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The dominance of a particular grass, weed or shrub can indicate the soil fertility status. Example is the striga weed thrives in infertile soils. It is suppressed by application of organic or inorganic fertilizers.
Observing methods, however does not tell us which nutrients are in the soil and what quantities or proportions. Generally dark-color soils are assumed to be fertile and high in soil organic matter; and in most cases symptoms observed in a plant could be a result of nutrient deficiencies, disease or pest damage. It is therefore, important to start with close examination of the plant leaves, stem and roots to check for the insect or signs of diseases.
There are some general clues to nutrient deficiencies; visual observation assume that what is deficient in a crop is symptomatic of what lacking in the soil, and observation are made on the leaves, but sometimes on stems. A nutrient deficiency is suspected when the plant shows any of the following characteristics:
• Very poor initial crop
• Stunting in early growth.
• Restrict or abnormal root growth.
• Maturing too early or too late.
• Growth markedly different from crops growing close by poor quality products, appearance, taste and moisture content.
• Leaf symptoms that may be point to deficiencies of specific nutrients.
There are terms that can be used to describe nutrient deficiency conditions:
Chlorosis is the generalized yellowing of the leaves caused by direct or indirect effect of the deficiency on the process of photosynthesis. When photosynthesis and chlorophyll (the green pigment) are reduced, other colored compounds can then become dominated and more visible. The leaves turn white if there is drastic reduction of chlorophyll.
This is the death of the entire plant or parts of the plant that are affected by the deficiency. The affected tissues become brown and die.
• Inter-veinal Chlorosis.
Inter-veinal Chlorosis the leaves become yellow in the between, but the veins themselves remain green, in grasses forming stripes.
Firing means yellowing followed by rapid death of the lower leaves of the plant starting from the bottom like the effect of fire.
Nitrogen is an essential component of amino acids, and thus of all proteins, and is needed for all cell division and reproduction. Plants which are deficiency in Nitrogen are generally stunted and with very light green or yellowish (chlorotic) Chlorosis of the entire leaf blade are followed by necrosis and leaf drop. This yellowing usually appears first on the lower leaves as the nutrient in the lower leaves is trans-located to the new growing areas of the plant in case of insufficient supplies from the roots.
The lower leaves may drop off before the top most leaves have lost their intense green color. However, this should not be confused with yellowing due to lack of sun light needed for photosynthesis. Plant growth is reduced and/or stunted.
Phosphorus is needed for cell division and for reproduction; therefore deficiency of phosphorus causes slow growth. Phosphorus promotes root establishment and formation of flowers. Phosphorus deficiency the leave become purple or dull bronzed (especially on the underside of the lower leaves) petioles and stems. The older leaves are affected first because phosphorus tends to be trans-located form the older to new growth leaves when in short supply.
Not all crops show signs of stunting or delayed maturity, and fruits drop pre-maturely. Flowering and fruiting are limited and plants are slow to form new roots and establish. Seedlings may not develop post emergence, cotyledons may yellow, shrivel and fall off.
Potassium increases the plant vigor and disease resistant and producing strong, stiff stems. Potassium also promotes production of sugar, starches, and oils, increase size of the grain and fruits and improves the overall quality of the crop.
Potassium deficiency result in reduced vigor increased disease problems, as well as small fruit with thin weak skins. Deficiency leaves are usually curled and show deficiency symptoms first, and in severe cases the dead areas may fall out leaving ragged edges in maize and other gasses, firing starts at the tips of the leaves and proceeds down to the leaf stalks, midrib green.
Plants tend to bend and break over and lodge due to weak stems or leaf stalks and show general stunted growth. Seeds and fruits become small and shriveled.
Magnesium is a key element in chlorophyll and is therefore essential in the photosynthesis process. Deficiency symptoms appear on older leaves in the latter part of the growing season.
Magnesium is a constituent of chlorophyll, so deficiency results in the yellowing (Chlorosis) of the leaves, between the veins, and then the leaves turn reddish purple as deficiency become severe. Necrosis may develop between the veins or along the leaf margin or tips, curling or dropping of the leaves may also occur.
Calcium is a part of cell walls and regulates cell wall construction; also cell walls give plant cells their structural strength. Calcium is abundant in the soil, and it is usually difficult to notice a deficiency unless critical limits are attained. In calcium deficiency leaves may show spotted or nicrotic areas near the midrib.
In tomato fruit, deficiency is manifested as blossom end rot; calcium deficiency will also show up as weak stems as well as reduction of any new growth from the growing parts.
Sulphur occurs in certain amino acids and all proteins; it is a part of the flavor component in mustard and plants in the onion family. Sulphur deficiency symptoms are similar to those of nitrogen; however with diminishing supply of sulphur, small spindly and yellowish younger leaves stand in contrast to the normal older leaves.
In zinc deficiency plants, terminal growth is affected first, causing severe retardation of leaf and shoot growth and resulting in small leave, short internodes, and rosettes of the leaves or excessive side branches. L eaves may also become bronzed or develop rusty brown flecks and it is easily recognizable in maize wit in two weeks of emergence. The upper leaves show a broad band of yellowing on the either or both sides of the midrib. As deficiency become severe, leaf edges crinkle and reddish purple pigment is noticed.
Stunting and shortening of the internodes can also be noticed, small thin yellow leaves as well as yields are symptoms of zinc deficiency.
Manganese is crucial in plants enzyme systems, and for the synthesis of chlorophyll and assimilation of nitrate. Manganese deficiency is manifested by a mooted chlorotic appearance on the leaves (brownish, black or grey spots net to the veins) and growth is stunted. There may be patches with stunted growth and pale green to yellow foliage within an apparently healthy crop.
In the more acute stages of manganese deficiency light green parts may become white and nicrosis develop near the edges and the tips of the older leaves and extend to the base, causing leaf death. Twig growth is also reduced in most fruit crops.
Boron affects the absorption of other nutrients and is important in sugar transport within the plant, and it also affects the germination of the pollen tube once the pollen grains land on the stigma. Without this germination, the pollen is not available, seeds are not fertilized and fruits abort.
In boron deficiency, terminal growth shows resetting die back, discoloration and failure to elongate, leaves may curl, wrinkle, thicken or become brittle, petioles or stems maybe corky, cracked, or show water-soaked dead areas. Fleshy parts of fruits, tubers, or roots may show brown flecks, nicrosis, cracks and dry rot. Multiple buds may develop with a lack of boron, leave are small, and heart rot.
Molybdenum helps the plant use the nitrogen and it is also essential for nitrogen fixation by nitrogen fixing bacteria in legumes. Plants lacking sufficient Mo, become nitrogen deficiency and the leaves turn pale green and have rolled or cupped margins (leaves may fail to unroll) tubers may fail to develop and seeds or glumes of the grains do not fill out. The symptoms are easily noticed in cabbage/brassica group where leaves become cup shaped presenting rolled appearance. The plant then twists in latter stage.
Chlorine is present in plants in small quantities and takes part in the photosynthesis reactions in which oxygen is liberated, and it is seldom deficiency under natural conditions, Chlorosis then wilting, and death of the leaves are some of the signs of deficiency.