142 CONTINUOUS CROPPING 



very freely to an application of phosphatic manure, 

 and seldom requires much nitrogen. Again, the most 

 important ingredient in a potato manure is potash, 

 while turnips require, as a general rule, very little 

 potash, but must have a liberal supply of phosphates. 



Just one step further, and we have the principle of 

 manuring on the tip of the fingers. A saying, as true 

 as it is old, states that " one man's meat is another 

 man's poison." Exactly the same apphes to plant 

 nutrition. 



Pork steak is very nourishing, but feed a dyspeptic 

 on pork for a week, and at the end of it, he won't want 

 any more food — a coffin will better supply his needs. 

 In the same way, if a boggy pasture be deficient in 

 phosphates and assuming the pasture to be of a very 

 acid nature — ^that is to say, short of lime — a dressing 

 of superphosphates which consists essentially of 

 phosphates, will not increase the yield of pasture, but 

 will decrease it ; whilst on the other hand, basic slag 

 which again consists essentially of phosphates, would 

 on such land produce marvellous results. 



The explanation is, that the phosphates of slag exist 

 in a different chemical condition from those contained 

 in superphosphate, and the former manure contains 

 a certain amount of lime which helps to neutrahse the 

 acidity of soils. 



This brings us to a consideration of the second part 

 of the subject. 



Let it, before we go further, be clearly understood 

 that the value of artificial manures depends entirely 

 upon the amount of phosphates, potash, and nitrogen 

 they contain. Some artificial manures, such as super- 

 phosphates, contain phosphates alone ; others hke 

 muriate of potash contain potash alone. Nitrate of 

 soda and sulphate of ammonia only contain nitrogen, 

 while slag contains phosphates and lime. 



