There are a number of steps to follow when preparing the main tobacco lands for the crop. With the cultivation of all types of tobacco the seedlings are first raised in seedbeds away from the main crop and then planted out into the lands. The soil must therefore be well prepared to receive the young plants.
2. LAND PREPARATION
This must be done early in the year. Tobacco is planted out in September to October and ploughing should be done the previous February to March, before the end of the rainy season. The reason for this is to plough the land when it is still wet, so that the organic matter which is being ploughed in has time to decompose and release any nitrogen which is formed during the breakdown process. Nitrogen is a critical factor in tobacco production and the aim of the farmer should be to have as little organic or natural nitrogen retained in the soil. The farmer is then able to control the amount of nitrogen received by the crop through his fertiliser applications. Early ploughing allows for the release of organic nitrogen before the crop is planted, and also conserves moisture in the soil, another important factor. If ploughing is delayed until the rains have stopped, for example from April onwards, it should not be undertaken until the soil has completely dried out. In this case, the breakdown and release of organic nitrogen will be variable and uncertain.
Land should be ploughed to a depth of 30cm to allow plenty of loose soil for the construction of ridges. When the land is being prepared for planting out the young tobacco plants, any lime required should be broadcast and the furrows disc harrowed and rolled to produce a fine tilth.
The next step is to ridge up the land and at the same time apply the basic fertiliser dressing. Tobacco is grown on ridges for the following reasons:
To provide a good depth of fine soil for the young plants, so that they get a good start after being transplanted from the seedbed;
To give the correct distance between the rows so that the mature plants are not touching each other, causing damage to the leaves, and to allow easy access to the crop during harvesting;
To facilitate drainage of the soil during the rainy season;
To mark out the plant stations and excavate the hole where the young seedling is to be planted;
To allow each plant station to be fumigated against eelworm before the seedling is planted;
To allow the fertiliser to be placed at the correct depth and in the right place for maximum effect; and
To allow for the cultivation of the crop while it is growing.
The ideal ridge should have a broad base and be flat or slightly convex on the top. A ridge that comes to a sharp point or shape at the top should be avoided.
Figure 1: An Ideal Ridge
Once the ridge has been formed, the centre line should be marked and the holes for the plants made along this line.
The distance between ridges should be 1,2 metres, which is far enough apart to avoid excessive damage to the leaves during growing and harvesting, and not far enough apart to waste land. Large, early planted crops should have a wider spacing and smaller, late‐planted crops should have a closer spacing. Where the crop is being grown on lands with contours, the ridges should be drawn at a 45⁰ angle to the contour to assist in soil conservation and prevent runoff.
Figure 2: Ridges according to the contours
The basic application of compound fertiliser is applied in the ridge. The methods of doing this are as follows:
Applied with small measures or cups to the planting holes. Care must be taken to avoid the roots of the seedlings coming into direct contact with the fertiliser; and
Broadcast over the soil before ridging, and rely on the ridging operation to mix fertilizer and soil together and place the fertiliser in the right place.
The ideal place for the fertiliser is on either side of the plant roots, but not in direct contact with them.
Figure 3: Placement of fertiliser in the ridge
The holes where the young seedlings are to be planted are dug with a hoe along the top of each ridge. The distance apart between these holes sets the plant spacing and plant population of the crop, and this has a marked effect on the type of tobacco produced.
Closely spaced plants will produce thinner leaves, lighter colours, lower nicotine content and reduce the yield per plant, but can increase the yield per hectare. In a good season, with good growth, it will have very little difference on the yield, but a marked difference on the quality and grades of the tobacco. In a poor season, with poor growth, such as a drought year, the grades will be very little different but the yield will be reduced.
Recommended spacings of tobacco plants are as follows:
Light and Dark air‐cured
450 ‐ 600 mm apart, giving a plant population of 15 000 plants per hectare
Flue‐cured and Burley
450 ‐ 500 mm apart, giving a plant population of 17 000 plants per hectare
Fumigation should be carried out two weeks before planting on sandy soils, and three weeks before planting on heavy soils. The objective is to kill any eelworm in the soil. The chemical used is Ethylene Dibromide (EDB) and it is applied into the planting holes using a special injector gun. It is important to push the injector pipe into the hole to the correct depth before operating the plunger.
