Basics of mineral nutrition and chemical composition of vegetable crops

The influence of nitrogen, phosphorus and potassium on crop development

Nitrogen, phosphorus and potassium are essential nutrients that determine the development and yield of vegetable crops. Nitrogen is necessary for rapid green mass accumulation, which is critical for leafy vegetables. Phosphorus stimulates the development of root systems during the early stages of the growing season and accelerates the transition of plants to flowering and fruiting. Potassium regulates cell conditions, ensuring plant hardiness, cold resistance and resistance to a number of common diseases.

Nitrogen is the basis for the synthesis of proteins, nucleic acids, nucleoproteins, chlorophyll, alkaloids, phosphatides and vitamins. It directly controls metabolism, is part of enzymes and ensures photosynthesis. Sufficient nitrogen nutrition in grain legumes significantly increases protein content. With a deficiency of this element, plants show a sharp slowdown in growth, and both yield and produce quality drop.

Phosphorus stimulates rapid growth of the root system during the early life stages of plants and accelerates their maturation. Under its influence, protein breakdown processes in leaves and the transfer of breakdown products to reproductive organs and grain are accelerated. Phosphorus nutrition also improves the water regime by increasing the hydration of cell protoplasm, which reduces moisture consumption and helps crops withstand drought. In the autumn, this element promotes the accumulation of sugars, ensuring the successful overwintering of winter and perennial crops. In the marketable part of the produce, it increases the sugar content in fruits, vegetables and root crops, as well as starch in potatoes.

With phosphorus deficiency, plant growth is suppressed, internodes are shortened, and leaves acquire an atypical purple color. This leads to a sharp decrease in yield. Timely monitoring of starvation symptoms helps the agronomist to quickly adjust the top dressing scheme in the field.

Plant chemical composition and nutrient element balance

For full development, plants require a whole complex of chemical substances. More than 70 elements are found in plant organisms, but only a few pass the strict test for essentiality. Without the necessary elements, a plant is physically incapable of completing its life cycle. Conditionally essential elements are not indispensable, but they have a proven positive effect on crop growth.

• Elements in plants — more than 70
• Essential elements — 20
• Conditionally essential elements — 12

The group of essential elements includes hydrogen, sodium, potassium, copper, magnesium, calcium, zinc, boron, carbon, nitrogen, phosphorus, oxygen, sulfur, molybdenum, chlorine, iodine, manganese, iron and cobalt (H, Na, K, Cu, Mg, Ca, Zn, B, C, N, P, O, S, Mo, Cl, I, Mn, Fe, Co). They cannot be replaced by other chemical compounds in the plant life cycle. Conditionally essential elements — lithium, silver, strontium, cadmium, fluorine, selenium, aluminum (Li, Ag, Sr, Cd, F, Se, Al) and others — improve general physiological indicators.

The requirement of crops for nutrients is not uniform. Macronutrients such as nitrogen, phosphorus, potassium and calcium are contained in plants in amounts ranging from hundredths of a percent to whole percentages. Micronutrients — molybdenum, manganese, copper, zinc, boron, cobalt and others — are required in microdoses ranging from thousandths to hundred-thousandths of a percent.

Knowing the precise chemical composition of plants allows for understanding the peculiarities of yield formation and assessing nutritional conditions. Below are the indicators of mineral element content in the ash of the marketable part of major vegetable, fruit, berry crops and potatoes. These data help to more accurately calculate nutrient uptake and adjust top dressing for the target yield.

Average content of mineral elements in the ash of the marketable part of produce, % of ash mass

Crop	K₂O	P₂O₅	CaO	MgO	Na₂O	Fe₂O₃	Cl
White cabbage	42	10	16	5	4	1,0	10
Cucumber	50	12	7	5	4	1	10
Tomato	45	10	6	6	17	2	7
Table beet	40	15	10	8	10	—	—
Onion	35	10	22	4	5	2	3
Potato	60	16	3	5	2	—	—
Apple	52	10	4	4	4	1,2	—
Pear	52	12	5	4	4	1,0	—
Sweet cherry	48	11	4	4	4	0,4	—
Plum	55	3	4	3	3	0,6	—
Peach	54	14	3	3	4	0,8	—
Apricot	55	9	3	3	2	1,6	—
Strawberry	34	14	18	9	2	1,5	—
Blackcurrant	44	19	9	4	5	0,7	—
Raspberry	38	20	11	8	3	0,5	—

Potassium is one of the key elements determining the quality and marketability of the harvest. In field conditions, its sufficiency directly affects the drought resistance and winter hardiness of plants, and also strengthens stems, reducing the risk of lodging. It is important for the agronomist to monitor the potassium balance to ensure proper crop development at all stages of the growing season.

