624 



SCIENCE 



[N. S. Vol. XXVTII. No. 723 



genous fertilizer on one side does not stray 

 six inches over the boundary. Again, on 

 the Rothamsted wheat field the plots are 

 20.7 feet in breadth and were separated by 

 unfertilized strips only about a foot in 

 breadth; in 1893 each plot was sampled 

 down to a depth of 7.5 feet and the amount 

 of nitrates was determined in each succes- 

 sive sample of nine inches in depth. The 

 amount of nitrates found was in each case 

 characteristic of the supply of nitrogen to 

 the surface of the plot, and right down to 

 the lowest depth there Avere no signs of the 

 proportions approximating to a common 

 level, as they would have done had any 

 considerable amount of lateral diffusion 

 been taking place. Considering that the 

 plots are only separated by a foot or so 

 of soil and each had been receiving its 

 particular amount of nitrogen for forty 

 and in some cases for fifty years, the sharp 

 differentiation of plot from plot in the 

 amount of nitrates at a depth of seven feet 

 is sufficiently remarkable and is evidence 

 that the movements of the soluble salts in 

 the soil are confined to up and down mo- 

 tions due to percolation and capillary up- 

 lift, and take place laterally only to an 

 insignificant extent. 



From these considerations we may con- 

 clude that when a fertilizer is mixed with 

 the soil each particle will establish round 

 itself a zone of a comparatively concen- 

 trated solution to which the plant's roots 

 will be drawn by the ordinary ehemiotactic 

 actions, and that these zones will extend 

 but a little way into the generally much 

 less dilute mass of the soil water, because 

 of the slowness of the diffusion process. 



That some such state of things prevails 

 in the soil may be surmised from the com- 

 mon farming experience of the benefits 

 derived from sowing the fertilizer close to 

 the seed in such a case as superphosphate 

 and turnip seed, where the fertilizer is not 

 injurious to germination and the young 



plant is specially dependent on being 

 rapidly pushed into growth in the early 

 stages. Again, the intimate way in which 

 the feeding fibrous roots of a plant will 

 surround and cling to a fragment of fer- 

 tilizer in the soil, such as a bone or a piece 

 of shoddy, shows that some other actions 

 are at work in the soil than the mere feed- 

 ing of the plant upon the nutrients con- 

 tained in the soil solution. 



Whitney and Cameron 's theory also sup- 

 poses that the plant itself exerts no solvent 

 action, whereas it has often been supposed 

 that the roots excrete substances of an acid 

 nature which exert a solvent action upon 

 the soil particles. In this direction an ex- 

 periment of Sachs's has become classical: 

 he took a slab of polished marble and set it 

 vertically in a pot of soil in which beans or 

 some kindred plants were gi'own. After 

 the plants had been growing for some 

 time the contents of the pot were turned 

 out and the slab of marble washed, where- 

 upon the polished surface was found to be 

 etched wherever the roots had been grow- 

 ing in contact with it. A polished slab of 

 gypsum similarly treated shows a raised 

 pattern wherever the roots have protected 

 the surface from the solvent action of the 

 general mass of water in the soil. Al- 

 though Sachs himself attributed the etch- 

 ing to the action of the carbon dioxide 

 which is always being given off by the 

 roots, it has also been set down to fixed 

 acids excreted by the root hairs, and deter- 

 minations have been made of the acidity 

 of the sap of the roots with the idea of 

 differentiating between the solvent power 

 of various plants. The roots of germi- 

 nating seedlings are also found on occasion 

 to redden blue litmus paper and un- 

 doubtedly may excrete substances of an 

 acid character, but the behavior of seed- 

 lings, which are building up their fresh 

 tissue out of the broken-down reserve 

 materials contained in the seed, is essen- 



