Specifics of mineral nutrition and fertilizer application rates for vegetable crops

Vegetable crops are the most demanding consumers of nutrients in the field. Compared to grain crops, they remove almost twice as many nutrients from the soil. At the same time, their root system is poorly developed and significantly inferior to field crops in terms of absorption volume. To achieve the planned yield, an agronomist must maintain an optimal concentration of nutrients directly in the root zone.

Groups of crops by volume and rate of nutrient consumption

The nutritional needs of vegetables depend on the duration of the growing season, growth rates, and the depth of root penetration into the soil. Based on the total quantity of elements removed per unit area, crops are divided into four groups:

• High removal: medium and late-maturing white cabbage, carrots, table beet, swede, potatoes, and celery.
• Medium removal: tomato, cauliflower, onion and leek, early cultivars of cabbage and carrots, asparagus.
• Low removal: lettuce, spinach, and other leafy greens.
• Very low removal: radish.

When planning top dressing, it is important to consider not only the total removal per season but also the rate of consumption per unit of time. Early-maturing crops assimilate nitrogen, phosphorus, and potassium very intensively. Lettuce, spinach, and radish absorb nutrients 2–6 times more actively per day, although their total removal with the harvest is 4 times less than that of cabbage. The basic indicator for calculating fertilizer doses remains the removal of mineral substances per unit of marketable produce.

• NPK removal by cauliflower — 245–394 kg per 10 t of harvest
• NPK removal by pepper — 120–165 kg per 10 t of harvest
• NPK removal by potatoes — 150–160 kg per 10 t of harvest
• NPK removal by eggplant — 115–155 kg per 10 t of harvest
• NPK removal by radish — 110–20 kg per 10 t of harvest
• NPK removal by early white cabbage — 110–115 kg per 10 t of harvest

Most vegetable plants consume the most potassium, followed by nitrogen, and the least amount of phosphorus. However, onions, melons, Brussels sprouts, peppers, eggplants, and tomatoes remove potassium and nitrogen in approximately equal amounts. Peas, beans, and sweet corn are the exception: they absorb 1.5–2.0 times more nitrogen than potassium.

How nutrition changes by plant development stages

Nutrient consumption changes according to plant development stages. Germinating seeds do not need external fertilizers, as the embryo uses reserves of the endosperm or cotyledons. The consumption of mineral substances begins only with the transition to root nutrition, but in the first period, the absorption volumes are very small.

With the growth of the root system and above-ground biomass, nutrient absorption increases sharply. Plants become more resistant to fluctuations in the concentration of the soil solution. Nevertheless, the assimilation of each specific element and their ratio in nutrition change unevenly until the very end of the growing season.

During the period of vegetative mass accumulation, there is active nitrogen absorption. Potassium and phosphorus consumption increases during the formation and growth of product organs in biennials, and before budding in fruit-bearing plants. The supply of easily accessible forms of phosphorus and potassium during these phases determines the final productivity and crop increase.

By the end of the formation of the product organ (vegetative or generative), plants practically no longer need an additional inflow of mineral substances from the soil. During the ripening period of the harvest, additional growth of product organs is carried out through the movement of plastic substances from the stems and leaves. Thus, the absorption of mineral nutrition elements by vegetable plants occurs throughout the entire growing season of the plant, but their ratio changes.

The greatest removal of mineral substances occurs during the period of intensive formation of product organs, when biomass increases most actively. It is important to know that:

• nitrogen is the basis for the formation of vegetative mass and biomass in general;
• potassium promotes the formation and transport of carbohydrates;
• phosphorus is a determining factor during the transition to the reproductive life stage of plants.

It should also be remembered that the general pattern described above regarding the change in the ratio of absorbed mineral nutrients during the growth and development of vegetable crops has some exceptions due to the individual characteristics of the crops. According to Z.I. Zhurbitsky, transplants of cabbage absorb the most nitrogen, slightly less potassium, and very little phosphorus, while tomato plants of the same age consume more potassium and less nitrogen, with some increase (compared to cabbage) in the volume of phosphorus assimilation.

