Why Composting Works So Well for Colorado’s Clay Soils
By Richard F. Hennessey
Last month we detailed a step-by-step guide to backyard composting. This month, let’s discuss how humus, the end product of composting, particularly improves Denver’s heavily clay soils to improve soil quality, plant productivity, and moisture infiltration and retention, resulting in reduced water requirements for our gardens and lawns.
To explore how humus accomplishes these feats, let’s review some basics about soils and soil water. We’ll then focus on how humus benefits Denver’s clays soils.
Soil Basics
Soil is the mixture of minerals, organic matter, gases, liquids and hosts of organisms on the earth’s surface that serve as a medium for the growth of land plants. Soils typically are a 50 percent mix by volume of particles of decomposed rock and sediment and 50 percent by volume of the spaces between those mineral particles.
Here in the West, the particles mostly contain mineral solids together with a small amount of organic matter. The spaces between the particles are filled with air and water.
Healthy plant growth depends on a correct balance among the soil’s particles, air and water.
Soil Texture
Soil is the eventual result of the weathering of solid rock and accompanying biological activity cause by various environmental influences.
Soils are classified along a continuum according to their texture, which is largely determined by the nature of their parent material or rocks. At one extreme is sand, the largest of soil particles: its texture is coarse and gritty, and its grains are irregular, do not fit well together, and are actually visible to the naked eye.
At the other extreme is clay, the finest of soil particles: its texture is hard, almost brick-like when dry and rubbery and sticky when moist. Its parent material is shale. A clay particle is microscopic, less than 1/12,500” and can be as much as 2,000 times smaller than a grain of sand. In the middle of the continuum between sand and clay is silt, which shares some of the characteristics of both sand and clay: its texture can be powdery, slippery or smooth but not sticky and its particles are spherical in shape.
Loam, which is considered the best soil for texture, is 20 percent clay, 40 percent sand and 40 percent silt, by weight.
A soil’s texture influences its properties, such as porosity, permeability, shrinking and swelling, water retention and vulnerability to erosion. These properties determine what plants will or will not thrive, as well as plant nutrition and water use.
For example, the compacted character of Denver’s clay soils not only inhibits root penetration and but also hinders drainage leading to both water runoff and reduced moisture retention. By contrast, moisture easily penetrates sandy soils but passes through it all too quickly, because the sandy soil has poor water retention capability.
Soil Structure
Distinct from a soil’s texture is its structure. Structure relates to how a soil’s individual mineral particles (sand, silt or clay) clump together into larger and larger aggregates and distinct shapes.
Soil structure is influenced by freezing-thawing, expansion-contraction, wetting-drying, compacting forces, the action of expanding roots, working the soil when it is wet, and even too much tilling. While a soil’s texture is innate and does not change, its structure can be changed.
Ideally, a soil’s structure should be granular, with crumb-like particles no larger than 1/2”, which is small enough to maintain good contact with seeds and roots. In contrast, the aggregates of predominantly clay soils tend to be larger – blocky, prism-like or columnar – and closely bound together, resulting in poor water and root penetration. However, since soil structure can be changed, humus can dramatically modify the structure of clays soils.
Soil Water
Water is essential for plant growth. There are forces that hold water in soil and determine its availability to plants.
J.I.Radale’s Encyclopedia of Organic Gardening (1971) describes soil water as occurring in three forms: hygroscopic, capillary and gravitational. Gravitational water is the water that normally drains from the soil under the influence of gravity after a rain or irrigation. Having drained away, it is unavailable for plants. Some water becomes chemically bound strongly to soil particles and is unavailable to plants. This is called hygroscopic soil water.
Finally, there is a less tightly bound film of water that remains in the spaces between the soil particles. It is this capillary water, also containing dissolved minerals, that plant roots absorb and on which plants largely depend for their moisture.
If drainage is poor due to the soil’s texture and structure, the gravitational water causes soggy, unproductive soil. On the other hand, if the soil’s texture and structure cause excessive drainage, capillary water will decline sooner and plants will suffer from drought.
Therefore, a soil’s ability to hold water against the pull of gravity but to do so without making the soil soggy is very important. Humus helps achieve this balance.
How Humus Amends Denver’s Clay Soil
Clay soils, due to their texture, possess certain dominant properties relative to other soils. More than any other soil, clay possesses high water-holding capacity, high levels of soil organic matter, and high ability to store nutrients.
On the other hand, clay soils present significant difficulties for gardening purposes: poor aeration, high compactability, slow drainage and susceptibility to runoff, slow decomposition of organic matter and poor suitability for tillage after rain.
The marvel of humus is that it not only overcomes these difficulties, it also augments the very desirable properties of clay soils.
Clay is a source of fertility. A soil’s fertility is measured in part by its cation exchange capacity (CEC). (A cation is an ion with a positive charge, produced by the loss of one or more ions.) Sand has the lowest CEC and clay the highest. This is because many minerals in the soil – e.g., calcium, magnesium and potassium – carry a positive ionic charge and adhere to the negatively charged surfaces of clay particles.
Plant roots stimulate a process in which the positively charged mineral nutrients are released to the soil solution for the roots to absorb. This occurs when hydrogen ions (from acids released by the roots) in the soil solution displace the mineral ions from the clay particles.
Since clay particles are so numerous and finely divided, they provide an enormous amount of surface area for binding and releasing these plant nutrients, while also keeping them from being washed away. The point is that clay, more than any other soil, holds the potential capacity to boost fertility.
Humus releases the clay soil’s capacity for fertility precisely because humus changes the clay soil’s structure. By infiltrating and breaking up the clay aggregates, humus builds a crumbly soil for good contact with seeds and aeration of the roots. Adding humus also exposes more of the surfaces of the numerous, fine clay particles for the biological activity that stimulates plant growth.
For these same reasons, clay soils amended with humus have improved water infiltration and water retention, some say resulting in as much as a 20 percent reduction in water consumption.
Finally, humus is a storehouse of mineral nutrients that are released gradually in the soil, as microorganisms continue to decompose organic matter.
For all these reasons, composting is worthwhile, productive and interesting, and especially beneficial to Denver’s clay soils. If you want to learn more about composting, from May through October, free classes are provided at Gove Community Gardens, on Colorado Boulevard between 13th and 14th Avenues, typically held on Wednesday evenings, 6 – 8 p.m. and Saturday mornings, 9 -11 a.m. You can also visit check the Denver Urban Gardens’ website at dug.org, call 303-292-9900.
Richard F. Hennessey is a retired Denver attorney and resident of Park Hill since 1968. He is a participant of the 2014 Master Composter Training and Outreach Program sponsored by Denver Urban Gardens and Denver Recycles. Part 1 of this series, Unraveling the Mysteries of Composting – A Step-By-Step Guide, appeared in the July issue of the GPHN. Mr. Hennessey again expresses appreciation to Judy Elliot, Education Specialist with Denver Urban Gardens and lead trainer for the Denver Master Composter Training and Outreach Program.