Any time of the year is a good time to improve garden soil. But what makes good soil? It must meet the basic needs of any growing plant — first, a firm substrate that roots can use to anchor the aerial plant body; second, a supply of minerals for growth; and, most importantly, a consistent supply of available water.
The anchoring part is sort of a slam dunk. Anything falling between the two extremes of solid rock and a liquid will support a root structure to some extent. Of course, there is a gradient; dense clay won’t allow roots as easy an entry as loose sand. Here’s where the rest of the soil’s attributes come into play. Mainly, these have to do with the size of the mineral particles that define its texture. Extremely small particles are found in clay soils, and relatively large particles, in sandy soils, with every possible gradation in between. Clay soils also bind essential nutrients more closely than sandy soils. The water-carrying capacity of soils is also directly tied to the average size of the mineral particles; clay soils have little space between their particles, so less is available for water; sand, with its large, irregularly shaped particles, can sequester much more water.
Beyond the mineral structure of soil, organic matter is also absolutely key to good soil. The fertility of soil depends on the activities of a myriad of organisms (that can also contribute in many ways to soil structure). Organic matter provides microscopic organisms, bacteria, fungi, nematodes, and more with a suitable habitat. Most of these organisms are engaged in the decomposition of such organic matter. In this digestive process, they alter the organics and contribute various exudates. Humic acid is one such addition and is important for the overall health of the soil. Along with some of the larger fauna that inhabit the soil, like earthworms, these critters function as mini-shredders, leaving behind uniformly sized particles.
These organic bits act as wedges between the clay particles, allowing for water and air penetration. In sandy soil (with large pores between particles that can fill up with lots of water), organic particles increase the water-holding ability of the soil. Add organic matter to feed the soil dwellers, and get soil with a better texture. If the soil supports a healthy mix of these organisms and an adequate supply of organic matter, there should be little need for additional fertilizer. Nitrifying organisms transform this stored material to nitrate available for use by plants. The more organic matter in the soil, the more nitrogen there is that can be converted into plant nutrients.
While almost any organic material incorporated into the soil will help, well-made compost will also have many of the beneficial soil organisms already in place and thriving, so it would always be a good first choice. Well-rotted animal manures and forestry by-products like ground fir bark are good but should not be added in great quantity unless composted first. Once the system nutrient cycling is up and running, maintaining a layer of mulch on the soil surface will give these friendly microorganisms a constant source of food.
Now is a good time to get down and dirty; take stock of the state of the soil, and make a plan to improve its fertility and water-holding capacity. This subterranean stratum is just as fascinating and important as any other.