NOTE- this article was first published in February 2013. We have decided to republish in response to questions we have been recieving about the effect of floods on crops and soil.

In a series of experiments carried out between 1975 and 1978 a sandy loam and a clay soil were used to assess the effect of waterlogging on winter wheat.

The researchers state that the treatments were selected to represent “the most extreme likely to occur in field conditions in Britain i.e. waterlogging to soil surface for up to 16 days in October, 42 days in January and February and 6 days in May”. How prescient as the extreme now seems to have become the norm.

Soil oxygen levels

The most immediate effect of waterlogging is that soils rapidly lose oxygen. In these experiments, looking at a sandy loam, oxygen concentration dropped dramatically once waterlogged in comparison to its freely-drained control. This comparison was not so obvious in the clay soil as even in the freely-drained control the natural swelling of the clay led to a drop in oxygen concentration. However, when waterlogged the oxygen concentration dropped to similar values in both soils (see diag). This drop is as a result of water replacing oxygen in the soil pores. If these anaerobic conditions persist levels of carbon dioxide, methane and volatile fatty acids increase in the soil. The net result is that plants shift their metabolism from aerobic respiration to anaerobic fermentation. This is a much less efficient process and plants need to compensate for the deficit in energy by using up their carbohydrate reserves.

Leaf growth and development are affected adversely by waterlogging. In another study an 83% reduction in leaf area in wheat was reported and attributed primarily to nutrient deficiency. Waterlogging also causes the plant’s stomata to close and photosynthetic rate has been shown to decline to values lower than those of well-aerated plants 3 days after waterlogging in wheat.

Nutrient uptake and 'Take-all'

Uptake of nutrients such as nitrogen, potassium, and phosphorus is often inhibited under waterlogged conditions. One study reported that in barley, the concentrations of total levels of N, P, and K in leaves declined, respectively, to 75, 69, and 77% of the levels in aerated controls in just 48 hr.  Take-all has also been shown to increase in waterlogged soils.

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In the diagram above the dotted lines represent the oxygen concentration in waterlogged soils and the solid lines those in the freely-draining controls.

Yield

Wheat’s ability to survive long periods of waterlogging is dependent on the growth stage of the crop at the time of waterlogging. Plants that have already started to tiller before the soils become saturated show little loss in shoot numbers irrespective of soil type. However, plants that had not started to tiller before waterlogging occurred showed a decline in shoot numbers. In the experiments on different soil types carried out in the late ‘70’s yields on both soil types were depressed by waterlogging in December by 7% on sandy loam and 16% on clay. Late winter (Jan/Feb) waterlogging reduced yields by about 12% on both soils. Yield losses were attributed to fewer ears, smaller grain weight and fewer grains per ear.

Oilseed rape

Oilseed rape, growing in a sandy loam soil in lysimeters, was subjected to waterlogging to the soil surface at different stages of growth. Brief periods of waterlogging in December and January (10 days) or in May (10 days) resulted in slightly shorter plants but hardly affected yield. Waterlogging in December/January for 6 weeks in cold (1–2°C) weather, slightly restricted leaf development and delayed flowering, but yield was unaffected. At the end of waterlogging for a similar duration between January and March (when the mean temperature was about 6°C), the height and leaf area of the plants was approximately halved, the number of senescent leaves was almost doubled and flowering was accelerated; at harvest in July these plants were 17% shorter and yields of seed, oil and straw were 14, 17 and 23% less, respectively, than for plants in freely-drained soil. During the prolonged waterloggings short fleshy white roots developed slightly below and on the soil surface.

Sources

J. Sci. Food Agric. 1980, 31

Plant Stress 2 (1) 2008

Journal of Hydrology 267 (2002)

Journal of Flood Risk Management 2 (2009)

Journal of the Science of Food and Agriculture (1980)