78 CROP PRODUCTION AND SOIL FERTILITY 



plants as the source of energy in the manufacture of foods. Under 

 field conditions duration of light seems more important than inten- 

 sity. Some plants require much sunlight while others do better 

 when shaded. Crops which manufacture much starch and sugar 

 require much sunlight. In order to increase the succulence and 

 delicacy of such crops as asparagus, cauliflower, celery, lettuce 

 and radish, they are sometimes grown under half -shade. In case 

 of cauliflower more desirable heads are produced through shading 

 brought about by bringing together and tying the leaves in the 

 form of a head. Shade is employed in forcing rhubarb, in gin- 

 seng culture, and has proved of much importance in pineapple 

 culture in Florida, 



The sugar beet is a crop requiring abundant sunlight together 

 with plenty of moisture, especially during early growth. These 

 conditions are found most favorable in the irrigated districts of 

 the Rocky Mountain and Pacific States — hence in these districts 

 the sugar content of beets is usually higher than those grown 

 elsewhere. 



Shade a Factor in Weed Killing. — Many have experienced 

 much difficulty or failed entirely in trying to establish a lawn under 

 heavy shade, even when all other conditions were favorable. Lack 

 of sunlight under such conditions is the cause of failure. Weeds 

 may be killed through the use of so-called '^smothering crops.'' 

 These are fast-growing crops which quickly rise above the weeds 

 in thick growths, and in so doing shut out the sunlight from the 

 weeds beneath. 



It is connnonly observed that a field growing a cultivated crop 

 after good clover or alfalfa sod is exceptionally free from weeds, 

 except when weed seeds have been introduced through manure 

 apphcation. The explanation of this lies mainly in the fact that 

 the thick growth of clover or alfalfa killed the millions of small seeds 

 which started to grow, mainly by depriving them of sunlight. 



Weeds May Smother Crops. — Many a seeding of alfalfa, or 

 planting of potatoes, for example, has produced no returns because 

 weeds were allowed to get ahead of the young plants, and thus 

 they were robbed of sunlight as well as of moisture and plant 

 food elements. 



One Crop May Deprive Another of Sunlight. — Lodged grain 

 usually kills out the grass seeding. This killing is due mainly to 

 the exclusion of sunlight. It is very important that seedings of 

 clover and alfalfa, in particular, should not be entirely robbed of 



