Requirements of vegetable crops for soil moisture and humidity

Vegetable crops are much more demanding regarding soil moisture than field crops. This is because their succulent produce organs consist of 70-97% water. At the same time, the roots of vegetable plants absorb moisture poorly, and the bulk of it is consumed for temperature regulation and the transport of nutrients.

A lack of soil moisture leads to a drop in yield and a deterioration in produce quality: vegetables become coarse and lose physiologically active substances. However, waterlogging is also harmful. An excess of water makes fruits watery, reduces sugar content, and impairs their suitability for canning.

• Water in produce organs — 70-97%
• Expenditure on transpiration — 98%
• Retained in the plant — 2%
• For dry matter formation — 0.1-0.2%

Soil moisture regimes for different crop groups

To obtain a high and quality harvest, it is necessary to maintain an optimal moisture level in the root zone. In relation to water, vegetable crops are traditionally divided into four main groups. Their requirements for the content of available soil moisture differ significantly.

These limits may fluctuate depending on the cultivar and growing conditions. It is important to remember that a drop in moisture below optimal values always leads to a decrease in the intensity of photosynthesis and a loss of plant productivity.

Physiological causes of varying water demand

The varying water needs of vegetables are explained by the strength of their root system and the intensity of transpiration. The main indicator of evaporation is the transpiration coefficient, which shows how many grams of water a plant spends to form 1 g of dry matter. For white cabbage, this coefficient is 250-600, for watermelon — 576-600, for tomato — 500-650, and for cucumber — over 700.

Under field conditions, the water consumption coefficient is also taken into account — the volume of water in cubic meters spent per ton of harvest (m³/t). With an increase in yield and an improvement in the level of agrotechnology, this indicator decreases. Due to strong dependence on climate and soil fertility, it is not used for crop classification.

It is more reliable to divide vegetable plants according to the balance between water absorption by roots and its expenditure. Based on this principle, four types of crops are distinguished:

• Cabbage (white, cauliflower, Chinese, kohlrabi), cucumber, eggplant, lettuce, radish, celery, spinach, and other salad crops. Their roots are poorly developed, absorption is slow, and transpiration is high. These crops are very demanding regarding irrigation.
• Tomato, carrot, parsley, melons and gourds, bean, asparagus. They have powerful roots and regulate transpiration perfectly. They effectively extract water from the soil and consume it economically.
• Onion, garlic, bunching onion, and other bulbous crops. They possess a weakly developed root system but consume moisture very economically. High soil moisture is vital for them during the first half of the growing season.
• Beet. Has well-developed roots and is capable of absorbing water even at high salt concentrations. At the same time, it consumes moisture very intensively and always responds well to irrigation.

The water demand of vegetables changes at each stage of the growing season. Maximum soil moisture — up to 90% of FC — is required by seeds during the germination period. High humidity is also necessary for young plants at the start of growth and immediately after transplanting. Their root system is still weak and concentrated in the rapidly drying top layer of soil, although the overall water consumption of the plants at this time is small.

Throughout the season, the need for moisture fluctuates. The entire growing season is divided into three stages: before the formation of productive organs, the phase of their growth, and crop maturation. The lowest soil moisture is maintained at the first and third stages. The peak of irrigation should occur during the phase of active crop formation — this is a critical period for all vegetable crops.

Air humidity and agricultural practices for managing water balance

Air humidity directly affects water evaporation by leaves (transpiration) and pollen viability. Each group of vegetable crops has its own requirements for this indicator. Any deviation from the optimum immediately affects the growth rates, development, and health of the plants.

• Air humidity for cucumber, lettuce, spinach, dill, celery — 80–90 %
• Air humidity for cabbage crops, onion, pea — 70–80 %
• Air humidity for tomato, pepper, eggplant, bean — 60–65 %
• Air humidity for cucurbits (watermelon, melon, pumpkin) — 45–55 %

To maintain an optimal water regime in the field, an agronomist needs a set of technological practices. First of all, for vegetable crop rotation, plots are selected where natural conditions make it easy to regulate humidity. To manage the soil water balance during the growing season, special operations are carried out:

• Snow retention and runoff regulation. In regions with stable winters, snow is retained in the fields, and in case of heavy precipitation in late autumn and winter, water runoff is prevented.
• Profiled soil tillage. Cutting beds and ridges allows for rapid drainage of excess water during waterlogging or irrigating plants by furrow irrigation during dry periods.
• Regulating stand density. Planting density is calculated so that the reserves of natural or irrigation moisture are guaranteed to be sufficient for the plants until full maturity.
• Short-term spring mulching. Applied at the start of the season to protect the soil from excessive evaporation.

Shelterbelts and catch crops help protect plantings from excessive evaporation, improving the microclimate in the field. On large areas, land reclamation systems for drainage or artificial irrigation are used. In greenhouses, the humidity regime is regulated using special irrigation systems or by spraying water from hoses in accordance with crop requirements.