Problems

In general terms, wet soils are defined as soils where saturation with water is the dominant factor determining the nature of soil development and the types of plant and animal communities living on the surface [2]. Wetness occurs whenever the water supply by rainfall, overland flow, and seepage from upslope exceeds the water loss by drainage, throughflow, and evapotranspiration. Poor drainage usually is associated with soils that have low permeability. Often, soils are shallow and/or overlie an impermeable barrier such as rock or very dense clay pan. The impermeable subsoil prevents the water from moving downward and the natural subsurface drainage system from functioning properly. Many of these areas have flat topography with depressions and, in some instances, the areas may lack adequate drainage outlets. Representative examples include the basin clay soils of fluvial deposits, pseudogley soils of central and eastern Europe, planosols of the semihumid or semiarid tropics, vertisols of the semiarid tropics, and glacial till soils.

Experiments that have been conducted to determine the effect of soil wetness on crop yield generally show a reduction in yield at shallow water-table depths (see Fig. 4.2). Frequency and duration of high water tables as well as the crop sensitivity to waterlogging

Table 4.1. Relative tolerance of plants

Tolerance

High Groundwater

Level

Levels

Waterlogging

High

Sugarcane Potatoes

Plums Strawberries

Broad beans

Several grasses

Medium

Sugar beets

Citrus

Wheat

Bananas

Barley Oats

Apples Pears

Peas

Blackberries

Cotton

Sensitive

Maize

Peaches Cherries Raspberries Date palms Olives

Source: Modified from [4].

Source: Modified from [4].

Figure 4.2. Yield depression as a function of mean water-table depth during the growing season for two types of soil. Source: [3].

also have a relevant impact on yield (see Chapter 5, Section 5.5). Table 4.1 gives some indications of the relative tolerance of some plants to waterlogging.

Wet soils are an obstacle to man's use of land resources for cropping and, in general terms, for economic development. However, wetlands play a major role as a natural resource, which may be superior to the benefits from agricultural resources. To bring these soils back into beneficial agricultural use, they must be reclaimed. To this end, it is necessary to eliminate the depressions, to provide sufficient slope for overland flow, and to construct channels to convey the water away from affected areas.

Table 4.2. Drainage coefficients for typical agricultural basins

Drainage

Water

Slope of

Coefficient

Region

Crop

Source

P5a

land (%)

(s-1 • ha-1)

Netherlands^

Grasses, crops

Rain

31-44

0.05-0.20

1.0-1.3

Yugoslavia0

Grasses, crops

Rain

52-64

0.25-0.50

3.5

Sudan

Cotton

Furrow irrigation

64-90

0.05-0.10

4.0

Japan

Rice

Flooded

80-115

0.05-0.20

5.0

Tanzania

Sugarcane

Sprinkler irrigation

145-165

0.20

7.0

a 24- to 28-hr rainfall for a recurrence interval of 5 years. b Subsurface drainage. c Subsurface and surface drainage. Source: From [6].

a 24- to 28-hr rainfall for a recurrence interval of 5 years. b Subsurface drainage. c Subsurface and surface drainage. Source: From [6].

Growing Soilless

Growing Soilless

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