This element performs many important physiological tasks in a plant. It regulates key life processes on which final productivity depends. The effect of potassium on a crop is manifested at several levels simultaneously:

• promotes the normal course of photosynthesis and redox processes;
• stimulates the synthesis and accumulation of vitamins and enzymes;
• increases plant resistance to drought, low temperatures, and infectious diseases;
• increases the sugar content in fruits and starch in potato tubers;
• strengthens stems, reducing the risk of crop lodging;
• participates in the construction of complex organic compounds and is a constituent part of many proteins.

Chemical composition of vegetables, potatoes, and fruits

For accurate calculation of fertilizer application rates, it is necessary to know the initial chemical composition of the crops being grown. The ratio of water, nitrogen, and carbohydrates in tissues determines not only the nutritional value but also the plants' requirements for nutritional conditions. These indicators vary significantly depending on the specific type of produce.

• Total nitrogen content in garlic — 7.0%
• Maximum sugar content (grapes) — 19.5%
• Highest tissue moisture (cucumber) — 95%
• Share of other carbohydrates in potatoes — 17.7%

Crop	Water, %	Total Nitrogen, %	Sugars, %	Organic acids, %	Pectic substances, %	Fiber, %	Other carbohydrates, %
Cabbage: white	91	1.8	4.0	0.3	0.3	0.8	2.0
Cabbage: cauliflower	90	2.5	3.0	0.1	0.35	1.2	2.0
Cucumber	95	0.8	1.5	0.005	-	0.5	1.0
Tomato	94	0.6	3.0	0.5	0.1	0.2	1.0
Carrot	88	0.9	6.6	0.1	0.4	1.0	-
Sweet pepper	88	1.5	6.0	0.2	-	1.0	2.0
Eggplant	93	0.9	3.0	0.2	-	1.0	1.5
Onion	85	1.6	9.0	0.2	-	0.6	2.0
Garlic	61	7.0	5.0	0.2	0.3	1.0	25.0
Potato	75	1.25	0.9	0.2	0.7	1.0	17.7
Apple	88	0.07	10.5	0.7	1.0	1.0	-
Pear	88	0.07	10.8	0.2	1.0	0.8	-
Cherry	88	0.15	8.7	1.8	0.3	0.5	-
Strawberry	78	0.18	16.5	1.4	1.0	1.2	-
Gooseberry	77	0.14	6.0	2.0	1.1	2.3	-
Blackcurrant	85	0.20	7.0	2.5	1.5	2.0	-
Grape	79	0.12	19.5	0.7	0.8	0.5	-
Orange	88	0.15	6.4	1.4	0.8 (in peel up to 4.5)	2.0	-
Lemon	88	0.15	2.5	5.8	1.0 (up to 7)	2.0	-

The vitamin content in the marketable part of vegetable and fruit crops determines their nutritional value and directly depends on cultivation conditions. The table shows the average vitamin composition indicators for the main cultivated crops:

Crop	β-carotene, mg	C (ascorbic acid), mg	B9 (folic acid), mg	B1 (thiamine), mcg	B2 (riboflavin), mcg	PP (niacin), mg	B6 (pyridoxine), mg	B5 (pantothenic acid), mg
Cabbage white	2.0	30	1.1	0.1	0.07	0.7	0.1	0.40
Cucumber	2.0	5	0.7	0.04	0	0.2	0	0.20
Tomato	4.0	30	1.0	0.1	0	0.5	0.1	0.53
Carrot	10.0	5	1.5	0.1	0.04	1.0	0.1	1.0
Sweet pepper	10	200	0.046	0.05	0.08	0.98	0.29	0.32
Lettuce	3	—	1.2	0.03	0.08	—	0.18	0.65
Parsley (leaf)	10	150	5.2	0.04	0.05	—	0.18	0.70
Potato	0.1	20	2.0	0.1	0.05	1.05	0.1	0.90
Apple	1.0	18	1.4	0.08	13	0.05–0.5	0.1	0.03–0.2
Pear	0.1	5	8.0	0.05	10–100	0.16	0.17	0.025
Plum	0.05–0.6	0–10	—	0.05–0.2	40–90	0.35	—	0.13
Cherry	0.50–1.1	15	18.0	0.06	16–65	0.35	—	0.12
Strawberry	0.03–0.45	70	25.0	0.035	20–80	0.50	—	0.07–0.23
Blackcurrant	0.01–0.24	200	1.4	0.05	40	0.30	0.17	0.40
Redcurrant	0.01–0.15	180	0.003	0.05	20	0.25	0.1	0.60
Grape	0.18	3	1.0	0	0	0.18	0.08	0.05

