There's more to soil than meets the eye. To some, it's simply dirt, always waiting to be scrubbed away. However, to gardeners, soil is alive, teeming with billions of organisms whose collective personal agendas contribute to bountiful crops in the garden. Although invisible without powerful microscopes, the bacteria, fungi, insects and assorted microbes are hard at work, breaking down organic matter and releasing the nutrients that nurture your plants. It's the organic materials that give soil its ability to hold water and nutrients until the rootlets can take them up as needed.
Humans can't resist tinkering with this system, in the endless search for more and better harvests. It's easy to interfere with these relationships and to throw off the balance of nutrients, however, as we casually spread a winter's supply of wood ashes or endless bags of fertilizer on the soil. The goal of fertilizing should be more rational, based on the results of a soil test. This doesn't have to be done twice a year, or even yearly. An adequate test, performed every other year, is plenty. The changes tend to take place rather slowly, and, unless you really go overboard, most breakdowns of organics take two or three years.
Rather than randomly spreading whatever materials happen to be available, consider their purpose: to maintain the levels of organic matter, to maintain proper soil pH or to add nutrients as plants require.
For instance, "clean tilling" practices tend to burn out organic matter, as does the overuse of inorganic fertilizers. Instead, we should try to replace vital soil ingredients. A general guideline is to work in at least one inch of organic materials, spread over the entire garden, per year. We can use one pound per square foot of cow or horse manure, but be prepared to deal with plenty of weeds from the seeds carried along in the manure. When using dried manure, use only one-fifth the amount.
For my garden, good-quality clover or alfalfa hay is even better. Spread it over the garden surface and chop it up with the mower for easy breakdown. The best-quality hay isn't cheap, but it is easy to spread and chop.
A cheaper way to go is to plant "green manure," and chop up the sod to add organics to your soil. One can set aside half the garden each year, and alternate between the sod crop and the vegetables. Allow clover to develop all summer, or use buckwheat, followed by rye in the fall as a cover crop. In a large garden, it can take Herculean efforts to turn under a well-developed green manure crop. It does wonders for the soil, but it can be heavy going.
Prepared organic fertilizers, like bone meal, crab meal and greensand can be used to help correct a soil deficiency. In a large garden, they can be expensive, but most do not have to be applied every year. With the rock powders, you won't see a big difference in the soil test until time and the microbes have had two or three years to do their work, depending on soil conditions. Of course, the effects last much longer than the chemical fertilizers. These must be used often, and are so soluble that they are easily washed out of the soil. The chemicals can be expensive, and can burn the plants if one uses too much.
All organic matter, used on the garden, is not created equal. Some doesn't decay readily, and some materials rob the soil of nitrogen as it decays. The speed at which this process takes place depends on the carbon-nitrogen ratio. This is 20-1 or 30-1 in manure and legumes, and these decay rapidly. In straw and hay, the ratio drops to 100-1, or 200-1 in sawdust and bark. It's the teeming billions of soil microorganisms that do the work, but they need both the carbon and nitrogen to grow. Simply put, the carbon provides the energy, and the nitrogen is a source of body protein. Therefore, the microorganisms can grow only as fast as the amount of available nitrogen allows, but they need the carbon to do it.
If the tiny creatures do not get enough nitrogen, they will lock it into their bodies and die. This delays the natural breakdown process that should hurry along the flow of nutrients to your plants. Therefore, if you add such materials as sawdust, straw, hay and leaves to your garden, add some high-nitrogen materials at the same time. This will allow the process to proceed without slowdown. On the other hand, if there is too much nitrogen and not enough carbon to balance it, the microbes will break down the humus to get enough carbon to balance out the nitrogen. Therefore, your soil will become lower in organic matter.