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close this book Soils, Crops and Fertilizer Use
close this folder Chapter 4: Seedbed preparation
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View the document Some handy seedbed skills for intensive vegetable production

Some handy seedbed skills for intensive vegetable production

In this section, we'll cover these useful seedbed skills:

• Seedbed layout for intensive vegetable gardening

• Double-digging

• Dealing with heavy clay soils . Dealing with soil crusting problems . Making up a soil mix for nursery seedboxes


Western vs. Intensive Seedbed Layout

A typical Western-style vegetable garden, with each of its rows wide enough to walk down, makes poor use of space. In this type of garden, the rows are anywhere from about 60-90 cm apart, even though the plants' foliage and root spread may cover only a portion of this (see Fig. 4-6a). In fact, the usual row spacings recommended by most county extension offices in the U.S. (e.g. 90 cm for peppers and cabbage, 76 cm for turnips, beets, and lettuce, 60 cm for radishes) result in a garden with only about 30 percent of its space occupied by the plants and their roots; the rest of the area is essentially walking space! Such a layout wastes valuable land, water, and fertilizer; it also requires more time and labor for weeding and watering. In addition, providing a walkway between every row greatly promotes harmful soil compaction.

The bed-and-alley system of intensive gardening is an alternative to the wide-row system and may offer a number of advantages under non-mechanized conditions (the system isn't suited to animal- or tractordrawn operations once the beds are planted). Here are the features and possible benefits of of the bed-and-alley system:

• The garden is arranged into beds of closely-spaced plant rows; walkways (alleyways) are placed only between the beds instead of between each row (see Fig. 4-6b).

• The beds' widths can range from about 100-130 cm, which enables all the plants and bed area to be easily reached without walking in the bed. The beds (they can be flat, raised or sunken, depending on soil and rainfall conditions) are best kept to 10 meters or less in length so that alleyways can be run up and down, as well as across the garden to facilitate access. The alleyways can be as narrow as 50-60 cm, although occasional wider ones (up to 100 cm) are useful for wheelbarrow traffic.

Figure 4-6: Wide-row gardening system (A). Bed-and-alley system (B).

Since there's no need to walk in the beds, row spacings can be based solely on what's needed for plant spread, which is usually much less. For example, cabbage rows can be 3045 cm apart (vs. 90 cm), leaf lettuce rows 1520 cm apart (vs. 75 cm), and carrot rows 5-10 cm apart (vs. 75 cm). In the case of root and leaf vegetables, the between-row spacing can be as narrow as the in-therow spacing. Depending on the crop, anywhere from 1.5 to 15 times more plants can be grown per sq. meter of land (bed and alley area included) with the intensive system.

Using 120 cm-wide beds and 60 cm-wide alleyways, the intensive system will give a landuse efficiency of about 65 percent, compared to about 30 percent for a typical wide-row garden. In other words, such an intensive layout would consist of 65 sq. meters of actual bed area and 35 sq. meters of alleyway area per each 100 sq. meters.

• Aside from the savings in space, labor, and water, the intensively-spaced plants eventually form a "living mulch" with their leaves that effectively shades most of the bed's surface. This reduces water losses from surface evaporation, helps suppress weeds, and cools the soil (beneficial for roots of cool-season crops like cabbage grown in hot weather). (Note that under high-rainfall, humid conditions, such close plant spacings may promote the development and spread of some plant diseases, especially fungal leafspots.)


What is Double-Digging?

As we've seen, most land preparation methods loosen the upper 15-20 cm of soil (the topsoil). Double-digging is a method of loosening, as well as improving/enriching, a much deeper layer of soil down to 40-60 cm which includes some of the subsoil, too.

Benefits of Double-Digging

Under appropriate conditions, double-digging may have some important benefits:

• Yields may be increased due to better and deeper root growth and improved aeration and water availability. In very hot areas, it may allow roots to grow deeper into cooler, more hospitable soil.

