounts of soil organic matter in soils can help to coat the soil particles (whether they be sand, silt, or clay). Coating of soil particles facilitates aggregation, which provides pores and channels in the soil that allow rainfall or irrigation water to pass through the soil. Aggregation also reduces the runoff of water and nutrients, and soil erosion. Soil aggregates also protect the soil from compaction so it is easier for plants to grow.
- Supply of energy to soil microorganisms: Organic matter is the main source of food for several microorganisms (bacteria, fungi, actinomycetes). When fresh organic matter (e.g., plant tissues, organic wastes) is added to the soil, these microorganisms start the decomposition process. During this process, nutrients are released, soil aggregates are formed, and humus is created. Organic matter also acts as an energy source to specialized soil bacteria so that they can convert nitrogen gas in the atmosphere to plant-available nitrogen.
- Protects Water Quality and the Environment: Organic matter acts like a sponge for water. More water goes into the soil, and less water runs off the soil surface. Because surface water is reduced, pesticides and fertilizers are retained in the soil instead of washing into nearby streams and rivers. In addition, soil organic matter binds selected harmful pollutants like residual pesticides and trace elements so that these constituents cannot escape from the soil.
Research reveals that plant deficiency diseases usually are less severe in soils that that are well-supplied with organic matter. Evidence shows that that organic matter increases the vigor of plants, but various soil microorganisms become more active in the presence of an abundance of organic matter. For example, certain kinds of fungi that live in decaying organic matter have been found to kill harmful nematodes.
So what does this all mean for turning my vegetable garden into a mean production machine? Research points out that a small increase in the percentage of organic matter in the soil can result in more productive crops. For example, a study in Michigan demonstrated potential crop-yield increases by about 12% for each 1% increase in organic matter. Researchers in Maryland observed an increase of approximately 80 bushels of corn per acre of when organic matter increased from 0.8% to 2%.
It is estimated that each 1 percent increase of organic matter helps the soil hold 20,000 gallons more per acre, or doing the math, a little less than ½ gallon per square foot (20,000 gallons/43,560 square feet=0.4591 gallons per square foot). In theory, this means that for every 1 percent increase in the organic material in my 5′ by 200′ raised bed, the soil has the capacity to hold an additional 459 gallons of water!
How to Build Organic Matter in Soils?
The amount of organic matter in soil is the result of two processes: the addition of organic materials (roots, surface residue, compost, etc.), and the loss through decomposition. Soil organic matter is continuously produced and broken down by living soil microorganisms, insects, and worms as they consume it for food. Microbial activity and decomposition rates of organic matter are enhanced in warm, wet conditions.
Regular additions of organic residues are needed to maintain a consistent amount of organic matter in soils. In a natural ecosystem, this addition is achieved by the constant recycling of organic matter as plants and animals leave residues or die. In an urban landscape, this cycle is often disrupted when plant trimmings and residues are removed and sent to the landfill.
For a gardener, the most common and rapid way to build soil organic matter is to add plant or animal residues back to the soil. Grass clippings and leaves contribute nutrients and organic matter if these organic sources are directly returned to the soil. Alternatively, these materials can be composted (along with plant clippings and some food waste) and then returned to the soil. Another option is to purchase organic amendments (e.g., composted manures, biosolids, or yard waste) from a lawn and garden store or a local composting facility. Animal manures are also an excellent source of nutrients and organic matter and can be successfully added to soils once manures have been well composted. Avoid adding raw manures to the soil in your landscape as these uncomposted materials may burn plants and may also have an offensive odor. For more information on using manures, check out The Garden Shed article on “Manure”.
Another often overlooked way of adding organic matter to the soil is the use of cover crops. Cover crops provide a relatively easy way to add organic matter to the soil. The mat of roots formed by the cover crop is often more valuable than the above-ground biomass. Both winter (annual rye) and summer cover crops (buckwheat) can be used. For more information on cover crops, see The Garden Shed article titled “Cover Crops.”
Keep in mind that whenever the soil is tilled, it can be detrimental to the structure of the soil. Tilling exposes organic matter to air and increases the rate that organic matter mineralizes. It also increases the risk of organic matter being lost by erosion.
Building soil organic matter is a slow and gradual process. First, the amount of residue and organic matter need to be increased. This extra amount will increase the species and diversity of macro– and micro– organisms in the soil. These organisms then will actively decompose organic matter. It may take a decade or more for organic matter levels to significantly increase. Fortunately, the beneficial effects of the changes in organic matter can be seen after just a few additions of organic residues/compost.
Generally, the greater the organic matter level, the better the overall soil tilth or soil quality, as nutrient- and water-holding capacities increase, and improved aeration and soil structure enhance root growth. Increasing the percent of organic matter in a soil can reduce the need for watering, improve plant vigor, and increase crop yields.
Thank you for stopping by The Garden Shed this month, and we hope to see you again next month!
“Soil Test Note #1 – Explanation of Soil Tests,” Virginia Cooperative Extension, Publication Number 452-701, https://pubs.ext.vt.edu/452/452-701/452-701.html
“Anions and cations in plants, oh my! But why do we care?” Michigan State University. http://msue.anr.msu.edu/news/anions_and_cations_in_plants_oh_my_but_why_do_we_care
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“What Does Organic Matter Do In The Soil?” Noble Research Institute, https://www.noble.org/news/publications/ag-news-and-views/2001/august/what-does-organic-matter-do-in-soil/
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“Organic Matter in Soil,” USDA, https://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/newsroom/?cid=nrcs143_023543