PRINCIPLES OF PLANTING AND CULTIVATION 35 



statement that while these three factors are essential, they must 

 be present in definite relations to one another. For certain seeds 

 it is necessary that the soil be only slightly warm, while for other 

 seeds it is necessary that it be comparatively hot. It is also neces- 

 sary that the moisture be present within certain limits ; that is, the 

 soil must be neither too wet nor too dry. There is, then, a certain 

 definite relation existing between these three essentials. If the 

 soil is too moist it contains, as a rule, a deficient amount of air, so 

 that the removal of moisture and the application of heat tend to 

 increase the amount of air in the soil. The changes which occur 

 during a single season in the relation between these different fac- 

 tors heat, moisture, and air give us a clew to the reason why 

 certain crops do best when planted early in the season and why 

 others do best when planted late in the season, when the moisture 

 content of the soil is comparatively low and heat is high. In gen- 

 eral, we know that such plants as lettuce, radishes, cabbage, cauli- 

 flower, spinach, etc. grow best during the cool, moist parts of the 

 year. Hence spring and autumn, during which these conditions 

 are most prevalent, are the normal seasons for the growth of these 

 plants. Such plants as tomatoes, eggplants, corn, and beans, how- 

 ever, require a higher degree of heat and do best if planted later 

 in the season, after the soil has warmed up. Under artificial condi- 

 tions in the greenhouse and in the hotbed, the gardener has these 

 factors almost entirely under his control. If he has a greenhouse, 

 the heating plant can be operated so as to maintain the degree of 

 heat desired. Water can be applied or withheld to give the proper 

 humidity to the soil, and as the amount of water in the soil deter- 

 mines very largely the amount of air, the water displacing the air, 

 the proper balance between these two factors can easily be main- 

 tained. Under field conditions, however, water is practically the 

 only factor which can be controlled. Irrigation provides a way of 

 supplying a lack of moisture, and tile draining serves as a means 

 for carrying off any excess of moisture. Indirectly irrigation and 

 drainage influence soil temperature as well as its air and water 

 content. Drainage by removing excess moisture renders cold, late 

 .soils earlier and otherwise more congenial for plant growth. 



Seed sowing. Seeds are sown by machinery whenever practi- 

 cable. In planting certain crops, such as squashes, it is not possible 



