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The Pamphlet Collection of Sir Robert Stout: Volume 73


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This element is of immense importance as part of the mineral food of plants. Like nitrogen, it is in the more choice parts of plants that it is chiefly found, and in those plants which are most highly valued as food for man. There is as much as 10lb. of it in 50 bushels of wheat, 80 bushels of oats, 60 bushels of barley, 6 tons of potatoes, 20 tons of turnips or mangels, 2½ tons of oaten hay or 3 tons of meadow hay, 18cwt. of beans or peas, and smaller quantities in the green grasses of our pastures. It is from plants that animals get the phosphorus, in the first instance, where-with to build their bones. The plants get it in the soil; and it comes into the soil from its original storehouse in the rocks, in which it exists mainly as phosphate of lime.

Phosphate of lime when pure contains one-fifth of its weight of phosphorus. It occurs, sometimes in immense quantities, in rocks of every geological age, from the oldest Laurentian and Cambrian rocks of Canada and the Silurians of Norway—both far below the old English coal-measures—down to the fossiliferous Tertiary coprolitic rocks of Norfolk and Cambridge, in England.

The necessities of the British farmer, when he found his fields becoming less and less productive as time went on, drove him to distant countries in search of the phosphates which a long series of grain crops had withdrawn from the soil. Time and distance and wide expanse of seas were no obstacle to his enterprise, and he found the object of his pursuit in vast quantities in the guanos accumulated by countless generations of birds in the islands of the sea; in the phosphorites of Estremadura, Nassau, and Bordeaux; in the apatites of the Laurentian rocks of Canada and the metamorphic schists of Norway; in the sombrerite of the West Indies, the land and river phosphates of the Carolinas and Florida, as well as at Home in the coprolites of the Tertiary formations of England. Not satisfied with these rocky sources of the precious phosphates, he earned for himself a malodorous name by digging up and shipping off to England the bones that lay entombed on the battlefields of Leipzig, Austerlitz, and Waterloo, and harrying the catacombs of Sicily in the same unholy crusade. By-and-by the despoiled peoples found out their mistake, and John Bull's quarrymen were warned off these phosphatic fields. He still, however, commands the sea, and his fleets are on every ocean, streaming in a straight line to London, Liverpool, and Glasgow with heavy cargoes of mineral and guano phosphates from wherever they are to be found. Besides these guanos and mineral phosphates there are large quantities of phosphate of alumina, which have been used as fertilisers, and smaller quantities of the phosphates of iron and lead, which are of no manurial value.

Phosphorus itself—the waxy, pale-yellow, translucent substance that is so largely used in these colonies for poisoning rabbits and in the manufacture of lucifer matches—is made, so far as I know, in only two factories: one near Birmingham, in England; the other at Lyons, in France. Considering the large and permanent market for it here, and the abundance of the raw material—bones and sulphuric acid—I have often thought its manufacture would be a very appropriate and profitable adjunct to some such manufacturing industry as the chemical and manure works of Messrs. Kempthorne, Prosser, and Co., at Burnside. The by-products in its manufacture have also a considerable value. The process is as follows:—

1. Raw bones are crushed and boiled to remove the fat, which is then skimmed off and sold to the soapmaker.

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2. The bones are then digested in superheated steam, which extracts the 30 per cent, of gelatine that they contain, and this is sold to the glue-maker.

3. The residue of the bones is then roasted and thus converted into bone earth, or bone ash, or phosphate of lime, of which the formula is Ca3P2O8. and which contains one-fifth of its weight of phosphorus.

4. This bone ash is now digested with semidilute sulphuric acid in tar-lined wooden tubs, which has the effect of converting them into sulphate of lime and soluble phosphate of lime by this equation:—
  • Ca3P2 + 2H2SO4 = 2CaSo4, + CaH4P2O8;
which means that 310 tons of bone ash acted on by 196 tons of sulphuric acid will make 272 tons of dry sulphate of lime and 234 tons of soluble phosphate of lime. To make these quantities quite clear, it only requires to be stated that Ca means 40 parts of calcium; P, 31 parts of phosphorus; O, 16 parts of oxygen; S, 32 parts of sulphur; and H, 1 part of hydrogen—these numbers representing the atomic weights of the elements named.

5. The mixture of sulphate of lime and phosphate of lime thus got is boiled down to a small bulk, when, on standing, the sulphate will be deposited as a sediment, the phosphate remaining in solution.

