CROPPING SYSTEMS FOR TOBACCO 15 



different cropping systems produced little or no effect on the content of soil organic 

 matter. Certain differences between the initial and final organic matter content 

 of soils under different treatments will be noted, but lack of consistency in most of 

 them suggests that the differences shown may be due to little, if any, more than 

 chance. The organic matter has not been materially affected where no means of 

 maintaining it have been employed, as shown by the no-cover or check plots. 

 Likewise, the use of a rye cover has maintained the organic matter. There is con- 

 sistent indication of an upward trend of organic matter content wherever timothy 

 has been used either as a cover or as one crop of a rotation. A slight increase is 

 to be noted for clover in rotation. On the other hand, a downward trend is indi- 

 cated with red top cover and manure. 



T.\Bi.E 7 — Effect of Cropping Systems on Organic Matter* in Tobacco 



Soils. 



(Average percentage of organic matter basetl on oven-dry weiglit of soils) 



Organic Matter 

 Treatment 1923 1932 



No cover 2.45 2.36 



Manure 3.02 2.81 



Manure and timothy cover 2.66 2.96 



Timothy cover 2.64 2.96 



Rye cover 2.45 2.49 



Red top cover 2.87 2.75 



. ^ . ^ > old 2.38 3.14 



Rotation with timothy - ^ ,. - .^ 



) new 2.65 3.45 



) old 2.83 2.90 



♦Calculated from organic carbon using factor 1.724. 



Rotation with clover f -, rv-, t in 



new 3.03 3.1U 



In discussing the question of the value of organic matter in a cropping system, 

 consideration should be given to the type of organic matter determined and 

 changes which it may have undergone in the soil. As usually determined, and as 

 was done in this experiment, soil organic matter consists of the more stable and 

 less active forms of carbon compounds. Succulent cover crops, crop residues, and 

 animal manure decompose comparatively rapidly in soils having optimum mois- 

 ture, good aeration and a temperature such as prevails during the summer months 

 in this region. Cultivation of the soil serves to hasten the process. It may be 

 expected that moderate quantities of organic matter added to the soil in the form 

 of cover crops, crop residues, or fertilizer will largely di-sappear in the course of one 

 season; and this is what the data appear to have proved. What has been deter- 

 mined is the relatively inert, stable organic matter remaining in the soil, and not 

 the intermediate decomposition products which may or may not have been 

 beneficial. 



It does not necessarily follow, therefore, that since a given addition of organic 

 matter has not materially changed the quantity of stable organic matter that it 

 has not been beneficial or detrimental. It appears that both effects have been 

 demonstrated in the cropping systems studied. Red top cover has produced 

 both a high yield and good quality of tobacco. Timothy cover has only slightly 

 improved yield and has had no effect on quality. Tobacco grown in rotation with 

 timothy has been poor in both yield and quality, but here there is a complication 



