Farmer's Weekly (South Africa)
Growing potatoes: Part 2
To produce a good yield, the crop requires moist, loose, highly fertile soil without clods.
Begin by ensuring that the soil is damp when you till it; this will help prevent deterioration of the soil structure. Cross-rip the land to a depth of at least 50cm. Deep ripping is important, as the potato plant has a relatively weak root system and is sensitive to compacted soil.
After ripping, plough the soil to a depth of at least 30cm. Incorporate all crop residue to ensure that any plant material will decompose completely. Use a disc to break up clods and level the soil.
Immediately before planting, irrigate the land lightly to ensure that the soil is moist and any remaining clods break up during tilling.
soil chemistry
A well-prepared seedbed comprises a 15cm-deep layer of loose, finely crumbled, moist soil. To obtain this, till the land with a springtined cultivator with roller, a rotavator or a disc harrow.
Potato productivity is influenced by soil chemistry (pH, soluble salts and inherent fertility), texture (the proportions of clay, silt, sand and organic matter) and physical condition, especially compaction.
To determine the chemical status of the soil, arrange for a soil test before planting. Potatoes are adapted to a wide soil pH range, but you may have to modify the pH to optimise production. This in turn can affect nutrient availability and the activity of soil pathogens.
Potatoes require highly fertile soil. Before planting, sample the land extensively to determine fertility. For every 2ha to 5ha to be planted, take between 20 and 30 topsoil samples (each 20cm deep and weighing 500g) and five subsoil samples (each between 30cm and 60cm deep).
Mix the topsoil samples thoroughly and take one composite sample of 1kg for testing. Do the same with the subsoil samples, but use a 500g sample. If the soil varies noticeably across the land, take a separate sample of each type and have it tested. Note where the different samples were taken.
When you send soil samples to the laboratory for analysis, provide information about irrigation practices, planting date, cultivar and target yield.
applying fertiliser
Potatoes have a poor root system, so the fertiliser should be applied at the same level (or just below) as the seed tubers in the furrow during planting. The seed tubers and fertiliser should not come into direct contact with each other.
Where top dressing is recommended, apply it on both sides of the plant just before ridging, then ridge and irrigate. • Source: ‘Grow potatoes profitably’ in ‘Production Guideline for Winter Vegetables’. 2013. Agricultural Research Council.
safe and do, in fact, reduce parasitoid numbers. This is not too much of a problem, provided the insecticide kills more leaf miners than parasitoids; this will enable the wasps to build up their numbers again and take control.
Consulting for a farmer some years ago, I noted that the parasitoids were increasing on his beet lands. On one land, however, they were nowhere to be found. Asked what had happened, the farmer said a chemical representative had assured him his product would not kill the wasps.
I looked up the product. The wording stated that it was ‘soft on predators’, but clearly that did not apply to parasitoids.
Today, many farmers I visit have stopped using harsh chemicals. The result is that the parasitoids are taking control, and the crops no longer have a leaf miner problem.
• Bill Kerr is a vegetable specialist and a breeder of a range of vegetables. Email him at farmersweekly@ caxton.co.za. Subject line: Vegetable production.