6. It is then filtered through a haircloth filter, which allows the phosphate to pass through in solution, while the precipitated sulphate is retained on the haircloth.

7. The phosphate solution is now evaporated to dryness, mixed with charcoal, and heated redhot in an air-tight retort, when two-thirds of the phosphorus present passes over and is received and condensed in cold water, the other third remaining in combination with the lime. The equation is

3CaH4P2O3 + 10C = 10CO + Ca3P2 + 6H2O + 4P;

which means that 702 tons of soluble phosphate of lime and 120 tons of charcoal will, when distilled, yield 280 tons of carbonic oxide, 310 tons of bone ash, 108 tons of water, and 124 tons of phosphorus.

Returning now to phosphate of lime as a manure. It is, in all its forms—apatite, phosphorite, coprolites, guano, bones, or bone-earth—insoluble in pure water. Now, when it is considered that plants cannot take in by their rootlets any solid substances, it is evident that it will be necessary to dissolve this insoluble phosphate of lime before the plant can make any use of it. How is this to be done? It is effected partly by the carbonic acid in the moisture of the soil and of the air. The soil itself gets its carbonic acid partly from the rain-water, that has washed a little of it out of the atmosphere, and partly by the decay of dead animal and vegetable matter. This carbonic acid in the soil-water converts the insoluble tricalcic phosphate (commonly known as phosphate of lime) first into the dicalcic phosphate, and then into the monocalcic phosphate, both of which are easily dealt with by the living rootlets of the growing plant. The equations which show this action of the carbonic acid on the phoaphates may be put thus:—
(1.)Ca3P2O + CO2 + H2 = Ca2H2P2O8 + CaCO3;
which reads—310 parts of insoluble or tricalcic phosphate acted on by 44 parts of carbonic acid in the presence of 18 parts of moisture will produce 272 parts of dicalcic phosphate and 100 parts of carbonate of lime; and
(2.)Ca3P2O8 + 2CO2 + 2H2O = CaH4P2O8 + 2CaCO3;
which means that 310 parts of insoluble tricalcic phosphate, acted on by page 11 88 parts of carbonic acid and 36 parts of water, will make 234 parts of monocalcic phosphate and 200 parts of carbonate of lime. Besides this solvent action of carbonic acid, however, there is another agency at work to dissolve the phosphates and thus make them fit for absorption, and that agency is the rootlets (root-hairs and terminal spongioles) themselves. These plant-organs get the credit of secreting and extruding from their microscopic openings a liquid (probably of an acid nature) that attacks and dissolves these phosphates and other substances required for plant-food. One is familiar with the extraordinarily intimate manner in which these root-hairs and thread-like roots cling to and embrace and enwrap the minute loose particles of soil, pushing in their slender filaments into every opening, and grasping the sod and every part of it with such a clinging tenacity that it is often a difficult matter to separate them. Now, these live root-organs are all the time busy searching out suitable food, secreting a liquid to dissolve it when found, and drawing it up in solution, to pass it on to the laboratory of the plant, where, under the action of light, &c., it is digested, assimilated, and suitably built—like bricks into a building—into the structure and living substance of the plant itself.

It was the great Liebig that first came to the assistance of plants in their difficulty with these insoluble phosphates. That pioneer and prince of agricultural chemists, just fifty-four years ago, proposed that we should share with our growing crops the task of dissolving the insoluble phosphates; or rather that we, with our ready command of acids and other chemical agents, should take that task on ourselves and present to the plants their phosphate food in a condition in which they could at once take it. This Liebig did by his receipt for the manufacture of superphosphate of lime by the action of strong sulphuric acid (oil of vitriol) on bones. Two years after, the great English agricultural experimentalist, Sir John B. Lawes, of the renowned duet of "Lawes and Gilbert," and who is now offering for competition in Otago challenge shields for the best-managed farms, extended the same sulphuric-acid treatment to the mineral phosphates—the apatites, the phosphorites, and the coprolites—with entire success. Thus was created one of the greatest industries of modern times. There are now extensive artificial-manure works by the dozen in every great city of Europe and North America; while we here in New Zealand have them established and grinding and dissolving the phosphates in all our important towns in both Islands.

In England and on the Continent of Europe an immense quantity of coprolites is thus manufactured into a valuable manure, and sold in all the markets. There is good reason to believe that in New Zealand, also, there are large quantities of these mineral phosphates awaiting discovery, which would possibly be hastened by the offer of a bonus to the explorer.

