RESPECTIVE VALUES OF ORGANIC AND INORGANIC MANURES. 225 



and potash into solution. It has been proved that mineral phos- 

 phates only work well on a soil containing plenty of organic matter ; 

 this is because the carbonic acid gas generated by the bacteria 

 enters into the water and dissolves the phosphates, otherwise they 

 are very ineffective. 



It is clear, therefore, that we are absolutely dependent on the 

 bacteria in the soil, and especially is this the case with the intensive 

 cultivator who uses the soil as a manufacturing medium to convert 

 the fertilizers he applies into crops. 



As I have said, bacteria feed on humus and must have it, anjl 

 we therefore see how necessary it is to use organic manures that supply 

 this all-important ingredient. 



On the other hand, the mineral manures, as we have seen, not 

 only contain no humus, but by the acid residues they leave in 

 the soil (especially superphosphate and sulphate of ammonia), and 

 by the bad effect they have on its physical condition, they are 

 actually harmful to bacterial life. 



So that from this aspect also we see that the organic manures 

 are far preferable to those obtained from mineral sources. 



In spite of these great advantages, however, I do not deny that 

 the minerals have their uses. For rapid results, in cases where we 

 want to stimulate the plant into very quick growth, we must use 

 the mineral forms of nitrogen — such as nitrate of soda, nitrate of 

 lime, sulphate of ammonia, &c. — because they require to undergo 

 practically no change before being taken up by the plant. They 

 produce an unnaturally quick, soft, and sappy growth, just the thing 

 we want to avoid in permanent crops like fruit, but, on the other 

 hand, just the thing we require in green crops like cabbage, lettuce, 

 &c., because besides giving us earliness they also produce a tender 

 leaf, though, if this is overdone, the produce is apt to be unwhole- 

 some as food. 



There is probably also another reason why minerals are of con- 

 siderable value, and this is the subject of very interesting investigation 

 by the American agricultural chemists. 



It is now generally admitted that all plants, as a result of their 

 growth, give rise to a certain toxin or poison in the soil, the toxin 

 being peculiar to the particular crop, and, as a rule, a poison only to 

 that crop. In some kinds of plants this is much more noticeable 

 than in others ; probably two good instances are strawberries and 

 clover, which, as we know, are difficult to grow continuously on 

 the same site. At the other extremity we have wheat and man- 

 golds, which Rothamsted shows us can be grown for many years 

 in succession on the same soil. 



These toxins have probably a great deal to do with the advan- 

 tages of rotation, as this system gives the soil time to rid itself of 

 these substances, which we must remember are poisonous to the 

 one crop only. 



Intimately connected with this matter is the recent work of 



