Cultivation

Potato is grown for food in about 100 countries, under temperate, subtropical and tropical conditions. In temperate zones, potatoes are planted in spring and harvested in late summer. In tropical areas, the best yields are obtained at altitudes of 1 800 to 2 500 m (5 900 to 8 200 ft) or, at low altitude, when the crop is grown during the coolest months.

The main limiting factor on potato production is temperature. Optimum yields are obtained when mean daily temperatures are 18 to 20°C (64 to 68°F), while a night temperature of below 15°C (59°F) is required for tuber initiation. Tuber growth is sharply inhibited in temperatures below 10°C (50°F) and above 30°C (86°F).

Potato cultivars are grouped into three basic types: early (which produce tubers for harvesting within 90 to 120 days of planting), medium (120 to 150 days) and late (150 to 180 days). Early varieties bred for temperate climates require a day length of 15 to 17 hours, while late varieties produce good yields under long and short day conditions. In tropical climates, varieties must be adapted to tolerate short days.

Growing potatoes involves extensive ground preparation. The crop is usually grown not from seed but from "seed potatoes" - small tubers or pieces of tuber sown to a depth of 5 to 10 cm. The planting density of a row of potatoes depends on the size of the tubers chosen, while the inter-row spacing must allow for ridging of the crop. Usually, about two tonnes of seed potatoes are sown per hectare.

For rainfed production in dry areas, planting on flat soil gives higher yields (thanks to better soil water conservation), while irrigated crops are mainly grown on ridges. To protect soil, control weeds and reduce losses from pests and diseases, potato is usually grown in rotations of three or more years with crops such as maize, beans and alfalfa. Crops susceptible to the same pathogens as potato (e.g., tomato) are avoided in order to break potato pests' development cycle.

Land and water.  Potatoes require well-drained and well-aerated soil. The use of chemical fertilizer depends on the level of available soil nutrients - volcanic soils, for example, are typically deficient in phosphorus - and in irrigated commercial production, fertilizer requirements are relatively high. However, potato can benefit from application of organic manure at the start of a new rotation.

The soil moisture content must be maintained at a relatively high level. For best yields, a 120 to 150 day crop requires from 500 to 700 mm (20 to 27.5 inches) of water. In general, water deficits in the middle to late part of the growing period tend to reduce yield more than those in the early part. Where supply is limited, water is directed towards maximizing yield per hectare rather than being applied over a larger area.

Because the potato has a shallow root system, yield response to frequent irrigation is considerable, and very high yields are obtained with mechanized sprinkler systems that replenish evapotranspiration losses every one or two days. Under irrigation in temperate and subtropical climates, a crop of about 120 days can produce yields of 25 to 35 tonnes/ha (11 to 15.6 tons per acre), falling to 15 to 25 tonnes/ha (6.6 to 15.6 tons per acre) in tropical areas.

Yellowing of the potato plant's leaves and easy separation of the tubers from their stolons indicate that the crop has reached maturity. Depending on the scale of production, potatoes are harvested using a spading fork, a plough or commercial potato harvesters that unearth the plant and shake or blow the soil from the tubers.