Frosts

the falling of the air temperature below 0°C in the evening or night when temperatures during the day are above freezing. They are caused either by the invasion of cold air masses from other regions, generally from the arctic (advective frosts), or by radiant cooling of the soil surface and vegetation at night (radiation frosts). In most cases, however, both preliminary advection of a mass of cold air into a given region and subsequent night radiation that cools the soil and, through it the air, to subzero temperatures play a part in the occurrence of a frost. Such frosts usually occur in the central zone of the USSR in spring (until mid-June) and fall (beginning in the second half of September).

Frosts can substantially lower yields of field, vegetable, and fruit crops. Their destructive effect on crops is due to the direct action of low temperatures on living cells: it freezes water out of the cell sap, causes the formation of ice crystals in the interstitial spaces, and dehydrates the protoplasm. The hardiest plants, early spring grain crops and early-planted beans, can withstand a brief frost ranging from -7° to - 10°C; soy, Italian millet, and Chinese bellflower are moderately hardy and can withstand temperatures of -4° to -3°C; corn, millet, sorghum, potato, and rustic tobacco can tolerate frosts from -2° to -3°C; kidney bean, rice, cotton, melons, sesame, peanut, and buckwheat are tender plants whose shoots are injured by temperatures ranging from -0.5° to — 1.5°C. The reproductive organs are particularly sensitive to frosts.

The introduction of early-ripening crop varieties into regions with a short frost-free period, the breeding of plants for hardiness, the use of potassium fertilizers, timely cultivation, and the correct choice of the sowing site, taking the micro-climate into account, are very important in combating frosts. The best known and most popular method is smudging, which is widely used to protect flowering fruits and shoots of heat-loving vegetables in the central zone and southern regions of the USSR. Open heating, in the air between plants by the combustion of oil and coal in special heaters, is used on citrus plantations in the subtropical regions of the USSR. Lemon and orange trees are sometimes covered with three layers of gauze. The heating of plantations by electric heaters or warm-water or steam units is effective but costly and is used only for valuable produce.