Soil is to plants as water is to fish. It is so vital to the very life and well being of the plants you grow. Just like polluted water kills fish, polluted or unhealthy soil can likewise kill the plants that reside in it. It is therefore imperative, that we understand the vital importance of soil and what makes it healthy. It is an entire universe of its own, beneath our feet, with a food web of predatory/prey organisms all of which help promote plant growth needed to sustain higher forms of life. I will try not to get too academic but provide useful information about soils that will make you a better and more productive gardener, while respecting and preserving our frail balance of nature.
First, some facts about soils. Good balanced soil comprises 50% solids and 50% pore space. The solids are 45% mineral and 5% organic matter. Pore space comprises about 20-30% water and 20-30% air. Soils have physical, chemical, and biological elements. The best soils are those that balance these three elements equally.
Within on cubic centimeter (1/4 tsp) of soil, you might find 1 earthworm, 30 nematodes, 100 arthropods, 5,000 protozoa, 30,000 algae, 200,000 fungi, 4,000,000 actinomycedes, and 90,000,000 bacteria which form a food web – guess who’s at the bottom! The healthy smell of earth comes from the actinomycedes in the soil. Nematodes are often seen as bad guys as some cause root galls which can harm plants but 90% of nematodes are beneficial.
Thirty percent of the biomass of a plant is in the roots. An area surrounding every root part by 1-2 millimeters is known as the rhyzosphere. Whereas roots occupy 1% of topsoil, the cumulative space occupied by the rhyzospehere is 10%. Bacteria are 23X greater within the rhyzospere than in bulk soil and fungi are 250X greater in this area. Average soil pH in bulk soil in central Texas is 7.5 but within the rhyzosphere, it is 4.8 and levels of iron, Manganese and Zinc (micronutrients) are much higher. Biological nitrogen fixation is caused by bacteria in and around the roots of nitrogen fixing plants such as peas, alfalfa, peanuts, and other legumes. These plants pull nitrogen from the air and fix it into the soil in useable form.
Microrhyzal fungi live in the roots of plants and produce flagella into the soil which add to the rhyzosphere and absorb nutrients from the soil converting them into usable forms for the plant. They are often barely seen as clusters of filament type structures surrounding a root.. They also produce exudates that cause the soil to aggregate allowing porous space for air and water in the soil.
Organic matter in soils range between 1 – 6% of which humus comprises 60-80%, living biomass is 10-20% and decaying matter is 10-20%. Organic matter in soil links the physical, chemical and biological elements together to promote healthy soil. Compost is one of the most beneficial supplements to add to any soil. It is basically decomposed organic matter that is resynthesized into usable form of nutrient for plants. A good recipe for compost is 3 to 5 parts carbon (dead leaves) to 1 part nitrogen (fresh foliage) which is well aerated and turned frequently while decomposing. Heat built up during decomposition generally kills any weed seed, while not harming bacteria.
So, feed your soil organically. Cover crops where feasible can be used such as nitrogen fixing crimson clover during winter, or mulching with alfalfa hay, or buckwheat as a cover crop during warm months (July – Sept.). If this is not feasible, add compost. Any cover for soils is beneficial. Mulches will decompose over time and also keep soils warm and moist which helps microbiological activity in the soil.
Beneficial micro-organisms living in root and leaf biospheres perform multiple functions: symbiosis, biocontrol, mutrient accdess, and improve soil structure. The number and diversity of soil micro-organisms is both an indicator and driver of soil fertility, biocontrol and soil functions. The soil food web is a community of soil organisms that compete for carbon resources in a predator/prey food chain. This foodweb retains and cycles nutrients, builds soil structure, sporesses diseases and pests, colonizes plant surfaces, produces plant-growth promoting substances, and decomposes toxic compounds – all good and beneficial things. Bacteria dominate soils in areas of invasive weeds and turf grasses whereas fungi dominate areas with trees, vines, and forests.
Agricultural and urban gardening practices (such as use of chemicals and synthetic fertilizers, tillage, compaction) degrades the soil foodweb. Biological soil management practices (compost, cover crops, mulch, no-till) rebuilds soil foodweb complexity. Consider this when preparing your garden beds and landscaped areas. Practices like mulching and leaving lawn clippings to decompose, minimal disturbance to soil areas when planting, providing organic cover, avoiding the entry of toxic substances to soil, all will help produce healthy soil without use of dangerous chemicals and fertilizers which can harm the soil foodweb and even the water that all life depends on.
For further information, an excellent “Soil Biology Primer” is available for your reference on the internet.