XIII.] SULPHATE OP AMMONIA 231 



into a liquid by absorption of water from the air. 

 Although it is so soluble in water, it can be applied to 

 the soil without any danger of washing out, because it 

 there interacts with the humus and clay, and is converted 

 into substances which are for the time insoluble. It 

 should not, however, be used as a manure long before 

 the plant is ready to take it up, because it is readily 

 converted by the bacteria we have spoken of before into 

 nitrates which do wash out of the soil Sulphate of 

 ammonia is therefore best employed as a top-dressing 

 or for root crops, in which case it is put in the soil at the 

 time of year when there is very little danger of any 

 washing out. It is found by experience that when a 

 concentrated nitrogenous fertiliser is needed, sulphate of 

 ammonia is most suitable for shallow-rooted crops, like 

 barley, swedes, turnips, potatoes. On chalky soils and 

 on very heavy clays it is the most useful nitrogenous 

 fertiliser, but it should not be used on light sands nor on 

 peaty and other soils which have any tendency to get 

 sour. Being such a concentrated fertiliser, only small 

 quantities are, as a rule, required. It should be 

 remembered also that when used as a top-dressing it 

 will scorch and kill the foliage of any green plant on 

 which it happens to rest This is not because it is 

 poisonous, but merely because any soluble salt in contact 

 with the green leaf of a plant will draw water from the 

 tissues, and eventually kill the leaf by so doing. For 

 this reason sulphate of ammonia mixed with sand is 

 often used to kill out weeds on lawns ; when sprinkled 

 over the lawn the soluble material lodges in the crowns 

 and rests on the broad leaves of weeds like plantains, 

 buttercups, and daisies, eventually killing them, whereas 

 it does not touch the upright leaves of grasses but slips 

 down to their roots and acts as a fertiliser. If a satis- 

 factory result is to be obtained, however, the weather 



