Growing potatoes successfully depends to a great extent on the quality of the soil to get a better crop. It really needs to be that mythical combination of ‘free drained, yet moisture retentive’, that’s so often suggested in books and magazines. Adding well-rotted compost is the best way to improve the soil to get it in good condition for potatoes, but this is done before planting.
It’s easy to think of the crop as forming on the roots, but in fact potatoes are stem tubers which are formed at the end of sideshoots that grow from the stem of the potato plant. And this means that while the roots underneath tap down for moisture in the soil, the developing spuds need an open soil if they are to expand readily.
Why are potatoes earthed up
This is one of the reasons for earthing up the developing plants. Loose soil is drawn up around the stems to promote the formation of abundant side-shoots and to exclude light to keep the potatoes from turning green which indicates chlorophyll – the truest indication, if any were needed, that they are made up of stem tissue.
It is often suggested that earthing up is done a couple of times to cover the stems but this can over-loosen the soil, allowing it to dry out and draw water away from the roots. And, given a heavy shower of rain or hand watering, the mounded soil can wash away from the plants.
Covering the ground around potatoes
A useful way of protecting the mounded soil from drying out and washing down flat is to use a surface mulch to cover the ground – particularly around main crops. Straw or preferably dried hay is put down between the rows to cover the earthed-up mounds and this performs a couple of functions.
Firstly it acts to shade the soil, to prevent it overheating when the sun is directly overhead in summer and thus reduces evaporation from the soil surface. And secondly, it intercepts heavy rain or applied water to lessen its impact and washing down of soil.
The potato plants themselves benefit from such cossetting. The roots underground stay moist, so that they are able to keep the stems supplied with essential water to swell the developing crop. And the side-shoots and potatoes themselves are less likely to lose moisture in hot weather which means that they will develop steadily without a check to growth that could cause cracks and brown marks in the flesh.
Watch out for potato blight
Once the potatoes have formed, it then pays to keep a watch out during warm, wet weather. Such conditions can encourage the potato blight fungus, so it would be a good idea to draw the straw or hay right away from the plants. This will reduce humidity around the foliage of the plants in which the blight spores are able to develop.