• In some cases, double dug plots may produce the same yield on less area (1/2 or less in some cases); this means that less land, and, more importantly, less water and labor may be needed.

• Where double-digging results in more production on less land it may make it feasible to use other possibly beneficial practices like crop rotation and green manure crops. (These 2 practices are explained in Chapter 8.)

Limitations of Double-Digging

• Double-digging is not a "cookbook recipe" for increased yields. Like many so-called "improved" practices, it should first be tested locally before promoting it.

• It requires much more time and labor than standard land preparation (probably at least 3 times as much), and therefore is seldom feasible on anything but small plots.

• If the subsoil is very acidic (below pH 5.0-5.5), it may require liming to enable root development there.

• Double-digging will not solve subsoil drainage problems caused by a high water table.

Situations Most Likely to Benefit from Double-Digging

Double-digging is best suited where crops are grown in beds (flat, raised, or sunken) that have alleyways around them. Such a double-dug bed will not be recompacted by foot, animal, or machinery traffic, as would be the case with a wide-row, open-field plot. Beds are most likely to respond well to double-digging under the following conditions:

• Dense, compacted! clayey subsoils

• Deeper-rooted crops like tomatoes, eggplant, okra, squash, and beans may benefit more than shallow-rooted crops like onions, lettuce, cabbage, and radish. However, even some shallow-rooted crops can put down roots 40-50 cm deep with good soil conditions.

• Hot-dry climates where increased root penetration is likely to improve water availability and allow roots to enter cooler soil.

How to Double-Dig

The best way to learn double-digging is to do it. A shovel is sufficient, but a digging fork or pick may come in handy for very hard subsoils. Here are the steps:

1. Stake out a 1 x 4 meter bed.

2. Start at one end and dig out a trench 30-40 cm wide to the depth of your shovel blade. Carry the soil to the other end of the plot and deposit it just outside as in Figure 4-7a.

3. Now add a layer of compost or manure at least 2 cm thick to the bottom of the trench. Work it into the subsoil, ideally to the depth of the shovel blade. It's not necessary to invert the subsoil, as long as the organic matter can be mixed into it. Although some gardening manuals give dire warnings about mixing different soil layers, this is unlikely to cause any problems. Soil conditioners like rice hulls or peanut hulls can be added along with compost or manure; however, if used by themselves, they're likely to cause a temporary nitrogen tie-up. (Refer to the section on soil conditioners in Chapter 8.)

NOTE: Double-digging will be largely ineffective unless you add manure, compost or some sort of soil conditioner to the subsoil; otherwise, it may soon recompact and also will remain infertile, thus discouraging root growth.

4. Now dig the next 30-40 cm-wide trench, but this time throw the soil into the trench you made in Step 2. (See Figure 4-7b.)

5. Repeat Step 3 with the soil in the bottom of the 2nd trench.

6. Continue this sequence until you reach the end of the bed.

7. After loosening and enriching the subsoil in the last trench, fill in the trench with the soil that you carried to that end of the bed in Step 2. (See Fig. 4-7c.)

FIGURE 4-7: Double-digging a bed.

A shortcut: When double-digging a series of adjacent beds, you can avoid carrying the soil from the first trench to the other end of the bed (see Step 2 above) by using the method shown in Figure 4-8.

How often to double-dig?: Double-digging a plot once a year should be sufficient if the bed-and-alla, system is used, since it will help avoid recompaction. It may take several years of annual double-digging to measurably improve the soil; after that, you may be able to do it less often.

FIGURE 4-8: Shortcut method for double-digging adjacent beds.


Tillage, bed construction, and planting can become very difficult or even impossible when clayey soils are too wet or too dry. This is because they have a very narrow moisture range within which their filth (workability) is satisfactory. Aside from adding sand and organic matter (see Chapter 2), there are several other techniques that will help:

, Whenever possible, allow overly wet, clayey soils to dry out to the point where filth again becomes satisfactory before attempting to work them. Otherwise, you will actually worsen the soil's condition.

