LETTUCE 213 



phate or perhaps untreated phosphatic rock with stable 

 manure at the barn or as it is thrown from the railroad 

 cars. 



Various forms of commercial fertilizers are sometimes 

 used in the forcing of lettuce. Greenhouse plants are 

 easily injured by excessive applications of chemicals, 

 such as nitrate of soda, acid phosphate and muriate of 

 potash, and large amounts should not be used at any 

 time. Sayre draws the following conclusions from fer- 

 tilizer experiments made at the Indiana Experiment 

 Station : 



"In regard to the experiments last year which were reported at 

 the meeting of the Society of Horticultural Science, the items of 

 principal interest were as follows : None of the fertilizer treatments 

 except manure were beneficial, but the manure plots were greatly 

 superior and indicated that manure was by far the best and most 

 economical fertilizer. Our report was chiefly concerned with the 

 effect of various fertilizers on the nitrogen content of the plant. 

 An analysis of the plants shows that the chemical composition, at 

 least in regard to nitrogen, was appreciably affected by the fertility 

 of the soil, and could be modified by the addition of chemical fer- 

 tilizers. The addition of phosphorus to the soil tended to decrease 

 the percentage of nitrogen in the plant, and the application of nitro- 

 gen in addition to phosphorus tended to offset the phosphorous effect 

 and raised the nitrogen content of the plant, but there is a definite 

 limit to which the nitrogen content can be raised. Nitrogen alone 

 slightly decreased the nitrogen content of the plant as might be ex- 

 pected from any element added in excess. Nitrogen unquestionably 

 tended to promote leaf growth, while phosphorus tended to hasten 

 maturity." 



Most growers of lettuce apply lime to the beds about 

 once a year. See Fig. 72. 



Preparation of soil. The preparation of greenhouse 

 soils has been fully discussed in Chapters V and VI. 

 Stable manure is generally applied for the first crop in 

 the fall, and, if desired, additional amounts for subse- 

 quent crops. It may be well decayed, though some 

 growers prefer fine manure that is comparatively fresh. 



