Fertilizer facts and folklore.
Despite this array, choosing the right fertilizer can be simple if you know what fertilizer ingredients can do for plants. On these six pages, we explain why plants need fertilizer, why there are so many products, how to select the right one, and how to use it.
To find out how various fertilizers affect plant growth, bloom, and fruiting, Sunset asked researchers at California Polytechnic State University at San Luis Obispo to test various fertilizer formulas; results are illustrated in the four big photographs above. The tests revealed several fertilizer beliefs to be myths. We also interviewed fertilizer manufacturers, soil scientists, and horticultural authorities to separate fertilizer truth from folklore. Nitrogen, phosporus, and potassium: the primary nutrients
You may have read or heard that nitrogen in a fertilizer produces green growth, that phosphorus generates flowers, and that potassium promotes overall plant health. This oversimplification is misleading. Plants need all three of these primary nutrients to produce green growth, good bloom, and good health. Without any of these three elements (and 10 others in smaller amounts), a plant won't grow and eventually will die.
But you don't need to fertilize with equal amounts of all three primary nutrients. The crucial element is nitrogen, referred to by its chemical symbol N. Plants need a lot of it, and nitrogen from some fertilizers gets quickly moved by water through the soil out of roots' reach. So, one way or another, nitrogen needs to be replenished frequently.
Plants use nitrogen to form proteins, chlorophyll, and enzymes needed for plant cells to live and reproduce; nitrogen combines with other nutrients to let them work. Without adequate nitrogen, leaves yellow from tip toward stem, the plant yellows from bottom up, and growth is stunted.
To learn how much nitrogen is needed by various plants and when to apply it, consult the chart on page 122.
Of the other two primary nutrients, phosphorus (P) is used by plants to form nucleic acids to assist in producing early growth, roots, and seeds. Starved of phosphorus, a plant will be stunted, leaf tips will brown, some leaves will turn purplish, and seeds won't develop properly. Potassium (K) helps move sugars and starches throughout the plant and is necessary for the plant to grow roots, resist diseases, and produce fruit. Without potassium, leaf edges will brown, plants will grow slowly, and fruit will stay small.
Most fertilizers contain nitrogen along with varying quantities of phosphorus (listed on the label as phosphoric acid) and potassium (listed as potash). A fertilizer containing all three primary nutrients is called a complete fertilizer. By law, the label must tell how much of each element is present--in a series of three numbers known as the NPK formula; for details on how to interpret the label, see page 123.
Conventional garden wisdom, including advice that has appeared in Sunset, is usually to apply a complete fertilizer. As you'll read elsewhere in this report, nitrogen is the only one of the three primary nutrients that moves down through the West's alkaline soils. The other two do not move down through soil and therefore require special handling.
Phosphorus, when spread on the surface as fertilizer and watered, binds chemically to the mineral particles in the top inch or two of soil; it may never reach the roots. For phosphorus to have any effect, it must reach the root zone: dig it in, apply in a deep trench, or use stakes or tablets dropped in holes. Phosphorus in alkaline soil is less available to plants; Western soils are alkaline except in areas where rainfall exceeds some 20 inches annually (mostly west of the Cascades and in some parts of the Rockies).
Fertilizing in containers is a different matter. Most gardeners use a soilless mix for container plants. When fresh, such as a mix usually includes a small amount of fertilizer, but it gets used up in a month or less. In containers, it's important to use a complete fertilizer that includes minute amounts of the 10 additional elements or micronutrients plants need. A controlled-release fertilizer that lasts for several months (see box, page 123) is excellent for use in containers. How complicated is fertilizer shopping?
Of the 169 fertilizers we found, those labeled for tomatoes are a good example of the whole fertilizer picture. Each of the 20 tomato foods (including vegetable foods that mention or picture tomatoes on the label) has a different NPK formula.
