Urban Agroecology

Soil is the foundation–literally and figuratively–of good agricultural practices. It is a complex substance, composed of a mineral substrate (often described as clay or sand), partially decomposed organic matter (humus) and living organisms. Let’s first take a look at the soil ecosystem model.

Energy, crucial for all organisms, is gained, used and lost by soil ecosystems. Some energy is added to soil ecosystems by plants, which photosynthesize sunlight into carbon compounds. As plants grow, their roots spread through the soil, bringing in energy from above the surface. While plants live they typically lose leaves and other plant parts, which fall to the soil surface where they begin decomposing. Similarly, when plants die they begin to decompose. Some of this decomposing organic material and energy leaches into the soil. Other organic material is taken underground by animals that live underground. All of this adds energy to the soil. Depending on the nature of the soil and water flows, some of this may also be leached out of the soil.

All soil has a mineral substrate. This is, in fact, how soils are generally classified by experts. For our purposes, what matters is the basic texture of the soil, usually described as sand, loam or clay. The key factor is the size of particles. Very large particles, like rocks and gravel, are too large by themselves to be useful as soil. But, smaller particles, small enough to be considered sand, are where we start defining soil. As particles get smaller, their number per unit volume increase. While the surface area per particle decreases, the increase in number of particles increases the total surface area. This has two effects, one good, one bad. The good effect is that there is much more surface area for plants and other organisms to use to obtain mineral nutrients. The bad effect is that there is more surface area for the particles to touch each other and bind together. In the extreme, very fine particles bind together to form clay. This reduces the available surface area for organisms to obtain nutrients. The ideal particle size is somewhere in between the coarse sand particles and the fine clay particles.

Organic matter in the soil provides another source of nutrients. As mentioned above, organic matter also provides the energy necessary for most of the organisms in the soil. But this organic matter has other useful properties, too. Organic matter tends to retain water. This helps especially in sandy soils, because sand’s large particles tend to allow water to drain through very quickly. In clay soils, organic matter tends to separate the clay particles and hold them apart, increasing the available surface area of clay particles and allowing air and water to flow between the particles. This is why compost, composed of organic material, air and water, is so valuable for soils. It helps both sandy and clay soils, but in different ways. Adding compost also helps offset the natural loss of organic material in the soil as that material decomposes.

Generally, water is necessary in soil in moderate amounts. Too much water and the soil becomes water logged, air cannot flow through the soil and most organisms cannot survive. Too little water and surface plants cannot get sufficient water to conduct photosynthesis and to grow. Air flow is necessary for respiration, the process by which most organisms convert energy stored in organic materials to energy they can use. Oxygen (O₂) from air is necessary for respiration and CO₂ produced during respiration must be carried away.

All organisms have preferred environmental conditions, in particular temperature, acidity and salinity. Soil temperatures vary by hour, season and depth, plus daily weather variation affects temperature. Temperatures just a few feet under the surface show very little variation, but close to the surface tend to reflect ambient air temperatures. However, soil temperatures lag air temperatures, both by day and by season. This is particularly important when considering seed germination. Many seeds require a minimum level of heat to germinate and this can be approximated by soil temperature. Because soil temperatures lag air temperatures, it is not the daily air temperature that determines when a seed will germinate, but the soil temperature, which will come later in the season. Most soils are neutral or slightly acidic. The actual pH can be critical for specific plants. For example, blueberries prefer a slightly acidic soil. Most plants, and this applies to virtually all plants that urban growers are likely to grow, don’t like saline soils. Various mineral salts make it difficult for most plants to grow. This is a problem to pay particular attention to in areas that require heavy irrigation with high levels of dissolved minerals.

In summary, soils are an active ecosystem of microorganisms and other (usually small) organisms that live on imported energy, a mix of mineral and organic nutrient sources, a matrix of mineral and organic matter that allows both air and water to flow in moderate amounts, has a temperature that approximates but lags air temperature, and usually has neutral or slightly acidic pH and is low in salinity.