CHARACTERISTICS OF DIFFERENT SOILS 209 



soil to another, and plants may be used as guides in deciding 

 upon the nature and fertility of a given piece of land. 



The photograph shows typical samples of turf taken from 

 two of the experimental plots at Rothamsted, which have been 

 receiving a different kind of manure every year for half a 

 century before the photograph was taken. The great contrast 

 in the character of the vegetation in the two plots has been 

 brought about solely by a difference in the nature of the nitrogen 

 compounds supplied ; one compound has favoured certain species 

 of grass which have become dominant, while in the other case 

 a different set of species have been brought to the front by the 

 particular manure that has been given so repeatedly. 



It is not so much that the variations in composition, chemical 

 or physical, between different soils are sufficient to prevent the 

 growth of a given plant in one place while encouraging it in 

 another ; on the contrary, the differences are so small and the 

 adaptability of most plants so great that, as we see in gardens, 

 the same plants will flourish on almost all varieties of soil. In 

 nature, however, the wild plant has not the scope and freedom 

 possessed by the garden plant ; food is scarcer and there is an 

 enormous competition for it, under which conditions a given 

 species only requires to receive a slight advantage over its neigh- 

 bours to become dominant, while the least disadvantage will 

 rapidly cause it to be pushed off the field altogether. The 

 plant lives in a state of fierce competition, neighbours are 

 encroaching on every side, some by their roots trying to rob 

 it of food and water, others by their superior height depriv- 

 ing it of light. Every year the crowding is intensified by the 

 vast number of new seeds that are shed and by the way each 

 plant colony tries to push into fresh ground. How severe this 

 competition becomes may be seen from the fact that our domes- 

 ticated plants, either of farm or garden, are rarely able to get a 

 footing outside cultivated land, and indeed only continue to 

 exist there as long as they are kept free from the competition of 

 weeds. Even wheat, with its vigour and adaptability to all 

 sorts of conditions of soil and climate, is soon crowded out by our 

 native weeds, as may be seen from the following account of an 

 experiment at Rothamsted. " In 1882 about an acre of the 



VOL. V. 14 



