Moisture Stress in Soybeans

Prolonged dry conditions can reduce soybean yield at any stage of development, but there are two stages that are the most susceptible. Sufficient water during germination and during the reproductive growth stages is critical. Dry soil conditions during germination will reduce plant stands and can lead to thin, patchy, and uneven stands. Moisture stress during the reproductive stages will lead to fewer, smaller, and poor quality seeds. Severe moisture stress is evident by leaf wilting and under extreme conditions may lead to plant death.

A lack of moisture during the vegetative growth stages (May and June) is usually not yield reducing if enough moisture was present for emergence. Under these conditions plants grow deep, vigorous root systems searching for water. These extended roots provide excellent nutrient uptake when rainfall occurs later in the season.

The crop is the most susceptible during the reproductive stages of growth. Soybeans will begin to flower during the last week of June. Once the plant starts to flower it needs significant water right through to seed fill.

Soybean yield is a function of the number of plants/acre, pods/plant, seeds/pod, and the size of the seed. Each of these yield components is fixed in the above order during specific timeframes of plant development. Moisture stress confined to a specific growth stage reduces the yield component that is established during that phase. On the other hand water abundance during a specific phase enhances a specific component. Stress early during the reproductive stages reduces seed number. However, if moisture returns that reduction is offset by enhanced individual seed size or mass. Stress later during the reproductive phases greatly depresses seed mass, the last yield component to be fixed during reproduction. See table #1

Table #1. Yield, Number, and Individual Mass Response to Moisture Stress.

Means followed by different letters are significantly different, LSD (P = 0.05). Reaper and Purcell, 1999.

Varieties grown in Ontario have an indeterminate growth habit. This means they have an overlapping vegetative and reproductive growth period. This results in an extended flowering period lasting from the end of June into early August. This developmental trait protects yield potential during periods of stress. Yield that may be lost due to flower abortion early in the reproductive stages can be compensated for by flowers and pods that will be produced later in the season. In other words a good rain at the end of July may provide just as much seed set as having a few small showers during early flowering.

If plants go through the entire flowering period with insufficient moisture significant yield losses will occur. Even if water is supplied past flowering yield losses cannot be regained.

Stress Induced Nutrient Deficiencies

Inadequate water reduces nutrient availability, nutrient uptake and movement within the plant. One of the most evident water stressed nutrient deficiencies is potassium (K). K deficiency leaf symptoms often become evident in dry soils even if soil K levels are adequate. In dry conditions the roots are unable to take up K from the soil.

Another serious consequence of moisture stress is the reduction of biological nitrogen fixation. The high protein concentration of soybean grain requires a large amount of N.
A 50bu/ac crop of soybeans will remove 210 lbs of N per acre. Plants that lack water will shut down N fixation in the root nodules. One of the reasons soybean yields are lower in dryer years is because the plant has inadequate nitrogen.

Increased Susceptibility to Disease and Insects

Plants weakened by stress are also more susceptible to disease and insects. In dry years soybean cyst nematode (SCN) above ground symptoms become more evident. Plants are short, stunted, and yellow. The yield impact of SCN is worse in dry years because plants are unable to compensate as much as they could when moisture is prevalent. Also, insect feeders like soybean aphids do considerably more harm in a dry year than in a wet year. Aphids suck juices from the plant causing stunting and reduced seed yield if populations are high.

Aphids prefer plants that show K deficiency and will cause yield reductions if left uncontrolled on these plants. See figure #1.

Strategies for Success

There are no management strategies to alleviate moisture stress once it occurs. Irrigation is impractical for most producers. The following practices have been shown to lighten the impact of moisture stress:

1. Planting deep enough for adequate germination moisture
2. Early planting for greater root depth and density
3. Establishing a full canopy early in the season through narrow rows
4. Good weed control to reduce competition
5. Good crop rotation
6. Careful management of disease and insects
7. A good fertility program

Figure #1. Plants showing K deficiency due to moisture stress. High aphid populations are also present on these plants