On Wednesday morning I gave a little talk about building soil fertility using organic gardening methods. I was pretty surprised by the large turnout, but one feature of the talk was a demonstration of converting a recommendation for inorganic fertilizers (like 10 pounds 10-10-10 per 1000 sq ft, plus 15 pounds 15-0-15 per 1000 sq ft) from a UGA Soil Test Report to organic fertilizer ingredients. It turns out that a lot of people were interested in seeing someone put this math into action.
The Georgia Extension Service offers a handout on this conversion, so the demonstration wasn't anything I had to work especially hard to put together. The hardest part is choosing the ingredients to use in the conversion, since the list of possible sources is quite large, and some of them aren't widely available.
It turns out that the plant-a-row-for-the-hungry garden this year is going to need about 50 pounds of cottonseed meal per 1000 sq ft and 10.2 pounds of Sul-Po-Mag per 1000 sq ft to meet the requirements of "medium feeders," which describes most garden crops. "Heavy feeders" like tomatoes will need an extra (full) dose partway through the summer, and "light feeders" - mostly legumes - will need half as much as the other crops.
If building soil fertility were as easy as dumping in a few bags of amendments, though, we would all have perfect gardens! Unfortunately, to get really root-friendly soil, a little more is involved.
The biggest hurdle is working in enough organic matter that - in heavy clay soils - the clay particles are held apart so roots can grow and water can move freely through the soil, and that -in sandy soils - the big sand particles are held together enough that the water doesn't run right through and leave the roots in a desert after a few hours in the sun. The organic matter helps hold nutrients, helps maintain more even moisture levels, and it also contributes nutrients - especially the micronutrients that are missing in most bags of 10-10-10.
Getting more organic matter into heavy clay soils and into sandy soils is easy to manage in small gardens. A few bags of Nature's Helper (or similar organic amendment) and a couple of loads of compost from a pile in the backyard, with the addition of some mulch to keep the weeds down, is just about enough.
For larger gardens, getting enough organic matter into the dirt is more problematic. It is possible to buy soil amendments by the truckload, but that can be expensive. I, for one, would not like to be able to commiserate with the guy who wrote that "$64 tomato" book!
For people who can't really buy enough organic matter to dig into their soil, there are cover crops, also known as green manures. I grow one of these, hairy vetch, most winters in at least one of my garden beds. At the plant-a-row-for-the-hungry garden, we have grown Austrian Winter Peas for this purpose. And the guy who wrote The Joy of Gardening, Dick Raymond, plants his dwarf peas and bush beans in blocks, then turns the plants back into the soil after they've produced, giving them a dual purpose.
One of the great things about cover crops is that they conserve nutrients by tying them up in the actual plants. It's hard to for nutrients to wash away when they are firmly rooted to the ground! If the crop is a legume, in the bean and pea family, there is the bonus of ending up with more nitrogen in the soil than was there originally.
We found, at the plant-a-row-for-the-hungry garden, that we weren't really able to add enough organic matter as mulch (we used leaves that had been saved from the fall) to keep the soil in good condition. Over time, productivity of the garden fell, but switching to organic fertilizers and to using a cover crop made a huge difference. In my own garden, I've used mostly organic methods all along, but I haven't made as full a use of cover crops as I could have. This year, I think that will change.