24 BULLETIN 1004, U. S. DEPARTMENT OF AGRICULTURE. 



These figures represent a high degree of correlation and point out 

 that the quantity of water used by the crop certainly exerts a pre- 

 ponderant influence on the yield. The fact that the quantity of 

 water used correlates more closely with the total weight of crop 

 produced than it does with the yield of either grain or straw is one 

 that would be expected. It was not anticipated, however, that so 

 close an agreement would exist between the two correlations of the 

 yield of grain and water use and the yield of straw and water use as 

 was shown by this study. It is, of course, necessary that there should 

 be a yield of straw before a yield of grain can be produced, and it 

 was thought probable that the total water use would correlate much 

 more closely with the yield of straw than with the yield of grain or 

 with the total weight. The fact that the three correlations agree 

 very closely strikingly illustrates a statement previously made. This 

 statement was that when affected by drought the wheat crop seems 

 to spend its last energy in producing grain and that if there is any 

 chance at all it will produce some yield of grain. This study indi- 

 cates that a high yield of straw means a high yield of grain and that 

 a low yield of straw means a low yield of grain. There have been 

 a few cases when exceptionally favorable weather enabled wheat to 

 fill so well that the yield of grain was out of proportion to the yield 

 of straw. These years are very infrequent, and as a whole the yields 

 of grain and of straw are nearly proportional. 



It will be noted in Table 3 that in most years the precipitation 

 has greatly exceeded the stored water used from the soil. In nearly 

 every case a high precipitation means a high yield and a low precipi- 

 tation a low yield. Very little relation is shown between the quan- 

 tity of stored water used and the yield. There are two reasons why 

 this is the case. In the first place the quantity of water stored in 

 the soil is never enough to produce a good crop. Rainfall is neces- 

 sary to supplement the soil moisture stored and a large quantity of 

 stored moisture may be associated with a low rainfall, or vice versa. 

 Another reason why the water used from the soil has little relation 

 to the yield lies in the fact that the reduction of the soil water 

 content is dependent upon the sufficiency or insufficiency of the rain- 

 fall. In years of heavy precipitation there may be an actual increase 

 in the quantity of stored water in the soil at harvest over the quantity 

 present at seeding time, and in these years a negative correlation 

 between the water used from the soil and the yield is shown. This 

 condition has been observed, but is very infrequent in dry farming. 



That the stored water used from the soil actually plays an im- 

 portant part in the yield of the crop was shown by correlating 

 precipitation with yield. 



The correlation between the precipitation and the yield of grain 

 was 0.74±0.03, that between the precipitation and the yield of 

 straw was 0.60 ±0.04, and that between the precipitation and the 

 total yield of grain and straw was 0.73 ±0.03. These correlations 

 are distinctly lower than those obtained in correlating the total 

 water used and the yield. This makes it certain that though precipi- 

 tation is the deciding factor in the yield of wheat the quantity of 

 water stored in the soil in many cases is of great value in determining 

 yields. 



That the quantity of stored water is of importance in determining 

 yields was shown by another study. Comparison of two plats each 