• Share of magnesium in organomineral compounds — about 50 %
• Iron content in tissues — hundredths and thousandths of a %
• Maximum level of vitamin C in produce — 200 mg

Physiological role of meso- and microelements

Sulfur is a structural component of vegetable oils, and is also contained in such important-for-life amino acids and compounds as thiamine and biotin.

Calcium is critically important for the development and fruiting of crops. In young plants, it is concentrated in the cell protoplasm, and as they age, it moves into the cell sap, where it is deposited in the form of water-insoluble salts of oxalic, sulfuric, carbonic, and phosphoric acids.

In a plant organism, calcium performs three key functions:

• participates in building cell walls: by binding with pectic substances, it forms the middle lamella, which glues the walls of adjacent cells together;
• maintains the structure of protoplasm, keeping its colloids in a gel state (coagulated form) and regulating viscosity;
• neutralizes excess organic acids.

Magnesium is contained in plants in three forms: as part of organomineral compounds, chlorophyll, and as free cations or mineral salts in the cell sap.

The physiological significance of magnesium is as follows:

• it is part of chlorophyll and directly ensures photosynthesis: with magnesium deficiency, this process slows down, which blocks the synthesis of carbohydrates, proteins, and fats;
• activates enzymes responsible for the transfer of phosphoric acid from ATP to molecules of sugars, amino acids, and other compounds to form ADP.

Iron accumulates primarily in the non-marketable part of the harvest — the roots. It is part of respiratory enzymes and redox metabolism, and also participates in the synthesis of chlorophyll, although it is not directly included in its molecule.

Manganese is present in all living plant cells and regulates key life processes:

• it participates in redox reactions due to its easy transition from a lower valence state to a higher one and back;
• it activates photosynthesis, respiration, and also accelerates the formation of chlorophyll and the accumulation of ascorbic acid;
• it is part of the structure of oxidative enzymes (oxidases) and accelerates the synthesis of proteins and carbohydrates;
• it influences nitrogen metabolism, helping the plant to reduce nitrates to ammonia.

Microelements regulate key biochemical processes in plants, directly affecting yield and the marketability of produce. Proper management of microelement nutrition allows one to control metabolism, increase the immunity of crops, and enhance their resistance to adverse weather conditions.

Zinc activates plant respiration and stimulates the synthesis of proteins, vitamins, and growth substances (auxins). This element is essential for the development of a strong root system and protection against pathogens. Zinc is part of the enzyme carbonic anhydrase, which activates the decomposition of carbonic acid in tissues.

Copper is a component of enzymes and proteins that regulate the oxidation of phenols and the hydroxylation of monophenols. It improves carbohydrate and protein metabolism and increases chlorophyll concentration, general hydration, and the water-holding capacity of tissues. By regulating phenolic growth inhibitors, copper increases the resistance of stems to lodging, and also protects crops from drought, frost, and heat.

Molybdenum is directly involved in nitrogen metabolism. It is a component of nitrate reductase — an enzyme that converts nitrate nitrogen into an ammonia form for the subsequent synthesis of amino acids and proteins. In case of its deficiency, nitrates and nitrites accumulate in leaves and protein metabolism is disrupted, which leads to a loss of harvest. Molybdenum fertilizers effectively increase protein content, vitamins, and chlorophyll in produce.

Molybdenum plays a special role in the nutrition of grain legumes (clover, alfalfa, peas, beans, vetch). It is part of the nitrogenase enzyme, which fixes atmospheric nitrogen, and significantly increases the efficiency of nodule bacteria.

