By Dave of Darlington (with apologies to the H.D.R.A.)
The very dry summer of 2006 had an adverse effect on many vegetable crops, especially potatoes, which need a constant level of moisture in the soil to grow well. A dry soil not only checks the growth of the plants, retarding tuber development, but also increases the incidence of common scab, a fungal disease that normally only affects the skin of the tubers but which, in dry soil, can penetrate deeper into them and seriously impair their quality.
Even when, as in some areas in 2006, the drought ends in August and gives way to wet weather, thus supplying water to the soil in time to swell the tubers of at least the main-crop potatoes, the problem is not over. A sudden change from dry to wet soil can cause hollow-heart. This is a defect involving a hole in the centre of the tuber that is not apparent from the outside and so can escape grading and quality control. It is a particular nuisance in baking potatoes, because then it may not be discovered till the tubers are actually being eaten.
So, for all these reasons, it is important in a potato crop to keep the soil continuously damp. In dry weather we can irrigate, but, if the potatoes are being grown in the traditional way (in ridges), irrigation is not always very effective. The water can simply run off the sides of the ridges into the furrows, by-passing the potatoes altogether, especially when, as often happens in very dry weather, the soil has become water repellent.
Retaining the vital moisture
We can get round this problem in one of two ways. One is to grow the potatoes on top of the soil under deep mulch, as we did at Organic Growers of Durham and which I described in detail in a previous article . (I naïvely thought that we had invented this method, but I recently found a description of a very similar technique in a booklet on organic potato-growing published in 1983 by the Henry Doubleday Research Association , to whom I sincerely apologise for the unwitting plagiarism.) If this method is used, provided the soil is damp in the spring when the mulch is applied, it will remain so throughout even the driest of summers, thus avoiding the above-mentioned problems.
Make ridges redundant
However, some amateur gardeners may not be able to get hold of enough mulch to use this method, so what are the alternatives? One is to grow the potatoes in a flat bed instead of in ridges. In dry weather this method will usually give a higher yield of better quality potatoes, because, firstly, a flat bed will dry out more slowly that ridges and, secondly, if we have to irrigate, more of the water will reach the roots of the potato plants than in a ridge system. Also, being a zero-tillage (no-dig) technique, the flat-bed method causes less soil disturbance and uses much less time and energy than the traditional ridge technique.
The method is quite simply to grow the potatoes in an undug flat bed. The soil should be in good condition and not be compacted, so avoid walking on it by working from the side of the bed. This method may not work on a heavy soil. The seed potatoes should be planted in the soil to a depth of around 15 cm. On a garden scale this can be done with a trowel. Steps will have to be taken to control weeds until the potato foliage canopy has developed. This can be done either with a shallow layer of mulch or with a cover crop. However, in the latter case it may be necessary, early in the season, to cut back the cover crop to prevent it out-competing the potatoes.
Finally, if you have not already done so, you may like to try out some of the potato varieties specially recommended for organic growers by the Newcastle University organic potato project, especially the Scottish variety Lady Balfour, which is not only resistant to blight, eelworm and scab, but is also delicious and culinarily versatile.
Footnotes Growing Potatoes in a Zero Tillage System, in issue no. 6, p. 36, of this magazine  Potato Growing the Organic Way, by Pauline Pears, publ. in 1983 by the Henry Doubleday Research Association
This article appeared in Growing Green International magazine Num 21 (Summer 2008), p7.