Many of the experiments carried out in the field in cooperation with Iowa farmers on some of the more extensive soil types in the state have been under way for 10 or more years. The average results secured on the Carrington loam, the Clarion loam, the Carrington silt loam, the Grundy silt loam, the Grundy silty clay loam, the Tama silt loam, the Muscatine silt loam, the Clinton silt loam, the Marshall silt loam and the Waukesha silt loam are presented and discussed in this bulletin.

The results secured show that crop yields may be increased on many Iowa farms by adopting better methods of management and treatment of the land. The Iowa system of soil management, which has been developed from extensive experimental work and has been tested on many farms, points the way toward securing greater crop yields per acre and keeping the land permanently productive.

The fertilizing value of farm manure and green manures is well known, but the results given, in this bulletin and in other publications emphasize the importance of keeping up the supply of organic matter in soils.