Soybeans

Soy is one of the oldest plants used in agriculture. More than 6.000 years ago, it was cultivated together with wheat and rice in China. The formation of the biological properties of soybeans was greatly influenced by the monsoon climate characteristic of the region of its origin. The heavy rainfall during flowering contributed to the survival of only closed-flowered (cleistogamous) forms, which led to strict self-pollination and fertilization within the same flower. In this regard, small and unattractive to insects flowers have formed, very few insects can pollinate them. The cross-pollination rate of soybeans is low – about 0,1%, which allows growing different varieties of soybeans and their reproductions without the risk of re-pollination in a single field with little spatial isolation.

Soy is significantly more resistant to various fungal and bacterial diseases compared to other legumes, which could also be a consequence of the long evolution of cultivated soy’s ancestors in humid climates, which contributed to the selection of the most resistant forms.

The long census has significantly changed this species, making it more productive – the size of seeds and beans has repeatedly increased, the beans have become indestructible. From climbing plants with a height of 3-5 meters, soybeans turned into a stem form 0,5-1,5 meters high with simultaneous ripening of beans in the rows, resistant to lodging, with improved biochemical composition. Scientific breeding later made it possible to create soybean varieties suitable for mechanized harvesting and a wide range of growing conditions, including temperate climates with limited heat resources and long summer days.

Currently, there is a wide variety of soybean varieties, which differ greatly in the requirements of key life factors, the height of plants, the length of the growing season, and so on. Therefore, without paying attention to the differences between varieties, let us briefly consider the most common biological characteristics of soybeans.

Soy is a short-day crop, and most strains are very sensitive to the photoperiod extension. Usually, an increase in the light day leads to a slowdown in plant development, a postponement of the start of flowering to a later date, a sharp drop in flowers and an increase in the overall growing season. This can vary the overall height of the plants, the length of the internodes, the number of leaves, flowers and beans and the resulting productivity of the plants. The optimum daylight duration of any given variety is related to its location of excretion, which is why most soybean varieties are adapted to very narrow belts of latitudes. Usually, for every 1 to 2 degrees of latitude, a new variety is needed that is better adapted to the length of the day in a particular area. Therefore, when choosing the right variety, it is more suitable from breeding centers that are located near your premises.

In terms of the intensity of lighting, soy is classified exclusively as a light-sensitive crop. When the light intensity is reduced by 50%, the number of nodes, beans and seeds in the plants decreases drastically. After the leaf cover closes in soybean plants, only the upper layer leaves receive high-intensity light. The light intensity under the canopy is only 2-3% of the total light intensity. The formation of lower tier beans occurs mainly due to the influx of assimilates from the upper tier. Therefore, the noxiousness of weed vegetation is much higher in the early stages of soybean development than in the late stages. In the early stages, the weed vegetation takes away most of the light from soybean plants, while in the late stages, on the other hand, the leaves of soybeans take away most of the light from the weeds.

Traditionally, soy is considered a heat-loving crop. However, in recent decades, a large variety of cultivars has been created that can form a complete seed crop at the sum of active temperatures (above 10 °C) 17-22 °C. In all agroclimatic zones in Belarus, this indicator is even higher. When choosing a variety for sowing, it is not enough to be guided only by the sum of the active temperatures of this variety, since the varieties also have a specific reaction to the length of the day. When creating Belarusian varieties, breeders proceeded from the need to use soy as an all-season crop that dumps the heat of the entire growing season. As a rule, the extremely ripe varieties from Sweden (“Fiskebi”, “Bravella”) and Ryazan (“Mageva”) in temperate climate zones (Northern Black Earth and South Black Earth) are too short, cannot harvest more than 2 tons per Hectares form and are characterized by very large stubble losses during harvest.

Soy’s heat requirement increases during the developmental stages – from germination to flowering, while decreasing during the bean filling and ripening stages. With optimal warming of the soil (20-22 °C), shoots appear 5-6 days after sowing, and with insufficient sowing (12-14 °C), this period can be extended up to 20 days. When seeds germinate under optimal conditions, field germination typically increases. The lack of heat and moisture negatively affects the friendliness of the shoots and leads to their thinning.

Soy is most demanding for heat during the flowering phase. In our terms, this period falls at the end of June or July. Usually, even the earliest maturing strains suffer at temperatures below 16 °C. A temperature of 14-16 °C is sufficient for the ripening of the seeds, at lower temperatures this phase extends somewhat. In conditions of a natural limited thermal resource, the adaptability of cultivated varieties to this factor plays a large role.

In terms of frost resistance, soy is superior to other heat-loving crops, probably due to its origin from Northeast China, where a prolonged spring with frequent frosts is common. Bean sprouts do not die in frosts in -2 or -3 °C. With frosts in -5 °C, the primordial and triple leaves die off within 10 hours, while the cotyledons remain viable. Later, new triple leaves may grow from the cotyledon sinuses. Similar observations (the formation of new leaves instead of dead ones) are made with prolonged frosts for 5-7 consecutive nights. Soy is most sensitive to frost in the branching and budding phase. Short-term frosts of b -3 °C can destroy plants and buds. Autumn frosts usually do not adversely affect soy and in some cases accelerate the ripening process through natural drying.

