Increased usage of potatoes, particularly in making French fries and crisps, has made the crop one of the most important in the country.
Potatoes are mainly grown in counties in Rift Valley, in particular Narok and Nyandarua. It is, thus, important to understand how to grow them successfully.
The crop needs deep fertile, loose and free-draining soils. Soils with a high sand content are very suitable as long as they are fertile enough.
The crop requires a pH of 5 to 6. If it is lower than that, it is recommended you add the necessary amount of lime before planting.
Like cabbage, potato is a highland crop, which needs cooler conditions and sufficient water supply. Under cooler conditions, potatoes can be grown year round.
Always buy certified seeds or use your own that are disease-free but don’t use those some people hawk around after picking from market places.
Potatoes are extremely susceptible to diseases that are spread through the seed potato and then introduced in your soil, which will ruin it for many years.
To start the planting process, spread the seeds in a single layer in crates and keep them under semi-light conditions until they start to sprout.
Place the potatoes in ridges and cover with compost and topsoil. The trenches/ridges should be 75cm apart and a spacing of 30cm within the row is recommended.
When the plants are about 15cm tall, start “earthing them up” by bringing the soil back against them or even top-dressing with compost so that only the top leaves remain visible.
As the plants mature, keep repeating this until the potatoes grow on ridges. This will encourage new growth of side roots that will continue to produce more potatoes and thereby increase your yield drastically.
Water regularly if you grow your potatoes outside the rainy season as they need about 500mm of water during their growth cycle.
Water stress will lead to reduced yield. During the early growth phase, the soil should be kept moist constantly to a depth of 15cm.
After the plants start flowering, reduce irrigation to once or twice per week.
Potato is a crop with a high potassium (K) demand of 140kg per acre, with most of it being used up during the tuber bulking stage. Nitrogen is taken up by potatoes at a rate of 97kg per acre and needed mainly during the early growth stages.
Phosphorus (P), on the other hand, is only needed at 12kg per acre. Potassium can be supplied by the soil, but that can only be determined by a soil test. Wood ash and sheep manure are good organic sources for the nutrient.
Potatoes sprout and emerge within 15 to 30 days after planting. Tuber initiation starts again between 15 to 30 days after emergence. This stage coincides with the flowering of the plant.
At this stage, the plant has the highest nutrient needs. Therefore, fertiliser application is split into “before planting” (especially nitrogen) and top-dressing after six to eight weeks.
This should coincide with the weeding or earthing up to incorporate the fertiliser into the soil quickly and to bring it close to the plants for easy uptake.
Potatoes mature 90 to 120 days after emergence. Harvest when the plant has died as early crops cannot be stored. Dig up the tubers carefully to avoid damaging them and let them dry a little in rows before you gather them.
The former is caused by soil-borne bacteria (Ralstonia solanacearum) and is favoured by high temperature and wet soil while the latter is caused by a fungus (Phytophtera infestans) and is favoured by cold and wet conditions.
Bacterial wilt also attacks tomatoes and is hosted by a large number of weeds. The major symptom (wilting) becomes more severe under dry conditions.
It starts from the lower leaves, rapidly moving upwards and eventually killing the entire plant.
The disease can be identified by brown rings in potatoes when cut open. Use of certified seeds, crop rotation and hygiene are the only ways to prevent bacterial wilt.
The first symptoms of Late blight are spots that enlarge and kill the entire leaf. The whole plant eventually rots.
In weather conditions that favour a blight outbreak, fungicides should be applied regularly. Again, hygiene in the field, which include removal of harvest residues, is a must to control blight.
However, the most destructive insect pest in potatoes is the potato cyst nematode (Globodera rostochiensis and G. pallida).
Unlike free-moving nematodes, these are plant-parasitic nematodes, which means, they attach themselves to the roots and tubers of the potato plant, and feed on them.
Nematode damage weakens the plants and makes them more susceptible to drought stress or nutrient deficiencies in the soil.
The first symptom is stunted growth. Nematodes cause small wounds on the roots through which other diseases like wilt and viruses enter the plant.
Control of nematodes always has to be a multi-angled approach. Start with certified seeds. Practice crop rotation. Ideal crops are maize or green beans or fallows planted with green manure like Crotolaria or alfalfa.
Test the soil for nematode infestation as soon as you see the first signs. Chemical control is done by nematicides, either as fumigants or non-fumigants.
