Microelements are chemical elements necessary for the normal life of living organisms (plants, animals and humans) and used by them in micro amounts compared with other mineral nutrients. Despite this, their biological role is extremely large, since they are used in building the enzyme systems of organisms. Their shortage in the soil does not lead to plant death but can cause a decrease in the speed and completeness of biochemical processes and plant disease, which largely affects yield size and quality.
According to Rothamsted Research Station, the microelement composition of a crop is considered normal from the point of view of its food and feed value if it contains: iron and manganese 25 each, boron and zinc 10 each, copper 5, molybdenum 0.5, iodine 0.2, cobalt 0.1 mg per kilogram of dry matter. With the intensification of farming, when removal of microelements grows owing to a considerable rise in the yield of the crops grown, and where application of organic fertilizers and «raw» potassium fertilizers has practically ceased, the importance of using microelements grows
On the importance of microelements in mineral nutrition
Currently the role in mineral nutrition, methods of determining content in soil and plants, supply grades, fertilizers and methods of their application have been reliably established for the following elements: boron, iron, manganese, copper, molybdenum, zinc, cobalt.
As for other elements found in the catalogs of microfertilizer sellers, the question is ambiguous. How can one offer to apply elements whose role is not established, when no laboratory in Ukraine can determine their content in any object of the environment?
The importance of individual chemical elements (microelements) in mineral nutrition, signs of acute deficiency
Most microelements are components of enzymes that act as biological catalysts in biochemical reactions in the plant. However, their role in the plant is much broader. Microelements influence the formation of biocolloids and the direction of biochemical processes. In particular, manganese regulates the ratio of divalent and trivalent iron in the cell. The iron-manganese ratio should be greater than two. Copper protects chlorophyll from destruction and promotes better uptake of nitrogen and phosphorus. Boron and manganese strengthen photosynthesis after the resumption of vegetation of winter crops.
Crop needs for microelements
- Boron: rapeseed, sugar and table beet, cauliflower, Brussels sprouts and white cabbage, kohlrabi, celery, apple, cherry, plum, cereal crops;
- Copper: cereal crops, salad cabbage, sugar and table beet, carrot, spinach, green onion;
- Manganese: cereals, beans, peas, soybean, cucumbers, lettuce, radish, black radish, spinach and table beet;
- Molybdenum: soybean, legume crops, cereal crops, sugar beet, head lettuce, spinach, cauliflower;
- Zinc: corn, beans, vegetables, apple, pear, potato, grape.
Signs of boron shortage
Boron is necessary for meristem development, so the need for it exists throughout the whole growing season. Boron plays an important role in cell division and protein synthesis; it is a necessary component of cell walls. Under the influence of boron, the synthesis, movement and accumulation of carbohydrates in plants improve. With boron shortage sugars accumulate in the leaves and move poorly into the marketable part of the crop. Characteristic signs of boron shortage are: death of growing points, formation of voids in roots, disruption of plant vascular tissues, and disruption in the formation of reproductive organs. Research has established that monocots (all cereals) are less sensitive to boron shortage than dicots, and the highest boron content in the plant is in the flowers.
Boron promotes better calcium uptake in plant metabolism. Often it is boron shortage that inhibits calcium supply to plants even though its content in the soil is quite sufficient. It has also been established that on soils with a higher potassium content, boron accumulation in plants increases. Boron shortage leads not only to lower yield but also to lower quality. External signs of deficiency: growth stoppage, chlorosis of the tip, then complete death of the growing point, flower drop, empty flowers, ovary drop. Boron is an element that is poorly redistributed between old and young leaves, so it is desirable to maintain the optimal boron content in plants throughout the growing season by foliar feeding. The most sensitive crops: sugar beet, rapeseed, sunflower, vegetables, fruit, grape. Less sensitive: corn, grain legumes.
Signs of iron shortage
Iron. Among microelements iron has a special role. Its content in plants reaches hundredths of a percent, followed by manganese, the zinc concentration is thousandths of a percent, and copper does not exceed ten-thousandths of a percent. Iron takes an active part in redox processes in the plant that take place in respiration and photosynthesis. Iron takes direct part in chlorophyll biosynthesis, so one of the signs of iron shortage is chlorosis (paling of young leaves). With acute iron deficiency plants may die.
Grape and fruit crops are the most sensitive to iron shortage.
Signs of manganese shortage
Manganese activates numerous enzymes, especially those involved in photosynthesis and the synthesis of carbohydrates and proteins. The physiological role of manganese in plants is linked with its participation in redox reactions, photosynthesis, respiration, and carbohydrate and protein metabolism.
Signs of manganese deficiency in plants are most often observed on carbonate, over-limed, and also peat and other soils with pH above 6.5.
