Management of Orchard Soils. 143 



But if you are going to make use of a cover crop to supply nitrogen, you 

 mv;st, in order to manage it intelligently and with continuously good results, 

 understand the changes that take place in a green manure before it becomes 

 useful as plant food. For present purposes, the nitrogen in plants may be said 

 to exist in the form of protein, a complex substance found in all plants and 

 animals. Under certain circumstances protein is capable of nourishing the 

 plant directly without undergoing change, as where a seed stored with protein 

 nourishes the young plant which springs from it ; but this only when the con- 

 nection is closer than that between the roots and the soil. Most of its life 

 the plant must depend upon its roots for nitrogen, and to these protein as 

 such is absolutely useless. To become available the protein must go through 

 a process of decomposition or breaking down into simpler substances until the 

 nitrogen is converted into the soluble salt known as nitrate, and this is accom- 

 plished by the activity of bacteria which are present in vast numbers in all 

 fertile soils. Several species of these bacteria are necessary to produce nitrates 

 from organic matter, or proteids (that is, protein and substances like it), for no 

 v.ne species can convert the one directly into the other. One species lives upon 

 prctein, and forms as a by-product ammonia. Now another common bacterium 

 in the soil can live upon nothing but ammonia, so as soon as this is formed 

 by the first sort it is promptly seized upon and consumed by this second sort. 

 The ammonia feeders also produce a by-product, i. c, nitrites, salts different 

 from nitrates only in having a little less oxygen in them, but neverthless 

 useless to plants. Finally, there Is a group of bacteria that feed only upon 

 nitrites, and their important by-product is nitrates, the food so highly useful 

 to all plants. From this description you might think the conversion of proteids 

 Into nitrates a long and tedious operation, but really, after the first change 

 has been made, the others follow rapidly, so that little time Intervenes between 

 the formation of ammonia and its transformation into nitrates. Ammonia Itself 

 is readily soluble in water, and may be tised by plants without further change, 

 though as a matter of fact but little is so used : nitrates seem by far a more 

 acceptable form. 



As we are so absolutely dependent upon these tiny organisms — it wotild take 

 a thousand or more laid end to end to reach across the head of a pin — it naturally 

 liehooves us to know something of their mode of life, or at least under what 

 conditions they thrive best. Three things are essential for the activity of 

 these bacteria — moisture, warmth, and air. The absence of any of these, or 

 excess of the first two. means suspension of all functions, and in extreme 

 cases, death. They have the same need of air, or oxygen, that we have, 

 namely, for respiration ; and just as a person grows drowsy and inactive in 

 an air exhausted of oxygen, or is smothered to death when completely deprived 

 of it, so these organisms grow more and more sluggish as the supply of oxygen 

 diminshes, and perish if it is wholly withheld. The effect of lack of oxygen 

 is most strikingly seen in peat bogs and wet marshes, where the continual 

 presence of standing water h;is excluded the air to such an extent that the 

 decomposition of the vegetable remains has been almost comijletely suspended, 

 and the black, sodden mass of moss, leaves, weeds and other plant remains 

 may lie there for centuries in a state of preservation, finally passing to the 

 permanent condition of coal. It is the first chapter of the same story when 

 you turn a cover crop under on a heavy soil, leaving it to settle down into an 

 air-excluding cover ; there will be enough change to turn the material black. 

 l>ut at that stage it will remain, in a sort of mummified condition, adding no 

 more richness to the soil than if It were stone. So see to it that the bacteria 

 who work for you have air enough when you engage in green-manuring. 



Heat, too, they must have. Their activity ceases at the freezing point, 

 and is only barely perceptible at 40 degrees, but from this point increases with 

 the rise of temperature up to 100 degrees, from which it diminishes to 130 

 degrees, where it again ceases. The optimum temperature, or point at which 

 they work most actively, is seldom or never reached in this latitude, but the 



