This is a study of the effect which crop plants have on the growth and yields of those crops which follow. The study began in 1907; this bulletin reports the results from 1930 to 1942. Millet, mangels and rutabagas generally seem to be harmful to crops which follow them. Carrots, mangels and millet were least favorable of all crops to mangels. Potatoes did poorest when following potatoes, rutabagas, or millet. Rutabagas yielded low following rutabagas, mangels and millet. Onions were unfavorably affect by preceding crops of mangels, cabbage, or rutabagas. There is no simple explanation for the effect of crops on those which follow. The relationships are complex and interdependent, and are associated with the physical, chemical and microbial conditions of the soil. Well fertilized crops in moderately to strongly acid soils apparently produce unfavorable conditions for a succeeding crop for one or more of the following reasons: 1. They deplete basic nutrients with a consequent increase in H-ion concentration, accompanied by possibly harmful concentrations of aluminum or other elements. 2. Unless organic matter is provided, an adequate quantity of water stable aggregates is not maintained so that the soil becomes compact and possesses a poor physical condition unsuitable for maximum crop growth. 3. After certain crops, conditions are more favorable for the growth of plant pathogens and the development of seedling root rot. 4. The chemical, physical and biological conditions of the soil are intimately related and interdependent so that an improvement in one may produce some improvement in the other
