The 1939 Iowa Corn Yield Test

"In order to keep Iowa soils from becoming less fertile and from being washed away, it is necessary that every farmer develop and carry out a program of soil and moisture conservation for his farm. This program need not be complicated. Any Iowa farmer can, by following practical methods, keep his soil fertile and in place. Every acre of a farm should be planned to produce its maximum income in whatever crop it is adapted to produce. Soils on level areas can be kept fertile by proper crop rotation and necessary soil treatments. Rolling land should be handled the same way except that as it becomes steeper other practices must be adopted to keep the soil from washing away. Some lands must remain in pasture, while those less fertile areas which are becoming eroded should be planted by trees. Many rolling Iowa lands farmed to intertilled crops are losing topsoil in spite of the use of long rotations having the minimum amount of intertilled crops. Other practices, such as contour tillage, strip cropping and terracing, are being recognized by Iowa farmers as necessary and practical These practices alone will not solve the problem of soil and water loss. By combining all of the necessary principles and practice of soil and moisture conservation as the apply to each individual farm, Iowa land can be kept productive." -- p. 323