662 



THE SOUTHERN PLANTER. 



[November 



that they only take up their food in a solu- 

 ble form. He has even shown that the re- 

 tentive properties of soils are not so abso- 

 lute in their nature but that the soil may 

 be freed of its ammonia by repeated wash- 

 ings. He admits, at the same time, that the 

 degree of solubility of ammonia in a reten- 

 tive soil is very difficult to fix. One of his 

 experiments shows that the ammonia is not 

 absolutely fixed, but tends to diffuse itself 

 equally over continguous portions of the 

 same soil. A quantity of earth which had 

 absorbed a portion of ammonia was put at 

 the bottom of a flower -pot placed in a saucer, 

 and another portion of the same earth, but 

 free from ammonia, was put above it. The 

 two were only separated by a thin canvass. 

 On watering the whole from below it was 

 found, at the end of eleven days, that about 

 one-triird of the ammonia had passed 

 through the canvass, and diffused itself in 

 the soil in the upper part of the flower- 

 pot. 



After all the experiments, then, which 

 have been made of late years by Liebig, 

 Way, and others, and misinterpreted by 

 them, we are forced to go back to the doc- 

 trine put forth by Liebig in 1850, that the 

 absorbent power of soils is the result of sur 

 face attraction. S. W. Johnson, in the 

 paper already referred to, while still cling- 

 ing to the idea of the action being chiefly 

 chemical, supplies some curious facts to 

 show that it is so far mechanical. His de- 

 ductions, however, in a great measure lose 

 their interest since Brustlein has given to 

 the world the results of his decisive experi- 

 ments. One point, however, he touches, 

 which deserves from us some attention, re- 

 garding the manner in which plants take up 

 their food. 



We had formerly stated that the old doc- 

 trine of vegetable physiologists, that plants 

 had no power of selecting their food, but 

 merely took up what was in solution in the 

 same manner as the wick of a candle, must 

 be abandoned since Way's experiments 

 showed that their food existed almost in an 

 insoluble form in the soil. Since that time 

 Liebig and many others have admitted that 

 such must be the case, although they have 

 not attempted to indicate how a function of 

 this nature can reside in the roots of plants. 

 Before the appearance of Liebig's recent 

 Letters on Modern Agriculture we gave an 

 explanation which was conceived to be most 



consistent with analogous phenomena.* In 

 these Letters it appears to us that Liebig 

 has served to complicate the whole question 

 by supposing that the laws regulating the 

 absorption of the food of land and water 

 plants must be totally different. There is 

 not the least necessity for such an hypothesis, 

 as the same law that might regulate the one 

 might also regulate the other. This weak 

 point in the views of Liebig has already been 

 commented upon by Brustlein, Johnson, and 

 others, and arguments have been founded 

 upon it to show that plants only take up 

 what is soluble. The former remarks in his 

 paper in the Annates de Physique et Chi- 

 mie : — 



In taking into account the small quantity 

 of ammonia which exists in arable land, and 

 its diffusion in the soil, though its solubility 

 is extremely small — knowing, besides, that 

 the reaction of the active alkalies, except 

 the volatility, are identical to those of am- 

 monia, it appears probable enough that 

 plants choose the greater part of their food 

 from dilute solutions, in which is found the 

 nitrogenous element. It is not doubtful that 

 it is so. Aquatic plants afford the proof of 

 it, and the beautiful experiments of M. 

 Boussingault have established that a plant 

 acquires a complete development in a soil 

 formed of pure sand and quartz previously 

 calcined, having for manure nitrate of 

 potash, phosphates, and alkaline ashes. Un- 

 der these conditions the plant is then neces^ 

 sarily obliged to take up its food in solution. 



The fact of plants thriving vigorously in 

 sandy soils when abundantly supplied with 

 moisture, has always been sufficient to con- 

 vince us that the double silicates of alumina 

 and potash were not, as Way assumed, abso- 

 lutely necessary in preparing the food of 

 plants. Nor are we to suppose with Liebig 

 that what agriculturists term the ¥ burning 

 of jyoung plants" f in sandy soils by concen- 

 trated manures, can at all be ascribed to 

 their food being presented to them in solu- 

 tion. There are no evils arising from solu- 

 tions, if sufficiently diluted. In rainy sea- 

 sons, sandy soils are greatly benefited by ap- 

 plications of nitrates and ammoniacal salts, 

 but the plants "burn" more readily with 

 these applications in dry weather. The ab- 

 sorption of food being greatly facilitated 

 when it is presented to the roots in a soluble 



* Journal of Agriculture for January 1859. 

 | Letters on Agriculture, p. 38. 