Figure 4: Hand Soil Injector and Placement
If at planting time, the holes still smell of EDB, planting should be delayed as this residual chemical will kill the young seedlings.
Lime: The best pH for tobacco is 5.0 to 5.5.
Soils which are more acid than this will:
tie up phosphate in the soil in forms which are unavailable to the plant; reduce the quality and grades of the tobacco leaves; and
reduce the yield of the crop
This is caused mainly by the acidity interfering with the uptake of nutrients by the plant.
Soil samples should be sent to a local laboratory as a rough guide.
The following amounts of lime will raise soil pH by 0,5:
|Sandy Soils||¼ ‐ ¾ ton/ha|
|Sandy Loams||¾ ‐ 1 ton/ha|
|Heavier Soils||1 – 3 ton/ha|
Phosphorus: The recommended rates for phosphorus are as follows:
|Rotated, well fertilized soils||50 kg/ha|
|Sandy loam soil (Burley)||50 kg/ha|
|Clay loam soil (Burley)||60 kg/ha|
Phosphorus should be applied before the seedlings are planted out, and incorporated in the ridges. Phosphorus deficiency causes slow growth, narrow dark green leaves on the plant and a poor quality, dark cured leaf.
Potassium: The normal dressing of potassium is 45 kg/ha for burley and flue‐cured tobacco, 85 – 95kg/ha on soils that release little potassium. This is applied to the lands before planting out.
Potassium is taken up by the plant early in its growth and the demand for potassium is during the 8 weeks after planting out. Potassium has marked effects on the plant, including the following:
Increases the drought resistance in the plant; Gives better burning quality to the cured leaf;
Increases the general vigour of the growing plant and delays the flowering and ripening; and Increases resistance to some diseases.
Potassium is applied to tobacco in the form of Sulphate of Potash rather than Muriate of Potash (KO), which contains chlorine. Small amounts of chlorine improve the quality of tobacco, but large amounts lower the quality.
Nitrogen: An important nutrient in tobacco production, as it affects both the quality and the yield.
The recommended rates for nitrogen application to the lands before planting out are as follows:
|Early ploughed (Burley)||80||‐125 kg/ha|
|Late ploughed (Burley)||120 ‐ 160 kg/ha|
|Sandy soils (Flue‐cured)||15||– 40 kg/ha|
|Sandy loams (Flue‐cured)||10||– 30 kg/ha|
These figures are for crops planted 2 ‐ 4 weeks before the expected start of the rains.
Further top dressings of nitrogen can be given while the crop is growing, depending on the growth of the crop and, above all, the colour of the leaves of the plant. A yellowing of the leaves indicates a shortage of nitrogen. A top dressing of Ammonium Nitrate can be given 2 ‐ 3 weeks after the transplanted plants have started growing, and a further dressing of Nitrate of Soda 4 ‐ 5 weeks later, if required. Nitrogen should not be applied in doses larger than 55kg/ha in one application. Much will depend on the amount of rain falling on the crop in the early part of the season, because heavy rain leaches nitrogen out of the soil before the plant can use it. If there is a spell of heavy rain (50 ‐ 75 mm in a week) during the 4 ‐ 5 weeks after planting out, up to 50 kg/ha of Ammonium Nitrate can be applied. If further heavy rains occur, 50 kg/ha of Nitrate Soda should be given, but no top dressing should be done later than 8 weeks after planting out. Tobacco should not be top‐dressed with nitrogen after 8 weeks from planting, as this will negatively affect the ripening and quality of the crop.
The effects of nitrogen applied to the crop in the correct quantities are:
It speeds up the rate of growth of the plant; It increases the total yield of the crop;
It causes the plant to produce larger, thinner leaves; It increases the moisture content of the leaves;
Because the plant grows more quickly, it matures earlier and; It reduces the levels of carbohydrates in the leaves.
A deficiency of nitrogen causes pale green leaves and poor growth in the plant, with a marked reduction in yield.
Magnesium: Applied to the lands with the lime before the crop is planted out. As magnesium isassociated with the chlorophyll in the green leaf, a deficiency causes yellow looking leaves.
Boron: Deficiency causes the growing point of the plant to go light green and die. Boron is included
in all recommended tobacco fertilisers. Excess Boron is very poisonous to the plant.
Sulphur: Deficiency causes discolouring of the leaves and consequently sulphur is supplied in allrecommended tobacco fertilisers.