154 I^K. SCIILEIDEN'S THEOllY OF AGBICULTURE. 



exist, or where they are exceeded, the species will not grow ; 

 and in the case of cultivated plants, certain conditions are neces- 

 sary for the perfect success of each species or variety. It is 

 possible that different conditions may be requisite or desirable 

 at different stages of growth, a point which requires attentive 

 study, as also in what order and in what quantities the con- 

 stituents of organic materials become free under the influence of 

 weather or decay, and thirdly, in what proportion, at different 

 periods of the year, and in different stages of vegetation, sucli 

 dissolution and decay take place. 



If vegetable mould cannot be regarded as the actual nutriment 

 of plants, with reference to their organic constituents, yet some 

 of its constituents are probably not without importance for the 

 supply of their inorganic elements. Besides that many salts are 

 changed when in contact with decaying humus into easily soluble 

 carbonates or more salutary to vegetation, there ai'e two acids 

 contained in humus, the crenic and apocrenic, which, through 

 their remarkable properties, are in a condition to supply a great 

 quantity of inorganic matter to plants, without, at the same time, 

 overwhelming them with a great mass of organic substance. 

 Both acids are pluribasic ; the apocrenic, for instance, is 5 basic, 

 that is, it contains I equivalent of acid, and 5 of the base ; 

 therefore double and triple salts are easily formed ; and if but 

 one equivalent of ammonia be present, the salts are easily soluble. 



It must be remembered, that this condition must be reversed, 

 if the organic substances of the ground were really destined for 

 the organic nourishment of plants. 



h. The physical nature of the soil must be in a proper 

 relation to its chemical constituents. Sand, lime, clay, and 

 humus are the elements of the question. These, if properly 

 proportioned, will satisfy the necessary conditions of vegetation. 

 About 40 or 50 per cent, of clay seems the best proportion. A 

 greater quantity makes the ground too stiff, and also, from its 

 retention of water, too cold. With less than 10 per cent, it 

 will be too light or poor. Humus is here, however, of the greatest 

 moment. Its very loose consistence acts with extraordinary 

 rapidity on the texture of clay, while its attraction and reten- 

 tion of water serve in the most wonderful degree to bind sand 

 together, as appears from the fertility of the red soil of Cuba, 

 which is naturally free from humus, and the Brazilian and Nortli 

 American forests, which consist of nothing else ; so that, under 

 favourable circumstances, clay may replace humus, and iuunus 

 clay. The following table shows, in the most striking way, tiiat 

 the fertility of soil is proportionally quite independent of the 

 humus which it contains: — 



