Agronomic principles

Water Requirements

Water availability is important, especially in two development stages:

Germination-emergence (VE-VC) and Flowering (R1-R2) – Grain filling (R5)

During the germination-emergence, both the excess and the deficit of water are harmful for obtaining good uniformity in plant population. 

Soybean seed needs to absorb at least 50% of its weight in water to ensure good germination. In this phase, the soil moisture should not exceed 85% of the maximum available water capacity and not less than 50%. 

Water requirement of soybean is increasing with plant development, reaching a maximum during flowering and grain filling (7 to 8 mm/day), decreasing after that period. Expressive water deficits during flowering and grain filling may cause physiological changes at the plant, such as stomata closure, winding sheets and, consequently, premature leaves and flowers dropping, pods abortion and yield losses. The total soybean water requirements (in order to obtain maximum yields) varies between 450 and 800 mm/cycle, as affected by climatic conditions, crop management and cycle duration. In order to minimize water deficit effects, it is recommended to use adapted cultivars to region and soil condition; sowing in the right time (less climatic risk) and on adequate soil moisture conditions; adopt management practices that improve soil water storage.

Requirements in temperature and photoperiod

Soybean grow better under temperatures around 20°C and 30°C; the best temperature for growth and development is around 30°C. 

The sowing should not occur when soil temperature is below 20°C, because it may have negative effects on germination and emergence. The suitable soil temperature ranges for sowing varies from 20°C to 30°C, and 25°C is the best temperature for a fast and even initial growing. 

Soybean vegetative growth is small or zero at temperatures less than or equal to 10°C. Temperatures above 40°C have adverse effect on growth rate, causing flowering disturbances and decrease of pod retention. Water deficits further increase the problems of high temperature. 

Soybean flowering is only induced under temperatures above 13°C. The differences observed in flowering date, in different years, with same cultivars, sowing time and location are due to temperature variation. Thus, early flowering occurs mainly due to higher temperatures, potentially decreasing plant height. Insufficient water/light during the growth stage may intensify the problem. Differences in flowering date between cultivars, in the same location and sowing conditions is explained by differential response of cultivars to day length (photoperiod).

High temperature can accelerate ripening. When it is associated to high humidity periods, high temperatures may cause low seeds quality, and when associated to low humidity, it makes the seeds more sensitive do mechanical damage during harvest. Low temperatures at the harvest stage, associated with the rainy season or high humidity, may cause harvest delay, as well as greens stems and leaf retention. The adaptation of different cultivars to specific regions depends, in addition to the water and temperature requirements, to its photoperiod requirements. The sensitivity to photoperiod is different among cultivars, i.e., each cultivar has its critical photoperiod, above which the flowering is delayed. Therefore, soybean is a “short-day” plant. The adaptability range of each cultivar varies as it travels towards north or south. However, cultivars with “long juvenile-period” have wider adaptability, allowing its use at different latitudes and sowing times.