Only before the formation of the harvest organs do plants of both crops absorb more potassium than other nutrients (as with all vegetable crops). In cultivars with a long fruiting period, the absorption activity of nitrogen and potassium alternates as stem growth and the formation of new generative organs occur.

Therefore, one must know the specific nutrient requirements of each crop to ensure the most favorable conditions for yield accumulation at minimal fertilizer costs.

Micronutrients are of great importance for the growth and development of vegetable plants. The manifestation of their positive effect is especially noticeable with optimal provision of macronutrients (nitrogen, phosphorus, potassium). The deficiency or absence of certain micronutrients (boron, manganese) leads to the shedding of buds and fruit set, which is associated with disturbances in the formation of pollen grains. Micronutrients such as molybdenum, cobalt, copper, iron, magnesium, and others are components of enzymes and other metabolic regulators that determine the activity of growth, organogenesis, and plant development.

Vegetable plants react differently to changes in the concentration of the soil solution. There is a pattern: the younger the plant, the worse it tolerates an increase in the concentration of mineral substances. But in general, vegetable crops tolerate a concentrated nutrient solution poorly, which is explained by the relatively low suction force of their root hairs (compared to grain crops). Therefore, vegetable growers must constantly remember: despite a high demand for mineral nutrients, vegetable plants should not be exposed to an excess of salts in the soil solution.

The least salt-tolerant are carrots, turnips, cucumbers, radishes, garlic, onions, and beans, which reduce yield at 0.1% salinity and are strongly inhibited at 0.4%.

Moderately tolerant, withstanding salinity up to 0.4–0.6%, include tomatoes, cabbage, peppers, and eggplants; relatively salt-tolerant are beets, zucchini, squash, and watermelons, which react negatively to an increase in soil solution concentration exceeding 1%.

The harvest of vegetable crops largely depends on the level of soil acidity. Most of them prefer a slightly acidic or neutral environment. Some vegetable plants tolerate soil acidification down to pH 5.5 (carrots, squash, tomatoes, beans, peas, etc.), and some even down to pH 5.0 (sorrel, rhubarb, chicory, watermelons, fennel, potatoes). But an environment with pH 6.8–6.0 is more favorable for all of them.

In acidic soils, plants absorb nitrogen, phosphorus, potassium, magnesium, calcium, and molybdenum worse. In alkaline soils, there is a deficiency of zinc, manganese, iron, and copper.

Table 6 — Classification of vegetable crops by reaction to soil acidity (according to various authors)

Organic fertilizers are of great importance for vegetable plants. First of all, it is important that they increase soil fertility and improve its physical condition.

However, it should be especially noted that with the enrichment of soil with organic matter, its absorption capacity increases significantly, preventing the occurrence of an increased concentration of the soil solution even at relatively high application rates of mineral fertilizers. For vegetable crops sensitive to an increase in salt concentration, this is of great importance. Therefore, organic fertilizers not only enrich the soil with mineral nutrients and the air with carbon dioxide but are also the primary means of creating optimal conditions for the nutritional regime of vegetable plants. This is why most vegetable crops react positively to the application of such fertilizers, especially when combined with mineral fertilizers.

It is necessary to consider the varying responsiveness and reaction of vegetable crops to organic fertilizers:

Organics, dose calculation, and nutritional specifics in greenhouses

Fresh manure and other fresh organic fertilizers are perfectly utilized by late-maturing white cabbage cultivars; cucumber plants also respond positively to them. However, applying such organics directly under other crops is not recommended. Beet and tomato effectively utilize the residual effect of manure, so their placement in crop rotation after late cabbage provides a significant yield increase. For early-maturing crops and cultivars, it is better to apply humus or compost, as they show practically no response to manure applied in the autumn.

To create an optimal nutritional regime and prevent the excessive accumulation of nitrates in vegetables, use the balance method for calculating fertilizer doses. Although all calculation methods have margins of error, the balance approach allows for obtaining an NPK ratio closest to optimal. Based on these calculations, a feeding system is formed, which is then applied in split doses throughout the growing season.