Manganese shortage resembles iron shortage, but besides chlorosis, gray, gray-green or brown spots appear on the lower part of the leaf. Signs of manganese starvation in dicots are the same as with iron shortage, only the green veins stand out less sharply on the yellowed leaves, and brown necrotic spots appear. Leaves die faster than with iron shortage. Manganese shortage sharpens at low temperatures and high humidity. The most sensitive crops are sugar, fodder and table beet, oats, potato, apple, sweet cherry.
Signs of copper shortage
Copper is part of many enzymes linked with redox processes in the plant.
By sensitivity to copper shortage, plants are arranged in the following order: wheat, barley, oats, flax, corn, carrot, beet, onion, spinach, alfalfa, cabbage. Moderately sensitive: potato, tomato, red clover, beans, soybean. Copper shortage often coincides with zinc shortage, and on sandy soils also with magnesium shortage. Copper plays a large role in photosynthesis.
Copper plays an important role in plant uptake of nitrogen, especially ammonium nitrogen. Copper shortage delays the synthesis of peptides, amino acids and proteins. Copper shows fungicidal properties, and therefore raises plant resistance against fungal and bacterial diseases, reduces cereal infection with smut, and raises plant resistance to brown spot.
Signs of copper shortage most often appear on peat and acidic sod-podzolic soils of light texture. Plant disease symptoms linked with copper shortage show as whitening and drying of the tips of leaf blades. With severe copper deficiency plants begin to tiller strongly but do not head. On fruit crops copper shortage shows as drying of branch tips (dieback), and on leaf blades of stone fruits interveinal chlorosis appears.
Signs of molybdenum shortage
Molybdenum in the plant is part of the complex of enzymes linked with the plant's nitrogen uptake and its transformations. The need for molybdenum for plant growth has been established. With molybdenum shortage nitrates accumulate in plant tissues as a result of inhibited conversion of this form of nitrogen into compounds of protein nature. Molybdenum takes part in carbohydrate metabolism in the plant, and in phosphorus uptake and its participation in the synthesis of vitamins and chlorophyll, and influences the intensity of redox reactions. Molybdenum is part of the enzymes nitrite and nitrate reductase, which are responsible for the transformation of nitrogen in the plant, so molybdenum is very important for legume crops, which are able to fix nitrogen from the atmospheric air with nodule bacteria. Molybdenum shortage leads to deep disturbances in plant metabolism. The most sensitive to molybdenum shortage are: grain legume crops, legume grasses and fruit crops. The need to feed plants with molybdenum usually arises on acidic soils where it is less available.
Visually molybdenum deficiency shows as a light green or even yellow-green colour of leaves, while the leaves themselves become narrower and their edges curl inward and die. Leaf petioles have a red-brown colour.
Signs of zinc shortage
By sensitivity to zinc supply, crops are divided into three groups.
- Very sensitive: corn, flax, hops, grape, fruit crops;
- Moderately sensitive: soybean, beans, faba beans, peas, sugar beet, sunflower, vegetables, berry crops;
- Slightly sensitive: oats, wheat, barley, rye.
The physiological role of zinc in plants is very important and varied. It is part of many enzymes responsible for plant life and health (carbonic anhydrases, ribulose phosphate dehydrogenases, peroxidases, oxidases, polyphenol oxidases). Zinc raises plant resistance to drought, heat and cold. With zinc shortage, phenolic compounds accumulate in different parts of plants, which harm the plant and reduce resistance to plant disease. The importance of zinc for plant nutrition lies in its participation in protein and carbohydrate metabolism in plants, in particular in the accumulation of proteins, sugars and starch in the crop.
Zinc deficiency in plants most often appears on sandy, carbonate and peat soils, and also with very high phosphorus content in the soil. Its shortage is reflected most strongly in seed formation and less strongly in the formation of vegetative organs of plants.
With zinc deficiency, chlorotic spots appear on plant leaves, which over time become pale green and then turn white. In fruit crops «rosetting» appears. With zinc starvation, fewer flower buds are laid down on fruit crops and yield drops sharply. Among field crops, zinc shortage most often shows in the early growth phases of corn as whitening of the leaf tip or of the whole plant. In legumes it shows as chlorosis on the leaves.
Signs of cobalt shortage
Cobalt is needed to strengthen the nitrogen-fixing activity of nodule bacteria. It is part of vitamin B12, which is contained in nodules on legume roots and has a positive effect on the activity of the enzyme hydrogenase and also strengthens the activity of nitrate reductase. Cobalt influences the accumulation of sugars and fats. Cobalt influences chlorophyll synthesis and stimulates processes in the Krebs cycle.