Tuesday, September 16, 2008

Soil nutrients and fertilizers

A couple months ago when strawberries were still in season we drove by a popular conventional strawberry farm and were struck by how thickly the strawberry plants had been planted. There must have been fully ten times as many plants per acre as in our strawberry patch. Sometimes we forget how radically different it can be to farm with synthetic fertilizers and intensive irrigation. Unlike our plants, the strawberries at that conventional farm didn't have to rely on the fertility of the soil; all the macro-nutrients (e.g. N-nitrogen, P-phosphorus, and K-potassium) could instead be synthesized from fossil fuels and other mined materials and fed to the plants through irrigation lines, limited only by the depth of the farmer's pocketbook and the depth of the mines. The organic ideal, in contrast -- not to be confused with "USDA organic" as legalistically defined to suit big business -- requires that nutrients be continuously recycled through natural processes like excrement, shedding, death, and decay. On the surface those processes may not sound like things we want to associate with our food production. Poop and dead animal parts are things we in our "developed" consumer society pay big business to make disappear for us; they're certainly not treasured nutrient sources. And so we've cut ourselves off from any knowledge of or responsibility for the nutrient essentials of organic agriculture. Meanwhile one of the costs of maintaining the consumer's illusion of disconnectedness from these natural processes is that animals have been concentrated in extremely unhealthy ways. All sorts of pollution to soils, water, and air have resulted. What, then, is the alternative? How can nutrients be recycled organically? There are some first order nutrients that nature can recycle apart from human transfer of materials to the farm: water, oxygen, carbon dioxide, and atmospheric nitrogen. Bacteria that live in the root systems of leguminous plants can, for instance, convert atmospheric nitrogen into forms of nitrogen useable by plants. All these natural processes work within limits, of course. Most of the plant-necessary nutrients aren't in the air, though, and so nature on her own has very little means of recycling those other nutrients when farmers sell those nutrients and they leave the farm in the form of produce or eggs or meat. One of the macro-nutrients that's especially of concern to our circumstances on this farm is phosphorus. Everything we sell out of the garden or from our chickens or livestock contains phosphorus, for example, and when we sell those things we're parting with the phosphorus they contain (along with all sorts of other nutrients). We could go on for a while without replenishing those nutrient losses, but the fertility and productivity of our soils would all the while decrease. Therefore we try to bring as much organic matter back to the farm as possible. Plant matter like hay mulch, for example, has wonderful attributes, but plant matter is relatively low in nutrients like phosphorus. Manure contains very roughly ten times the phosphorus of vegetable matter, and animal parts, particularly bones, contain something like a hundred times the concentration as plant matter. Of course, manure from our own animals is a wonderful organic fertilizer, but whatever manure we use in the garden must first be robbed from the pasture, and so as much as we're selling things off the farm we need to be bringing nutrient-rich materials back to the farm in order to maintain the fertility of our soils organically.

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