A principal challenge of cultivating a healthy and productive vegetable garden is maintaining an adequate level of organic matter in the soil.
Decomposed organic matter, humus, — not hummus, which is ground up chickpeas — serves many essential functions in promoting plant growth. It binds soil particles together into crumbs, increasing the porosity of clay and silt soils, enhancing their drainage and ability to absorb water and making them more permeable to plant roots. Organic matter also acts like a sponge, absorbing and retaining moisture, which makes it beneficial to drought-prone sandy soils. As it decays, organic matter releases nutrients into the soil that nourish plant growth.
Humus can be remarkably stable, persisting in the soil for centuries in some cases. Yet it is nothing to take for granted, especially in a vegetable garden. Unlike a wild forest or grassland where the supply of organic matter is constantly renewed by the deposit of new plant debris on the soil surface, a vegetable garden is regularly depleted of organic matter by the harvesting and removal of crops. If you till the soil in your garden, mixing air into it, you speed up the decomposition of the humus. This leads to a short-term burst of fertility, but in the long run, if you don’t replace the organic matter lost in this fashion, you will impoverish the soil. Likewise, reliance on synthetic fertilizers will also, over time, deplete the humus content of the soil because these products supply no organic matter.
A soil test, easily obtained by sending a soil sample to your state agricultural university testing laboratory, will reveal the percentage of organic matter in your garden soil. The organic content will likely vary with your soil type. Sandy soils, for example, are typically low in organic matter, and an organic content of 2 percent in such a soil is generally considered good. Clay and silty soils are better at retaining organic matter, and with these, you may aim for an organic content of 5 percent or even more.
Blanketing the garden with an organic mulch such as shredded leaves or clean straw will help to boost the organic content of the soil as the mulch decomposes. I have a large garden, however, and I find it expensive to purchase sufficient mulch. I used to collect and shred fall leaves for this purpose, but at age 68, I find this task challenging.
In recent years I have compensated by buying pick-up loads of decomposed landscape waste from a town composting facility, but I’ve stopped doing that because of the risk of introducing invasive Asian jumping earthworms into my landscape. Egg cases of such worms may be introduced into the composting process with debris from infested gardens and can survive any but the most thorough, high-temperature processing. I have a friend whose garden was infested this way, and the invasive worms, which strip the soil of any organic debris on its surface, have had a disastrous effect on her garden ecosystem. I’ve also used composted manure from a nearby horse farm to add organic matter to my garden, but this led to a bumper crop of weeds. Horses are not picky in their grazing and any seeds remain viable after passing through their digestive systems.
This year I’m going to rely on an organic gardening technique that has been practiced for millennia in many regions of the world; I’m going to plant “green manure.” When the time comes to harvest and remove the summertime crops, I’ll replace them with a sowing of hairy vetch. This is a legume, a type of plant that partners with rhizobia bacteria in the soil to transform nitrogen gas from the atmosphere into a natural fertilizer. To make sure there is sufficient rhizobia for the vetch, I’ll treat the seed before planting with a commercially available inoculant.
The vetch will sprout in the early fall and over-winter to resume growth in the spring. By covering the garden in this way, I’ll protect my soil from erosion, keep the existing nutrients from washing away, and crowd out weeds. In late spring, two to three weeks before I want to plant my summer crops, or just as the vetch is coming into bloom, I’ll mow it down and till it into the soil. This will boost not only the organic content of the soil but also its fertility. What’s more, the roots of the green manure should penetrate down into the soil, loosening it and providing a deep dose of organic material as they decay.