On corn plots in Iowa, South Carolina, Ohio and Minnesota which were treated with oat straw either plowed under or on the surface and with two levels of N, measurements were made of soil temperature at the 4‐inch depth, and these measurements were correlated with dry matter yields of corn early in the season. The data from Iowa, Minnesota and Ohio support a theory that early season corn growth is decreased by low temperatures caused by a mulch of crop residues. The data from South Carolina, where soil temperatures were considerably higher than in the three other states, showed that the mulch did not appreciably influence the growth rates. The data from all four of the states agreed in a general way with data of Lehenbauer who found that for a range of temperatures from about 50° to 86° F. there is an increase with temperature of seedling growth rate; whereas around 86° to 90° F. there is little influence; and from about 90° to 111° F. there is a decrease. From the soil temperature measurements taken at a 4‐inch depth it was shown how temperatures for other depths could be deduced from a theoretical expression based on heat flow theory. A detailed example of the use of the heat flow theory in studying the mulch problem in practical experimentation is presented.