190 LECTURES ON 



had little influence on the wheat crop, but at once raised the turnip 

 field to a considerable degree of productiveness. These facts, borne out 

 by the quite general result of practice, indicate the conclusion which 

 some eminent authorities have unhesitatingly adopted, that soluble 

 phosphate of lime exercises a specific action on the turnip, indepen- 

 dent of the actual need of this plant for phosphates. There are, 

 however, such grounds for doubting this doctrine that, until further 

 investigations give us more complete data for judgment, a decision 

 must be suspended. 



Some recently described experiments of Mr. Lawes on the effect of 

 fertilizers upon meadows are very interesting. He found that when 

 a manure consisting of phosphates and sulphates of lime, potash, 

 soda, and magnesia was applied to grass land, the development of 

 clover was at once astonishingly increased; while, when nitrogenous 

 manures were used, either alone or in addition to the above mixture, 

 the true grasses maintained the mastery. 



The attempt made not long since to manure, plants with mixtures 

 representing what is taken off the field by a crop, turned out unsatis- 

 factorily, as the facts we have instanced make evident such a scheme 

 must; and we are led every day more and more to seek explanations 

 of the anomalous effects of manures in their indirect action. 



The most familiar instance of indirect action is that of gypsum or 

 sulphate of lime. In contact with carbonate of ammonia, with so 

 much water as to make the mixture wet, an interchange of ingredients 

 takes place, so that sulphate of ammonia and carbonate of lime are 

 formed; and Liebig accounted in part for the beneficial operation of 

 gypsum by assuming that it thus "fixed" the volatile carbonate of 

 ammonia of rains and dews, and held it in the soil for the use of 

 vegetation. 



On the other hand, Boussingault showed that when the mixture of 

 sulphate of ammonia and carbonate of lime, from being wet, dries so 

 far that it is only moist, like the soil is ordinarily, the reverse decom- 

 position ensues, and the ammonia once fixed, is unfixed. While we 

 can conceive of circumstances in which both these properties come 

 into play, beneficially or otherwise, it must be remembered that the 

 more late discovered absorbent power of the soil sets these effects of 

 gypsum quite out of the account in nearly all cases. 



Humus, which, in the form of peat or swamp muck, or as resulting 

 from the decay of litter and the carbonacious ingredients of the ex- 

 crements of cattle, is a most common and useful manure, doubtless 

 accomplishes more by indirect than by immediate action. It is the 

 most energetic absorbent of ammonia, as carbonate (according to 

 Brustlein, not of other salts) is the source of carbonic acid in the 

 soil, thus, by its presence, setting in operation the endless train of 

 changes whose result is the solution of mineral matters, and by its 

 hygroscopic character it assists to maintain the proper physical con- 

 dition of the soil. 



Lime, which is one of the greatest renovators in use in agriculture, 

 is, in a similar manner, of more indirect than immediate effect. Its 

 influence is especially manifest in fluxing the insoluble stores of plant- 