The action of the sulphuric acid on the phosphates has already been described, and the equation given, when explaining the process for the manufacture of phosphorus. The equation is here repeated:—
  • Ca3P2O8 + 2H2SO4 = CaH4P2O8 + 2CaSO4

The mixture of 234 parts of soluble monocalcic phosphate and 272 parts of sulphate of lime shown on the right-hand side of this equation is what is known among farmers as superphosphate of lime, or soluble phosphate of lime. There is, however, one remark to make about this sulphate of lime. It is not exactly as 2CaSO4 that it appears in the manure, but 2(CaSO4, 2H2O)—that is, 272 parts sulphate of lime combined page 12 with 72 parts of water; and it is a curious thing that this water is not there as wet water, but water so combined with the dry sulphate that it actually makes a part of it, and the substance with all that water really in it is just as dry as though the water were not there at all. The sulphate of lime without the water would be identical with "plaster of Paris," and the same sulphate with the water in it would be the stucco made by rapidly rubbing up water with the "plaster"; 272 parts of the sulphate of lime, indeed, absorb 72 parts of water in the superphosphate vats, ami this behaviour on its part dries up the mixture, and so makes it fit for the market. The more carbonate of lime the raw phosphate contains the greater will be the proportion of this drying sulphate of lime in the superphosphate, and the nicer will it be to handle, and the more porous will it be, and therefore the more rapidly will it be dissolved when applied to the land. For these reasons some guano superphosphates containing originally much carbonate of lime or coral are preferred, although their intrinsic value (from containing so large a proportion of sulphate) is less than that of stronger superphosphates made from bones.

Superphosphates therefore differ in three respects from insoluble phosphates—(1) in that they are an artificial or manufactured article; (2) that they contain much or nearly all of their phosphate in a soluble condition, and therefore instantly available for plant-food; and (3) that they contain necessarily a large proportion of sulphate of lime, which is also soluble, and provides for the plant both lime and sulphur in a soluble condition.

There are, however, great differences in the manurial value of superphosphates, and it would be a great mistake to suppose that one superphosphate is just as good as another. I have analysed in Dunedin within the last year a superphosphate fairly worth £11 per ton, and a superphosphate worth not more than £3 per ton, and superphosphates worth well nigh every price between these two extremes. It would not therefore be a bad thing for some farmers when buying their superphosphates to get a guarantee of its composition, and, if doubtful of its quality, to stipulate with the vendor to have a sample analysed at his expense. The cost of an analysis under the present Manures Adulteration Act is only 7s. 6d., and, even though the purchaser himself were to pay the amount, it might in some cases be money well spent. The points in which one superphosphate differs from another are, first, in the proportion of soluble phosphate in it. In this market I find the extremes are 36 per cent, down to about 8 or 9 per cent, of this valuable constituent. Now, as the price of soluble phosphate is 4s. per unit per ton, it is evident that the value of this item is £7 4s. per ton of manure in the first case, and from £1 12s. to £1 16s. per ton of manure in the other. Second, superphosphates differ in the proportion of nitrogen which they contain. Nitrogen is worth about 15s. per unit per ton: that is to say, a manure containing 1 per cent, of nitrogen is worth 15s. a ton for its nitrogen alone, without reckoning the value of its other constituents; and a manure containing 6 per cent, of nitrogen is worth £4 10s. for its nitrogen alone; and so on for other proportions of nitrogen. A manure containing 3 per cent, of nitrogen is described as containing 3 units of nitrogen; if it contains 10 per cent, of nitrogen it is described as containing 10 units of that element. A "unit" therefore I means a "per cent." Fourteen parts of nitrogen make, by fermentation or microbe action in the soil, 17 parts of ammonia, and it is generally under the name of ammonia that manure nitrogen is recognised by vendors and buyers.

The cause of the difference in the amount of soluble phosphate lies in the quantity and strength of sulphuric acid employed in its manufacture.

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The more acid the more soluble phosphate. Hence the advantage those manure-makers possess who make their own acid with which to make their superphosphates, as against those who have to pay a big price for the acid to those who often enough are their rivals in the superphosphate trade.

The proportion of nitrogen in the superphosphates depends on the proportion of that element in the raw phosphate which has been used in the manufacture.