Boron, cobalt, and silicon: reproductive functions and protection against stress

Boron is responsible for the synthesis, accumulation, and transport of carbohydrates and proteins in the plant. Under its influence, the outflow of sugars to fruits, sugars to the root crops of sugar beet, and starch to potato tubers is accelerated. Boron also stimulates the formation of nodules on the roots of legumes, supporting atmospheric nitrogen fixation.

In seed production, boron plays a decisive role: it stimulates pollen germination, accelerates development, and increases the number of flowers, fruit sets, and seeds. A deficiency of the element leads to mass shedding of unfertilized flowers and a sharp decrease in seed productivity. In horticulture, boron helps prevent the drying out of fruit tree tops.

The following crops have the greatest need for boron:

• sugar and forage beet;
• potato;
• cabbage;
• flax;
• clover and alfalfa.

Cobalt is concentrated in the generative organs of plants, accumulates in pollen to accelerate its germination, and is also localized in the nodules of grain legumes. The element is part of vitamin B12 and participates in DNA synthesis and cell division. Due to its variable valence, cobalt actively participates in redox processes and significantly increases the drought resistance of crops.

Silicon acts as a protective barrier against stresses of various origins. It increases plant resistance both to biotic factors (damage by diseases and pests) and to abiotic threats — temperature shock, drought, and imbalance of mineral nutrition.

The role of silicon in plant nutrition and protection

Silicon directly affects tissue strength, root system development, and the overall resistance of field crops to stress. Optimizing silicon nutrition helps plants absorb other elements more fully and protects them from unfavorable environmental conditions. During the growing season, this element performs a complex of critical physiological functions.

• Provides mechanical strength, strengthens the walls of epidermal cells, and prevents stem lodging.
• Improves nitrogen and phosphorus metabolism in tissues, and increases the uptake of boron and other microelements.
• Reduces the toxic effect of heavy metal excess in the soil.
• Stimulates the development of the root system and increases its overall activity.
• Increases leaf area and activates the synthesis of plastid pigments, enhancing photosynthetic efficiency.
• Increases resistance to drought and temperature stress through the accumulation of the amino acid proline in leaves.
• Stimulates the production of flavonoids in leaves, activating redox processes.
• Participates in the synthesis of proteins and individual amino acids.
• Acts as a natural antiseptic, providing pronounced antibacterial, antiviral, and fungicidal effects to strengthen immunity.
• In low concentrations, stimulates crop growth and improves harvest quality by controlling the ratio of protein to non-protein nitrogen.
• Regulates the activity of key plant enzymes.

Visual diagnostic methods and symptoms of nitrogen deficiency

With balanced nutrition and a sufficient amount of humidity, heat, and light, crops develop normally and maintain a healthy appearance. Any deficiency of nutrients disrupts metabolism, which is quickly manifested in the external appearance of the crop. Based on the nature of these changes at different stages of the growing season, one can determine the cause of the suppression and adjust the agricultural techniques.

Since metabolism is most intensive in leaves and growing points, signs of nutrient deficiency appear there first. It is here that the deficiency is noticeable earliest, which allows for timely measures to be taken. At the same time, it is important to remember the time lag between the start of starvation and its external manifestation.

Most often, in field conditions, plants experience a nitrogen deficiency. Common signs of nitrogen starvation include stunted growth, a depressed appearance, reduced leaf and flower size, and chlorosis — a pale or yellow-green color of the vegetative mass. In cases of acute nitrogen deficiency, premature leaf drop, accelerated plant maturation, and a sharp drop in yield are observed.

For accurate diagnosis, it is necessary to distinguish nitrogen starvation from other pathologies and natural aging. In field conditions, one should be guided by the following distinctions:

• Reaction to moisture: with a simultaneous deficiency of nitrogen and moisture, plants wilt during the daytime. If only nitrogen is lacking, leaf wilting does not occur.
• Localization of symptoms: in the case of nitrogen deficiency, the old lower leaves turn yellow first (as nitrogen is redistributed to the young growing organs), and then the young ones. With iron deficiency, on the contrary, the upper young leaves turn yellow first.
• Pattern of yellowing: with nitrogen deficiency, chlorosis starts from the leaf veins and adjacent tissues. During natural aging, the space between the veins turns yellow first, while the veins themselves remain green for some time.

