What you should know about fertilizers:
How much material to use varies with the nutrient content of the material. The age of the material is also a factor; when organic material decays and is rained on, they lose nutrients, especially potassium and to some extent nitrogen. Fresh manures may cause plant injury, especially when large quantities are used.
Examples of Natural Fertilizers
Cottonseed meal is a byproduct of cotton manufacturing. As a fertilizer, it is somewhat acidic in reaction. Nutrient content varies slightly but generally contains 7 percent nitrogen, 3 percent phosphorus (P2O5), and 2 percent potash (K2O). Nutrients from cottonseed meal is readily available to plants in warm soils, and it poses little danger of burning. For general garden use, apply 2 to 5 pounds per 1,000 square feet. The nitrogen is slowly made available. Cottonseed meal is frequently used for fertilizing acid-loving plants such as azaleas, camellias, and rhododendrons.
Dried blood is a byproduct from beef processors. As blood is drained from recently killed animals, it is dried and ground into a powder. It is a rich source of nitrogen and supplies certain essential trace elements including iron. The gardener must be careful not to use more than the amount recommended on the label.
Animal manure is also a complete fertilizer but is low in nutrients. Manures vary in nutrient content according to the animal source and what the animal has been eating. A nutrient ratio of 1-1-1 is typical. The amount of nitrogen in manure varies but is normally around 0.5 percent (poultry litter is higher in nitrogen). This is 1/20 of the nitrogen found in an equal amount of 10-10-10 fertilizer. Commonly available manures include horse, cow, chicken, and sometimes sheep. The actual nutrient content varies widely: the highest concentration of nutrients is found when manures are fresh. As it is aged, leached, or composted, nutrient content is reduced. Fresh manure should not be used where it will contact tender plant roots. Manures are good soil conditioners. Manures should be incorporated into the soil to conserve nitrogen. If left on the soil surface, up to 25 percent of the ammonia nitrogen can be lost within 2 days and 75 percent or more can be lost within 1 month after application.
Sewage sludge is a recycled product of municipal sewage treatment plants. Two forms are commonly available: activated and composted. Activated sludge has a high concentration of nutrients (approximately 6-3-0) and may contain significant amounts of lime. Composted sludge is usually sold in a dry, granular form for use as a general purpose, long lasting, nonburning fertilizer. Composted sludge is used primarily as a soil amendment and has a lower nutrient content (approximately 1-2-0). There is some question about the long term effects of using sewage sludge products. Heavy metals, such as cadmium, are sometimes present in the sludge and may build up in the soil. Possible negative effects vary, not only with the origin of the sludge, but also with the characteristics of the soil where it is used. It would be desirable to have the sludge analyzed for heavy metals.
Rock phosphate is ground rock that contains as much as 30 percent phosphate. It is not in a readily available form and is released very slowly.
Granite dust contains about 5 percent potassium but little of it is available for plant growth.
Greensand contains 6 percent potassium. It is also available very slowly.
Wood ashes are often used as a soil amendment. They contain potash (potassium), phosphate, boron, and other elements. Wood ashes can be used to raise soil pH; use twice as much wood ash as limestone for the same effect as lime. Ashes should not come into contact with germinating seedlings or plant roots as they may cause root damage. Spread a thin layer during the winter and incorporate into the soil in the spring. Check pH yearly if you use wood ashes. Never use coal ashes or large amounts of wood ash (no more than 20 pounds per 1,000 square feet), as toxicity problems may occur.
Soil pH
Soil pH is a measure of the hydrogen (acid-forming) ion activity of the soil solution. The scale contains 14 divisions known as pH units. It is centered around pH 7, which is "neutral." The term neutral means that the acids and alkalines are present in equal concentrations. Values below 7 constitute increasing acidity (acid range of the scale), and values above 7 make up the alkaline range.
The pH scale is not a linear scale but a logarithmic scale. A soil with a pH of 8.0 is ten times more alkaline than a soil with a pH of 7.0. A soil with a pH of 4.0 is ten times more acidic than a soil with a pH of 5.0 (100 times more acidic than a pH of 6.0). A slightly acidic soil pH is generally considered ideal for most plants in North Carolina.
The major impact pH extremes have on plant growth is the availability of plant nutrients. For example, if the pH drops as low as 5.0, the phosphorus combines with soil iron and is not readily available.