130 Fertilisers [pt. hi 



up. or at any rate not retained, as a whole, by the plant 

 but a decomposition takes place, the nitrate part being 

 kept by the plant while some of the soda remains in the 

 soil as sodium bicarbonate. This reacts on some of the 

 potassium compounds in the soil, Uberating a certain 

 amount of potash which then becomes available for the 

 plant; this has been demonstrated by actual field experi- 

 ments at Rothamsted, and is also illustrated by the 

 experiment on p. 135. But the bicarbonate also acts on 

 the clay, converting it into the sticky defioeculated 

 state, and on a heavy soil this action becomes rather 

 serious, causing much damage to the tilth. A suitable 

 remedy is found in dressings of lime or addition of sul- 

 phate of ammonia to the nitrate. The student who is 

 interested in the history of the subject will find that, in 

 the old papers, nitrate of soda is sometimes called a 

 "scourge," and some of the older farmers still retain a 

 dislike to it. This idea probably arose partly from its 

 harmful effect on the texture of a heavy soil and partly 

 from its effect on hay land. It encourages a very good 

 growth of top grasses and may be used A\dth great ad- 

 vantage whenever hay is sold. But the heavy crop 

 naturally draws on the soil phosphates, and unless those 

 are replenished at the same time the soil becomes im- 

 poverished and the crop ultimately falls off in quantity, 

 while weeds and poor grasses appear and bring down the 

 quality. Grass land should never as a regular course be 

 fertilised with nitrate only, but should periodically re- 

 ceive the other necessary fertilisers. 



Nitrate of soda readily washes out of the soil and 

 must therefore not be applied until it is needed. It 

 is best put on as a top dressing when the plant is up : 

 when this course is adopted the loss in a wet season is 



