Don’t tell a soil scientist about the “dirt” in your garden, and don’t call the dust of Mars “soil”, as NASA press briefings generally do. The soil scientist’s outrage may be feigned, but the point will be that soil is much more than something you sweep from your kitchen floor or wash from your hands, and it’s not simply eroded planetary crust.
In a very real sense, soil is alive and evolves—biologically, chemically and physically. It is always changing; it is always growing. It is essential for all terrestrial plant life, and without the food, feed, fiber, and, of course, oxygen that we derive from plants, we and the rest of the animals could not live.
Soil is the home of a vast array of life forms that function in concert with each other and contribute to the functionality and health of soil. In addition to plants, there are a plethora of animals—mostly invertebrates, various arthropods and worms—and there are fungi and bacteria. The number of bacterial cells alone estimated to inhabit the Earth’s soils is staggering—2.6 x 1029. If you’ve forgotten scientific notation, that’s the number 26 followed by 28 zeros. In terms of cellular mass (fixed organic carbon), this is estimated to be about equal the mass of all plants on Earth. All these creatures add to soil and change it through interactions with each other cycling nutrients through complex food webs and directly with soil in chemical and physical ways.
Soil is formed over time through interactions among the parent material (planetary crust, rock), climate, topography (surface shape and features), and biological activity. Soil is a mixture of four different elements—minerals, organic matter, water, and air; minerals and organic matter are solid and comprise about half of a soil’s volume.
Soil has texture. Texture (in part) controls how water moves in soil, affects chemical reactivity and nutrient availability, and plays a role in the potential for erosion of a soil. Texture is determined by the relative proportions of three soil components—sand (2–0.05 mm), silt (0.05–0.002 mm), and clay (<0.002 mm). Sands, silts, and clays are as varied as the parent material from which they come.
Three basic soil types exist: sands, clays and loams. Sandy soils have more than 70% sand and do not retain water or nutrients well; clay soils have 25 to 35% clay and will retain nutrients but drain poorly. Loam soils have the good features of both sands and clays (and also contain silt). The ideal garden (or agricultural) soil is a loam composed of roughly equal amounts of sand, silt and clay. You can determine the texture of your soil and, by inference, its characteristics through the USDA Soil Texture Calculator (see http://soils.usda.gov/technical/aids/investigations/texture/) or by feel (see http://soils.usda.gov/education/resources/lessons/texture/).
Soil has structure. Soil structure is the arrangement and size of the solid particles of the soil and the pore spaces between them. Soil structure too is influenced by the parent material from which the soil developed, and physical–chemical and biological processes are involved in its formation.
Positively charged ions (cations) such as calcium, magnesium, and aluminum initiate binding soil particles together (“flocculation”) into aggregates. Clay particles (platelets) have lots of negative charges and begin to aggregate into microscopic clumps called “floccules” as cations collect between platelet surfaces and bind them together.
Burrowing worms, plant roots, and fungal hyphae create spaces between aggregates; their secretions and exudates further bind those aggregates together. Fungi and other microorganisms also deposit storage materials in the soil that act as organic glues. These along with plants, animals, and their waste and decay products comprise the organic material in soil. Fully decomposed organic matter is called humus. Humus also contributes to binding soil particles together and provides nutrients for plants and the other soil organisms that are essential for healthy soil.
Given sufficient time, most soils will develop stratified layers or “horizons”. Horizons result from chemical weathering, break down of organic matter, and the movement and deposition of humus, mineral particles, and other chemical substances from the upper layers of a soil to the lower layers as water moves through the soil profile. In agricultural fields, the layer familiar to most people makes up the “top soil”, which is brought to the surface and mixed with upper soil layers by plowing. Most soils have a distinct profile or sequence of horizontal layers that can be seen in road cuts or washed out gullies.
Soil health is fundamental to the productivity of your garden. The basis of the idea of soil health is that soil is a living, complex and dynamic environment that does more than hold up your plants (the soil scientist’s refrain). A healthy soil has the capacity to infiltrate water and cycle nutrients to feed growing plants. The more diverse its fauna and flora, the more fully functioning and productive the soil is. There are lots of relatively simple things you can do to manage and improve the health of your soil to increase its productivity.
First, the less you do to disturb the soil, the happier it will be. You often need to work or till soil in your garden (distribute fertilizer or work in soil amendments, suppress weeds, whatever), but too much tillage is a bad thing. Tillage disrupts soil structure and soil organisms. Tillage and too much traffic over soil can compact it. In compacted soil, water infiltration is reduced, runoff (and erosion) is increased, and as a result productivity is decreased.
Consider also that soil is populated by plants and organisms that evolved there together. Long ago they adapted efficient and sophisticated ways of working together to produce and cycle nutrients (food webs). In an ideal state, it is a functioning and self-regulating system. But no system is at equilibrium for long, and occasional inputs of fertilizer or pesticide may be necessary to maximize plant growth and health. But excessive inputs can disturb or entirely disrupt food webs, and this reduces the health of your soil and hence its productivity. Before you fertilize, determine whether your garden (or lawn) needs it. There are soil testers on the market for less than $20 that will give you a good guess about your soil’s nutrient status (http://www.burpee.com/gardening-supplies/garden-growers/electronic-soil-tester-prod001200.html); alternatively, your local county ag agent can advise you on where to get your soil tested professionally.
As much as possible, keep your soil covered. Mulch is a good way to do this. Mulch insulates the soil, reducing rapid temperature changes. It protects soil from the effects of raindrops, which breakup soil aggregate structure, and it retains moisture and reduces water loss. Mulch suppresses weed growth (reducing the need for tillage) and provides habitat for members of the soil food webs. Its breakdown products (ultimately humus) enrich the soil. For your plants themselves, mulch reduces freeze–thaw cycles, frost heaving, and the like. Many municipalities convert their green waste to mulch and deliver the finished product for a nominal charge. Garden stores sell it too.
Cover crops (or green manure) have many of the same benefits as mulch, but in addition they actively encourage and support soil microorganisms. In natural environments, many soil organisms exist in a near-starvation state or go through boom–bust cycles that follow the seasons. Live roots growing throughout the year support and maintain stable populations of soil organisms that utilize the sugars secreted by plant roots and return the favor by translocating minerals and water to plants. At the end of the season, the cover crop can be worked into the soil to provide additional biomass that will be broken down and recycled.
Even in the Northeast, hit last weekend by a snow storm, it’s not too late to plant a cover crop. Burpee has several cover crop mixtures tailored to different growing zones.
The key to improving soil health (and the productivity of your garden) is to remember that soil is a living, changing system that you manage with a light hand. Increase your garden soil’s above- and below-ground diversity with mulches and green manures and as many different kinds plants as makes sense. With time, this will create a more fully functioning and resilient soil that will be reflected by the plants in your garden.