The required high rates of nutrients must be distributed over several applications in accordance with the plants' needs during different growth phases. A special approach is required in protected ground, where high plant productivity and enormous biomass cause an increased need for nutrition. At the same time, the limited volume of the root zone in greenhouses necessitates the use of highly fertile artificial soils rather than ordinary soil.

• Organic matter content in greenhouse soils — from 20–30% to 100%
• Split application of mineral fertilizers — in 3–4 stages
• Frequency of top dressing in greenhouses — once every ten days

Even the richest greenhouse soil cannot immediately accommodate the entire volume of required salts due to the risk of a dangerous increase in their concentration. Agronomists must constantly monitor the state of the substrate and conduct regular top dressing. An alternative option is vegetable growing on inert substrates with a constant supply of nutrient solutions of a specified acidity and concentration.

Cultivation and preparation of soils for vegetables

The high requirements of vegetable crops for mineral nutrition are satisfied only in fertile soils with good physical and chemical properties. If natural soil fertility is low, the soil is improved using available reclamation techniques. Under the conditions of the Southern Federal District, almost any soil type can be adapted for vegetable farming with proper preparation.

Waterlogged organic soils in the deltas of the Volga, Don, and Kuban rivers require the highest costs for cultivation. They have increased acidity and are rich in nitrogen and calcium, but are deficient in available phosphorus and potassium. After drainage and loosening, the organic matter in them decomposes actively with the accumulation of humus, which allows for high yields when applying phosphorus and potassium fertilizers. Due to slow warming, such moisture-capacitive areas are cold and are not suitable for early vegetable farming.

Light sandy soils are characterized by low water-holding capacity and excessive aeration, which causes organic matter in them to burn instantly, and mineral salts are quickly leached out. Commercial vegetable farming on such fields is possible only if irrigation is mandatory. In addition, sandy areas require annual enrichment with organic and mineral fertilizers.

Heavy clay soils contain a large supply of nutrients but have poor structure and warm up slowly. To improve their physical properties, organic matter and calcium-containing materials are added to the tillage layer, after which they are used for cold-hardy vegetable crops. Getting an early harvest on clay fields will not be possible even after carrying out land reclamation.

Developing podzolic soils for vegetables requires significant initial investment. However, due to good natural moisture availability, these lands allow for consistently high yields. Preparation of podzolic areas for vegetable crops includes several mandatory sequential steps.

1. Subsoiling to increase the depth of the tillage layer.
2. Liming to neutralize excess acidity.
3. Systematic application of organic and mineral fertilizers.

Vegetable crops are sensitive to soil type and humidity levels. The best choice for establishing commercial plantings will be loams, especially chernozems and light loamy soils. If artificial irrigation is set up on such lands and calculated doses of fertilizers are regularly applied, you are guaranteed to obtain a high harvest of vegetables of any maturity period.

Due to the high moisture demand of vegetables, crop rotation is usually located near water sources — in river floodplains or lowlands near lakes. Floodplain fields with fertile alluvial soil are the most popular, however, growing conditions on them are heterogeneous. When planning planting, be sure to consider the topography of the specific site, the mechanical composition of the soil, and the groundwater level.

How to distribute crops by floodplain zones

Depending on the distance from the river channel, the floodplain is divided into three zones with different conditions for plant cultivation:

• Channel part. Consists of coarse sandy deposits; groundwater here is deep. These are the least fertile areas of the floodplain, but they are suitable for vegetable production if regular irrigation is organized.
• Central floodplain. The most fertile zone with an optimal composition of river sediments. This is the best place to locate the main vegetable plot for mid- and late-maturing crops.
• Terrace-adjacent part. Characterized by low relief and heavy, waterlogged soil prone to swampiness. These lands can only be used for planting after extensive land reclamation work.

To obtain early products, it is safer to use the first or second supra-floodplain terraces. For planting, choose areas with a slope toward the sun. On such slopes, the soil warms up a week earlier than on the plain, while maintaining the ability to quickly supply water for irrigation.