Saturday, August 28, 2010

Feeding the Dirt

At the Plant-a-Row-for-the-Hungry (PAR) garden, productivity has generally trended downward over the last few years. There have been some extenuating circumstances (drought of historic proportions; floods of historic proportions; and what may be the hottest summer on record). However, it is possible that something else is going on, too.

One suggestion tossed out is that the soil is losing fertility. This is a garden for which soil tests are done every year, and the recommended amounts of fertilizer are applied every year. However, plants need more nutrients than just the nitrogen, phosphorus, and potassium (N-P-K) that are in most commercially available fertilizers.

The garden is mulched every year, and the mulch (usually leaves) is an additional source of nutrients, but this might not be enough.

The other PAR gardeners all know that I tend to not use those standard N-P-K formulations, so I have been asked about how I feed my garden's dirt. The answer is, essentially, that it isn't 100% the same every year.

There are organic gardeners who use one specific combination, in specified amounts, every year. An example is Steve Solomon's fertilizer recipe that was featured in a 2006 issue of Mother Earth News. That first link is to the article that explains Mr. Solomon's reasoning behind the development of his recipe. This page contains the actual recipe.

In general, the LONG article that precedes the recipe makes some very good points about the benefits of the various ingredients, and it may be that any garden that uses this for the first time, after years of only N-P-K additions, will experience a drastic rise in productivity just from the addition of so many essential micronutrients that hadn't been available to the plants before, but the recipe doesn't take into account different soil types and conditions. Over time, using this recipe without regard for soil test results could become a problem.

For example, the recipe calls for a specific amount of limestone, preferably dolomitic. While this ingredient does add calcium and magnesium, both of which may be in short supply initially, repeated applications will have the effect of raising the soil's ph. Mr. Solomon recommends that gardeners not worry about soil ph, that it is not as important as we have all been led to believe, but the agricultural research seems to say otherwise.

In general, most garden crops perform best within a ph range of 6.0-6.5. As the ph rises to 7 and beyond, the plants become less able to take up other essential nutrients. This would be bad. At the PAR garden, we are already fighting a too-high ph, so switching wholesale to Mr. Solomon's fertilizer formula with the inclusion of limestone might not be such a great idea.

The essential nutrient phosphorus is supplied primarily by bone meal in Mr. Solomon's recipe. In my own garden, the phosphorus level tends to be a bit high. Too-high a phosphorus level can also interfere with plant-uptake of other nutrients, so my switching straight to Mr. Solomon's recipe could be less than helpful for my garden, too. It doesn't need additional bone meal any time soon.

However, the article that comes with Solomon's recipe is well-worth reading for the information about the possible ingredients to use in "mixing your own" and to get an idea of the general proportions and strategies for choosing ingredients.

In my own garden, to keep fertility up in spite of the incredible demands I make on my garden's dirt (two to three crops per year for much of the garden), I rely on a mix of strategies.

Like Mr. Solomon, I add quite a bit of organic matter each year. This serves as an additional source of micronutrients, but it also helps the soil in many other ways. Some of the organic matter added to my garden is compost from my own pile out back; sometimes I go get a truckload of mixed stable bedding & manure; sometimes I add bagged soil conditioners like Nature's Helper; sometimes I buy bags of composted humus and manure, or other compost, to add to my garden.

I also mix up my own fertilizer, kind of like Mr. Solomon's but taking my own soil's needs into account. So far, this always includes some kelp meal, which is a great source of many micronutrients in addition to the potassium for which it is a primary provider. For the other ingredients, though, I follow (approximately) the guidelines in this Extension Service bulletin about converting a standard fertilizer recommendation to an organic fertilizer amount.

That Extension Service bulletin includes a helpful list of potential ingredients, along with how much of what major nutrient each contains, so that it is possible to choose from among several options.

Since most of these ingredients are from plants or animals, more than one nutrient will be supplied by each, so the additive characteristic of a mix of such ingredients should be considered. For example, cottonseed meal is a source of N, but it also provides P and K. A garden that needs more N and less P & K might want to select a different source of N.

I also know that some nutrients in particular are not abundant in the local clay soils. One of these is magnesium, and one is boron. When I am mixing up my fertilizer, I add tiny amounts of Epsom Salts (magnesium) and 20 Mule Team Borax (boron). By tiny, I mean, for example, amounts of boron that would be the equivalent of one tablespoon per 100 square feet of garden.

I'm pretty good at math, but I'm also lazy, so my amounts are always approximations, but it all seems to work.

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