PROCEEDINGS OF THE CHEMICAL DEPARTMENT. 191 



less easily accessible to the organs of the plant. To obtain a satis- 

 factory knowledge of these different forms of combination is a 

 most difficult problem. A soil is not a definite chemical com- 

 pound, but a mixture of a great variety of substances, of very 

 variable composition. We easily recognise in it sand and clay, 

 which form its chief bulk ; but that which we call sand often 

 consists to a large extent of the comminuted fragments of the 

 rocks from which the soil is derived ; and under the term clay, we 

 comprise a variety of substances which, though possessing in a 

 more or less marked manner the character of plasticity, are really 

 often very different in composition and in properties. A perfect 

 analysis would seek to separate these different substances ; but as 

 this is practically impossible, the chemist is obliged to content him- 

 self with such an investigation as may serve to classify the nutri- 

 tive constituents into groups of more or less ready accessibility. 



It is generally admitted, that that part of a soil which resists the 

 action of strong acids is entirely useless for the time to the plants 

 which grow upon it, but to what extent those which are extracted 

 by acids are available, is not so clear. That the whole of them 

 are not equally so is very obvious, for the slightest attention to 

 the effect of an acid upon the soil shows that a part of it quickly 

 and easily dissolves, while another portion requires the long con- 

 tinued action of the acid, aided in all probability by heat. 



There can be no doubt at all that the portion of the soil which 

 dissolves most easily is that by which plants are mainly fed ; 

 and hence it was formerly the practice, in complete analyses 

 of soils, to determine the proportion of the constituents which 

 dissolve in water, although it was noticed that this quantity 

 was always small, and did not differ to any material extent, even 

 in soils which differed greatly in fertility. At the period in 

 question, it was supposed that plants could only assimilate those 

 substances which had dissolved in water, and hence peculiar 

 importance was attached to this part of a soil ; but when it was 

 discovered that soils, and especially the most fertile soils, had 

 the power of withdrawing from solution, and actually bringing 

 into an insoluble state those very substances on which the plant 

 is most dependent, the accurate determination of the substances 

 soluble in water lost the interest which formerly belonged to it, 

 and chemists ceased to occupy themselves with what is a very 

 troublesome and laborious part of a soil analysis. The analysis 

 was, therefore, confined to determining under one head all the 

 substances which could be extracted from the soil by the action 

 of an acid, invariably hydrochloric acid, and under another those 

 which resisted the action of this solvent, and which can only be- 

 come soluble by virtue of those slow and gradual changes, to 

 which even the most intractable substances are liable under the 

 long continued action of air and moisture. 



