LEGUMES AND GRASSES 



45 



tative matter or decomposition 

 products had little or no influence 

 on the proportion of water-stable 

 aggregates greater than 0.84 mm., 

 but they increased the proportion 

 of medium-sized water-stable ag- 

 gregates, decreased the proportion 

 of water-stable aggregates less than 

 0.02 mm., decreased soil cloddiness, 

 and increased wind erosion. He 

 concluded that far greater pro- 

 tection from wind erosion would be 

 derived from maintenance of vege- 

 tative materials on the soil surface 

 than from mixing them into the 

 soil to increase soil aggregation 

 during the initial stage of their 

 decomposition. 



Summary 



Yields of all crops on green- 

 manure fallow were less than those 

 on ordinary fallow at Hays 

 and Colby. Yield reductions were 

 caused by reduced moisture storage. 

 Green-manure fallow crops had 

 used part of the moisture normally 

 stored by ordinary fallow. There 

 was no cumulative benefit from 

 their use. 



Crop rotations and fertilizer 

 treatments have been studied at 

 Manhattan since 1915. Grain yields 

 on unfertilized plots declined with 

 time in all but the alfalfa rotation. 

 In both legume and nonlegume 

 rotations, there was an upward 

 trend in wheat yields with time 

 where commercial fertilizer or ma- 

 nure was used. In all rotations 

 except the 16-year rotation that 

 included 4 years of alfalfa, there 

 was a downward trend in corn 

 yields where commercial fertilizer 

 or manure was used. There was 

 a downward trend in alfalfa yields 

 regardless of fertilizer treatment. 



At Manhattan, alfalfa depleted 

 available soil moisture to a depth 

 of 25 feet. Two years of fallow 

 with normal precipitation restored 

 depleted soil-moisture reserves. 



From 1919 to 1928, continuous 

 cereal cropping failed to restore 

 soil-moisture reserves depleted by 

 alfalfa. However, from 1938 to 

 1949, 12 years of cereal cropping 

 resulted in water being stored to a 

 depth of at least 22 feet. From 

 October 1941 to October 1945, 

 total precipitation exceeded long- 

 time totals by 36.6 inches. Soil- 

 moisture samples were not taken, 

 but most of the water probably 

 was stored during this period. 



At Manhattan, sweetclover re- 

 duced subsoil moisture to a depth of 

 9 feet in 1 year and 14 feet in 2 

 years. Bromegrass did not deplete 

 the soil moisture below 4 feet. 



Soil nitrogen and carbon changes 

 have been measured periodically 

 at the Hays, Colby, and Garden 

 City Branch Stations since 1916. 

 At Hays, green-manure rotations 

 have been less effective than con- 

 tinuous small grains in conserving 

 soil nitrogen and carbon. At Colby, 

 green-manure rotations caused ni- 

 trogen and carbon losses as great 

 as continuous row crop. At Garden 

 City, green-manure rotations were 

 intermediate between continuous 

 small grain and continuous row 

 crop in soil nitrogen and carbon 

 losses. Reasons for the differential 

 effects of green manure are not 

 known. 



At Manhattan, none of the crop 

 rotations or fertilizer treatments 

 maintained the nitrogen and carbon 

 content of the soil at its original 

 level. Green manures reduced car- 

 bon and nitrogen losses. Commer- 

 cial fertilizer had a similar but less 

 marked effect. 



Alfalfa grown continuously at 

 Manhattan increased the supply of 

 nitrogen and carbon in the plow 

 layer of soil at the rate of 0.71 

 percent and 0.43 percent per year, 

 respectively. Alfalfa added to the 

 supply of soil nitrogen for 19 years — 

 the duration of the experiment. 

 Wheat grain yields were increased 



