In gardening, whether you’re talking about pest control or fertilization, “organic” generally means natural. The word “organic” refers to organic matter, the linchpin of organic gardening and, traditionally at least, all good gardening. Organic matter is material derived from what is or was once living.
Plants are most hungry for three nutrients — nitrogen, phosphorus and potassium — and organic fertilizers can supply them just as synthetic, or chemical, fertilizers can.
The three prominent numbers on any fertilizer label indicate the percentages of these three nutrients that the bag contains. In the world of organics, many different sources exist for each of these major nutrients.
Let’s begin with nitrogen, because it’s the nutrient needed in greatest amounts and the one most readily lost from the soil.
The richest organic sources of nitrogen are manures, ground-up animal parts — blood meal, feather dust, leather dust — and seed meals — soybean meal, cottonseed meal. Nitrogen concentration in any manure varies not only with the kind of animal, but it also varies with the kind of bedding used, the age of the manure and how it was stored.
Ground-up animal parts and seed meals generally have the highest concentrations of nitrogen. Because they undergo some processing, they are more consistent in their nitrogen concentration.
Phosphorus and potassium
The major organic sources of phosphorus include, again, certain manures, as well as bone meal and pulverized rock phosphate.
Rock phosphate is a phosphorus-rich rock that is ground into fine particles that release their phosphorus slowly and over the course of many years. Hence, only infrequent applications are needed. I spread some every 10 years. Colloidal phosphate is especially fine, so releases its phosphorus somewhat more quickly.
Manures contain some potassium as well as nitrogen and phosphorous, but richer sources of potassium include seaweed, wood ashes, and the minerals greensand and granite. Wood ashes are alkaline so should not be used on rhododendrons, azaleas, camellias, mountain laurels and other plants that thrive only in very acidic soils.
Some commercially available organic fertilizers are blends of one or more individual organic fertilizers, so they can offer a balance of all three major nutrients. Particular blends vary in their nutrient concentrations and in how quickly these nutrients are available to plants. For instance, blood meal, bone meal, seaweeds and wood ashes release their nutrients into the soil relatively quickly, while leather dust and pulverized rocks release their nutrients slowly. Read the label carefully if your plants need food quickly.
The Cadillac of balanced organic fertilizers is compost. Besides offering a wide spectrum of nutrients, especially when made from a wide spectrum of raw materials, compost also is a good source of organic matter. So good, in fact, that its nutrient concentrations are not high enough for compost to be legally sold as “fertilizer” — it must be classified as a “soil amendment.”
Nonetheless, compost spread over the ground can provide all the nourishment that even the hungriest plants need for a season. All that organic matter that hitchhikes along with the nitrogen has far-reaching benefits, indirectly bolstering biological activity in the soil and helping it retain air and moisture.
Sometime each year, usually in autumn or early spring, all the beds in my organic vegetable garden get blanketed with an inch of compost. Nutrients in compost are available slowly over the span of a number of years, so yearly additions keep my plants very well nourished.