12 FERTILIZERS 



and hard to manage, and bakes easily and while 

 sand alone is too porous to retain moisture, the 

 addition of a little clay with sand makes the proper 

 balance for retaining moisture and for friability. 



15. Humus, decayed organic matter, is absolutely 

 essential as it is the source of the necessary nitrogen. 

 It is a fundamental truth in connection with this sub- 

 ject that there is no fertility without humus. It also 

 influences favorably, as nothing else will, the soil's 

 mechanical condition and moisture content, besides 

 supplying essential plant foods. 



The humus which plants contribute to the soil 

 not only furnishes all of the soil nitrogen, except 

 that artificially added, but gives life to numerous 

 forms of bacterial life, with which every healthy soil 

 is teeming. We cannot discuss this subject here (86- 

 89), but it will suffice to say that, without the 

 contribution made by plants to the soil, the micro- 

 scopic forms of life could not exist and they are now 

 regarded as essential to fertility. Indeed, it is pos- 

 sible to inoculate soils with beneficial bacteria, and 

 improve their fertility. 



A soil may contain sand, clay and humus and yet 

 lack some essential plant food, hence the last division. 

 But as a rule, sanrl, clay and humus in proper pro- 

 portions will, for a time at least, supply all the 

 requirements of plant life. 



The plant requires of the soil that it furnish the 

 ten named soil derived elements, and the soil requires 

 of the plant that it supply humus and such of the 

 air derived elements as are necessary to its own 

 health and fertility. Just what amounts of hydro- 

 gen, oxygen, carbon and nitrogen are absorbed 



