Nitrogen, phosphorus and potassium
Nitrogen is the main component of protein, nucleic acid and phospholipid, and also an important component of protoplasm, nucleus and biofilm, which plays a special role in life activities. Therefore, nitrogen is called the element of life. Nitrogen is also involved in the composition of enzymes, many coenzymes and auxiliary groups such as NAD+, NADP+ and FAD. Nitrogen is also a component of some plant hormones such as auxin, cytokinin and vitamins such as B 1, B2, B6 and PP, which play an important role in regulating life activities. In addition, nitrogen is a component of chlorophyll, which is closely related to photosynthesis. Because nitrogen has the above functions, the amount of nitrogen will directly affect the division and growth of cells. When the nitrogen fertilizer supply is sufficient, the plants are luxuriant in branches and leaves, tall in stature, strong in tillering (branching) ability and high in protein content in grains. In addition to carbon, hydrogen and oxygen, nitrogen is the most necessary element in plants, so special attention should be paid to the supply of nitrogen fertilizer in agricultural production. Commonly used fertilizers such as human excrement and urine, urea, ammonium nitrate, ammonium sulfate and ammonium bicarbonate mainly supply nitrogen nutrition.
When nitrogen is lacking, the synthesis of protein, nucleic acid, phospholipids and other substances is blocked, and the plants grow short, with few branches and tillers, small and thin leaves, few flowers and fruits, and are easy to fall off; Nitrogen deficiency will also affect the synthesis of chlorophyll, make the branches and leaves yellow, and the leaves will be premature or even dry, resulting in a decline in yield. Because of the great mobility of nitrogen in plants, the nitrogen in old leaves can be transported to young tissues for reuse after decomposition, so when nitrogen is deficient, the leaves turn yellow and gradually rise from the lower leaves, which is a remarkable feature of nitrogen deficiency symptoms.
When there is too much nitrogen, the leaves are large and dark green, soft and loose, and the plants are white and long. In addition, when there is too much nitrogen, the sugar content in the plant is relatively insufficient, and the mechanical tissue in the stem is underdeveloped, which is easy to cause lodging and pests.
(2) Phosphorus
Phosphorus is mainly absorbed by plants in the form of H2PO-4 or HPO2-4. The adsorption capacity of these two forms depends on the soil pH. When the pH is less than 7, H2PO-44 is the majority. When pH > 7, there is more H2PO-4. When phosphorus enters the root system or is transported to the branches and leaves through xylem, most of it is converted into glycophospholipids, nucleotides, nucleic acids, phospholipids and other organic substances. Some of them still exist in the form of inorganic phosphorus. The distribution of phosphorus in plants is uneven, with more growing points in roots and stems, more young leaves than old leaves, and richer fruits and seeds.
Phosphorus is the main component of nucleic acid, nucleoprotein and phospholipid, which is closely related to protein synthesis, cell division and cell growth. Phosphorus is a component of many coenzymes, such as NAD+ and NADP+, which participate in photosynthesis and respiration. Phosphorus is a component of AMP, ADP and ATP; Phosphorus is also involved in carbohydrate metabolism and transportation. For example, in the process of photosynthesis and respiration, the synthesis, transformation and degradation of sugar mostly react after phosphorylation; Phosphorus also plays an important role in nitrogen metabolism. For example, NAD+ and FAD participate in nitric acid reduction, while pyridoxal phosphate and pyridoxamine phosphate participate in amino acid transformation. Phosphorus is also related to fat transformation, and fat metabolism requires the participation of NADPH, ATP, CoA and NAD+.
Because phosphorus participates in many metabolic processes, its content is high in the meristem with the most vigorous life activities, and phosphorus application has a good effect on tillering, branching and root growth. Because phosphorus promotes the synthesis, transformation and transportation of carbohydrates, which is beneficial to the growth of seeds, tubers and tubers, the yield increase effect of potato, sweet potato and cereal crops after phosphorus application is obvious. Because of the close relationship between phosphorus and nitrogen, the role of phosphorus fertilizer can not be fully exerted when nitrogen is lacking. Only the combined application of nitrogen and phosphorus can give full play to the role of phosphate fertilizer. In a word, phosphorus plays a great role in the growth and development of plants, and it is the second most important element after nitrogen.
