1844. 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



109 



AGRICULTURAL CHEMISTRY. 



By Professor Brande, F.R.S., &c. 

 Lecture IV.— Delivered at the Royal Institution, Feb. 17, 1844. 

 (Specially reported for this Journal.) 

 Phosphoric acid, once considered unimportant, is requisite for most crops, 

 but especially for wheat and grass. It is generally added to the soil in com- 

 bination with lime, as bone dust, the earthy part of bone consistinj; almost 

 entirely of phosphate of lime. Phosphorus, the base of phos|ihoric acid, is 

 never found in a free state In nature, being obtained Ly distilling a mixture 

 of phosphoric acid and charcoal. It is a pale yellow semi-transparent solid, 

 remarkable for its easy combustibility, luminous in the dark, owing to its 

 slowly burning. When set fire to and a tall glass jar placed over it, flakes 

 of a beautiful white substance are collected, which is phosphoric acid, the 

 phosphorus having combined with the oxygen of the air. These flakes will, 

 in a short time, abstract moisture from the air, and become liquid. If this 

 liquid be then added to a solution of lime, phosphate of lime, or bone earth, 

 falls as a fine white insoluble powder. When bones are burned, it is this 

 which is left as the while ash, the animal matter having been destroyed. But 

 as phosphate of lime is soluble in acids, the animal part may be obtained in a 

 separate state by digesting bone in weak muriatic acid ; the bone remains in 

 its original shape, but is then as flexible as cartilage, the whole of the earthy 

 part being removed. In this state it is that bones are employed to make 

 animal glue and portable soup. Formerly it was supposed that it was to this 

 part that the virtue of bone manure was due ; but it is found that bones are 

 nearly as efficient after the animal part has been destroyed, as for instance, 

 after they have been distilled for the manufacture of hartshorn, or ammonia. 

 Since this has been clearly established there has been great demand for phos- 

 phate, search has been made to ascertain whelher any large natural supply 

 of this substance could be discovered, as it is frequently found in a mineral 

 state. For this purpose Dr. Daubeny made a pilgrimage to Spain, and in 

 Estramadurahe found a large quantity known as Phosphorite, from its giving 

 off a pale blue light when heated ; but as there is no water conveyance from 

 thence, and the roads are bad. the expense would be too great. It is present 

 in nearly every soil, anj is even to be found in chalk, as the following 

 analysis of the Brighton chalk will show. 



10000 



In slate, phosphoric acid sometimes exists combined with alumina, and oc- 

 casionally the surface of the slate is found covered with a crystalline mass of 

 phosphate of alumina, or wavellite, as mineralogists term it. When this dis- 

 integrates, to form a clay soil, the phosphoric acid will get diffused, and thus 

 become one of the elements of the food of the plant. In the chalk, which is 

 agreed by most geologists to be the debris of organic matter, the phosphoric 

 acid has, no doubt, had its origin from theshellsuf come of the loweranimals. 

 An interesting observation has been made on this subject, viz. that the shelly 

 or bony structure of the lowest animals consists wholly of carbonate of lime; 

 that as they increase in complexity of structure, a little phosphate of lime Is 

 found, and in that of the animals highest in the scale of creation, phosphate 

 has entirely replaced the carbonate of lime. From this fact, the absolute 

 necessity for phosphoric acid in the soil is evident; for the growing animal, 

 browsing in tlie field, is c ntinually adding phosphate of lime to his increas- 

 ing bone ; from whence can it come ? As he eats nothing but plants, it must 

 be in their structure, and analysis proves it to be so: then, again, as the 

 plant derives all it contains from the earth, air, and water, it must be in one 

 or other, and the analysis of all fertile soils shows that it is invariably present. 

 These remarks applying to phosphoric acid, are applicable to all the inorganic 

 constituents of animals or vegetables. But it is also requisite that they should 

 be in a soluble state, or the plant cannot take them into its circulation, and 

 frequent disappointment has arisen in the use of bone dust, from there having 

 been nothing in the soil to dissolve the phosphate of lime ; it is by some 

 imagined that plants excrete from their roots acid substances to render soluble 

 the substanci s around them. Experiments have been made by dissolving the 

 phosphate in an acid previous to mixing it with the soil, and very beneficial 

 results have been obtained. The Duke of Richmond found that a much larger 

 crop of turnips was thus obtained, and some agriculturists have slated that 

 one pound of bone dust mixed with acid is as efficacious as 61b. wilhuut. The 

 propoilions advisable are, to take 1001b. of bones, and alter breaking small, 

 to add to tlicm about 501b. of sulphuric or muriatic acid, they being the 



cheapest, and about three cwt. of water, which will give a solution of sub- 

 phosphate of lime and free phosphoric acid. In order to get it into a con- 

 venient form for application. It is advisable to sprinkle this liquid over 

 some substance which will absorb it, such as saw-dust ; if to this be added 

 some silicate of potash, a most fertilising agent is obtained. A manure which 

 is now being very extensively employed in this coutry, and the demand fur 

 which is daily on the increase, viz,, gunno, appears to owe its qualities prin- 

 cipally to the phosphate and other salts which it contains. It is the excre- 

 ment of sea birds, and is obtained in immense quantity. The amount of In- 

 organic matter it contains may be ascertained by burning a known weight in 

 a silver crucible, and weighing the ash which remains. 



The quantity of inorganic material requisite to be added to the soil will 

 depend upon two things, the quantity that the plant absorbs to form part of 

 its system, and also upon what part of the plant is removed from the field, as 

 it has already been seen how greatly the various parts of a plant differ in the 

 quantity of their inorganic constituents. Hence is evident the importance of 

 ploughing in all the parts of the plant not taken to market, especially the 

 leafy parts. It is even, in sandy soils, found advisable to burn the plants 

 which have grown there, such as heath and furze, and to return the ashes, 

 which, by this means, in a few years render it fit for the growth of more 

 profitable crops. The great difference in the quantity of the salts wdiich 

 various plants appropriate, will be rendered very evident by the two following 

 tables : — 



Composition of Three Soils. 



This will serve to explain why those plants which contain but few of these 

 salts are said to form good fallow crops. It is remarkable, also, the invariable 

 proportion in which they are present in the same plant, although grown in a 

 different climate, and in a different soil. Sprengel analysed wheat from 

 several distiicts, and found them, in this respect, exactly to resemble each 

 other. 



A question has ari>en amongst vegetable physiologists « hether < nc sub- 

 stance usually present in a plant, can be substituted by another, and it has 

 been found to take place in some few instances. For instance, cases ha\e 

 occurred where plants which usually have one alkali present, when grown in 

 a soil where that alkali is not present, have been found to appropriate another, 

 but still preserving the .same amount. Indeed, some planis, when growing in 

 circumstances where they cannot obtain inorganic alkalis, will positively 

 form organic ones, in order to carry on their functions ; such is said to be 

 the origin of the morphia, in poppies, to which organic alkali the properties 

 of opium are due. It is well known that potatoes when stored up in a damp 

 cellar, will throw out shoots to a very great length. Now when these are 

 analysed, they are found to contain a vegetable alkali in great abundance, to 

 which the name of solanine has been given, not an atom of which is ever to 



