Volume 13, Bulletins 157-166. (157) Winogradsky Spontaneous Culture Method for Determining Certain Soil Deficiencies; (158) Production of Gum by Certain Species of Rhizobium; (159) Bacteriology of Butter: V. Studies on the Microorganisms in Churns; (160) Uses of Efficiency Factors in Analysis of Farm Records; (161) Variations in Swine Prices within Iowa Including a Study in Statistical Procedure; (162) Effect of Time, Depth and Method of Plowing Upon Yield and Eradication of Biennial Sweet Clovers; (163) Factors Affecting Experimental Error in Field Plot Tests with Corn; (164) Annual Fluctuations in the Price of Corn; (165) Relation of Moisture to Respiration and Heating in Stored Oats; (166) Relation of Moisture to Respiration and Heating in Stored Oats; (166) Pathogenicity and Seasonal Development of Gymnosporangium in Iowa