Potatoes are the fourth most important food crop in the world, after wheat, maize and rice. They are also the second highest vegetable producer of protein (second only to soya beans) and have a more balanced content of minerals and vitamins than any other major carbohydrate food crop.
By Arno Visser and Ineke Vorster
Potatoes are not only consumed by man and livestock, but are also used for industrial purposes. They may be boiled, fried (crisps and chips), baked, mashed and used in stews. They are also used as livestock feed and to make starch, spirits and industrial alcohol.
Climate
The potato is a cool weather crop that realises its highest yields and best quality in regions that have a temperate climate and a long average day length during the growing season. Diseases, pests and water stress at any time while the potatoes are growing are major constraints to production.
Planting Date
- This will depend on the climate of the area in which you are going to plant potatoes.
- Many cultivars don’t yield well if temperatures are too high during the period when they form new tubers and increase their size.
- In fact, temperatures higher than 29°C during this period can actually prevent tubers forming or increasing in size.
- The best way to establish when you should plant potatoes in your area is to talk to successful potato producers who are growing potatoes the same way you intend to do (eg dryland/under irrigation, with high/medium/low inputs).
- Speak to the local extension officer as well.
- Sometimes the local co-operative will also be able to help.
- Potatoes are easily killed by frost, so do not plant a cultivar that has a long growing period if there are only three months from the date of planting to the first frost.
Choice of Cultivar
Selecting the best cultivar to plant in a specific production area and for a specific purpose depends on several factors. These include:
- What grows well in areas that have the same conditions and planting date (eg irrigation vs dryland, high/medium/low input, temperatures during the growing season, day length and soil type).
- The purpose of producing potatoes (eg general household use, sale).
- Diseases prevalent in the area (eg late blight in the cooler and wetter areas, common scab).
- Length of the growing period (short: 70 to 90 days; medium: 90 to 110 days; long: 120 to 150 days).
- The availability of seed potatoes.
Soil Preparation
The aim of soil preparation is to break up any compacted layers, incorporate crop residues, control weeds and prepare a good seedbed.
Pre-planting tillage:
- The soil should be slightly moist when tillage takes place to prevent deterioration of the soil structure.
- Cross-rip the field to a depth of at least 50 cm.
- Deep ripping is important as the potato plant has a relatively weak root system and is very sensitive to compacted soil layers.
- After ripping, the field must be ploughed to a depth of at least 30 cm.
- This should incorporate all crop residues to ensure complete decomposition of any plant material before the potatoes are planted.
- Finally, a disc may be used to break any clods and level the field.
Seedbed preparation (directly before planting):
Irrigate lightly before preparing the final seedbed to ensure that the soil is moist at planting and that any remaining clods break up during tilling. A well-prepared seedbed consisting of a 15 cm deep layer of loose and finely crumbled moist soil, can be obtained by tilling with either a spring-tined cultivator with a roller, a rotavator or a disc.
Fertilisation
Soil properties that influence potato productivity include soil chemistry (pH, soluble salts and inherent fertility), texture (the proportions of clay, silt, sand and organic matter), and physical condition (especially compaction). Pre-plant soil testing is necessary to determine the chemical status. Potatoes are adapted to a wide soil pH range, but it may be necessary to modify the pH to optimise production because this can affect nutrient availability and the activity of certain soil pathogens. Potatoes require a high level of soil fertility, so soil that has low inherent fertility must be given high inputs of nutrients from organic or inorganic sources. Before planting, every field should be extensively sampled to determine inherent fertility and variations.
- For every 2 to 5 hectares to be planted, take 20 to 30 topsoil samples (up to 20 cm deep, 500 g soil per sample) and 5 subsoil samples (30 cm to 60 cm deep).
- Mix the topsoil samples thoroughly, and take one composite sample of 1 kg for testing.
- Do the same with the subsoil samples and take one composite sample of 500 g.
- If noticeably different soil types can be observed in the field, separate samples should be taken of each type, and tested.
- Remember to note where the different samples were taken.
- When you send soil samples to be analysed, you must also give information about irrigation practices, planting date, cultivar and target yield.
- Potatoes have a poor root system, so fertiliser should be applied at the same level – or just below – that of the seed tubers in the furrow during planting.
- Seed tubers and fertiliser should not come into direct contact, so plant the seed slightly to the side of the furrow or cover the fertiliser with a thin layer of soil.
- Where top dressing is recommended, apply it on both sides of the plant just before ridging, then ridge and irrigate.
Planting
- Potatoes should be planted when the soil is still moist (irrigate before planting), but not wet.
- Plant the tubers about 20 cm deep in a furrow to which fertiliser has been applied.
- Close the furrow with the soil removed during the opening of the furrow and if the soil has dried out, irrigate lightly.
- The spacing between rows for potatoes grown under irrigation may vary between 75 cm and 100 cm, depending on the size of the tractor and the setting of the implements used.
- When potatoes are grown without irrigation (rain-fed) the spacing between rows is usually increased to at least 1.25 m.
- The space between seed tubers in the furrow (row), depends on the size of the tubers.
- Small seed or “chats” are usually spaced 15 cm apart, while medium-sized seed (80 g to 100 g) may be spaced 30 cm apart.
Ridging
- Ridging (or hilling) is a cultivation practice during which the soil surrounding the young potato plants is used to build a ridge or hill in which the tubers can develop.
- This can be done mechanically or with hand implements, depending on the size of the planting and the availability of implements.
- Potato plants are ridged after they are well established (plants 20 cm to 25 cm high), to ensure that the developing tubers will be covered with enough soil to avoid sun damage and to protect them from attacks by pests and diseases.
- Ridging when the soil temperature is high can damage the plant stems and cause lower yields. Avoid this by irrigating lightly before ridging or immediately afterwards.
Harvesting
- Potatoes can be harvested as soon as the tubers have matured sufficiently to resist mechanical damage.
- In most cases, the skin of the tubers will be properly set (or matured) about 2 weeks after the plants have died.
- Readiness to harvest may be checked by digging up several plants from different areas of the field and rubbing the tubers with your fingers.
- If the skin rubs off, the tubers should be left in the soil for another 7 to 10 days to ensure that the skins have matured properly.
- The soil must be slightly moist when the potatoes are harvested.
- This will prevent excessive clod formation and subsequent damage to the tubers.
When potatoes are harvested with a potato lifter, you can limit mechanical damage to the tubers by paying attention to:
- The depth of the digging blade (to prevent damaging tubers).
- The speed of the digger chain – always adjust it to ensure that some soil remains on the chain bed.
- The dropping height behind the lifter, which should never be more than 50 cm.
Pick up potato tubers as soon as they have been lifted from the soil and take them out of direct sunlight. The keeping ability of potatoes is adversely affected by exposure to direct sunlight and high temperatures. Never use plastic bags to pick up potatoes. The tubers could “sweat” in the bags, which can cause rotting. Packaging and washing depends on your market. Establish the needs of your target market and adjust your harvesting, washing and packaging practices accordingly.
Rotation: Did You Know?
- Potatoes belong to the family Solanaceae.
- Tomatoes, brinjals, peppers, chillies and gooseberries also belong to this family, so they cannot be used in the same rotation as potatoes.
- These plants are sensitive to the same pests and diseases, so they could have a devastating effect on potatoes if planted with, or near to, them.
This article first appeared in Farming SA.
