A successful garden is all about soil. Soil provides the network to deliver air, water and nutrients to your plants.

So what is going on in garden soil?

Technically, soil is the unconsolidated mineral and organic matter in the upper layer of the earth’s crust. Weathering breaks rock down into mineral components. As plants and animals die and are decayed by fungi and bacteria, they are converted into humus, a carbon-rich organic material. Humus consists of long, hard-to-break chains of carbon molecules with large surface areas. These surface areas carry electrical charges that attract and hold mineral particles. The molecular structure of humus resembles a sponge with large and small spaces for soil microbes. Water, air and nutrients are transported to plant roots through this network of large and small pores.

Soil structure impacts soil productivity. Large pores (macropores) determine a soil’s permeability, the rate at which water moves through soil, and aeration, the capability to provide oxygen. Root and earthworm channels help to create these large pores. Because of gravity, irrigation water and rainwater infiltrate the soil and excess water drains through it via macropores.

Small pores (micropores), only a fraction of a millimeter in diameter, help soil retain water, resisting the effect of gravity. Most of the water in these micropores is available to plant roots.

The porosity (the amount of open space) of soil is affected by texture, degree of compaction and structure of the soil. Texture is determined by the ratio of sand, silt and clay in soil. Sand is the coarsest particle, silt is intermediate and clay is the smallest/flattest.

Pores between sand particles tend to be the macropores and pores between silt and clay particles tend to be micropores. Sandy soil has rapid water infiltration and water loss, has limited water-holding capacity and warms up earlier in the spring.

Silt and clay soils have high water holding capacity, reduced permeability and warm up later in the spring. Sand, silt, clay and organic matter bind together to form soil aggregates that provide structure to soil.

Soil structure can be damaged by compaction, over-tilling and tilling soil when it is too wet. Compaction squeezes macropores into micropores, creating horizontal aggregates that resist the transfer of water, nutrients and air. Severe compaction in the upper two feet of soil frequently is caused by heavy equipment during site construction and can damage or destroy trees. The collapse of soil structure results in less life in the soil and reduced soil fertility.

Good soil structure is key to good growing conditions. Good structure ensures adequate drainage as well as adequate plant-available capillary water and air circulation to sustain biological activity. In good soil there is space for soil biology to thrive.

Healthy soil is full of life. In addition to insects, earthworms and plant roots, soil has a diverse population of microorganisms. One-quarter teaspoon of topsoil can contain a billion microorganisms. When soil organisms break down plant materials and other organisms, then energy, nutrients and carbon dioxide are released, creating soil organic matter. Organic matter increases the diversity and number of beneficial organisms in soil.

Next week: “Understanding garden soil, Part II (the process).”

Soil organisms

Gardening practices including tillage, the use of fertilizers, manures, pesticides and crop rotation affect the diversity and population of soil organisms. Soil moisture, temperature and organic content affect the activity of soil organisms.

Beneficial activities of some soil organisms:

Mycorrhizae fungi infect plant roots increasing the uptake of nutrients from the soil.

Rhizobia bacteria convert atmospheric nitrogen into plant-available nitrogen.

Earthworms mix large volumes of soil and create macropore channels.

Voles and moles feed on insects and earthworms, however voles and moles also can be non-beneficial pests.

Judy English is a Washington State University-certified Clallam County Master Gardener.