186 THE YEAR-BOOK OF AGRICULTURE. 



another body that tends to unite with it. This opposition must be overcome before the two 

 will unite. 



23. All soils adapted for cultivation contain the mineral nutritive matters in both these 

 forms. Taken together they represent the capital of the soil ; the freely soluble parts are 

 the movable or available capital. 



24. The improvement enriching, making fruitful of a soil by proper means, but with- 

 out addition of mineral plant-food, implies a conversion of a part of the inactive, unavail- 

 able capital into a form available for the plant. 



25. The mechanical operations of tillage have the object to overcome chemical obstacles, 

 to set free and render directly useful the plant-food that is in insoluble chemical combina- 

 tion. This object is accomplished through the co-operation of the atmosphere, of carbonic 

 acid, oxygen, and water. This action is called weathering. The presence of standing water 

 in the soil, which cuts off the access of the atmosphere to the chemical compounds in the 

 soil, hinders the process of weathering. 



26. Fallow is the period of weathering. During fallow, by means of air and rain, car- 

 bonic acid and ammonia are added to the soil. The latter remains there when substances 

 are present capable of fixing it, i.e. depriving it of volatility. 



27. A soil is fruitful for a given species of plant when it contains the mineral substances 

 needed by that plant in proper quantity and proportion, and in a form adapted for enter- 

 ing it. 



28. When this soil has become unfruitful by continued use, by the removal of a series of 

 crops without replacing the mineral ingredients carried off, it will recover its productiveness for 

 this kind of plant by lying one or more seasons in fallow, if, in addition to the soluble and 

 removed ingredients, it had contained a certain store of the same substances in an insoluble 

 form, which, during the fallow, by mechanical division and weathering, are capable of 

 becoming soluble. By the so-called green manuring this result is effected in a shorter time. 



29. A field which does not contain these mineral forms of plant-food cannot become fruit- 

 ful by lying in fallow. 



30. The increase of the productiveness of a field by fallow and tillage, and the removal of 

 soil-ingredients in the crops, without a return of the latter, brings about, in shorter or 

 longer time, a state of permanent unfruitfulness. 



31. In order that the fertility of a soil be permanent, the removed substances must be 

 replaced at certain intervals ; i.e. its original composition must be re-established. 



32. Various species of plants require the same kinds of mineral food to their develop- 

 ment, but in unlike quantities, or at different times. Some cultivated plants need that 

 silica be present in soluble form in the soil. 



33. When a given field contains a certain amount of all kinds of mineral plant-food in 

 equal proportion, and in suitable form, it will become unproductive of a single species of 

 plant so soon as, in consequence of continuous cropping, any single kind of plant-food e.g. 

 soluble silica is so far exhausted that its quantity is insufficient for a new crop. 



34. A second plant which does not require this ingredient (silica, e.g. ) will yield one or 

 more crops on the same soil, because the other, for it necessary, ingredients, although in 

 changed proportions, (i.e. not in equal quantities,) are yet present in quantity sufficient for 

 its perfect development. 



After the second, a third kind of plant will flourish in the same field, if the remaining 

 soil-ingredients be enough for its wants ; and if, during the growth of these kinds of plants, 

 a new supply of the wanting plant-food (soluble silica) has been made available by weather- 

 ing, then, the other conditions being as before, the first plant will again flourish. 



35. On the unequal quantity and quality of the mineral ingredients of the soil, and on 

 the differing proportions in which they serve as food for the different kinds of plants, is 

 based the alternation or rotation of crops in general, as well as the peculiar method accord- 

 ing to which it is carried out. 



36. Other things being equal, the growth of a plant, its increase in bulk, and its perfect 

 development in a given time, stand in relation to the surface of the organs whose function 

 is to take up the food of the plant. The quantity of plant-food that is derived from the 



