A knowledge of plant nutrients is essential to successful gardening. Of all the elements present in the soil, only about 15 are needed for plant growth: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, manganese, iron, sulfur, boron, zinc, copper, and molybdenum.
Carbon, hydrogen, and oxygen are involved in photosynthesis. Carbon dioxide is a gas present in the atmosphere. It is used in photosynthesis and restored in the atmosphere by the oxidation of carbon-containing compounds, respiration, and the burning of fos sil fuels. Water contains both hydrogen and oxygen. Except in dry seasons, the quantity of water in the soil is normally sufficient for plant growth. Irrigation makes it possible to supply adequate amounts of water.
Substantial quantities of nitrogen, phosphorus, and potassium are used by plants. Soils are often deficient in these elements, which are usually added in the form of commercial fertilizers. It is important that the gardener understand the function of each of these major elements.
Nitrogen is a constituent of amino acids and proteins as well as of the chlorophyll molecule. Adequate levels of nitrogen ensure normal vegetative growth and a healthy green color. A deficiency of nitrogen results in stunted plants of yellowish green color. An excess causes rank, vegetative growth.
To maintain the proper nitrogen level in the soil, one should understand how nitrogen enters the soil. Approximately 80 percent of the air we breathe is gaseous nitrogen. This nitrogen is inert and cannot be used by higher plants. During electrical storms lightning fixes this gaseous nitrogen, and as much as 5 to 6 pounds of nitrogen per acre can fall in the accompanying rains. Bacterial nodules on the roots of certain legumes also fix the nitrogen in the soil air and convert it to protein in the cells of the bacteria. Certain free-living bacteria in the soil fix nitrogen, too. Called nitrogen-fixing bacteria, these function best in warm, well-aerated soils. Organic matter in the soil also releases available nitrogen as it decomposes. In the protein molecule, nitrogen is in the amine (NH2) form. This NH2 is further reduced in the soil to ammonia, NH3. Most higher plants cannot utilize the ammonia, which must be oxidized to the nitrate (N03) form before the nitrogen becomes available. Soil bacteria are involved in this conversion of proteinaceous nitrogen to the nitrate form. In a cold, wet spring, denitrifying bacteria can reverse this process and convert nitrates to ammonia. The amount of available nitrogen in the soil is very variable. Under natural conditions where no crops are removed from the soil, the nitrate level remains more or less constant. Where crops are removed, as in gardening, fertilizer is needed.
Phosphorus, a component of nucleic acid, is considered essential for flowering and fruiting. In soils it is usually present in a stable form as salts of calcium, magnesium, iron, and aluminum. At low and high pH's the phosphorus is tied up as insoluble salts. In the pH range from 5 to 7, which includes the pH of most garden soils, some phosphorus is soluble and available to plants. In such soils the available phosphorus is in the acid phosphate (HPO4) form.
Potassium serves a catalytic function in respiration and in carbohydrate metabolism. It is essential for healthy development of roots and stems. Levels of potassium are low in organic soils and fairly high in mineral soils. Potassium is present as soluble salts like potassium chloride. Continuous cropping necessitates adding phosphorus and potassium in the form of fertilizers.
The other elements are usually grouped as minor, or trace, elements. They are used in small quantities and are normally present in sufficient amounts to ensure normal plant growth, although deficiencies do occur occasionally. Some fertilizers contain these trace elements.
Calcium is a cementing substance that holds cells together. Most soils contain sufficient calcium to meet the requirements of plants grown, but sometimes on acid soils lime must be added to decrease the acidity.
Magnesium, manganese, and iron are all associated with chlorophyll. Magnesium is a constituent of the chlorophyll molecule, and both manganese and iron are catalysts in chlorophyll synthesis. Iron is usually present in adequate quantities, but in high lime soils it forms insoluble salts and is not available to plants. A chlorotic condition (yellowing of the leaves) known as iron chlorosis can develop in some plants grown on such soils. Clematis, planted near a house foundation, occasionally shows symptoms of iron chlorosis, because lime leaching from the mortar of the house foundation binds the available iron in an insoluble form.
The functions of the other essential elements are not too clearly understood. They are known to be essential for normal cell division and differentiation.
The use of commercial fertilizers in both farming and gardening has increased steadily. In the past these fertilizers have been rela tively cheap and have been the most economical means of maintaining soil fertility and productivity. When properly used, they give excellent results.
The analysis of the fertilizer is indicated on the bag. The percentage of nitrogen (N), phosphorus (P2O5), and potassium (K20) must be shown. A 10-10-10 fertilizer indicates that the fertilizer contains 10 percent each of N, P2 0s, and K20. The higher the numbers, the more concentrated the fertilizer. The selection of a fertilizer should be based on the crop to be grown. If you desire vegetative growth, a fertilizer high in nitrogen should be used. If you are growing a crop like tomatoes where fruits are the main concern, a fertilizer high in phosphorus is best. For potatoes, which store a great deal of starch in their tubers, a fertilizer high in potassium should be used.
