Water-Logging

Water-logging is one of the major problems of land degradation. An irrigated area is said to be waterlogged when the surplus water stagnates due to poor drainage or when the shallow water table rises to an extent that soil pores in the root zone of a crop become saturated, resulting in restriction of the normal circulation of the air, decline in the level of oxygen and increase in the level of carbon dioxide.

Water-logging is often accompanied by soil salinity as waterlogged soils prevent leaching of the salts imported by the irrigation water.

o                  EFFECTS OF WATERLOGGING :

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.

The growth of most crops is affected when groundwater is shallow enough to maintain the soil profile in the root zone wetter than field capacity. This excess water and the resulting continuously wet root zone can lead to some serious and fatal diseases of the root and stem. Working the soil when overly wet can destroy soil structure and thus restrict root growth and drainage further. The chemistry and microbiology of waterlogged soils is changed due to the absence of oxygen.

o      TOLERANCE LEVELS OF CROP TO WATERLOGGING :

Crops vary in their tolerances to waterlogging and a high water table. The table below presents the different tolerances of some crops.

Ø     HIGH TOLERANCE :  Sugarcane, Potatoes, Rice, Willow, Plum, Broad beans, Strawberries, some Grasses etc.

Ø   MEDIUM TOLERANCE : Sugarbeet, Wheat, Oats, Citrus, Bananas, Apple, Barley, Peas, Cotton,  Pears, Blackberries, Onion etc.

Ø   SENSITIVE : Maize, Tobacco, Peaches, Cherries, Olives, Peas, Beans, Date palm etc.

  

o      CAUSES OF WATERLOGGING :

§              The soil is inadequately drained.

§             The seedbed is above the  level of the drainage channels.

§             The field is not level.

§             Too much irrigation water has been applied which cannot drain sufficiently quickly.

§             Rainfall has been heavy.

§             The soil is naturally heavy with poor structure and inadequate pore space.

§             After heavy rainfall waterlogging can occur even in light soils because crusting seals the soil surface and prevents air from entering.

§             Not restricting irrigation supplies during period of no demands.

§             High sub-soil water table conditions

o                  MINIMIZING THE EFFECTS OF 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.

§                    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.

§                    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.

§                    IRRIGATION SCHEDULE :

Adjust the irrigation timetable to allow for rainfall events.