Wednesday, July 2, 2008
Summer & Mulch
The following are several excerpts from my book - Roots Demystified, Change Your Gardening Habits to Help Roots Thrive.
It’s time to tend to mulching. If you haven’t already – shame on you. But mulching now is still a blessing for the plants. First a bit about where the nutrients are absorbed.
Most soils will contain more humus near the ground surface, since the highest population of the soil organisms that decompose raw fiber into humus tend to hang out in the most aerobic zone of the soil, that is, in the duff or just under it. [See the enlarged illustration by clicking on the image. From: Roots Demystified.] The upper horizon of the soil is also the place where the most nutrients are liberated. Soil flora and fauna act as nature’s little fertilizing machines, using the creation of humus, among other processes, to liberate unavailable nutrients into a soluble form that can be absorbed by tiny root hairs—a process known as “mineralization.”
One study estimates the number of the bacteria in a gram of soil taken from upper layers of soil surfaces as ranging from 58 million to as many as 3–4 billion. Dig and test just three feet lower, and the bacteria numbers drop to as few as 37,000 per gram.
However, it’s not just the soil’s humus-clay-moisture complex that liberates nutrients. As mentioned earlier, plant roots, stimulated by the action of organisms in the humus, aid in the nutrient-release process by exuding sugars, organic acids, and other compounds to stimulate the microbial action in the rhizosphere.
Plants primarily absorb most of their nutrients in a chemical process called “ion exchange.”
This is a process in which ions (an atom or a group of atoms that has acquired a net electric charge by gaining or losing one or more electrons) are exchanged between a solution and an ion exchanger, i.e., an insoluble solid. Two notable ion exchangers are clay and humus, which are found suspended in the thin, moist chemical and biological activity in this thin layer of moisture converts nutrients into a soluble form that roots can absorb via ion exchange. Humus binds the clay particles so that the clay forms the aggregates that help maintain a more continuous pore space. The perplexing nature of a healthy humus-clay structure is that it both holds onto and releases many of the nutrients plants utilize.
What to do?
Mulch with compost. Be sure the compost you apply to your garden is thoroughly decomposed. A “finished,” properly-aged compost is no longer hot and makes no “steam” when turned or off-loaded from a commercial supplier. The finished material should have almost no recognizable pieces of the original compostable matter. It should also have the sweet smell of a forest loam. To achieve the goal of finished compost, you need to turn the pile two or three times (maybe even more) to incorporate oxygen into all the raw, composting materials. Then let the pile age, so that it develops a large cross section of microbes and other beneficial soil flora and fauna. The process may take up to one year, so plan in advance and always have a pile going for the following garden season. (Using worms to compost kitchen scraps is like a fast compost bin. The raw materials are quickly converted to castings—manure—that both stabilizes and inoculates organic matter better than unfinished compost.)
Commercial compost is often turned, but because of the surface area required by the large quantities made to meet high commercial demand, it is usually not cost-effective to both properly turn and age the compost. Thus, commercial compost is often sold in an unfinished state; beware if you have a load delivered, and it is still hot and steamy. Such compost should be allow to “mellow” until it has a dark, loamy feel, and it may require more turning.
If too much unfinished compost or fresh manure is added in great quantities, you risk the scourge of symphylans—nasty little critters that thrive in sandy loam soil, soils with a high level of organic matter and friable (crumbly) soil. Symphylans are 1/4-inch long and look like white centipedes. They eat the roots of many vegetable plants and are nearly impossible to banish by any organic method(s). However, a fallow period may be one option. Compulsive “Captains of Compost” are seeing more and more of this horrible pest. It can now be found in the northeastern, north central, and western United States. Beware of applying too much unsifted compost to your garden, especially in sandy loams. The addition of a layer of more than one to two inches may be too much. Keep the organic matter between three and five percent. Check with a lab report.
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