Most often, nitrogen starvation occurs on soils with low organic matter content: podzolic, red soil (krasnozem), grey soil (serozem), sandy, and eroded soils. The problem is also characteristic of areas with increased acidity or alkalinity, where beneficial soil microflora is suppressed. Under unfavorable external conditions, nitrogen deficiency can also occur on any other soil types.

When the soil loses available nitrogen: causes of deficiency

It is important for an agronomist to remember that visual nitrogen deficiency in plants is not always associated with its complete absence in the soil. Often, the element simply transitions into an unavailable form or is leached from the root zone under the influence of weather and agricultural techniques. To correct nutrition in time, it is necessary to monitor external factors and the state of the soil microflora.

Nitrogen starvation of crops in field conditions is usually provoked by four main causes:

• Excess of carbohydrates in organic matter. When plant residues with a wide nitrogen-to-carbon ratio (cereal straw, corn or mustard stalks) are applied to the soil, the soil microflora begins to develop rapidly. Bacteria actively consume available nitrate and ammonium nitrogen for fiber decomposition, temporarily binding it in their cells and taking it away from the crops.
• Low soil temperature. In the early spring period, the activity of microorganisms is strongly inhibited. Processes of organic matter mineralization and nitrate formation proceed sluggishly, which causes seedlings to experience hunger even in rich soils.
• Leaching regime and drought. Heavy precipitation or excessive irrigation leaches mobile nitrogen into the lower soil horizons. With drought, the opposite happens: due to intense water evaporation, nitrates rise to the very top dry layer, becoming inaccessible to roots in the parched zone.
• Sodding. Keeping soil under sod for a long time (for example, in orchard aisles) creates tough competition for nitrogen between crops and wild grass.

How to recognize nitrogen starvation by indicator crops

Not all crops react to nutrient deficiency in the same way. Some plants are the first to signal a problem by changing the color and shape of their leaves, which allows one to notice starvation at an early stage. Such indicators include potatoes, white cabbage, corn, apple, plum, blackcurrant, as well as weeds, for example, redroot pigweed.

In various crops, symptoms of nitrogen deficiency manifest specifically:

• Grain legumes (peas, soybeans, beans): leaves gradually lose their green color. First, the lower canopy turns yellow, then the upper, after which growth stops and leaves begin to drop.
• Potato: stems and leaves grow poorly, and the plantings appear light green or yellowish. By the budding and flowering stages, signs become most apparent: lower leaves lose chlorophyll along the edges, pale, and drop, while new leaves form small.
• Tomatoes: growth slows down sharply, leaves become small and thin, taking on a pale yellow-green shade. The veins on the underside of the leaf and the stems themselves turn dark red or bluish-red, harden, and become coarse. The root system turns brown and dies off, flowers drop, and fruits form small, hard, and bright red upon ripening.
• Cucumbers: lower leaves turn yellow, vines become thin, hard, and fibrous. Growth pauses, roots turn brown and die off. Few fruit sets occur; they are small, pale, and have a characteristic pointed tip.
• White cabbage: leaves and heads become smaller, the color turns yellow-green, and then the lower leaves take on a pink or purple shade. Symptoms often appear in early cultivars during cold, rainy weather both on transplants and in the field.
• Onion: leaves grow slowly, forming short and narrow, pale-colored. Drying starts from the tips, which turn brown and yellow, after which the entire plant is suppressed.
• Radish: the above-ground part and root crops grow slowly. Leaves are small and thin with weakened petioles, yellowing quickly. Instead of a bright color, the root crops turn out dull, reddish, and small.
• Fruit and berry crops: chlorosis begins from the base of the shoots and moves upward to younger leaves. During severe starvation, the tree sheds foliage prematurely, which turns crimson, orange, or red tones before dropping.

• in all species: slow development of buds in spring, small size of leaves and their departure from the shoot at an acute angle, loss of elasticity and lignification (stiffness) of growing shoots, brighter than usual fruit color with a low quantity of fruit;
• in apple: reddish, brown, or dingy shade of bark;
• in strawberry: poor development of runners, red pigmentation along the leaf edges, drying of lower leaves, brittleness of petioles;
• in raspberry: scorching of leaf edges and tips after yellowing and leaf fall;
• in gooseberry: the appearance of a purple and later dark rim on light green leaves and other signs.