6 An Account of the Soil [pt. i 



as possible. This is easy on paper but often difficult in 

 practice. 



Where no Hmiting factor is operating it is a general 

 rule that if any one of the necessary factors is increased 

 in amount there will be an increase in crop growth. This 

 is shown in Fig. 1 illustrating three pots of tomatoes 

 growing in the same soil, sown at the same time and 

 treated alike in every respect except one. The soil is 

 a very light sand ; in one pot there has been no addition 

 of plant food ; in the second the crop has received a dose 

 of manure, and in the third it has received a larger dose. 

 A similar result is obtained in the field as shown in Fig. 2 ; 

 the shortest wheat plant is a representative specimen of 

 the crop on the unmanured land ; the next plant shows 

 what happens when an almost but not quite complete 

 manure is added ; the third shows the marked gain when 

 one dose of complete manure is given ; next comes the 

 effect of two doses ; and the last shows the effect of three 

 doses. In all cases an increase in the amount of plant 

 food has led to an increase in the crop. 



Very similar results are obtained when the water 

 supply is varied. In Fig. 3 are shown tomato plants 

 growing in a good soil, sufficiently and equally manured, 

 and under the same favourable conditions of light, 

 temperature, air, etc. All the conditions, excepting one, 

 are the same for all pots: the water supply only varies. 

 When only little water is given the growth is poor in 

 spite of the presence of food and the favourable tempera- 

 ture and light conditions; when more water is added 

 there is better growth; finally with adequate water 

 supply growth is really good. 



But growth will not go on indefinitely. A fimit is 

 reached sooner or later beyond which the plant will not 



