36 



however, take into account the increase in live weight of the 

 sheep consuming it. 



The teaching of these Woburn rotation experiments is of 

 a very practical character. They proclaim the fact that on 

 land in high condition which merely implies that the 

 quantity of available plant food is for the time ample in 

 relation to the assimilating power of the crops the use of 

 cake-fed manure in a rotation may not only be without 

 immediate effect upon the crops, but may also leave no 

 valuable residue in the land. Some of the causes which limit 

 the effects of excessive applications of manure have been 

 already mentioned ; but why, we may ask, should the residue 

 of this cake manure produce no effect when the land was 

 subsequently left unmanured ? 



In looking at this question we have to bear in mind 

 that, in the case before us, the actual weight of manure 

 applied was probably nearly the same both in the case of the 

 cake-fed and of the maize-fed manure. Had the experiment con- 

 sistsd in a comparison of the effect of 10 tons and 20 tons of farm- 

 yard manure in a rotation, the more liberal manuring would 

 pretty certainly have shown a distinct after effect when the 

 land was cultivated without manure, for the larger quantity 

 of manure would longer have resisted the oxidising processes 

 always proceeding in the soil. The comparison is, however, 

 between two equal quantities of dung, both of them small 

 dressings, but one richer in soluble nitrogenous matter than 

 the other. Both of these dressings would disappear from the 

 soil by natural oxidation at the same time ; and if the pro- 

 ducts of their oxidation produce no effect on the crops during 

 the first two years, nothing is to be expected from them 

 afterwards. 



It is important always to remember that while the use of 

 oil- cake to enrich manure may produce a marked effect on the 

 crop to which the manure is immediately applied, it does not 

 appreciably increase the length of time through which the 

 effect of the manure will be perceived. The difference between 

 the first effect and the after effect will be greater with rich 

 manure than with poor. The characteristic of cake-fed 

 manure is that a larger proportion of its nitrogen is 



