Soybean 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 to 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 increases with plant development, reaching a maximum during flowering and grain filling (0.25 to 0.30 in/day), decreasing after that period. Excessive water deficits during flowering and grain filling may cause plant physiological changes, such as stomata closure and consequently, premature leaf and flower drop, pod abortion and yield loss. The total soybean water requirement (in order to obtain maximum yield) varies between 18 and 32 in per growing period, as affected by climatic conditions, crop management and growth period duration. In order to minimize water deficit effects, it is recommended to use region and soil adapted cultivars; timely planting (less climatic risk) on adequate soil moisture conditions; and adopt management practices that improve soil water storage. 

Requirements in temperature and photoperiod

Soybeans grow better under temperatures around 68°F and 86°F; the best temperature for growth and development being around 86°F. 

Planting should not occur when soil temperature is below 68°f, because it may have negative effects on germination and emergence. 77°F is the best temperature for quick and uniform establishment. 

Soybean vegetative growth is small or zero at temperatures less than or equal to 50°F. Temperatures above 105°F have an 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 55°F. The differences observed in flowering date are primarily driven by temperature variation. Thus, early flowering occurs mainly due to higher temperatures which 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 temperatures can accelerate maturity. Combined with periods of high humidity, high temperatures may cause low seed quality. Low humidity makes the seeds more sensitive to mechanical damage during harvest. Low temperatures at the harvest, combined with high moisture, may cause harvest delay as well as green stems and leaf retention.