There are high-nitrogen ones (23-19-17, 23-15-18, 12-6-6, and 8-5-5); high-phosphorus ones (7-40-6, 7-28-14, 18-24-6, 15-20-15, 6-12-6, and 5-10-4); high-potassium ones (18-18-21 and 9-4-17); some that are high in both phosphorus and potassium (10-20-20, 5-20-20, 5-10-10, 0-10-10, 4-8-6, and 3-8-7); and two that contain equal amounts of all three nutrients (14-14-14 and 15-15-15).
How can tomato fertilizers be so different? Fertilizer manufacturers say four factors determine what's sold as fertilizer: 1. Availability of raw materials and their cost. 2. The belief of many companies that any product should be different from all others. 3. Perceived consumer demand for convenience--fast label reading, fast shopping. 4. Plant nutrition needs.
Sunset's Cal Poly study indicated that many plants will tolerate a wide range of nutrients in varying amounts and still provide excellent growth, bloom, and fruit yield. In the tomato tests, fruit quality was virtually identical no matter which type of fertilizer was used. Two ways to go: specialty foods or "universal" fertilizers
Fertilizing with a series of specialty foods--for vegetables, rhododendrons, roses--is a workable routine, if cost and storage space are not considerations. If you have been doing this and your plants perform well, you may want to continue. It's important to remember that plants can't read labels. What you're after is the proper NPK, and hang what the labels say. It's perfectly all right to feed roses with "vegetable food" or indoor plants with "rose fertilizer"; any can be used as an all-purpose fertilizer. Compare prices: a ready-to-use product costing $1.40 a pound may compare to a fertilizer you mix yourself for 45 cents or less a pound. You can buy or make two "universal fertilizers to use on most plants:
In containers. In a soilless mix, use any fertilizer with an NPK ratio approximating 3-1-2--no matter what the lbel says it's supposed to be used for--such as 18-6-12 all-purpose food or 21-7-14 lawn food. You can combine products to achieve the ratio: for every 5 pounds of 21-8-8 indoor plant food, use also 1/2 pound of muriate of potash (0-0-60) to get a combination that approximates the 3-1-2 ratio. If your container mix contains real soil, follow directions for in-the-ground fertilizing below.
In the ground. A nitrogen-only fertilizer is effective and economical, since plants don't use up the other necessary plant nutrients until a year or more has elapsed. Examples of nitrogen-only fertilizers are 21-0-0 ammonium sulfate and 45-0-0 urea. (You may have to shop at a large garden center or feritlizer supply house to get these single-element products, known as simple fertilizers.)
If you rely on nitrogen-only feedings some or much of the time, there are several ways you can get the phosphorus and potash to the roots when you do apply them. One is to dig in lots of compost, manure, or other organic matter once a year--such material contains phosphorus and potash. Or apply fertilizers containing both nutrients in a trench or moat, as plant stakes, or with a root-feeding device. Still another way to get phosphorus and potash down where the roots are is to use an acid-producing complete fertilizer. It counteracts the tendency of alkaline soil to lessen the availability of the phosphorus. Acid fertilizers contain the statement on the label: Potential acidity equivalent to (number) pounds of calcium carbonate per ton. The higher the number, th more acid is produced.
Every year or two, add a micronutrient fertilizer to provide traces of the 10 other elements plants need. Using the chart at left
If you use a universal fertilizer, the label might not spell out how much to apply to all the plants you plan to use it on. The chart on the facing page tells how much fertilizer to use depending on the nitrogen source it contains. Six pounds of urea (45-0-0), for instance, will supply the same amount of nitrogen as 90 pounds of composted chicken manure (3-2-1). (Large amounts of manure can contain enough salts to be harmful to plants.)
Always water well after application. Don't let fertilizer land on leaves and stay there (except when foliar-feeding); if it does, rinse it off immediately.
Instead of following the directions in the when-and-how-to-apply column, you can use a single application of controlled-release fertilizer (nine-month or longer term) early in the growing season.
If the source of nitrogen in your fertilizer is ammonium nitrate, ammonium sulfate, or urea, it's safest not to exceed the recommended amounts or you could overdose the plant. The symptoms to watch for: burned-looking edges on older leaves or lawn blades that look greasy, then turn brown.
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|Date:||Apr 1, 1984|
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