42 



THE GARDENER'S ASSISTANT. 



wearing down of the subjacent rock, and of 

 organic matters derived from the decay of once- 

 living matters as described in the section re- 

 lating to that subject. For our present purpose 

 it is necessary to emphasize the fact that it is 

 chiefly, though not entirely, as a source of 

 water that the soil is important. By far the 

 largest ingredient in the composition of a plant 

 is water. Some plants, and some parts of 

 plants, contain much, others relatively little, 

 water; but in every case it is in large pro- 

 portion to the other ingredients, and in every 

 case it is derived mainly, but not exclusively, 

 from the soil. No solid substance, as a rule, 

 can enter the uninjured root-fibrils or root-hairs. 

 The water soaks through the cell-wall of the 

 fibril or hair, and thus gains access to the 

 interior of the cell and its contained protoplasm. 

 The water is thin, and readily permeates the 

 membrane; the protoplasm and cell-contents 

 are thick, hence a current is set up in virtue 

 of which the thin water from the soil soaks 

 through the cell-wall to mix with and dilute 

 the cell-contents. The water in the soil con- 

 sists of hydrogen and oxygen gases in com- 

 bination, in the proportions expressed by the 

 formula H. 2 0, the admixture of these gases 

 resulting in a liquid, water. 



The solid ingredients of the soil consist of 

 silica, alumina, oxides of iron, lime, magnesia, 

 potash or soda, ammonia, in combination usually 

 with various acids, as phosphoric acid, nitric acid, 

 carbonic acid, &c. Some of these ingredients, 

 being insoluble in water, are of importance to 

 the plant solely on mechanical grounds; they 

 make the soil light and permeable, or heavy and 

 impermeable to air and water, according as the 

 sand (silica) or the clay (alumina) predominates. 

 The draining, digging, trenching, harrowing, 

 raking, hoeing, &c, practised by cultivators, 

 have for their object the formation of a good 

 tilth, or the preparation of the natural soil in 

 such a way as to render its contents most freely 

 available to the roots, and to allow of the free 

 passage of air and water. Similarly, the forma- 

 tion of composts for potting purposes, such as 

 admixtures of sand, loam, and peat, are planned 

 with a view to meet the requirements of roots 

 of different characters. 



But some of the soil ingredients are impor- 

 tant on other than merely mechanical grounds; 

 they assist in the feeding of the plant. The 

 nature of the soil-food is ascertained by the aid 

 of the chemist, who, by analysis of the soil, 

 tells us what it contains, and by analysis, 

 especially of the "ash" left after burning the 



plant, shows us what mineral or incombustible 

 elements are found in the ash of the dead plant. 

 It has been before pointed out that the mineral 

 matters of plants are obtained exclusively through 

 the agency of the roots, and in a liquid con- 

 dition. Hence one important property of the 

 water in the soil is to dissolve certain of the 

 mineral or ash ingredients, and thus to render 

 them available for the nutrition of the plant. 



The analyses of the soil and of the plant 

 made by the chemists only show us what ele- 

 mentary substances exist in the soil and in the 

 plant respectively; they represent, as it were, 

 the constitution of the "dead " soil and of the 

 dead plant. But the soil is by no means the 

 inert substance it was once considered to be. 

 It is scarcely an abuse of language to say that 



I it is alive; at any rate, under certain circum- 



! stances, living organisms abound in it and great 

 chemical activity prevails in it. Moreover, 

 the iron and the potash and the acids do not 

 occur as such, but in combination, so that in 

 estimating the amount and quality of plant-food 

 in a soil, it is more important for us to know 

 how much nitrate of ammonia, or sulphate of 

 potash, or phosphate of lime the soil contains, 

 or the plant requires, than it is to know the per- 



! centage in the soil of the uncombined elements. 

 The Rothamsted experiments, carried out by 



! Sir John Lawes and Sir Henry Gilbert for more 

 than half a century, are invaluable from this 

 point of view. Plants of varying character and 

 different requirements are grown under as nearly 

 equal conditions as possible year after year, 



i on the same land, either without manure, or 

 with the addition of varying proportions of 

 different substances, potash, ammonia, phos- 

 phates, &c, alone or in combination. Soil and 

 plants under these varied conditions are analysed 

 periodically, the drainage water from the ex- 

 perimental plots is collected and analysed in 

 like manner, and the amount of rainfall, and 

 the aggregate amount of heat throughout the 

 year, ascertained. In this manner, independently 

 of the chemical analyses, the several crops tell 

 their own tale ; their vigour and productiveness 

 are indications that certain ingredients in certain 

 proportions are necessary for the full develop- 

 ment of some plants, different ingredients, or 

 different proportionate quantities, being required 

 for others. 



From these and similar experiments it has 

 been ascertained that the most important soil- 

 foods are combinations containing nitrogen, 

 potash, phosphoric acid, lime, and iron. These 

 are so greatly predominant in their influence 



