374 



PROFITABLE CROP PRODUCTION 



fertilizer added. For example, if a soil contains an abundant and 

 available supply of all the plant-food elements except phosphorus, 

 and all other factors and conditions are favorable, the yield, in a 

 large meas\n:e, will be proportional to the amount of available 

 phosphorus added in the fertilizer. No excess of the other elements 

 will make up for the shortage of phosphorus (Chap. VII, Fig. 33). 



The law of diminishing returns as it is applied to fertilizers 

 may be stated as follows : The first expenditure of a proper or needed 

 fertilizer is usually the most effective. Each additional increase in 

 application produces smaller and smaller returns j until a further 

 increase causes no increase in the yield. 



This is well illustrated by an experiment on wheat, begun in 

 1852, at Rothamsted, England, the oldest experiment station in 

 the world. Several adjoining plots received mineral fertilizers 

 such as phosphates and potash, in greater amounts than were 

 removed by the crops. In addition to the mineral fertilizers, a 

 nitrogen fertilizer was applied in different amounts. One plot 

 received no nitrogen fertilizer, another 200 pounds per acre, a 

 third 400 pounds, a fourth 600 pounds, and a fifth 800 pounds per 

 acre.'* The fertilizers were applied every year and each year 

 wheat was grown. At the end of the thirteenth year the following 

 average results were secured: 



Diminishing Returns from Nitrogen 



Treatments per Acre 



Average Yield 

 for 13 years 



Increase 



Increase per 

 200 lbs Nitro- 

 gen Fertilizer 





Bu 



Bu 



Bu 



Mineral fertilizers but no nitrogen 



18 3 







Mineral fertilizers + 200 lbs nitrogen 









fertilizer 



28 6 



10 3 



10 3 



Mineral fertilizers + 400 lbs nitrogen 









fertilizer 



37 1 



18 8 



85 



Mineral fertilizers + 600 lbs nitrogen 









fertilizer 



39 



20 7 



19 



Mineral fertilizers + 800 lbs nitrogen 









fertilizer 



39 5 



212 



05 



The fomrth column shows that the first 200-pound application 

 of the nitrogen fertilizer returned 10.3 bushels. Increasing the 

 application by 200 pounds caused an additional increase of 8.5 



* Equal parts of ammonium sulfate and ammonium chloride The Book 

 of the Rothamsted Experiments, 1917, pages 34 and 46. The experiment 

 IS still bemg contmued. 



