Plants need at least 15 different chemical elements for their growth processes. They require large quantities of carbon, hydrogen and oxygen, which they obtain from air and water. Nitrogen, potassium, phosphorus, calcium, magnesium and sulfur also are used in considerable quantity by the plants. These elements may not be present in sufficient supply or in the proper form. Whenever any element is in short supply for plant growth, it must be furnished by some type of fertilization for economical crop production.
In addition to the elements mentioned above, there is a group of elements, iron, copper, manganese, zinc, boron and molybdenum, that are all essential for normal plant growth but are needed only in small quantities. In some instances the nature of the soil limits their availability to the plant.
Soil reaction (pH) plays a vital part in fertilizer efficiency. It refers to the degree of acidity (sourness) or alkalinity (sweetness) of a soil. To have a simple numerical measure, the pH scale has been adopted. At pH 7.0 a soil is neither acid nor alkaline, but neutral. As values decrease from pH 7.0, soil acidity increases. As values rise above 7.0, soil alkalinity increases. Most crops grow best in the range pH 5.5 to 6.5. In some instances a low pH may be required for disease control.
Fertilizers differ in composition, depending upon how much of each of the different plant foods they contain and the source of these nutrients. A 100-pound bag of 3-8-8 analysis fertilizer contains 3 pounds of nitrogen, 8 pounds of available phosphoric acid and 8 pounds of water-soluble potash. The first figure always indicates nitrogen, the second phosphoric acid and the third potash.
In addition to the primary plant foods there are secondary plant foods. These are reported in a similar manner at the bottom of the fertilizer tag if they are guaranteed present. The commonly recognized secondaries include calcium, magnesium, copper, manganese, zinc, boron, iron, sulfur and molybdenum.
The general nutrient deficiency symptoms for the elements listed are:
Nitrogen deficiency: Severe dwarfing of the plant and a uniform yellowing of all the leaves. Yellowing starts on old growth and spreads to include the young leaves.
Phosphorus deficiency: Severe dwarfing of the plant and foliage an unusually dark green colour. There is a marginal leaf yellowing followed by dropping of foliage.
Potassium deficiency: A mottling of foliage and a marginal browning and dying of lower leaves.
Magnesium deficiency: The earliest symptom is a greatly reduced rate of growth. A chlorosis appears on the lower part of the plant. Yellowing is between the veins and veins remain normal green. Petioles are short, and the entire plant becomes severely stunted.
Calcium deficiency: Feeder roots die within 2 to 4 weeks. Death of the terminal bud follows. Plant is severely stunted and eventually dies.
Iron deficiency: The first symptom is a chlorosis between the veins of the top leaves of the plant. On some plants chlorosis becomes so severe that necrotic areas appear on the leaf, more generally on the margins and tip of the leaf and larger than in manganese deficiency.
Manganese deficiency: The top leaves become chlorotic between the veins. Manganese deficiency is not usually as severe as iron deficiency; the necrotic areas are smaller in size and are located in the middle of the leaf, and even the most minute veins remain green, and the leaf has a very netted appearance.
Sulphur deficiency: The veins of sulphur-deficient leaves are usually lighter than the rest of the leaves and the top leaves of the plant are affected first. Plants have a much slower growth rate.
Boron deficiency: Death of the terminal bud is characteristic. This causes the development of the lateral buds. The leaves on the top of the plant become thick and brittle and tend to roll in a half-circle from the tip toward the base.
Zinc deficiency shows as dwarfed growth (short internodes) and little-leaf, often giving a rosette effect.
Even under the most improved practices of soil management inevitably losses of plant food occur by fixation, leaching or erosion.
Fixation. Minerals applied to the soil in a soluble form sometimes are changed into insoluble or unavailable forms by chemical action in the soil. Factors promoting this fixation, particularly of phosphorus, are wetting and drying; low organic content of the soil; acid soils (pH 5.0 or lower); highly alkaline soils (above pH 8.0); and soils with high clay content. Fixation can be reduced by turning under manure, crop refuse or residues, or green crops: liming acid soils; drilling in rather than broadcasting fertilizer; using granular forms of fertilizer.
Leaching. Losses of nutrients by leaching vary with the soil and with the weather. Since organic matter acts as a storehouse of soil fertility, leaching losses can be diminished by maintaining a good supply of humus in the soil.
Erosion. The most fertile part of a soil is the topsoil, and this is the part which goes first in erosion. On bare, unprotected soil, heavy rains run away over the surface and carry with it some of the finer top-soil and a great amount of the soil nutrients.
(Information for the above was taken from Agricultural Extension Service Bulletin 177B, and the Queensland Agricultural Journal.)
DATE: September 1983
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