Soil drainage

Drainage refers to the soil's ability to get rid of excess water (water in the macropores) through downward movement by gravity. It is affected by topography, texture, filth, depth, and the presence of pans (compacted or cemented zones). Nearly all major crops need fairly good drainage so that their roots can obtain enough oxygen; some exceptions are rice and most varieties of taro (Colocasia esculenta).

Poorly drained soils adversely affect crop yields in several ways:

• Roots lack adequate oxygen, since the macropores are largely filled with water.

• Soil-borne fungal and bacterial diseases are encouraged.

• Nitrate nitrogen (a nutrient) is subject to loss by a process called denitrification (see Chapter 6).

• Manganese and iron may become soluble enough to injure plant roots.

Although clayey soils are more likely to have drainage problems, they also may occur on sandy soils in cases where the water table is close to the soil surface. (The water table is the upper surface of the ground water, below which the soil is completely saturated with water.)

How to Spot Drainage Problems

You and farmers can easily spot areas of poor drainage in a field. Here's what to look for:

Topography: Poor drainage is most likely to occur on level fields or in low spots where water tends to collect after a rain or irrigation. Soils with even a gentle slope seldom have drainage problems but are likely to have the opposite problem of excessive water runoff.

Presence of Hardpans or Claypans: A hardpan is a hardened, cemented layer a few centimeters thick, usually located in the lower topsoil or upper subsoil. It remains hard even when wet, and restricts drainage and root growth. A claypan is a thicker, dense clayey layer in the subsoil which will soften somewhat when wet. It still impedes drainage and root growth. Dig a pit to check for such pans.

Crop Appearance: Crops growing in poorly drained areas will be stunted and yellow compared to surrounding portions. Beware, though, that other conditions such as nitrogen deficiency and disease can produce these symptoms. Suspect poor drainage only when stunting and yellowing are associated with low spots or areas of standing water.

Standing Water: Any area where water ponds for a day or two after rainfall or irrigation is likely to be poorly drained.

Subsoil Color: Red, reddish brown, or Yellow subsoil colors indicate very good drainage. That's because the presence of sufficient air allows the soil's iron and manganese to remain in the oxidized form, indicated by bright colors. Dull greys and blues indicate a reduced state (little oxygen) which means poor drainage. Some soils in wet-dry climates have subsoils with alternate streaks of bright and dull colors. This color pattern is called mottling and indicates fluctuations in soil drainage (i.e. good in the dry season, poor in the wet season) caused by the seasonal variation in the height of the water table.

How to Test Soil for Poor Drainage

The hole test: Dig a hole 60-90 cm deep and fill it with water; allow it to drain, and refill it again. In a well drained soil, the water level should fall by 2-3 cm an hour and disappear in 24 hours. However, if poor drainage is being caused by a hardpan or claypan, this test won't be valid, as you will have overcome the problem by digging through them.

Checking for a high water table: Poor drainage in low spots is often caused by a high water table. Ideally, the water table should be at least 100 cm below the soil surface, at least during the cropping season. When digging, you can easily tell when the water table has been reached, since water will begin to pond in the hole.

NOTE: In some cases, a high water table can actually benefit crop growth by supplying water to the roots during long dry spells by upward capillary movement (as long as it's not high enough to affect drainage in the major root zone area). However, there's always a risk of poor drainage in wet years on such land.

Dealing with Drainage Problems

First, determine what is causing the drainage problem before deciding which of the methods below will be effective.

• Seedbed styles for poorly drained areas: Growing crops on raised beds or ridges can alleviate drainage problems that aren't serious. See Chapter 4.

• Breaking up pans: This can be done with digging hoes and picks or with tractor-drawn sub-soilers (narrow shanks that penetrate 40-50 cm deep). Hardpans can sometimes be permanently fractured and loosened. Claypans, however, tend to reconsolidate after being loosened, especially since they're usually moist and don't tend to fracture. On small plots, the best way to permanently loosen a claypan is by double-digging the soil and adding an organic soil conditioner to the pan area. (Double-digging is covered in Chapter 4.)

• Drainage ditches for surface water: These are shallow, wide ditches that follow the natural depressions in the field to conduct water away. Make sure the outlet is satisfactory, so one farmer's drainage problems won't be passed on to another.

• Drainage ditches for subsurface water: Shallow ditches remove only surface water. To remove excess subsurface water, deeper and more numerous ditches can be used. They will "pull" (attract) this excess water from the soil between them. These ditches are usually spaced about 15-45 meters apart, depending on the soil (the closer spacings for clayey soils) and are dug 30-60 cm deep. Top widths range from about 2-5 meters and bottom widths about 1.5-2 meters. Ditches with V-shaped sides permit the passage of farm machinery. Of course, the ditches must be designed to convey the water off the field and eventually into a natural drainage way such as a stream.

• Drainage tile or Plastic pipe: Drain pipe can be rayed 80-100 cm underground to drain saturated subsoils and conduct the water off the field. Short sections (30-40 cm long) of 10-12 cm diameter clay pipe can be laid end to end in a trench and covered with straw, building paper, or earth to facilitate water entry and retard plugging. Flexible, perforated plastic pipe may also be available for this purpose in your country. The pipe or tile are laid at a slight slope (about 25-50 cm per 100 meters of length) and lead to an outlet such as a ditch or canal. The distance between the tile or pipe lines varies from about 10-20 meters on clayey soils to 30-90 meters on sandy soils. If the land has a natural drainage way, running such an underground drainage line along this path can speed up the removal of water from these areas of accumulation.

• Land leveling will fill in depressions and lower high spots, although the high spots may end up losing lots of topsoil. Animal-drawn scrapers can be fabricated locally.

Seedboxes have special drainage problems: See Chapter 4.