MARYLAND WEATHER SERVICE 353 



of cases, alfalfa is the crop planted. The particular form of legume 

 must be selected to suit the local conditions of soil and climate. 

 Tobacco comes into the rotation in the section of the Zone along the 

 western shore of the Bay, and in the rotation as given above it may 

 be considered as substituted for the hay crop of the cycle, reducing 

 the rotation to three instead of four years; corn, wheat, tobacco, 

 being the order in this case, while corn, wheat, hay, hay was the 

 order before. 



The introduction of such crops as potatoes, into the general farm 

 rotation or of the several truck and canning crops in the truck areas, 

 causes a variation from the rotation order as outlined. This has 

 more or less importance as the area of the field is wholly involved 

 or only a portion of it receives the special crop. In the truck areas 

 the regular rotation of staple crops is less apparent, as the land is 

 under more continuous cropping and practically every crop receives 

 its share of fertilizer or manure. In the effort to keep the land in 

 crop the truck farmer rarely alternates the type of plants grown to 

 a degree sufficient to meet the theoretical conditions for rotation 

 suggested above. The trucker has this advantage over the regular 

 farmer, — much of his produce is green vegetation, as lettuce, 

 spinach, cabbage, etc., comparatively little developing to the point 

 of ripe seeds, as is the regular thing with such crops as corn and 

 wheat. It is the latter type of crop that is hard on the fertility of 

 a field, because of the amount of phosphoric acid taken in the ripen- 

 ing of the seeds. Green crops take very little of soil fertility from 

 the land, for the solid substance is largely derived from the air as 

 carbonic acid, and built into the cellular tissues of leaf or stalk, 

 the cells being filled with the juices of the plant, or with such other 

 material as starch, of a similar character to the cells themselves, as 

 regards composition. Such crops withdraw but little fertility from 

 the soil, as one can determine for himself by comparing the amount 

 of ashes left by a few pounds of cabbage or lettuce or some fresh 

 fodder crop with that left by the same weight of wheat or corn. 



There are other sources of loss in the fertility of one's farm land, 

 and one of these is the gullying and washing of plowed ground. 



When it is desirable to cultivate fields having steep slopes the 

 method of plowing known as "contour plowing" can be used to ad- 



