This database contains bibliographic descriptions of all Wageningen University PhD theses from 1920 onwards. It is updated on a daily basis by WUR Library.
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The Dutch Bulb Fields are situated on the Beach-ridge landscape between the Rhine Estuary and the Y-Polders in the mid-western part of North Holland and South Holland. Culture of some horticultural crops (tulips, daffodils) is important in this area because of the special requirements of these crops: rather coarse, calcareous, deep sandy and highly porous soils easy to penetrate by roots, without organic matter, without impermeable layers and a stable watertable. Continuous deep digging is inevitable. Soil formation (mainly decalcification) in the beach ridges over some 3000 years has caused non-calcareous topsoils and a slight podsolization; the soils have been improved by digging off sand. The area has been influenced by peat growth, marine and estuarine sediments and shifting sand. The soil survey resulted in a soil map, indicating soil quality. From the results ensued the recommendation of improving soil by suction dredging. This had to be planned and carried out very carefully, because the formation of humous and clayey layers in the new profile would make the soil completely unsuitable for valuable horticultural crops. Geological studies on Middle and Late Holocene geology and stratigraphy showed that the beach ridges were formed during and after the deposition of the Atlantic Old Marine Clay (4000-2000 B.C.).
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