Tree Planting in Problem Soils - Waterlogged Soils

Waterlogged Soils


These are the soils where water-table rises high and reaches within 3 feet of ground surface most of the time and thus affects crop roots. In extreme cases, the water-table rises even higher than ground surface. Waterlogging is very extensive problem in irrigated areas. It is caused by addition of huge amount of percolation and seepage water from canals, distributaries and water channels as well as inefficient irrigation practices and absence of any effective drainage system. The intensity of water logging is variable and depends on season, general land slope and soil porosity etc.
Waterlogging in Pakistan



Presence of gravitational water in the root zone for a long period of time excludes the soil air. Plant roots are therefore, unable to respire properly. Oxygen dissolved in soil water gets exhausted very soon because there is no new addition of oxygen to the standing water. In some cases, the ground water may pick up excessive amount of carbon dioxide and salts and intensify the problem of oxygen deficiency. The increased concentration of carbon dioxide in the ground water further complicated the situation.
Since respiration is one of the most basic physiological processes of plant which supplies energy for day today activities and growth, therefore, any disturbance in respiration of roots has overall depressing effect on all the life processes. Such plants lose vigour and are unable to withstand any adverse change in their vicinity.

The conventional approach to this problem has so far been to lower the water table by horizontal and vertical drainage. A further development to this approach has been minimizing the input of water at its source (lining of canals, realignment and straightening of pucca water structure) and in the field.  
A better way of overcoming this problem is to combine biological and engineering approaches together. As far as biological approach is concerned, the most important factor is to select suitable species and to modify the planting techniques. There are a number of tree species which can tolerate varying degree of waterlogging better than farm crops. Woody vegetation generally uses more water than grasses and forbes; desert grasses appear to be less efficient in water use than mesophytic trees, i.e. they consume more water for producing a unit weight of biomass. The above mentioned high consumption of water by trees is attributed to their following characteristics:
*      Higher leaf area index, crown raised high in the air, fully exposed to the wind and sun.
*      Extensive and deeper root system, utilizing a much larger volume of soil.
*      Longer growing season, which means longer transpiration period.
It is because of these reasons that tress lower the water table effectively and thus further improve the soil. The trees act very much like natural tube-wells (as well as air conditioners). It has been estimated that average trees growing in waterlogged conditions can transpire at least 30-40 kilograms of water per day in dry months. Considering long, dry, hot and bright season in Pakistan, the above figures of transpiration rate appear to be very conservative estimates. This much transpiration is equivalent to 50 cm of precipitation which in terms of drainage from the soil may mean 3 meter of subsoil water.

Regarding planting techniques the following points should be kept in view:
*      Mounds or ridges of appropriate sizes (one meter high) should be made to increase the distance between plant roots and water-table. The young plants can, therefore, grow unabated in their early life. During later growth stages when roots strike the water-table, these are able to survive there as a result of acclimatization. Although it sounds funny, additional water will have to be provided to young plants growing in water logged areas at the top of mounds or ridges during the first few months of their life.
*      Relatively large and vigorous plants should be used. Entire plants with or without ball of earth should be preferred.
*      Tree planting should proceed from least affected to most  affected areas.in first phase, one should plant to encircle the most affected areas by planting trees on least affected peripheral areas that surround it from all sides. With passage of time the most affected area will become smaller and smaller in size as a result of water consumption by trees. In the second phase the trees should be planted further towards the center of newly but partly reclaimed area. The tree planting thus continues to advance and the entire area gets reclaimed within a few years.
*      Water logging tolerant tree species be planted such as Salix tetrasperma (Hybrid Willow), Populus deltoides (poplar), Eucalyptus camaldulensis (Safaida), Terminalia arjuna (Arjan), Syzygium cuminii (Jaman) etc.





Location: Pakistan

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