Primary symptoms

Waterlogging causes lower leaves to turn bright yellow then die and upper leaves turn pale yellow through nitrogen shortage.

Summary

Waterlogging occurs when the soil is saturated with water. Heavy soils are most likely to waterlog. They have limited pore space through which water and air can move only very slowly. If the soil is saturated for too long oxygen is used up. Then roots stop growing and absorbing nutrients, stomata close preventing photosynthesis and soil denitrification commences. Because plants cannot absorb their nitrogen from the soil, they have to extract it from older leaves to support the growth of new leaves. During this extraction, old leaves become ‘nitrogen deficient’ and yellow during a period of waterlogging. Generally there is not enough nitrogen available in old leaves to support new tiller growth, so tillering does not occur.

Wheat deteriorates rapidly in waterlogged soils if temperatures are high; seedlings die within as little as 2 days. Later stages are more tolerant but can still lose a high proportion of their leaf area and yield. Waterlogging is avoided by ensuring that any water drains through the soil before it has time to stagnate. Wheat growing acceptably in mildly saline soils will not survive if waterlogging occurs.

Do you have too much water?

• After rain, do water puddles remain in the field for more than 12 hours? Examine the soil surface in areas where crop growth is poor. Is that soil very wet, perhaps covered with green patches of algae?
• Are some plants wilted at midday even though the soil is wet? Do those in the lower or damper areas look pale with yellow tips on older leaves? Does the crop appear to be nitrogen deficient even though fertilizer was applied?
• In those areas that have poor growth are the weeds a different species from those in apparently drier areas?
• Scoop up a handful of soil. Does it smell stagnant or fresh?

Causes of waterlogging

• The soil is inadequately drained. Is the seedbed above the level of the drainage channels and are any tied ridges allowing water to drain away?
• The field is not level. Can you see that there are low spots? Are these areas of poor growth?
• Too much irrigation water has been applied which cannot drain sufficiently quickly. Check records for amounts and timing.
• The soil is naturally heavy with poor structure and inadequate pore space.
• Rainfall has been heavy. Was the irrigation regime altered to allow for this?
• After heavy rainfall waterlogging can occur even in light soils because crusting seals the soil surface and prevents air from entering.

What you can do about waterlogging

• Nitrogen: Apply nitrogen after a period of waterlogging. It will make nitrate readily available and accelerate plant recovery.
• Weeds: Keep the field free of weeds to reduce competition for oxygen in the root zone.
• Cultivation: Consider a light cultivation if crusting occurs after intense rainfall. This will help aerate the saturated soil.
• Raised beds: If the soil is prone to waterlogging, consider adopting the raised bed system with its intrinsically good drainage.
• Leveling: Level the field, improve the drainage channels and put them closer together.
• Irrigation schedule: Adjust the irrigation timetable to allow for rainfall events.
• Deep cultivation: Use deep cultivation to increase soil pore space and break up any hard pans that might have developed. Pore space should be around 10% to avoid waterlogging.
• Green manure: Next season, grow and then incorporate a green manure crop to improve soil organic matter and pore space. Alternatively incorporate farmyard manure or crop residues.