PRECIPITATION 77 



rapid growth during the months of July and August, and, therefore, 

 it is during these months, while the corn is tasseling and forming ears, 

 that the greatest amount of rain is needed for the best growth of this 

 crop. The so-called small grains require their moisture earlier in the 

 season, since they make their growth and mature early. April is the 

 critical month for winter wheat, from the standpoint of precipitation, 

 and May and June are the important ones for oats. For these reasons, 

 the small grains are to quite an extent dependent upon the early 

 precipitation for their moisture, while it is the later rains which bene- 

 fit corn. While heavy May and June rains are needed for oats, they 

 may be detrimental to corn, in that they favor development of a shal- 

 low root system which is ill-fitted to withstand the frequent dry 

 weather of July and August. A very wet May or June means also 

 a poorer stand, vigorous growth of weeds, ineffective and insufficient 

 cultivation, and a puddling of the soils, which means baked and cloddy 

 ground when a dry spell arrives. The plants also tend to grow on too 

 large a scale, producing too great a proportion of stalks to roots. The 

 resulting condition of both plant and soil are such as to imfit them for 

 a dry summer. 



The accompanying charts show concretely the importance of pre- 

 cipitation and illustrate the foregoing discussion. They are based 

 upon the average yields of corn in Iowa for eighteen years 

 and the mean monthly temperatures and precipitations for the same 

 period. In each case, the heavy lines represent the normal yield, tem- 

 perature, and rainfall. Chart No. 12 shows the relation of yield to 

 the total rainfall of the growing season, for the months of May to 

 August inclusive. With a few explainable exceptions, the yield and 

 rainfall agree very closely. The years 1892, 1902 and 1903, show high 

 precipitation, with yields not correspondingly large, but the other 

 charts show that in each of these seasons there was an excessively 

 wet May or June, or both, accompanied by a low average temperature. . 

 In 1893, the yield was higher than the rainfall for these months would 

 account for, but it follows a very wet season and the April (1893) had 

 an unusually large amount of rain, which is not included in the total 

 which is plotted. In 1906, the largest average yield is shown with a 

 rainfall slightly below normal. The May, June and July conditions 

 of that year were nearly normal, while the critical month of August 

 was exceptionally favorable. The low yield of 1907 is accounted for 

 on the grounds of the very cold May and June, early frosts in the fall 

 and erratic distribution ol the rainfall. 



Charts 8 and 9 do not show close correlations between yields and 

 precipitations for May and June. The explanations for these discrep- 

 ancies have already been given. 



