Drainage of heavy soils [antikvár]

brahiaca; of heavy soils Synthesis by B.D. Trafford Ministry of Agrioulture, Fisheries and Pood Cambridge, United Kingdom 1. Xntroduotlon liide interest was shovm in this topio and fourteen contributions were received from Member Governments. This is due to the fact that in many of the European countries clay soils form a considerable part of the agricultural land. Th?ie foils pose a particularly avrkward problem because not only are they difficult and expensive to drain but are also not usually suited to high value cropping. Thus the high cost of the work which might be required on teotaical grounds frequently cannot be justified by the likely economic return. Clay soil ia a difficult problem because water movement is almost entirely confined to the oraoks and fissures in the soil and hence a soil which has a poor structure (i.e. has very few oraoki;) will have an extremely slow water movement or hydraulic conductivity. Many clay soils v7ithout an underlying permeable deep layer TOuld require a drain spacing of 2 to 5 m if the design were based on the hydraulic conductivity of the subsoil. Such intensive uork vould be so expensive as to be quite uneoonomic at today's prices and it is for this reason that many oountrins have been attempting to develop cheaper methods of tackling clay land. Basically tvfo approaches are possibles a. aocopting the low hydraulic ronductivity of the subsoil and not attempting to drain it, but forming the land surface so as to be able to remove all standing "later by surface runoff; or b. adopting some artificial means of improving the soil structure and hence hi'draulic conduotivity so that vider drain spacings may be used successfully. The rapporteur ha,s made a synthesis of the contributions received and a summary of the disousoions. In addition seven appendixes have been prepared v.'hich summarize the mate"i'il very briefly and provide a source for quick reference. Also, selected contributions have been reproduced in full. 2. Surface methods Vlhilst it was generally agreed that surface drainage methods, such as land bedding, made the use of modem mechanized techniques more difficult, several participants pointed out that these lou coot drainage techniques still had a useful place. In Israel land bedding had been successfully used for orchards but to a lesser extent with annual crops. Prtm the French raport it vas noted that good results had been obtained in some parts of North Africa. 3. Artificial backfill oomevrhat conflicting experiences were reported on the value of artificial backfill used viith the intention oi' either improving the general working of the drain by providing a pei^ meablo surround, or alternatively of providing an easy entry route for ^ater from the Gurfano la,Yers to the drain. In the Federal Republic of Germany artificial backfill is widely used irith sucoeosful ronulta, whilst in Spain problems had arisen v.ith cementation of backfill. Field measurements in the United Kingdom tended to show better working of the drain irith backfill but the differences vjere not large and hence the economics of adding the material uere doubtful. Israel reported oases where they felt artificial backfill would have been useful as the water movement in natursil backfill had been shown to be a limiting factor. Mention was made of the various materials in use and of the possibility of plastic materials such as Polystyrene (Styromull), but gravel, sand and similar naturally occurring materials appeared to be in most common use. The discussion showed that further experimental and research vor): is clearly needed on the subject of artificial backfill.