HISTORICAL AND INTRODUCTORY 11 



carbonic acid is being generated in the soil, for it enters the plant 

 root and affords extra nutriment over and above what the small 

 leaves are taking in. Hence a supply of humus, which continuously 

 yields carbonic acid, is advantageous. Further, the carbonic acid at- 

 tacks and dissolves some of the alkali compounds of the soil and thus 

 increases the mineral food supply. The true function of humus is to 

 evolve carbonic acid. 



The alkali compounds of the soil are not all equally soluble. A 

 weathering process has to go on, which is facilitated by liming and 

 cultivation, whereby the comparatively insoluble compounds are broken 

 down to a more soluble state. The final solution is effected by acetic 

 acid excreted by the plant root, and the dissolved material now enters 

 the root. 



The nitrogen is taken up as ammonia, which may come from the 

 soil, from added manure, or from the air. In order that a soil may 

 remain fertile it is necessary and sufficient to return in the form of 

 manure the mineral constituents and the nitrogen that have been taken 

 away. When sufficient crop analyses have been made it will be pos- 

 sible to draw up tables showing the farmer precisely what he must add 

 in any particular case. 



An artificial manure known as Liebig's patent manure was made 

 up on these lines and placed on the market. 



Liebig's book was meant to attract attention to the subject, and it 

 did ; it rapidly went through several editions, and as time went on 

 Liebig developed his thesis, and gave it a quantitative form : " The 

 crops on a field diminish or increase in exact proportion to the diminu- 

 tion or increase of the mineral substances conveyed to it in manure ". 

 He further adds what afterwards became known as the Law of the 

 Minimum, " by the deficiency or absence of one necessary constituent, 

 all the others being present, the soil is rendered barren for all those 

 crops to the life of which that one constituent is indispensable ". These 

 and other amplifications in the third edition, 1843, g ave n ' se to much 

 controversy. So much did Liebig insist, and quite rightly, on the 

 necessity for alkalis and phosphates, and so impressed was he by the 

 gain of nitrogen in meadow land supplied with alkalis and phosphates 

 alone, and by the continued fertility of some of the fields of Virginia 

 and Hungary and the meadows of Holland, that he began more and 

 more to regard the atmosphere as the source of nitrogen for plants. 

 Some of the passages of the first and second editions urging the neces- 

 sity of ammoniacal manures were deleted from the third and later 

 editions. " If the soil be suitable, if it contains a sufficient quantity of 