If bones, boiled or unboiled, be the raw material, they will import their own proper 4½ or 5 per cent, of nitrogen into the superphosphate made from them, where it will appear as something like 2½ or 3 per cent. If, on the other hand, the manure-manufacturer starts with steamed bones (digested with superheated steam under great pressure and at a high temperature), from which much of the nitrogenous gelatine has thus been removed, there will of course be a smaller proportion of nitrogen in the superphosphate, and its value will be lowered accordingly, unless, as is often the case, the gelatine be restored again to the sweltering mass in the superphosphate vats. If, again, the superphosphate has been made from the mineral phosphates—the apatites, phosphorites, and coprolites of Norway, Portugal, and England—there will be no nitrogen at all, and therefore no possibility of ammonia; unless some nitrogenous or ammoniacal substance or salt has been added, which I suppose is not often the case, judging from what is offered in the market.

There is another kind of phosphatic material in the manure market. It is a by-product in the Bessemer process for making steel. It is sold under various names—"basic slag," "basic cinder," "Thomas phosphate," "Thomas slag," being the more common. There are now over 700,000 tons of it made annually in Europe and America. It is made as follows: Most iron ores contain a small proportion of phosphorus, and the cast-iron made in the blast-furnace still retains the phosphorus of the ore. From this cast-iron, Bessemer steel is made by blowing air through it in the molten state in a kind of crucible called a "converter." For many years after the invention of this Bessemer process only those kinds of cast-iron were "converted" which contained little or no phosphorus, as that process left in the steel all the phosphorus contained in the cast-iron.

Thomas (hence the name of the manure), in order to remove the phosphorus from the molten cast-iron, lined the "converter" crucible with a mixture of burnt lime and magnesia, and also threw a quantity of the same mixture on to the surface of the molten charge in the "converter." This lime and magnesia mixture, as Thomas anticipated, had the effect of taking up the phosphorus and fixing it as a phosphate of lime, which, with the magnesia and some oxides of iron and manganese, floats on the heavy liquid metal, and is skimmed off, forming the "basic cinder" or "Thomas phosphate" manure referred to. The most valuable constituent of this "basic slag" is its phosphate of lime; but it also contains at first a considerable proportion of quicklime, which, after standing exposed to the air for some time, and especially after undergoing a long sea-voyage, becomes gradually changed into carbonate of lime. There is a peculiarity about the phosphate of lime of this manure. Being found to be more readily soluble in water than ordinary bone-earth or tricalcic phosphate (usually called insoluble phosphate of lime), it was suspected that it differed in composition from that compound, and, as the result of the analyses of many samples, agricultural chemists in England and Germany have proved that Thomas phosphate contains an extra or fourth molecule of lime, over and above the three molecules of lime present in the ordinary insoluble phosphate—that, in short, instead of containing 54 per cent, of lime it contained 61 per cent, of that constituent.

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This extra quantity of lime, it seems, makes the Thomas phosphate more easily dissolved by water than bonedust or guano or coprolites or an; other kind of ordinary or tricalcic phosphate.

The presence of the oxides of iron and manganese was at first objected to, as likely to be hurtful to young plants; but that objection is removed now by the oxidation of the lower hurtful oxide of iron into the higher oxide by the grinding and exposure to the air to which it is subjected before its application to the soil.

It is insisted on by the agricultural experts that this Thomas phosphate be ground to very fine powder before it is sent into the market. They require that it should be sifted through a sieve containing about three thousand six hundred holes to the square inch to bring out its soluble quality, and therefore its manurial value, to perfection. Many practical field experiments have been made in England and Germany with this phosphate against other phosphatic manures on many kinds of soil, and for different crops. These, it is claimed, while proving its inferiority to good superphosphates, have shown that for the immediate crop (the crop sown with it) it is superior to the insoluble phosphates—guano, bonemeal, ground coprolites, &c. Wagner, the great German authority, after many trials, strongly recommends it for cold, damp, sour lands. Its extra molecule of lime, acting much in the same way as quicklime would act, probably warms and sweetens the soil and helps to dry up this dampness by combining with the moisture and with the obscure acids to which the sourness is due. The same authority says that it is very nearly equal to superphosphate for the immediate crop.

The dark, unpromising appearance of this slag manure will prejudice farmers against it. It has been introduced into the colony, and I have analysed samples of it from Southland and Canterbury, as well as from Dunedin.

As there are many qualities of this manure, not differing much from each other in appearance, its proportion of phosphate of lime should be guaranteed to the purchaser, and an analysis should also state the degree of fineness, as upon this the activity and value of the manure greatly depend.

