Talks on Manures Part 54

"But is it as good?" asked the Deacon.

"Soluble phosphate of lime," said I, "is soluble phosphate of lime, and it makes no difference whether it is made from burnt bones, or from phosphatic guano, or mineral phosphate. That question has been fully decided by the most satisfactory experiments."

"Before you and the Deacon discuss that subject," said the Doctor, "it would be well to tell Charley what superphosphate is."

"I wish you would tell me," said Charley.

"Well," said the Doctor, "phosphate of lime, as it exists in bones, is composed of three atoms of lime and one atom of phosphoric acid.

Chemists call it the tricalcic phosphate. It is also called the basic phosphate of lime, and not unfrequently the 'bone-earth phosphate.' It is the ordinary or common form of phosphate of lime, as it exists in animals, and plants, and in the various forms of mineral phosphates.

"Then there is another phosphate of lime, called the dicalcic phosphate, or neutral phosphate of lime, or reverted phosphate of lime. It is composed of one atom of water, two atoms of lime, and one atom of phosphoric acid.

"Then we have what we call superphosphate, or acid phosphate of lime, or more properly monocalcic phosphate. It is composed of two atoms of water, one atom of lime, and one atom of phosphoric acid. This acid phosphate of lime _is soluble in water_.

"The manufacture of superphosphate of lime is based on these facts. The _one-lime_ phosphate is soluble, the _three-lime_ phosphate is insoluble. To convert the latter into the former, all we have to do is to _take away two atoms of lime_.

"Sulphuric acid has a stronger affinity for lime than phosphoric acid.

And when you mix enough sulphuric acid with finely ground three-lime phosphate, to take away two atoms of lime, you get the phosphoric acid united with one atom of lime and two atoms of water."

"And what," asked the Deacon, "becomes of the two atoms of lime?"

"They unite with the sulphuric acid," said the Doctor, "and form plaster, gypsum, or sulphate of lime."

"The molecular weight of water," continued the Doctor, "is 18; of lime, 56; of sulphuric acid, 80; of phosphoric acid, 142.

"An average sample of commercial bone dust," continued the Doctor, "contains about 50 per cent of phosphate of lime. If we take 620 lbs. of finely-ground bone-dust, containing 310 lbs. of three-lime phosphate, and mix with it 160 lbs. of sulphuric acid (say 240 lbs. common oil of vitriol, sp. gr. 1.7), the sulphuric acid will unite with 112 lbs. of lime, and leave the 142 lbs. of phosphoric acid united with the remaining 56 lbs. of lime."

"And that will give you," said the Deacon, "780 lbs. of 'dissolved bones,' or superphosphate of lime."

"It will give you more than that," said the Doctor, "because, as I said before, the two atoms of lime (112 lbs.) are replaced by two atoms (36 lbs.) of water. And, furthermore, the two atoms of sulphate of lime produced, contained two atoms (36 lbs.) of water. The mixture, therefore, contains, even when perfectly dry, 72 lbs. of water."

"Where does this water come from?" asked the Deacon.

"When I was at Rothamsted," said I, "the superphosphate which Mr. Lawes used in his experiments was made on the farm from animal charcoal, or burnt bones, ground as fine as possible--the finer the better. We took 40 lbs. of the meal, and mixed it with 20 lbs. of water, and then poured on 30 lbs. of common sulphuric acid (sp.g. 1.7), and stirred it up rapidly and thoroughly, and then threw it out of the vessel into a heap, on the earth-floor in the barn. Then mixed another portion, and so on, until we had the desired quant.i.ty, say two or three tons. The last year I was at Rothamsted, we mixed 40 lbs. bone-meal, 30 lbs. water, and 30 lbs. acid; and we thought the additional water enabled us to mix the acid and meal together easier and better."

"Dr. Habirshaw tells me," said the Doctor, "that in making the 'Rectified Peruvian Guano' no water is necessary, and none is used. The water in the guano and in the acid is sufficient to furnish the two atoms of water for the phosphate, and the two atoms for the sulphate of lime."

"Such is undoubtedly the case," said I, "and when large quant.i.ties of superphosphate are made, and the mixing is done by machinery, it is not necessary to use water. The advantage of using water is in the greater ease of mixing."

"Bone-dust," said the Doctor, "contains about 6 per cent of water, and the sulphuric acid (sp.g. 1.7) contains about one-third its weight of water. So that, if you take 620 lbs. of bone-dust, and mix with it 240 lbs. of common sulphuric acid, you have in the mixture 117 lbs. of water, which is 45 lbs. more than is needed to furnish the water of combination."