Due to its origin, soy responds to an improved supply of moisture to the plants. To form a crop unit, it uses more water than other legumes. In this case, the maximum water consumption falls at the phase of filling beans (in our conditions this is the end of July – August). Drought during this time reduces the soybean crop the most, and irrigation is most effective at increasing yields at this time. The soybean yield can be increased 2-4 times with sufficient irrigation. In conditions of unstable and insufficient moisture (on light soils), the most reliable are varieties that are well adapted to changing moisture conditions and can form an economically acceptable crop under such conditions. In general, soy can compare to other crops – for example with corn – be considered sufficiently drought tolerant.

Among the legumes it is only chickpeas inferior. Even under extreme moisture deficits (no rainfall throughout the season), soy can survive and produce a crop of up to 0,5 tons per hectare. In the stages of development, it can be found that before flowering, soybean is very drought tolerant (drought in May and June does not have a significant impact on seed yield), during flowering, bean formation and seed filling, it is relatively robust in the context of drought, they always retain their reproductive ability and respond to improved moisture supply with increasing generative productivity.

For the normal functioning of the root system of soybean, a favorable water-air regime in the root-based zone is necessary. Symbiotic nodule bacteria are aerobic microorganisms and require oxygen access to carry out biological fixation of atmospheric nitrogen. For these reasons, soybean is responsive to soil loosening, which is achieved during wide sowing by row-spacing cultivation. The critical lower threshold for the air content in the soil is 9%, and the optimal air supply to the root system is achieved at 15-22%. Air is needed not only for the roots to breathe, but also for the active life activity of nitrogen-fixing nodule bacteria. A sufficient oxygen content in the soil air is important: its low content dramatically slows down root growth. At the same time, soybean is relatively resistant to short-term oxygen deficiency in the soil and can withstand temporary flooding for 3-4 days.

The increased content of carbon dioxide in the soil air has a positive effect on the efficiency of photosynthesis, the increase in growth processes, nitrogen fixation and ultimately the yield. Normally, an increase in the concentration of carbon dioxide in the air layer can be achieved by introducing crushed straw from stubble precursors.

Soy consumes more mineral nutrients than other grains and legumes. It absorbs the batteries unevenly in their development phases, has the ability to assimilate nitrogen from the air, can use phosphorus and potassium from hard-to-reach compounds, and has the ability to redistribute (reutilize) their reserves from the leaf mass into seeds. Due to these properties, soy does not always respond to the use of mineral fertilizers. The amount of elements consumed from the soil depends on many factors: the biological characteristics of the variety, soil fertility, moisture conditions, nitrogen fixation activity, weather, intensity of photosynthesis and the size of the crop.

As shown by the results of experiments in different regions of the world, the consumption of soy nutrients for the formation of one ton of seeds varies significantly: nitrogen – from 64 to 95 kg, phosphorus – from 10 to 40 kg, potassium – from 22 to 76 kg. When calculating the optimal doses of fertilizers for the planned crop, it would be better to use the data of the closest experienced institutions that are in the same soil and climatic conditions and have studied the varieties of soybeans to be cultivated. Unfortunately, currently there is no such data for Belarus and Belarusian soybean varieties, due to the lack of better data, the average data must be used (see «Cultivation technology»). In general, the lower the natural soil fertility, the higher the responsiveness of soybean to mineral fertilizers.

The maximum absorption of batteries occurs during the flowering and ripening of the seeds. In these phenophases, 58 and 35% nitrogen, 60-65% and 31-36% phosphorus, 66-71% and 19-26% potassium are absorbed by soy. Only a small portion of macronutrients are used in the vegetative phase of plant development. The critical period for nitrogen application is the budding phase – flowering. The lack of nitrogen during this period can no longer be compensated for by additional application in later periods. In phosphorus nutrition, the critical period is the first month of plant life.

Nitrogen deficiency in vegetative soybean plants is manifested by lightening of leaf color and slowing of plant growth. The leaves of soybeans acquire a yellow-green color and decrease in size. The first tricolor leaf has a uniform light green color, the second has an uneven yellow-green color, the subsequent tricolor leaves have a focal yellow-green color.

With phosphorus fasting, soybean plants acquire a dark green color, but their growth is slowed, they are stretched, die early and turn completely brown. Brown patches of dead tissue quickly appear on primordial leaves. With further phosphorus fasting, the boring and dying of the leaves is also observed in the following layers.

Potassium starvation in soy is less manifest. There is a slowdown in plant growth, the appearance of yellowed areas on the edges of the lower leaves, their edges are twisted, dead tissue falls out.