What a confusion must exist in the minds of farmers about these four different kinds of phosphate of lime, all of which are in the market:—
1.There is the monocalcic phosphate, CaH4P2O8, contained in superphosphates, and constituting from, say, 9 per cent, to 36 per cent, of that manure, and all of it soluble very readily in water, and therefore immediately available for the crops of the current season.
2.Dicalcic phosphate, or reverted phosphate, Ca2H2P2O8, contained to a greater or less extent in old superphosphate, made from mineral phosphates, with a stinted supply of sulphuric acid. This variety of phosphate in respect of solubility holds a place intermediate between the soluble monocalcic and the insoluble tricalcic to be mentioned presently, In rainy seasons, or on wet, sour land, this dicalcic variety is by some people preferred to the more soluble and more acid superphosphate.
3.The tricalcic phosphate, or insoluble phosphate, Ca3P2O8. This is the kind of phosphate occurring in nature as coprolite, apatite, phosphorite, estremadurite, sombrerite, the guanos, raw bones, bonedust, bonemeal, &c. Though called "insoluble" it dissolves slowly, as already explained, in the carbonic acid soil-water, and thus gradually presents itself to the growing plants in a suitable form.
4.The "Thomas phosphate," "Thomas slag," "Thomas cinder," "basic cinder" (Ca4P2O9), or "basic Bessemer slag," described above as a by-product of the Bessemer steel works. This phosphate, in a state of page 15 fine dust, is more soluble in water, and therefore more active in the soil, than the tricalcic phosphate. Its free lime is also a good base for the operations of the nitrogen and fermentation microbes—those grand manufacturers of nitrites and nitrates in the soil.

In regard to the proportion of lime and phosphoric acid in these four kinds of phosphates, as they are offered for sale, with their moisture and organic matter and sulphate of lime (for those of them which contain this last ingredient), it may be said that the richest in phosphoric acid is the tricalcic, as it occurs in the Maiden Island, Abrolhes, and Coral Queen guanos, bonedust, bonemeal, and the mineral phosphates, ranging in all these from about 18 to 32 per cent.; Coral Queen, Abrolhes, and bonedust being about 24 or 25 per cent., and mineral phosphates up to 30 or 32 per cent. In all these kinds of tricalcic phosphates there is no sulphate of lime; and in the mineral phosphates there is no nitrogen. The lime in all of them ranges from 21 to 37 per cent., and both the phosphoric acid and the lime are in the insoluble state.

In point of phosphoric acid the "Thomas basic slag" comes next to the tricalcic, as it contains from 16 to 19 per cent, of that valuable constituent. Its proportion of lime ranges from about 18 to 30 per cent. The other two kinds of phosphatic manures—namely, the superphosphates (both the pure and partly reverted)—contain from 10 to 18 per cent, of phosphoric acid and very varying proportions of lime, a good deal of which is in a soluble condition as sulphate of lime and soluble phosphate of lime. For rapid effect the superphosphates are far ahead of the others, reverted phosphates and Thomas phosphate coming next, then the guanos and bonedust, and last of all the mineral phosphates. An admixture of Thomas phosphate with superphosphate has been tried with success on heavy, damp, and clay lands. Alone the Thomas phosphate is said to be inferior to mineral superphosphates on dry, light, chalky, or lime lands; but on clay lands it is said to be equal or superior to these.

The points which the farmer should get guaranteed in his phosphatic manure are—(1) The percentage of soluble phosphate of lime; (2) the percentage of reverted phosphate of lime; (3) the percentage of insoluble phosphate of lime; (4) the percentage of nitrogen or of ammonia (he can remember that 14 parts of nitrogen will make 17 parts of ammonia); (5) the origin of the manure, whether mineral, bone, or guano; (6) the state of division of the manure. All these six points should be taken into account in valuing a phosphatic manure. If the manure contains potash salts, the percentage of these should also be stated.

The process for ascertaining the proportion of phosphoric acid, lime, and nitrogen in phosphatic manures is scarcely suitable for these pages.