"The superphosphate produced from 620 lbs. of bones, therefore,"

continued the Doctor, "would contain:

Phosphoric acid} {142 lbs.

Lime } acid phosphate { 56 "

Water } { 36 "

Sulphuric acid } {160 lbs.

Lime } sulphate of lime {112 "

Water } { 36 "

Organic matter, ash, etc., of the bones* 335 "

-------- Total _dry_ superphosphate 877 "

Moisture, or loss 45 "

-------- Total mixture 922 lbs.

* Containing nitrogen, 23 lbs.

"There is a small quant.i.ty of carbonate of lime in the bones," said I, "which would take up a little of the acid, and you will have a remarkably good article if you calculate that the 620 lbs. of bone-dust furnish you half a ton (1,000 lbs.) of superphosphate. It will be a better article than it is practically possible to make."

"a.s.suming that it made half a ton," said the Doctor, "it would contain 14 per cent of soluble phosphoric acid, and 2? per cent of nitrogen."

"With nitrogen at 20 cents per lb., and soluble phosphoric acid at 12 c. per lb., this half ton of superphosphate, made from 620 lbs. of good bone-dust, would be worth $22.50, or $45 per ton."

"Or, to look at it in another light," continued the Doctor, "a ton of bone-dust, made into such a superphosphate as we are talking about, would be worth $72.58."

"How much," asked the Deacon, "would a ton of the bone-dust be considered worth before it was converted into superphosphate?"

"A ton of bone-dust," replied the Doctor, "contains 76 lbs. of nitrogen, worth, at 18 cents per lb., $13.68, and 464 lbs. phosphoric acid, worth 7 cents per lb., $32.48. In other words, a ton of bone-dust, at the usual estimate, is worth $46.16."

"And," said the Deacon, "after it is converted into superphosphate, the same ton of bones is worth $72.58. It thus appears that you pay $26.42 per ton for simply making the phosphoric acid in a ton of bones soluble.

Isn't it paying a little too much for the whistle?"

"Possibly such is the case," said I, "and in point of fact, I think bone-dust, especially from steamed or boiled bones, can be used with more economy in its natural state than in the form of superphosphate."

Superphosphate can be made more economically from mineral phosphates than from bones--the nitrogen, if desired, being supplied from fish-sc.r.a.p or from some other cheap source of nitrogen.

But for my own use I would prefer to buy a good article of superphosphate of lime, containing no nitrogen, provided it can be obtained cheap enough. I would buy the ammoniacal, or nitrogenous manure separately, and do my own mixing--unless the mixture could be bought at a less cost than the same weight of soluble phosphoric acid, and available nitrogen could be obtained separately.

A pure superphosphate--and by pure I mean a superphosphate containing no nitrogen--can be drilled in with the seed without injury, but I should be a little afraid of drilling in some of the ammoniacal or nitrogenous superphosphates with small seeds.

And then, again, the "nitrogen" in a superphosphate mixture may be in the form of nitric acid, or sulphate of ammonia, in one case, or, in another case, in the form of hair, woollen rags, hide, or leather. It is far more valuable as nitric acid or ammonia, because it will act quicker, and if I wanted hair, woollen rags, horn-shavings, etc., I would prefer to have them separate from the superphosphate.

CHAPTER x.x.xVIII.

SPECIAL MANURES.

Twenty five to thirty years ago, much was said in regard to special manures. Fertilizers were prepared for the different crops with special reference to the composition of the plants.

"But it was known then, as now," said the Doctor, "that all our agricultural plants were composed of the same elements."

"True, but what was claimed was this: Some crops contain, for instance, more phosphoric acid than other crops, and for these a manure rich in phosphoric acid was provided. Others contained a large proportion of potash, and these were called 'potash crops,' and the manure prescribed for them was rich in potash. And so with the other ingredients of plants."

"I recollect it well," said the Doctor, "and, in truth, for several years I had much faith in the idea. It was advocated with consummate ability by the lamented Liebig, and in fact a patent was taken out by the Musgraves, of Liverpool, for the manufacture of Liebig's Special Manures, based on this theory. But the manures, though extensively used by the leading farmers of England, and endorsed by the highest authorities, did not in the end stand the test of actual farm practice, and their manufacture was abandoned. And I do not know of any experienced agricultural chemist who now advocates this doctrine of special manures.