100 BRITISH PLANTS 



increases as we get into the deeper layers. During dry 

 weather water is being constantly brought to the surface 

 by capiUary attraction. The advantage of hoeing the 

 ground in dry weather depends upon the fact that by 

 so doing we cut off the upward water-columns an inch 

 or so from the surface and so prevent the rising water 

 from running to waste by evaporation. It also encour- 

 ages the plants to send down deeper roots into the soil 

 in search of water. The principles of soil-fertility and the 

 reaction of plants to fertilizers are as yet very imperfectly 

 known, and much of what we do in practice is the out- 

 come of long experience rather than the result of scientific 

 research. In general we know that a soil deficient in 

 plant-food is not likely to produce good crops, while a 

 soil rich in such material wiU do so ; also that some crops 

 take out of the soil certain ingredients which can be re- 

 placed by some form of manuring ; and, also, that certain 

 fertilizers benefit certain plants or have a general bene- 

 ficial action on their growth. Thus the chief functions 

 of the three great chemical fertilizers in common use are : 



1. Nitrates, by supplying nitrogen, encourage growth 

 of roots, stems and leaves. 



2. Potash, by its action in facilitating the processes 

 of photosynthesis, promotes the production of starches 

 and sugars, and is therefore usefully applied to potatoes, 

 root-crops and fruit-trees. 



3. Phosphates, by stimulating the vital activities, 

 encourage early and abundant flowering and fructifica- 

 tion. 



The choice and efficiency of manures depend upon many 

 other things than the mere putting into the ground of the 

 chemical substances which a plant needs. The soil is, 

 to a large extent, a living medium, a busy factory in which 

 aerobic bacteria are preparing plant-food. The most 

 important of these bacteria are those which convert 

 nitrogen-yielding substances into nitrates and those which 

 manufacture nitrates from atmospheric nitrogen (see p. 94). 

 The rate at which this nitrification goes on in the soil may 

 be regarded as the index of its fertility. An immense 

 number of aerobic bacteria occur in all fertile soils, and 

 anything that diminishes the number and activities of 

 these bacteria diminishes the fertility of the soil. The 

 proper nitrification of the soil is the first requisite. It 



