290 Dr, Playfair on the Chemical Principles [May 30, 



contains its ingredients either free or imprisoned, that is, either 

 soluble or insoluble. The action of air, rain, and frost, liberates 

 the imprisoned substances, rendering them available to plants. 

 The mechanical operations of the farm, ploughing, harrowing, clod 

 crushing, draining, &c., have this end in view. Jethro Tull ascrib- 

 ed all success to such operations, and we find as long ago as the 

 time of Cato, that the weathering effects on the soil were well 

 known. But sooner or later the fertile ingredients of a soil must 

 be removed by crops ; and to prevent sterility we supply, by manure, 

 what we take away by cultivation. The primary action of manure 

 must be to render to the soil that which is taken away ; or in other 

 words, to produce a constant condition of growth in that which 

 would otherwise be variable : but its secondary action often is, like 

 humus, to supply an excess of air food in order to gain time in 

 cultivation. Nutrition of plants must be directly proportional to 

 the quantity of the necessary ingredients, and inversely proportional 

 to the obstacles to their assimilation. 



The quantity of ingredients of earth food is constant for the 

 same crop, within certain limits, arising from the greater or less 

 development of particular organic substances in them. It is with 

 plants as with animals. Formerly experiments used to be made 

 with the latter by confining them to certain substances present in 

 food. Dogs were fed separately on starch, or gelatine, or sugar, 

 and they died, because all the conditions of nutrition were not 

 satisfied. So it is with plants. If a single necessary ingredient be 

 omitted, the plant cannot grow. A child could not be expected to 

 thrive, if bone earth were carefully kept out of its food ; it might 

 have flesh in abundance, enough to grow a little Hercules, or fat 

 enough for a cherub, but without phosphate of lime it would refuse 

 to grow. Exactly the same law rules vegetable growth. This, 

 expressed as a law of fertility, means that the body in minima 

 RULES the crop. If, for instance, bone earth be the body pre- 

 sent in least quantity, and potash, soda, lime, &c. be present in 

 excess, the extent of the crop will depend only upon the amount of 

 bone earth, and the amounts of the other substances taken up will 

 be exactly proportional to the limit of the former. All the bone 

 earth being removed, the excess of the other substances is of no use, 

 for the crop will refuse to grow. Add, however, an excess of bone 

 earth, the crop will grow to the extent of the next substance, in 

 minima, which may be sulphuric acid, or any other necessary 

 ingredient. 



After explaining these general laws of fertility, the establish- 

 ment of which are entirely due to Liebig, Dr. Playfair proceeded 

 to apply them to the recent experiments made by farmers, and 

 which were supposed by them to be irreconcileable with the pervail- 

 ing notions of agricultural chemistry. He drew attention to the 

 experiments of Mr. Lawes, who, aided by Dr. Gilbert, has carried 

 on conscientiously, and with a full desire to arrive at just conclu- 



