SOIL, MANURES AND WATERING. 371 



some cases have occurred where it has been as low as 

 twenty. Several causes contribute to make the results 

 vary so greatly. Generally it has been found that the 

 gain by sub-irrigation is less in the early part of the 

 winter than toward spring, owing largely to the compar- 

 ative quantities of water required at these dates. 



The character of the soil has something to do with 

 the results also. Surface watering compacts the soil, 

 but sub-irrigation does not, hence with a naturally heavy 

 soil it is somewhat difficult to give the needed quantity 

 of water when applied to the surface, without producing 

 a water-logged condition, and there is danger also of 

 inducing rot, no matter what the character of the soil. 

 When the lettuce becomes so large as to cover the sur- 

 face of the bed, the difficulty cf watering on the surface 

 is much increased, hence, uniform and thorough watering 

 at this critical period requires much skill and patience. 

 Care has been taken to give the surface-watered beds the 

 best of attention, but it has not been possible to water 

 them so thoroughly and well as by sub-irrigation, since 

 by the latter method the watering can be done as easily 

 at one period of growth as another, and the requisite 

 quantity of water more certainly supplied. 



The true explanation must await further experi- 

 ments, but at present the opinion may be ventured 

 that the amount of water and physical condition of the 

 soil have much to do with the results, and are probably 

 the most important factors. An important consid- 

 eration in the case of lettuce is the fact that there is 

 less rot among the plants on the sub-irrigated plots 

 than where surface watering is practiced. There is 

 a difference in waste in the way of trimmings, due 

 both to rot and soiled lower leaves, ranging from ten 

 to fifteen per cent in favor of the sub-irrigated. It 

 should not be inferred, however, that rot will not occur 

 where sub-irrigation is practiced, for it will appear if 



