ON THE YIELD OF HAY 



93 



due to phosphate varies within extremely narrow limits, and is not 

 dependent upon the rainfall. The curve for Tysea Hill is a perfect 

 limiting factor curve and indicates that some factor other than the 

 rainfall and phosphates is limiting the yield of hay. The absence 

 of any increase in yield due to phosphates in 1920 is curious. The 

 season was a particularly favourable one, and owing to the rainy 

 weather in July and the beginning of August the crop was not cut 

 until August 23rd. As will be shown later, at least half of the original 

 dressing of phosphoric acid applied in 1915 was still present in the 



16 

 14 

 12 

 10 



8 

 6 



1 



2345 



Inches of Rainfall, May 1 till Harvest 



FIG. 25. Influence of Rainfall on the Increase due to Phosphates at 

 Wendens and Butterfields O. 



soil in an available form in October, 1919, so that the negligible 

 increase of the treated plots over the untreated in 1920 cannot be 

 due to a deficiency in phosphates. It can only be concluded, therefore, 

 that with a high rainfall (9-34 inches from May 1st till harvest) and 

 a long growing period, no increase will be obtained from phosphates, 

 unless the second limiting manurial factor is first satisfied. The curve 

 for Martin's Hearne closely resembles that at Tysea Hill, and as the 

 two fields are on the same soil formation, have practically identical 

 chemical and mechanical compositions, and are only a short 

 distance apart ; the result at Martin's Hearne satisfactorily confirms 

 the conclusion that a second limiting factor comes into operation as 



