Organic Fertilizer by the Numbers
How much fertilizer to apply confounds many people; most gardeners simply guess. In the same way experienced chefs can develop a sense for how much of a spice to add to a recipe, gardeners do develop the ability to know if a plant needs a smidgeon, a handful or a shovelful. For the rest of us, though, it’s safest and best to follow directions on the package, or the results of a soil test from your local Cooperative Extension Office. The soil test is a must for anyone starting out.
Understand, too, that in organic gardening we don’t focus on N-P-K, nitrogen, phosphorus and potassium. Instead, we focus on living soil, which provides the nutrients that most plants need without the addition of a lot of extra fertilizer. Organic fertilizer, in fact, is soil food rather than plant food. The natural elements of organic fertilizer need to be consumed by the organisms in the soil prior to becoming “plant available.” And if that sounds like a complicated extra step . . . it’s not. This natural process is what makes the growth of forests and meadows possible without anyone running around applying fertilizer.
The benefit of a natural system is that very little supplemental fertilizer is required. Plants get the bulk of their nutrients from the living organisms in the soil. Having said that, what follows is a general breakdown of all the numbers involved in fertilizer, as well as sources of organic nutrients, for when your lawn or garden does need an extra boost:
IN BULK — When ordering loam, compost, manure, mulch or sand for your landscape, you may not be sure how much to order. These products are sold by the cubic “yard,” an old unit of measure based on the English system. A square box measuring 3-by-3-by-3 feet would contain a yard of a product, or 27 cubic feet in total. A typical wheelbarrow carries about six or seven cubic feet, or about four or five wheelbarrows per yard.
If your landscape project calls for three inches of compost per square foot of surface area, then a yard of compost would adequately cover an area measuring 10 feet by nine feet. If the project called for a light coating of a half-inch of compost over the lawn, then the coverage area of a yard would cover an area measuring 540 square feet, or 20-by-27 feet.
IN BAGS OR BOTTLES — Products registered as fertilizers are required by law to provide published analyses of available macronutrients, which are nitrogen (N), phosphorus (P) and potassium (K). These numbers, such as your 10-10-10 reflect the percentage of each of these nutrients in the container. In other words, a 20-pound bag of 10-10-10 contains two pounds each of N, P and K.
Right away, you may see that the way most fertilizers are sold is inherently problematic. A typical Northeast soil test might tell you the soil is deficient in nitrogen, but rich in phosphorus and only slightly light on potassium. Your 10-10-10 could provide the needed nitrogen, but it would oversaturate the soil with the other two elements. Leaching of excess nutrients is a major cause of soil and water pollution — and too much can lead to unhealthy, or even dead, plants. It is best to purchase fertilizers that only contain one of the major macronutrients; that way you can apply exactly the amount needed.
RAW PRODUCTS — Several naturally occurring substances contain macronutrients and allow you to fertilize your lawn and garden without potentially dangerous and harmful synthetic chemicals. Because these products break down and release their nutrients slowly in the soil, they are more forgiving in terms of quantity of application. If you double the required amount of synthetic 10-10-10 fertilizer, you may very well burn your plants to death. If you double the amount of compost, composted manure, bone meal or wood ash etc., you will be less likely to harm the plants or soil.
Having said that, it’s still useful to know something about the available nutrients in whatever you apply. Bone meal has a phosphorus factor of approximately 12, meaning a 10-pound bag contains about 1.2 pounds of phosphorus. It is quick to dissolve into the soil and will be fully available to the plants within six months. Rock phosphate contains about a 33 percent phosphorus, but is slow to break down — it will take three to five years to release all its nutrients. In other words, if the soil test calls for a pound of phosphorus per 1,000 square feet, you’d need to apply nine pounds of rock phosphate in the first year to actually get the pound of phosphorus into the soil.
Other organic sources of organic phosphorus are worm castings, alfalfa meal, guano and compost. None of these will typically contain significant percentages of phosphorus, however, and generally should not be relied upon to correct a phosphorus deficiency.
Nitrogen is probably the easiest macronutrient to find and to apply. Popular sources include blood meal, which contains about five to 14 percent nitrogen, and fish byproducts, which can range from five percent to 13 percent nitrogen. On lawns, where most nitrogen is needed — about four pounds per 1,000 square feet per season for bluegrass or Bermudagrass — the grass clippings themselves will add about two pounds of nitrogen per thousand square feet back into the soil.
Potassium can be the trickiest macronutrient to find and apply. Wood ash may contain from 4 to 10 percent potassium, but can raise the pH too high if you apply too much. About 20 pounds per 1,000 square feet per year is a general recommendation.
Greensand, with about five percent potassium, and seaweed are other common sources. Seaweed is one of the best overall fertilizers, and the basis for many of the organic fertilizer products currently on the market. Studies show it can contain anywhere from two to 12 percent potassium.