This crop can be planted anytime. For most varieties, artificial irrigation is only indicated if planted in the dry season. It is best to get the plant established before the major rains start to counter soil erosion: the developing roots help to anchor the plant and reduce soil erosion common on slopes; the emerging leaves help to reduce the impact of heavy rain around the vicinity of the plant, which otherwise hastens erosion. I normally plant 11-month crops April/May but intend to switch to 18-month crops with mungbean rotation. I have grown the crop over 22 months (see attached photo of me with a 23kg example taken some 4/5 years ago) before but the first-line processors (who chip the tubers and sun dry them before selling them on) are starting to measure the starch content. 18 months is optimum for tuber weight and starch content (dry matter percentage). It is therefore likely that I’ll start to plant mungbean in March (it has a propagation to harvest timeframe of 95-105 days) and then plant cassava in June. The cassava shall then be harvested in December of the following year. I shall plant only half my land in this fashion during the first year, and the other half the following year (the other half during the first year will be planted in April for an 11-month crop).
As a nitrogen fixer, mungbean (there are other crops you could choose from) puts around 23kg of nitrogen back into the soil per rai (equivalent to one 50kg bag of urea, 46-0-0, per rai costing nearly 1,000 baht these days). The beans are harvested (normally at a profit) and the plants are ploughed back into the soil, releasing the nitrogen and improving the soil structure by adding bio-mass. Should you not wish to grow the mungbeans yourself, it is common practice to allow someone else to do so for free – your land benefits, whilst they make the investment and take the risks.
This two-year cycle of growing cassava reduces the workload nearly in half, and consequently your costs. You should at least double your yield compared to a one-year cycle and more than double your profit compared to a one-year cycle.
One problem using this cycle presents itself: if you harvest in December you cannot use the stems six months later (they have a useful life of up to 3 months). This can be resolved by cutting the required stems in June from the other half of your land, which should now be 12 months old. The plant has another 6 months to recover from this – it has very little effect on the yield. You may be able to sell the December stems.
Having used only the 7-disc plough , along with a ridger for several years, my soil below resembles a layer of concrete. This assists erosion on slopes since water cannot easily be absorbed by this compacted layer. It also reduces the reservoir of moisture available to the plants in the dry season. On flat level areas, it reduces the effective drainage thereby increasing the risk of tuber-rot. The shallow tillage with such a compacted layer also acts as a physical obstruction to the growth of the tubers. The solution is to plough first with a 3-disc plough (which cuts deeper), then follow up (after a few days to allow weeds to die) by cross-ploughing with the 7-disc plough. (These disc sizes pertain to large tractors such as my Ford 6600.) However, I think it best to use the 3-disc plough only every second crop-season to prevent the compacted layer occurring.
Slopes should be counter ridged to prevent soil erosion. Water gullies shall still occur – you can try to minimize these by placing objects (e.g. stakes) in the gullies to slow (not to prevent) the water-flow. Whatever you do, slopes are going to present a challenge if consisting of sandy soil. Integration with crops such as mungbean, peanuts, etc. or even strips of grass will help, but they too reduce the area for cassava cultivation or (when confined to between normal-spaced ridges) compete with cassava for nutrients, reducing the cassava yield.
Stems are cut from mature cassava trees and then cut into pieces measuring approximately 20cm – these cut stems are called setts. These setts should preferably have at least 5 nodes. Stick them into the top of the (normally) ridged soil either vertically or at an angle. Of the 20cm, stick 10-15cm into the ground. Make sure the nodes are pointing up (just as they were whilst in the ground before cutting). Better results (in terms of sett survival rates) can be obtained by immersing the setts in a solution of water containing a root promoter the night before planting (normally utilizing a 200-litre drum).
The Thai recommendation is 80-100cm x 80-100cm. I follow local practice by planting approximately 50cm x 95cm (I say approximately because we don’t bother to use measuring sticks – the exact spacing is not that relevant). The total yield per rai is not affected. The closer spacing helps to reduce weeds (because of a greater leaf canopy), and soil erosion on slopes (the greater canopy reduces the impact of rainfall on the soil and the more extensive roots help hold the soil in place).
It is vital to keep your crop weed-free during the first 3-4 months. Cassava is a slow grower initially and cannot compete with weeds. Weeds will certainly reduce your cassava yield. You can spray the newly planted area during the first three days with a pre-emergence herbicide (I’ve never done so to-date) then spray paraquat around one month after planting and again as often as required during the first 3-4 months (keep the nozzle low to avoid spraying the cassava leaves).
50kg of “15-7-18” per rai one month after planting, and another 50kg of “15-7-18” three months after planting. This first application of fertiliser can be replaced, if you wish, by 1,000kg of aged-manure before planting – whilst better, it is impractical for most farmers and more expensive. Of course, any amount of aged-manure or compost before planting in addition to the chemical-fertiliser after planting as described can only help improve the soil.
Pests & Diseases:
Unlike other parts of the world, cassava in Thailand is not seriously afflicted by pests or diseases.
Where the tree stems are to be used or sold for planting, they should be cut and handled fairly carefully (so as to avoid damaging the nodes), de-branched (leaving the leaves and branches to be ploughed in later), bundled (tie with string), and transported to a site where you can safely keep them for up to 3 months. They should be untied and stood on the ground (nodes up), ensuring that each stem is actually in contact with the soil. Ploughing that small area of soil immediately before standing the stems will make moisture available to the stems. Although not vital, standing them where they can benefit from some shade, e.g. on the north side of a tree, should increase their storage life-span. The sooner you plant them, the higher the survival rate, the fewer replacements needed. You can occasionally slightly wet the soil should you think the stems are drying out too much.
To now harvest the tubers, use a tractor with a mouldboard plough attached to the 3-point hitch at the back to loosen the tubers from the soil. Do not expose more tubers than can be collected that day – the starch content rapidly reduces once the tubers are exposed. You will need one person per tonne per day to follow behind the tractor, haul the tubers out, separate them from the woody stem-base with a machete, and load them into your transport. The tubers should then immediately be transported to the processor. The processor shall weigh your loaded vehicle before and after unloading the tubers – you shall receive a ticket indicating both weights and the agreed price, and will probably have to come back after a couple of days (some times a week) to receive payment.
You can chip and sun-dry the tubers yourself on a large area paved with concrete – don’t bother. It is not worth the effort. The extra risks and labour involved do not justify the slight extra financial gain.