96 PLANT PRODUCTS 



system of drainage not merely is the water removed, but 

 also air is let into the soil. In certain particular cases, as, 

 for example, some of the fens, the soil may be alternately 

 wet and dry. In these fen districts it is not uncommon 

 that the level of the rivers and canals exceeds that of the 

 fields. In this particular case a ditch is dug between the 

 field and the river. The level of this is below the level 

 of the field, and considerably below the level of the river. 

 Into this main ditch branch channels run to carry the surplus 

 water, which is pumped into the river at a higher level. 

 When dry weather intervenes, it is only necessary to reverse 

 the action of the pumps, and let the water run back into the 

 ditch from the river, and thence into communicating channels. 

 Very large quantities of rank grass may be obtained by such 

 a method. Many of the grasses which normally have a 

 very bad name owe their lack of nutriment to a big develop- 

 ment of fibrous stalk. Under conditions of perpetual 

 moisture these grasses never mature, and are, therefore, 

 always moderately succulent. 



One of the results of bad drainage in a soil is a tendency 

 to accumulate poisonous materials. When the clay con- 

 stituents of a soil contain large amounts of soda, the soda 

 is removed from the clay by the action of water containing 

 carbonic acid, producing sodium carbonate, which defloccu- 

 lates the soil. One of the cures for this is drainage which 

 removes the soda salts. The application of gypsum to such 

 a soil converts the sodium carbonate into sodium sulphate 

 which washes away with rain, and leaves calcium carbonate 

 behind. The former is relatively harmless and drains 

 away in time, the latter is beneficial. Sodium sulphate 

 does not hinder the germination of seeds as much as sodium 

 chloride or sodium carbonate. This type of land is known 

 in America as the black alkali land, whilst in India it is 

 known as reh or usar. As on many poor soils, persistent 

 efforts at cultivation result in improvement. Where the 

 land has been steadily cultivated, maintained in an open 

 condition, and ample plant food given, the soil remains 

 fertile. Soils in the vicinity of rivers may need reclamation. 



