Fertilizers, Commonly Called Plant Foods
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Fertilizers add nutritious elements to the soil and, within limits, the richer the soil is in nutrients, the more a plant grows. Some of the elements are needed only in small amounts. Others, such as nitrogen, are required in considerable quantity. For the best plant growth, fertility should be "balanced". Too much fertilizer, or too much of any one element, can be as bad as no fertilizer at all.
Products used for fertilizing can be purchased wherever garden supplies are sold. Some are to be applied dry, others in solution. They are not poisonous and there is no harm in handling them, Instructions for their use are printed on the package containing them.
Content of Fertilizers
Every "complete" fertilizer, whether it comes in a big sack or small can, contains 3 major fertility elements. These are nitrogen, phosphorus and potassium-sometimes designated as N, P and K-and it is helpful to understand a little about them so that you can buy wisely. Stated very briefly, nitrogen encourages leaf growth, while phosphorus and potassium are helpful for flowering and fruiting.
Fertilizer bags tell the percentage of each of the 3 main nutrient ingredients. These are often mentioned as a series of 3 numbers such as 5-10-5. Each number stands for the percentage of one element, and they are always in the same sequence. The first always stands for nitrogen, the second for phosphorus, and the third for potassium. Thus a 5-10-5 fertilizer contains 5% nitrogen,10% phosphorus and 5% potassium. The larger the numbers, and thus the greater the percentage, the more powerful the fertilizer is.
When you are shopping for a lawn fertilizer, you want one that makes leaves grow, so it should contain a high percentage of nitrogen - 2 or 3 times as much as any other nutrient. Examples are 20-10-10 and 22-8-6. The same may be the case for vegetables in which leaves are the main crop, as spinach and lettuce.
For flower beds, you are after blooms rather than great amounts of foliage. So if you are buying a "complete" fertilizer to use on them, choose one in which phosphorus and potassium are equal to -or exceed- the nitrogen. Examples are 12-12-12 and 5-10-5. Secondary nutrients are needed for good plant growth, but in lesser proportion to the 3 listed above. Among these secondary ones are sulfur, calcium and magnesium. These are sufficient in most soils, and seldom are listed in ingredients in fertilizer, though they are there possibly as a result of manufacturing processes.
Finally, there is a group of minor elements required in only minute quantities. These are the so-called trace elements. Included are iron, zinc, copper, molybdenum, manganese and others. Lack of iron occasionally causes blanching (chlorosis) of lawn grasses where soils are very alkaline. Iron and other minor-element deficiency shows up more frequently in ornamental plants. Pin oaks often suffer iron chlorosis as do acid-loving azaleas and gardenias in alkaline soils.
Trace deficiency symptoms are difficult to diagnose exactly. Generally the first thing to do, if a minor-element deficiency is suspected, is to check the soil's pH. If it is either extremely acid or extremely alkaline, bring it toward the neutral condition or toward the condition preferred by the particular plant. With azaleas, for instance, you would increase acidity. Use methods described in the paragraphs on acid and alkaline soils earlier in this chapter. The trace elements may then become more "available" or soluble.
If the trace element is still lacking at a suitable pH, it may have to be added. To those soils that "tie up" or "fix" the trace elements, it may have to be added as the chelate, which is a chemical way of keeping it available; chelated compounds can be purchased at garden stores. Some "complete" fertilizers include trace elements; this is added insurance but perhaps unjustifiably costly since minor elements are usually adequate in most soils.