95 



Plants, however, differ considerably in their power of extract- 

 ing their supplies of necessary substances from the soil and those 

 which can tap supplies which are not available for other plants 

 may therefore develop normally on soils where the latter cannot 

 exist. As a general rule, the substances which the higher plants 

 seem to find most difficult to obtain in sufficient quantities are 

 nitrogen, phosphorus, and potassium, and these substances in 

 suitable combinations are, generally speaking, the most valu- 

 able manures for field crops, the soil being able to supply 

 a sufficiency of the other substances. Those plants which 

 have special difficulty in obtaining any one of these substances 

 from ordinary soil may therefore, if grown continually on the 

 same ground, be unable after a few years to obtain a sufficient 

 supply for their vigorous development and the yield of agricul- 

 tural crops may thus sink to Jrd the average produce. 



To show how plants vary in this respect, however, it should be 

 mentioned that, according to experience in Europe, although 

 turnips remove large quantities of nitrogen and potassium from 

 the soil, they as a rule only require a manure containing phos- 

 phorus, whereas barley and many other field crops usually 

 require an additional supply of nitrogen in the form of manure, 

 if a good crop is to be produced. Again, experiments extending 

 over 50 consecutive years have shown that, when barley :s 

 grown on one and the same soil continuously, only nitrogen 

 and phosphorus being given as manure and no potassium, 

 the crop will still be nearly normal and equal to that produc- 

 ed when a full manure of potassium, nitrogen and phospho- 

 rus is given. If grown continuously for the same period in 

 the same soil with no manure at all, the crop will sink to nearly 

 ^rd the normal, while if nitrogen is omitted from the manure 

 the yield sinks to less than \ the normal. 



Very little is known regarding the peculiarities of our forest 

 trees in this respect, but it is clear that we have to consider 

 not only the quantity of essential substances contained in the 

 soil and actually removed by the plants, but also the difficulties 

 which each species in nature experiences in availing itself of 

 the substances which are present in the soil, as on this 

 may depend the question of whether, or not, a certain species 

 can be grown continuously for long periods in the same soil. 



The root systems of different plants also develop in different 

 layers of soil, and on such facts in great measure depends the 

 theory of rotation of crops and the growing of mixed crops. 



Many plants are in nature found to take up large quantities of 

 substances which, so far as we can judge from water- culture Substances. 



