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Garden Fertilizers
The care and feeding of your garden crops

Fertilizer Basics

Organic garden fertilizers and their application.Fertilizer is a substance added to the soil to improve its fertility. Since a variety of elements contribute to the fertility of the soil, many individual elements and combinations of elements can be considered fertilizers.

What is fertile soil?

In brief, a fertile soil has good amounts of the major plant nutrients—nitrogen, phosphorus and potassium; a sufficiency of the micro nutrients (sometimes called trace minerals)—zinc, manganese, boron, iron, sulfur, copper, magnesium, molybdenum, and chlorine; an abundance of organic matter; and humus.

pH balance is important

To be fertile, the soil must also have a nearly neutral soil pH as well as good structure and drainage. It should also have the proper balance of Nitrogen, Phosphorus and Potassium.

Fertilizers and Organic Gardening

In the organic garden, natural fertilizers maintain and contribute to the improvement of all these necessary elements.

Nitrogen (N), phosphorus (P) and potassium (K)

Although the vocabulary of chemical agriculture dictates that fertilizers are only those substances that have measurable quantities of at least one of the major plant nutrients, organic growers hold to a much broader, more complex interpretation of soil fertility that recognizes the roles of organic matter and soil structure.

Natural materials as fertilizer

Unlike those farmers who rely on chemical fertilizers to supply crops with specific nutrients in forms that are immediately available, they use many different, organic fertilizers to maintain overall fertility. In this way, soil fertility and soil health are improved even as plant yields increase.

The Value of Nitrogen (N)

Nitrogen is a major element in plant nutrition. It is responsible for producing leaf growth and greener leaves. A deficiency causes yellow leaves and stunted growth. An excess produces an overabundant growth of foliage with delayed flowering; the plant is more subject to disease and its fruit is of a reduced quality.

Adding nitrogen to your soil

If you believe your soil is deficient in nitrogen, you can correct it by adding composting, manure or other nitrogen-rich fertilizers such as dried blood, tankage, cottonseed meal, cocoa bean and peanut shells, bone meal, or sewage sludge. Returning weeds, grass clippings and other garden wastes to the soil will add to its humus content and improve its nitrogen content at the same time.

The Value of Phosphorus (P)

All growing plants need phosphorus. It is a constituent of genetic materials and is important for proper seed development. Although the specific properties of phosphorus are not completely understood, a deficiency seems to cause stunted growth and seed sterility. Phosphorus is said to hasten maturity, increase seed yield, increase fruit development, increase resistance to winterkill and diseases, and increase vitamin content of plants.

Phosphate rock as a source of phosphorus

Phosphate rock, a natural rock product containing from 28 to 30 percent phosphorus, is the organic gardener's best source of phosphorus. When the rock is finely ground, the phosphate is available to the plant as it needs it. Phosphate rock is especially effective in minerally balanced soils containing plenty of organic matter. The bacteria that thrive in humusy soils secrete organic acids that promote the breakdown and availability of the phosphorus. Humus forms its acids slowly, releasing nutrients to the plants as they need them rather than in a single massive dose, as is the case with highly soluble chemical fertilizers.

Other phosphorous sources

Besides phosphate rock, other phosphorous sources are basic slag, bone meal, dried blood, cottonseed meal, and activated sludge. In most areas, barring any great deficiencies or excesses, a pound of phosphate rock for every ten square feet of garden space (two tons per acre) is a good amount to use once every three or four years. Dried blood, containing approximately 3 percent phosphorus, may be applied at the same rate, although allowance must be made for its substantial content ( 15 percent) of rather highly available nitrogen at the same time.

Phosphate rock is aided by manure

Phosphate rock is most effective when applied in combination with manure—about 25 pounds of manure for every ten pounds of phosphate. Put the manure on first, work the ground, then add the phosphate rock a month or two later. Sprinkle the ground phosphate on the soil just as you would lime, then work it into the top inch of soil. Spread on lawns the same way, one pound per ten square feet.

Bone meal

Bone meal is another source of phosphorus that makes sense on a small garden. Bone fertilizers have a phosphorous content of over 20 percent, but are slow to decompose and release their nutrients.

The Value of Potassium (K)

Potassium, the third major nutrient, is equally important to the strength of the plant. Often referred to as potash or potassium oxide (K,O), it helps in the formation of carbohydrates and is necessary for protein synthesis. In addition, it promotes early growth, improves stem strength and contributes to cold hardiness. It is also known to improve the keeping quality, color and flavor of fruit.

Potassium deficiency

Plants deficient in potassium are usually stunted and have poorly developed root systems. Their leaves, particularly the older ones, are usually spotted, curled or mottled and may even appear "burned" around the edges. Even before these symptoms appear, they produce low yields of crops.

Mineral fertilizers supply virtually insoluble potassium, which takes a very long time to dissolve into forms available to plants. Highly soluble sources of potash like muriate of potash (KCI) and potassium nitrate (KNO,,) may cause the plant to take up an excess of the element, which can restrict its ability to assimilate other essential nutrients. Since the natural mineral fertilizers become available more slowly, there is no chance of this occurring.

Sources of potassium

The organic grower has three sources of potassium at his disposal: (a) plant residues; (b) manures and compost; and (c) natural mineral sources, like granite dust, greensand and basalt rock.

Wood ashes

Included in the first category are wood ashes (6 to 10 percent potash), hay (1.2 to 2.3 percent) and leaves (A to .7 percent). The best approach is to combine both organic and mineral potash fertilizer—organic for short-term potash release and mineral for the longer period.


Many organic gardeners are turning to seaweed and seaweed extract to raise their potash levels, especially on porous, sandy soils. Seaweed also contains a moderate percentage of nitrogen. Since it is in organic form, it is not available to plants until decomposition has taken place. With the majority of seaweeds, however, this occurs rapidly in the soil, and it is not generally necessary to compost seaweed before adding it to the soil.

Granite dust

Granite dust is an excellent source of organic, slow-working potash. Its potash content varies from 3 to 5 percent. Granite dust should be applied at a rate of two tons to the acre. Ideally the granite dust should be mixed with phosphate rock and manure and turned under in the spring, before planting. Smaller applications call for 20 pounds to 100 square feet.


As a soil builder Greensand or greensand marl typically contains about 5 percent potash, although that extracted from especially rich deposits may average 1 or 2 percent higher. Being an undersea deposit, greensand contains most of the elements found in the ocean and is an excellent soil builder. Superior deposits of greensand contain 50 percent silica, 18 to 23 percent iron oxide, 3 to 7 percent magnesia, and small amounts of lime and phosphoric acid.
Greensand has the ability to absorb large amounts of water and provides an abundant source of available potash. Its minerals or trace elements are also essential to plant growth. Traditional recommendations call for an application rate of 25 pounds per 100 square feet but, since it is so slow to dissolve, it is best to combine it with manure and phosphate rock.

Trace elements

Other plant foods needed by crops in lesser amounts include calcium, magnesium, ;sulfur, iron, zinc, molybdenum, tin, and iodine. These trace elements are very important to proper growth of plants even though they are only needed in small amounts. Some, in fact, lave been found to serve as partial substitutes for other nutrients and to increase disease resistance in certain plants.