• Pre-watering dry, clayey soils a day or two prior to working them can do wonders for filth. Where the entire topsoil is very dry, apply at least 25 liters of water per sq. meter. Allow the soil to dry to a point where it has lost much of its stickiness and can be worked without producing clods that resist being broken down well by hoeing. You may also want to try doing the initial tillage of the soil while it's still dry and then applying water before breaking up the clods.

• Coarse materials like sand and rice hulls are excellent filth improvers for clayey soils. If in short supply, first prepare the bed for planting and then hoe in the material into the top 2-5 cm of soil. This will greatly improve the filth of this shallow planting layer and facilitate seed sowing and seedling emergence. Another way to efficiently utilize limited amounts of soil conditioners is to apply them in narrow strips centered over the rows.

• On clayey soils, seedling emergence from small vegetable seeds can be greatly improved by covering them with sand, compost, or sawdust instead of soil. Covering the seedbed with a pre-emergence mulch of hay or newspaper will help keep the soil moist for sprouting and allow for shallower planting.


Some soils high in clay or silt and in poor filth will form a cement-like crust at the surface each time it dries out. This can seriously affect seedling emergence, especially in the case of the blunt dicot seedlings (the ones that emerge with 2 seed leaves). Here are some suggestions for handling soil crusting:

• Make regular additions of compost or manure to maintain a healthy level of soil organic matter which helps counteract the tendency to crust.

• Cover small vegetable seed with sand or sawdust instead of the soil.

• Work in sand into the top 2-3 cm of soil.

• Apply a pre-emergence mulch to the soil after planting to help keep the soil surface moist and free from crusting. It also will absorb the impact from rain or overhead watering which encourages crusting by packing the soil. Straw or dried grass works well but must be removed as soon as seedlings begin emerging or they'll quickly become spindly and weak due to lack of sunlight. However, in some cases, such mulches do more harm than good by attracting harmful insects like crickets, ants, and termites. You can also try a "grow-through" mulch of light, fine material like rice hulls or sawdust which doesn't need to be removed at seedling emergence.


If you raise vegetable transplants using seedboxes, you'll quickly find out that straight soil, by itself, seldom works well as a growing medium. When confined to a shallow seedbox or pot, most natural soil becomes very poorly drained in the bottom half of the container, no matter how many drainage holes you make. Even lining the bottom with shells or pebbles won't help much. That's because there's no soil below to provide a suction force to draw the excess water away - gravity by itself just isn't strong enough. The same thing happens when you hang wet clothes on the line - a lot of water collects at their bottom end. So, with seedbox soil you'll want to make up an extra-coarse soil mix that will facilitate drainage yet still hold enough moisture. Another advantage of soil mixes is that they're lighter than regular soil which makes it a lot easier to move the seedboxes around.

Here are a few recipes for soil mixes for seedboxes or other containers - try out your own, too, and ask local farmers for their suggestions.

NOTE: As with in-the-ground nursery seedbeds, seed box soil mixes should be sterilized by heat or other method prior to planting to control diseases and nematodes.

• Mixing rice hulls with soil has proven very successful. One example:

Philippines horticulturists recommend mixing rice hulls with clay loam soil at a 2:1 ratio by volume and then adding 1 gram of single superphosphate (0-20-0) and 1 gram of 14-14-14 per liter at sowing. Another gram of 14-14-14 per liter is applied at 10 days and again at 20 days after sowing. Rice hulls have a strong capacity to tie up nitrogen and are also low in other nutrients, so that's why the chemical fertilizer is added. (N tie-up is explained in Chapter 6.)

• A 1:3 sand-compost mix. Rotted coconut husk fibers passed through 1/4-3/8 inch (6-10 mm) hardware cloth can be used to make a great instant compost.

• A 1:1:1 sand-soil-compost mix.