The fen silts bordering the Wash are among the most important soils for crop production in eastern England. Derived from marine alluvial deposits occurring naturally or as a result of reclamation, they form deep almost stoneless soils containing little coarse sand but much fine sand and silt.
Seven textural classes, ranging from loamy fine sand to silty clay, can be distinguished, but these can be grouped into (1) light silts, typically deep very fine sandy loams with a large available water-holding capacity but weakly developed structure; (2) medium silts, silty loams; and (3) heavy silts, silt loams or silty clay loams, less porous and more retentive of moisture than the light silts.
Formerly under grass, the silts were at first capable of growing good crops of potatoes with P fertilizer alone, but as their organic-matter content decreased, potatoes became increasingly responsive to N.
In 18 potato manurial experiments on silt soils in Holland, Lincolnshire, done between 1953 and 1963, the mean response to N was much larger than on most English soils, but response differed greatly from site to site and year to year; in most trials N was particularly effective in increasing tuber size and yield of ware. With long-continued use of P fertilizer, residues have accumulated and, particularly on light and medium silts, responses were quite small and differed little between sites; P tended to increase tuber numbers, and so to decrease ware percentage. Most silt soils are rich in K, and only small responses to K fertilizer can be expected, but on a few sites on the light silts, identified by soil analysis as being comparatively poor in K, potatoes responded well. Using these experimental results, recommendations are given for the manuring of potatoes on silt soils.
Cooking tests showed little consistent effect of manurial treatment on the amount or degree of tuber blackening.