Making the right seedbed for the crop, soil, and climate

NOTE: Seedbed preparation is very location-specific and varies with climate, soil type, crop, management level, and available equipment. Local farmers usually have good seedbed skills, so beware of tampering with local methods before thoroughly testing new ones.

What Kind of Seedbed?

There are basically 3 types of seedbeds: flat beds, raised beds, and sunken beds. The best type to use depends much more on the particular climate and soil conditions than on the crop.

Flat Beds

Flat beds are used where water availability is adequate and there are no drainage problems. In some areas, crops like maize, sorghum, beans, and potatoes are started out on a flat bed; as the season progresses, soil is thrown into the crop row to mound up the plants; this is called "hilling-up" and is done to control in-row weeds, provide support, and improve drainage. (Potatoes are also hilled up to keep the developing tubers covered with soil.) Hilling-up only works with plants that have enough stem height and leaf clearance to tolerate partial burial.

Raised Beds

Crops can also be grown on raised-up beds or ridges. They are especially advantageous for clayey soils under high rainfall or wherever else drainage is likely to be poor. They can also be used in many other situations. Where crops are furrow irrigated, raised beds or ridges are essential so that the water can flow down the furrows between them.

Height of raised beds: Raised beds are usually 10-30 cm high. The best height depends mainly on soil texture and moisture considerations. For example, raised beds are often 2030 cm high on clayey soils under high rainfall where poor drainage is likely to be a problem. On coarser-textured soil under the same conditions, bed height might be 15-20 cm. When raised beds are used in drier conditions, a bed height of 10 cm or leas may be best to avoid excessive moisture loss due to evaporation from the exposed sides.

Width of raised beds: Typically they are 100-130 cm wide.

Raised beds may have several advantages:

• Much better drainage compared with flat or sunken beds.

• They provide a double layer of topsoil, because they're made by dragging in topsoil from the surrounding alleyways. (Because of this, they're also likely to be looser than flat or sunken beds.)

• In temperate regions, raised beds warm up more quickly in the spring, which may benefit cold-sensitive crops and even permit earlier planting.

• Plants on raised beds are easier to reach when doing hand operations such as weeding and thinning.

Raised beds usually aren't a good choice during the dry season, because they dry out more quickly than flat or sunken beds; also, water tends to run off them and be lost into the alley-ways. These disadvantages can be partly overcome by mulching the bed with straw or rice hulls, making a lip around the bed's edge to reduce run-off, and by reducing bed height to 10 cm or less (see Fig. 4-4).

FIGURE 4-4: Two types of raised beds. Bed A is best suited to high-rainfall areas. Bed B has a lip around all 4 sides which helps prevents prevent water from running off (helpful in drier conditions).

Sunken Beds

In dry regions, especially on sandy soils with low water-holding capacity, vegetables can be planted in sunken beds (i.e. shallow basins) about 100-130 cm wide and 2-5 cm below the surrounding soil level. Sunken beds conserve water much more effectively than raised beds for 2 reasons:

• Sunken beds don't have the exposed sides of raised beds from where considerable moisture can be lost by evaporation.

• None of the applied water is lost by runoff.

FIGURE 4-5: A sunken bed. Depth shouldn't exceed about 4 cm.

One disadvantage of sunken beds is that some topsoil is lost in the usual method of construction. (They're made by pulling off soil from the bed area and placing it in the surrounding alleyways). This probably won't affect crop growth, as long as the topsoil is of normal depth let least 15 cm) and enough compost or manure is added. Here are 2 ways of building sunken beds without sacrificing topsoil:

• First take off the topsoil, and then replace it after removing enough subsoil to sink the bed enough.

• Make a "pseudo" sunken bed by mounding alleyway soil around the bed's borders. This will work well in clayey soil, but border dikes made of sandy soil may wash out when the bed is watered.

One variation of sunken beds is furrow Planting in which crops like maize, sorghum, and beans are planted in the furrow bottom between two ridges where soil moisture is higher and less easily lost. Soil can then be thrown into the furrow as the season progresses to control weeds and improve drainage if rainfall increases.