One of the most important agronomic practices that costs nothing but gives greater benefit is the depth of sowing. Optimum depth of sowing is needed for better availability of moisture, avoidance
of bird damage and quick and uniform germination of seeds. Depth of sowing varies form shallow (2-3 cm) to deep (10-12.0 cm) depending on the soil type and moisture condition. Generally optimum depth of sowing of most field crops is 3-5 cm. Deeper sowing is advantageous in sandy soils while in heavy soils shallow sowing is beneficial. Under dryland conditions, crop sown in deeper layers utilizes the moisture and germinates better. Very shallow sowing promotes quick germination of seeds but later the seedlings cannot withstand surface drying. Good soil contact is
very important with surface or shallow sowing to ensure efficient water uptake. This is generally improved by raking the soil after seed sowing. Bold seeded crops can be sown deep (up to 10 cm).
However, small seeded crops should be sown at a shallow depth of 3-5 cm. Seeds mainly depend on the reserve food for growth until it becomes photosynthetically self-sufficient. Larger sized
seeds contain considerable energy store and can grow for a longer time without becoming photosynthetically dependent. Hence larger sized seeds are sown deeper than the small sized seeds
with a limited energy store.
Sowing depth is also determined based on the mode of seed germination (hypogeal and epigeal).
In the hypogeal mode of germination, the cotyledons remain in the soil. Such seeds, if sown deep, neither delays nor retards germination. With the seeds of epigeal mode of germination, cotyledons emerge out of the soil. Such seeds, if sown deep, will fail to germinate. There is difference in coleoptile length between the varieties of the same crop.
Methods of Digging
Digging is dependent on the type of soil. For clay, digging is best undertaken in the autumn, leaving it exposed to the winter frost to break up the heavy clay sods. For light soils should be left until the early spring. If the soil has become hard and compacted, a fork may be the most useful tool. It is easier to insert into the ground than a spade and is effective at breaking up lumps and clods. You can opt for either a single or a double dig.
This means taking out a single spit (spade's depth) of soil. A single dig will be sufficient if the soil compaction is confined to the topsoil. Proceed as follows:
Lay a string line across the width of the area to be dug, giving you a 1 ft (30 cm) wide strip. Take out a spite of soil from one end, and place it in the corner diagonally opposite.
Facing this hole, and working backwards from it, drop a forkful of compost or well-rotted manure into the hole. Take out a spit of soil from the nest 1 ft (30 cm) of earth in your strip
and drop this on top of the organic material. Continue in this manner until you reach the end of the line and have dug a strip the width of your fork; you are left with a hole 1 ft (30 cm)
Move the string on a spade's width. Drop some organic material into the remaining hole in the first line. Take a spit of earth from the second line and drop it into the remaining hole in the first line. Continue in this manner along line two, and repeat until you have reached the end of the last line, dropping a forkful of organic material into each hole, until you reach the original spit of earth which will be waiting to be dropped into the last hole.
If you are using fine-quality, tilth-like compost, this is best reserved for the surface. If the soil is poor, you could incorporate some, and leave the rest for the surface.
Double digging is the taking out or working on two spits of soil, down to the subsoil level. For new ground, or for the soil of poor state or overgrown with weeds, digging is the arduous task. It will certainly be necessary if compaction has formed a hard pan at subsoil level. This will improve the structure of the subsoil. It is important, however, to take care that you do not allow the subsoil to be mixed with your topsoil. There are two ways to do a double dig.
The first is to take out your spit of topsoil as with a single dig. Then, before dropping in organic matter and filling the hole up with topsoil, use your fork to loosen the subsoil. This
will help break up the compaction and aerate the subsoil.
The second method is more thorough, and even more arduous. It requires a spit of subsoil to be excavated, after you have taken out the spit of topsoil. The technique is as follows:
- Proceed as for the single dig but start by removing two spits of topsoil to the far diagonal corner and also one spit of subsoil.
- Turn the exposed subsoil from hole two into hole one. Incorporate organic matter.
- Start hole three, taking a spit of topsoil and dropping it into hole one, over the organic matter. Take a spit of subsoil from hole three and drop it into hole two.
- Proceed right through (the area until the last two holes are reached.
- If weeds are being incorporated, take care to turn the topsoil spit over when incorporating so that any perennial weeds are buried at least 6 in (15 cm) deep.
Double digging, is hard work. In subsequent years, it should not be necessary if:
You maintain the organic matter with enough compost to encourage the present of worms, whose movements have the same effect as your digging
You take care not to trample the soil when it is wet, thus squeezing out the air and preventing water from draining away. In that event, you can look on it as a one-off tonic for poorly soil.
Application of organic mulch to the soil surface can avoid digging. This may a labor-saving device. It is a superior method of soil management because it avoids disturbance to its structure
and microlife. For this continued and generous application of organic matter to the surface is necessary.