March 30, 2009
What a Gardener Should Know About Fertilizers
Methods of fertilizer application vary somewhat with soil character, moisture present and kind of crop grown.
by Myron S. Anderson 1Lasca Leaves 20:14-15, 1970FERTILIZER IS A TERM that is not easy to define in such a way as to include all the materials sometimes added to soil for the improvement of plant growth. In the general trade, fertilizers for soil improvement fall into three groups, primary, secondary and minor constituents. The primary group includes constituents carrying the chemical elements nitrogen, phosphorus and potassium. Nitrogen is stated on the bag as the element N and is added to promote green plant growth. Phosphorus, stated as the oxide (P205), aids the health of plants, improves growth of roots and to a moderate extent hastens crop maturity. Potassium, also stated as the oxide (K2O), helps the plant to make better use of sunlight and also improves root growth.The secondary constituents include compounds carrying the chemical elements calcium, magnesium and sulfur. Calcium and sulfur are frequently included as a part of the compounds carrying the primary constituents. Thus some of the nitrogen may be in the form of ammonium sulfate that contains a substantial percentage of sulfur. Furthermore, a chemical compound may also have calcium and sulfur in its composition.In some areas soils are deficient in one or more minor elements such as iron, boron, copper, zinc, manganese or occasionally a few others. This lack of adequacy of certain elements in the soil may sometimes be detected by characteristic abnormalities in the appearance of plant leaves. Lowered crop yield as measured by both quantity and quality often results. Misshaped apples due to boron deficiency is a well-known example of the latter. These minor elements are so-called because the quantities present and needed are normally very small. Such constituents as compounds of copper and zinc, for instance, are usually stated as a few parts per million in a fertilizer mixture rather than by percentage as is the case with primary constituents.A GARDENER SHOULD learn to recognize the chemical composition of a mixed fertilizer by the symbols on the bag or box. One of the very commonly used fertilizers of relatively low analysis is designated as 5-10-5. This means that five percent of the weight of the material in the bag is nitrogen. This nitrogen is present as a constituent of one or more chemical compounds in the mixture. The middle number, in this case 10, designates the percentage of the mixture that is phosphorus expressed as the oxide, P205. This is not, however, the chemical form in which the phosphorus actually exists. The third number, 5, represents the potassium of the mixture. The five percent is actually the amount of potassium stated as the oxide.In a 5-10-5 fertilizer, the total quantity of primary plant nutrients thus adds up to 20 percent. The remaining 80 percent is made up of several items. The plant nutrients are in chemical combination with secondary elements or with other materials of non-fertilizing nature. A high-grade fertilizer material such as ammonium nitrate contains about 40 percent of nitrogen available to plants when placed in the soil. The remainder is a chemical carrier, not plant nutrients. A fertilizer such as 5-10-5 also contains materials known as conditioners. These are added to improve the physical condition of the mixed goods, especially to restrain caking. Inert material of low cast, called filler, is added to adjust the mixture to the total percentage stated on the bag.Methods of fertilizer application vary somewhat with soil character, moisture present and kind of crop grown. In many places it is well to apply about 10 to 15 pounds of a mixed fertilizer such as 10-10-10 to one thousand square feet of area. This is plowed or spaded in and the soil properly conditioned for planting. When seeds are planted or small plants transplanted a small handful of perhaps 5-10-5 fertilizer per linear yard is placed in a shallow trench about four inches to one side of the plant row at a depth of four inches. When a second crop is grown without preliminary plowing the rate of application of fertilizer may be somewhat less than that used for the first planting.Fertilizers of different grades are usually carried in garden stores. For best results a gardener frequentiy uses fertilizers of two or more grades. The 5-10-5 material has long been on the market and is widely used for growing many kinds of flowers and vegetables. The 10-6-4 grade is often recommended for use on lawns, while root crops usually respond well to 5-10-10. The 10-10-10 grade serves well to build up garden soil productivity.CARE SHOULD BE TAKEN to use proper amounts of fertilizer for the area involved. The method of application should be suitable. Commercial fertilizers improperly used may burn plants and cause poor germination of seeds.Longevity of the usefulness of fertilizers added to soil depends upon the rate and quantity of water added by rainfall or by irrigation, upon the character of the soil, upon fertilizer constituents dominant in the mixture and upon the quantity of fertilizing materials absorbed by growing plants.In many areas fertilizers may be purchased that are supplied with minor element compounds in adequate yet safe quantities. In the case of tree-crops the appearance of young leaves sometimes give a clue to the likelihood of a deficiency of a certain element. Sometimes a minor element can be better supplied by sprays than in fertilizer applications to the soil. Absorption of a minor element through leaves is often adequate for a current season.The fertilizer needs vary widely from place to place. A gardener should get as much reliable information as practical regarding the need of various constituents for different crops in the area of his garden. Some branch of a state university is usually in position to inform a gardener as to areas where minor element deficiencies are likely to occur and where other specific deficiencies may be expected. Sometimes this agency is in connection with the agricultural extension service, especially in the office of the county agricultural agent. The minor elements of fertilizers in a local garden supply store may provide some hint as to the likelihood of local deficiencies. One should read the chemical analysis on the fertilizer bag and buy the grade of goods that best suits the local garden situation.Dr. Anderson is a retired soil scientist with 40 years’ experience, much of it with the U.S. Dept. of Agriculture, as researcher and author.