148 



THE GARDENER'S ASSISTANT. 



be incapable of dissolving. The little root- 

 hairs, in particular, which cling so tightly to 

 the soil, are active agents both for absorbing 

 food from the soil water, and for dissolving 

 plant-food from the earth. 



If we bear in mind the enormous weight of 

 soil which covers every acre of land, and then 

 the small quantity of nitrogen, phosphoric acid, 

 or potash required for any one crop, we find 

 that there is a stock or store of plant-food in 

 every cultivated soil which it would take very 

 many crops to exhaust. Yet, on the other 

 hand, a large part of the elements of plant-food 

 contained in soils is present in such a form that 

 plants are unable to make use of it. An acre 

 of land may contain many thousand pounds of 

 phosphoric acid, potash, or nitrogen, and yet 

 be in a poor condition ; while a manurial dress- 

 ing, supplying fifty pounds of readily available 

 phosphate, or nitrogen, in the form of super- 

 phosphate, guano, nitrate soda, or ammonia 

 salts, may greatly increase its productiveness. 



The available character of the plant-food de- 

 pends very much on the description of the soil. 

 A much smaller proportion of plant-food will 

 render a sand fertile than would be required 

 in the case of a stiff clay. This results partly 

 from the far greater development of the plant 

 roots in a sandy soil, and partly from the differ- 

 ent condition in which the mineral food is held 

 by the soil. 



Hilgard has pointed out that the presence of 

 lime in a soil, especially when associated with 

 humus, much increases the availability both of 

 potash and phosphoric acid, so that smaller 

 quantities of these ingredients suffice when lime 

 is also present. 



Food can be taken up by the roots of plants 

 only when it is in solution, or in a condition 

 capable of being dissolved by contact with the 

 acid sap of the root-hairs. 



Matter which is in neither of these conditions 

 is for the time useless to the plant, though it 

 may afterwards become available by the chemical 

 action within the soil. Most of the ingredients 

 of soil are in a compound form, and are held 

 very tenaciously by the soil. This fact is really 

 of the utmost advantage, for if it were other- 

 wise many soils would lose their fertility by 

 heavy rains or by constant artificial waterings. 



The nitrogen contained in humus matter, such 

 as leaf-mould, peat-mould, and the like, is not in 

 a condition to serve as plant-food; to become 

 available, it must be converted into ammonia 

 and nitric acid. This is accomplished by certain 

 bacteria in the soil, to which reference has al- 



ready been made, the carbon of the humus being 

 at the same time oxidized to carbonic acid, 

 whereby heat is developed. This change of in- 

 soluble into soluble plant-food is always going 

 on in the surface soils of rich garden-moulds, 

 and as the nitrates are formed they are at once 

 taken up by the growing plant; or if there is 

 no plant at hand, then the soluble constituents 

 are washed away by the rains, and thus a con- 

 stant exhaustion of plant-food in soils that are 

 uncropped is being brought about. In rich 

 garden soils the production of available plant- 

 food is at its maximum, and so is also the waste 

 by drainage if proper care be not taken. 



Some Reasons for Tillage of Soils. — The opera- 

 tions of tillage and drainage of soils serve in 

 many important ways to make the conditions 

 of plant life more favourable, and to increase 

 the amount of plant-food which is at the disposal 

 of a crop. By tillage, aided by frost, and by 

 alternate drought and rain, the surface soil is 

 | pulverized, and brought into a loose, open con- 

 dition. The fine tilth thus obtained allows of 

 a rapid extension of the delicate root-fibres, and 

 also greatly increases the surface of soil particles 

 upon which the root-hairs feed. 



In spring-time our fields and gardens are often 

 in a state quite unadapted for sowing seed or 

 setting out plants. The soil is so compact at 

 the surface that the gardener cannot bury or 

 cover seed easily and uniformly, and if he 

 should fairly start the crops, they would, as a 

 rule, not thrive as he desired. The soil, how- 

 ever well it was loosened up last year, has settled 

 down so that the particles are in the closest 

 contact with each other. It is rain falling upon 

 newly-loosened tilth which especially aids in 

 compacting it. Not only does the falling weight 

 of the rain-drops actually beat down the sur- 

 face of the earth, but the filling of the pores of 

 the soil by copious rains partly or fully floats 

 the smaller grains and fine silt and clay, and 

 then, when the water runs off below, the whole 

 mass sinks by a nearer fitting together of the 

 light, mobile parts among each other and among 

 the heavier, less disturbed portions. The smaller 

 grains of sand and silt fill up the spaces between 

 the larger, and the microscopic rock-dust and 

 the invisible clay atoms run like mortar into the 

 finest cavities. Each heavy rain produces to 

 some extent a closer readjustment of the particles 

 of soils, and increases its compactness. 



The degree of compactness that ensues depends 

 somewhat upon the kind of particles which com- 

 pose the soil. A sand consisting of spherical 

 grains, all of uniform size., does not settle by 



