Know Your Soil
When you think about soil, pores might not be the first things that come
to mind. But soil is actually about 50% pore space. Its network of pores,
small and large, determines how well or poorly it will hold air and water,
both essential to plant health. Soil porosity depends on:
- Soil texture, or the relative proportions of sand, silt and clay
- Soil structure, or how soil particles are bound together
- Compaction, which can be caused by loss of topsoil, heavy equipment, foot and vehicle traffic, rain, or too much tillage
- Organic matter
The kind of mineral material-sand, silt, and clay-in a soil has a great
deal to do with its pores. Sand is the coarsest type of particle, clay
is the finest, and silt falls somewhere in between. A sandy soil will
have fairly big pores (macropores). A silty soil will feel more like flour,
and has more tiny pores (micropores) than macropores. As with silt soils,
a clay soil-very hard when dry but sticky and easily moldable when wet-will
have more micropores than not. If a soil is mostly macropores, it will
drain quickly but it won't be very good at holding water. A soil that
is mostly micropores will hold water well but will take a long time to
dry out, and won't provide much-needed oxygen to the roots.
Soil also contains organic
matter, the dead plant material that "glues" mineral particles
together. Unless you are trying to garden in a peat bog, all soils can
benefit from a dose of organic matter.
Organic matter is the food source for organisms that live in soil, from bigger critters like insects and earthworms, to tiny microorganisms such as bacteria, fungi, and protozoa. These creatures are essential to healthy plants because they break down materials into nutrients that plants can use. Many plants depend on naturally occurring mycorrhizal fungi to feed them nutrients, such as nitrogen. Microorganisms can also bind up or break down many pollutants.
Soil organisms are very sensitive to pesticides, so use integrated pest management methods (see The Ten Most Un-Wanted Pests for more information) to keep your pesticide use down.
In the Lake Whatcom watershed, the native soils are generally well-draining loams. However, over the past 30 years, it's become common practice to clear a parcel of land for development by stripping it of vegetation and its layer of topsoil. The soils on your property may have changed dramatically from what was originally there- for instance, organic matter, which is highly concentrated in the top few inches, has probably been lost; soil texture has likely been changed; heavy machinery may have compacted the soil, crushing pores. You may have a tough time growing plants in these conditions, so be prepared to spend some energy on restoring your soil's structure.
