450 PEACTICAL BOTANY 



Experimental Station, Harpenden, England. Some of these 

 began in 1848. Certain crops have there been grown year 

 after year upon the same soil. A barley field, which has 

 been unfertilized since the experiments began, produced in 

 the year 1849 a little over 40 bushels per acre. Each year 

 thereafter, with no fertilization, barley has' been grown on the 

 same field, and the yield has steadily decreased, so that dur- 

 ing the twenty years closing in 1909 the average per year 

 was less than 15 bushels per acre. Another piece of ground 

 was used for wheat, turnips, and clover, in rotation, with 

 three years given to each rotation, and was fertilized by use of 

 nitrogen and mineral fertilizers. Considering only the wheat 

 records, we have the following : In the first twenty years the 

 average yield of wheat for the years in which wheat was grown 

 was 35.3 bushels per acre ; in the second period of twenty 

 years the average yield was 32 bushels per acre ; and in the 

 third period of twenty years the average yield was 36.4 bushels 

 per acre. In the second twenty-year period one year of general 

 wheat failure materially reduced the average for that period. 



412. Soil improvement. Since plants use such large quantities 

 of the materials that are named in the tables given above, it is 

 apparent that any soil to be fertile must contain these materials. 

 Merely containing them, however, does not make a soil fertile. 

 They must be in the particular combination in which plants 

 can take them ; the soil must be of such a texture and physical 

 nature as to permit the processes through which plants secure 

 their foods. Such chemical substances within the soil as strong 

 solutions of injurious salts, if present in sufficient quantities, 

 will prevent the passage of materials into the plant. 



A fertile soil, then, is one that has the following requisites : 

 a favorable water content, a good supply of mineral substances 

 necessary to plant growth, freedom from harmful chemical 

 substances, and favorable texture and physical composition. 

 Many soils that once were fertile have become almost or quite 

 unproductive, either through exhaustion of food elements, or 

 through accumulation of harmful substances, and vigorous 



