DISCUSSION" OF PLAJSTT INDUSTRIES 



447 



409. Soils and plant nutrition. Soils differ widely in their 

 ability to support vegetation. E^'en the roots from one plant 

 may develop quite differently in different soils, as is shown 

 when the roots are arranged so that part of them grow iii 

 clean sand and part in rich loam (Fig. 348). A comparison 

 of plants of the same kind that have been grown in regions 

 that have different kinds of soil will show wide differences. 

 From the point of view of the growth of our economic plants, 





Fig. 348. Effect of quality of soil on growth of roots 



The cucumber plant shown in the figure was grown in a shallow box, one end of 

 which was filled with sand and the other with rich loam. The seed was planted 

 in the sand, quite near the partition (p) of mosquito netting, which separated the 

 sand from the loam. When the plant was one foot high the earth and sand were 

 washed away and the roots sketched. Those grown in the loam weighed nine 

 times as much as those in the sand. Three eighths natural size 



that soil is best which with the 'proper amount of cultivation 

 will produce the best and largest yield of plant material. Such 

 a soil is said to be fertile. 



There are now in progress many experiments relative to 

 the nature of soil fertility, and many disputed questions are 

 involved. Into these difficulties we shall not enter. How- 

 ever, a study of the way in which plant foods are built up 

 (Sect. 17), and of the chemical analyses that have been made 

 of plants and of soils, enables us to know some of the facts 



