1. During the years 1954–57 thirty field experiments were carried out on the magnesium requirement of main crop potatoes; eighteen covered a range of typical potato soils while the remainder were sited on sands and loams. In nine of the experiments the effect of heavy applications of potash on magnesium requirements was also investigated.

2. Magnesium application to the seed-bed as kieserite raised yield in four of the experiments; the largest yield increase was 2·15 ton/acre of tubers from 45·lb./acre of MgO. Apart from these four experiments there was also a small mean increase in yield over all the remaining sites.

3. Application of a very high rate of potash depressed yield compared with a normal rate, but only in one experiment could this be attributed to induced magnesium deficiency, and there was no indication of such an effect in the average results.