1 6 SOIL CONDITIONS AND PLANT GROWTH 



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 possible to draw up tables 

 showing the farmer precisely what he must add in any par- 

 ticular 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 quan- 

 titative form : " The crops on a field diminish or increase in 

 exact proportion to the diminution 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, gave 

 rise 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 con- 

 tinued 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 nece.ssity of ammoniacal manures were deleted from the 

 third and later editions. "If the soil be suitable, if it con- 



^ The underlying principle was not discovered by Liebig, having already 

 been enunciated by political economists of the Malthus School. He was, how- 

 ever, the first to apply it to plant nutrition. 