In the vegetable garden, where one grows a variety of crops in a relatively small space, it would be impractical to use a different analysis of fertilizer on each crop. In my garden I apply a 10-10-10 fertilizer over the entire garden when the soil is being tilled in early spring. By rotating the crops within the garden, the needs of each kind of vegetable are supplied. In a flower border, where bloom is the primary concern, or in a vegetable garden that has received liberal quantities of decomposed organic matter, a 5-10-10 fertilizer may be best. Lawn fertilizers are generally high in nitrogen.
More important than the analysis is the rate of application. A safe rule is to apply the fertilizer at a rate that will supply a pound of actual nitrogen per 1,000 square feet of soil surface. For a 10-10-10 analysis, one would use 10 pounds; for a 5-10-10 analysis, 20 pounds; and for ammonium nitrate (33-0-0), only 3 pounds per 1,000 square feet. It is the nitrogen in the fertilizer that "burns": too much nitrogen in the soil water causes reverse osmosis and the plant loses water, resulting in a "burned" appearance. By controlling the quantity used, the danger of "burning" is virtually eliminated.
A whirlwind type of spreader applies the fertilizer quickly and uniformly. With a little practice the proper setting can be determined in order to apply the right amount of fertilizer.
A soil test, which can be made by any soil-testing laboratory, gives a clear picture of the fertility level of the soil and serves as a guide for determining the analysis and quantity of fertilizer needed.
Some gardeners express much concern about the relative merits of so-called organic and inorganic fertilizers. Generally speaking, fertilizers derived from natural products like bone meal, blood meal, fish emulsion, soybean meal, rock phosphate, etc. are considered organic. Manufactured fertilizers like ammonium nitrate, super phosphate, potassium chloride, and blends of such ingredients are called inorganic. Fortunately, plants are less concerned about the source of the nutrients than gardeners are. For most plants, nitrogen is nitrogen so long as it is in the nitrate (NO3) form. In the soil most nitrogen regardless of source is converted to nitrate nitrogen, although some plants can use nitrogen in the ammonia form. Phosphorus enters the plant only in the acid phosphate form. The phosphorus in super phosphate is immediately available to plants. The phosphorus in rock phosphate must be acted on by soil acids before becoming available.
The chief advantage of organic sources of nitrogen is their longer period of availability. This may be an advantage or a disadvantage, depending on the season of the year. In early spring, when the soil is cold and wet, plants can suffer from a lack of nitrate nitrogen even with ample reserves of organic nitrogen present, because nitrifying bacteria are inactive. In the fall, organic sources of nitrogen keep trees and shrubs growing vegetatively and thus interfere with normal hardening processes that prepare the plants for winter.
The idea that chemical fertilizers poison the soil and kill earthworms and beneficial soil organisms is not well founded. If such fertilizers are properly used, with liberal applications of organic matter, the number of earthworms and beneficial organisms should actually increase. Earthworms are abundant in my garden, where I have used chemical fertilizers for years.
The purpose of mulching varies. A summer mulch is used primarily to control weeds and to keep the soil moist and cool. A
winter mulch is used to protect plants from extreme cold and to avoid sudden changes in temperature. Mulches are also used to delay the start of growth in the spring, to lengthen the growing season, to keep fruits clean, and to reduce leaf diseases of roses.
Many materials are used for mulches. Plastic and paper mulches have the advantage of being relatively inexpensive and easy to apply. Clear plastic helps warm the soil in the spring and get plants off to an earlier start. Special weed control treatments are needed where clear plastic mulches are used, or weeds will grow and thrive under the plastic and push it up. Black plastic excludes light and is effective in controlling weeds. Plastic mulches are used in northern parts of the region to lengthen the growing season for a wide variety of vegetables like sweet corn, melons, and tomatoes. Plastic is commonly used for everbearing strawberries to keep the berries clean.
Organic mulches are usually applied in early summer after the soil has warmed up. These organic mulches hold moisture and prevent high soil temperatures. They also help control annual weeds by excluding light needed for germination and reduce the spread of diseases carried by splashing rain. An organic mulch should be relatively cheap, readily available, and attractive looking. Many materials are used. Compost made from tree leaves is ideal. Chopped hay and straw are also good. Shredded tree bark is attractive and usually available from garden centers. Sphagnum peat is good looking but may be blown by the wind and once it dries out it is difficult to moisten. Coffee and tea grounds, peanut shells, and coconut fiber are also occasionally used. The advantage of organic mulches over plastic mulches is that they can be worked into the soil at the end of the growing season to improve the physical structure of the soil.
For a winter mulch, clean straw and marsh hay are commonly employed. Insulating materials in roll form can also be used, but these are rather expensive.
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