Phosphorus deficiency will affect cell division, reduce tillers, stop the growth of buds and young leaves, make stems and roots thinner, make plants shorter, make flowers and fruits fall off, and delay maturity; When phosphorus is deficient, the synthesis of protein is reduced, and the transportation of sugar is blocked, which makes the sugar content in vegetative organs relatively increase, which is beneficial to the formation of anthocyanins. Therefore, when phosphorus is deficient, the leaves appear abnormal dark green or purplish red, which is phosphorus deficiency.
Phosphorus moves easily in the body and can be reused. When phosphorus is deficient, most of the phosphorus in the old leaves can be transferred to the growing young leaves. Therefore, the symptoms of phosphorus deficiency first appeared in the lower old leaves and gradually developed upward.
When there is too much phosphate fertilizer, small focal spots will appear on the leaves, which is caused by calcium phosphate precipitation; Too much phosphorus will also hinder the absorption of silicon by plants and easily lead to rice diseases. Water-soluble phosphate can also combine with zinc in soil, reducing the effectiveness of zinc, so too much phosphorus can easily lead to zinc deficiency.
(3) Potassium
Potassium exists in soil in the form of KCl, K2SO4 and other salts, and dissociates into K+ in water, which is absorbed by roots. Potassium is in ionic state in plants. Potassium is mainly concentrated in the most vigorous parts of life, such as growing points, cambium and young leaves.
Potassium can be used as an activator of more than 60 kinds of enzymes in cells, such as pyruvate kinase, fructose kinase, malate dehydrogenase, succinate dehydrogenase, starch synthase, succinyl-CoA synthetase, glutathione synthetase and so on. Therefore, potassium plays an important role in carbohydrate metabolism, respiration and protein metabolism.
Potassium can promote the synthesis of protein. When potassium is sufficient, more protein will be formed, thus reducing soluble nitrogen. The distribution of potassium and protein in plants is consistent. For example, the content of potassium ion is also high at the growing point, cambium and other parts rich in protein. The content of potassium in seeds of leguminous plants rich in protein is higher than that of Gramineae plants.
Potassium is related to the synthesis of sugar. When barley and pea seedlings lack potassium, the synthesis of starch and sucrose is slow, which leads to the accumulation of monosaccharides. When potassium fertilizer is sufficient, the contents of sucrose, starch, cellulose and lignin are higher, but the accumulation of glucose is less. Potassium can also promote the transport of sugar to storage organs, so the storage organs rich in sugar (such as potato tubers, beetroot and starch seeds) have more potassium content. In addition, the phloem juice contains a high concentration of K+, accounting for about 80% of the total phloem cations. Therefore, it is speculated that K+ also has an effect on phloem transport.
K+ is an important component of cell osmotic potential. In roots, K+ is transported from parenchyma cells to the vessel, which reduces the water potential in the vessel and enables water to be transported from the root surface to xylem. K+ has a direct influence on the stomatal opening, as shown in Table 2-5. Potassium ion can enlarge procollagen, so applying potassium fertilizer can improve the drought resistance of crops.
When potassium is deficient, the stems of plants are weak, prone to lodging, drought and cold resistance are reduced, leaves lose water, protein and chlorophyll are destroyed, leaves turn yellow and die gradually. Potassium deficiency sometimes leads to scorching and slow growth of leaf margin. Because the middle part of the leaf is still growing rapidly, the whole leaf will form a cup-shaped bend or contraction. Potassium is also an element that is easy to move and reuse, so the lack of elements first appears in the lower old leaves.
Nitrogen, phosphorus and potassium are elements that plants need in large quantities and soil is easy to lack, so they are called "three elements of fertilizer". Fertilization in agriculture is mainly to meet the needs of plants for three elements.