POTATOHANDBOOK As a rule of thumb, a potato crop provides around 1 ton per hectare per 10 mm of water that it evaporates. stage, 25% to the foliage. Earliness is therefore a mechanism that helps the crop escape from the dry period that follows. When the crop is planted at the end of winter in the Mediterranean area in a wet soil, 60 mm of water is available for growth. It also receives an additional 180 mm from gradually decreasing rainfall, so its productive capacity is about 24 tons per hectare. With an early variety, the vigour of the growing tubers is so great that all the dry matter produced goes to the tubers at an early stage. As a result, nothing remains for the foliage that dies off within a few weeks. As a rule of thumb, a potato crop provides around 1 ton per hectare per 10 mm of water that it evaporates. In other words, the crop produces a ton of potatoes per 10 mm of water that is available from soil moisture, irrigation or rainfall. This figure is lower in relative ly-high wind conditions when the relative humidity is low, and at relative ly-high temperatures. At higher latitudes, such as northern Europe or southern Africa, with temporary dry periods alternated with rain, a late variety can absorb the total amount of water available over the entire growing season. It does this by adjusting the evaporation to the available amount of water. In addition, plants repel leaves when it is too dry and develop new layers of leaves when it rains again. If this crop was planted in the middle of spring with a reserve base of 60 mm and the average rainfall is 60 mm/month, the productive capacity is 24 tons per hectare. This is achieved 3 months after planting, based on a total water availability of 240 mm. After 6 months from planting the yield is 42 tons per hectare. Such yields are based on late-maturing varieties that initiate their tubers relatively late in the season. They distribute a relatively large proportion of the daily-produced dry matter to the foliage. When growers irrigate and the total amount of available moisture reaches 600 mm, the crop yield approaches 60 tons per hectare.’ ● Potato World 2019 • number 2 17 Pagina 16

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