Besides the mineral phosphates—apatite, coprolites, phosphorite, sombrerite, &c., all of which are true phosphate of lime—there is another, known as "rodunda" phosphate, so called from the name of the island on which it is found. It consists chifly of phosphates of alumina and iron. A modified variety of it occurs on Green Island, near Dunedin, and has been analysed. The phosphoric acid in it amounts to between 17 and 30 per cent., making it equal in that essential constituent to from 38 to 65 per cent, of phosphate of lime. Agricultural experts are not quite agreed as to the manurial value of this manure. Voelcker, the late great English authority, as the result of field experiments (similar to those which Mr. P. Patullo is carrying on in Otago and Canterbury), got from it in oats and peas larger returns than from the other manures he tried against it. Voelcker's results are tabled below:— page 16
With Oats.
Manure per Acre on Moderately-heavy Land. Crop per Acre.
Grain, in Bushels. Straw in Tons.
6½cwt. ground coprolites 65 1 3/5
5cwt. coprolites treated with sulphuric acid (mineral superphosphate) 72½ 2
10cwt. rodunda phosphate (like our Green Island phosphate) 78¾ 2
3½cwt. bonemeal, with sulphuric acid (bone superphosphate) 61 1/3 2
4½cwt. precipitated phosphate 66 1 9/10
No manure 60
3cwt. raw bonemeal 61 1/5 1 3/5
20 tons of dung (farmyard manure) 70 2/5 2 1/10
10 tons of dung and 5cwt. coprolites, with acid (superphosphate) 67 2
10 tons of dung, and 6½cwt. ground coprolites 62¾ 2
5 tons of chalk 72 2 1/7
3cwt. coprolites, with sulphuric acid (superphosphate), and 2½cwt. Peruvian guano 64¾ 2
With Peas.
Manure per Acre on Light Land. Crop per Acre.
Peas, in Bushels. Straw, in Tons.
No manure 39 2 1/7
5cwt. ground coprolites 42
5cwt. ground coprolites, and acid (mineral superphosphate) 44½ 3
5cwt. rodunda phosphate (like Green Island phosphate) 43 1/3
4cwt. precipitated phosphate 40½
3cwt. of raw bonemeal 43½ 2 1/7
3cwt. bonemeal and acid (bone superphosphate) 42¾
3cwt. coprolites superphosphate and 2½cwt. Peruvian guano 43

There was no information as to the percentage of phosphoric acid in the "rodunda" experimented on; and, as this manure ranges from 19 per cent, to 38 per cent, of that essential constituent (equal respectively to 40 per cent, and 82 per cent, of phosphate of lime), there is an unsatisfactory vagueness in the statement of the returns from it.

One thing about it is this: It must be ground to fine flour, passing through a sieve of sixty wires each way to the inch, or 3,600 holes to the inch, to bring out its best effects. This fine grinding, as already stated, is also essential to the success of the "Thomas basic slag." The rodunds page 17 or Green Island phosphate is not suited for treatment with acid. It cannot therefore be made into a superphosphate. The Green Island variety shows an advantage over the original "rodunda" in the matter of nitrogen, as there is no mention at all of that element in the rodunda, while in the Green Island there was found up to per cent, of nitrogen. This suggests a guano origin for this Green Island deposit.

The above results, published by the highest English and American authorities, show that, in money value, the rodunda phosphate (finely ground) is superior to the best kinds of mineral phosphates. The lecturer did not know what quantity of this manure is lying on Green Island; but, being so accessible to the market, it seems strange that it should be neglected. Fashions change in manures as in other things, and he had no doubt the Green Island phosphate would have its turn. He should like to see it tried, after passing through a 60-wire sieve, by Mr. Patullo in his field experiments. Before quitting the phosphates he would point out the quantities of the various kinds that are required for putting into the soil the phosphorus that the different crops take out of it, assuming that the yield per acre of the different crops is as follows: 50 bushels of wheat, 80 bushels of oats, 60 bushels of barley, 18cwt. of peas, 6 tons of potatoes, 20 tons of turnips or 20 tons of mangels (carted away and eaten elsewhere), 2½ tons of oaten hay, 3 tons of meadow hay. Or that 1,400 gallons of milk or half a ton of cheese has been produced per acre, or bone-material for a dozen sheep. In each of these cases about 10lb. of phosphorus will have been removed from the soil per acre. To restore the 10lb. of phosphorus per acre there will be required: Of Maiden Island guano, about 75lb.; Lacepede guano, 75lb.; steamed bones, 78lb.; Abrolhes guano, 80lb. to 85lb.; Coral Queen guano, 85lb. to 90lb.; bonedust, bonemeal (raw), 85lb. to 90lb.; Thomas phosphate, 100lb. to 130lb.; superphosphates (good quality), 110lb. to 140lb.; Chesterfield guano, from 90lb. to 140lb.; inferior superphosphates, 150lb. to 250lb.; farm-yard manure (dung), 3 to 4 tons.

These quantities take account only of the phosphorus to be replaced per acre, without regard to its state of solubility, and without regard toother fertilising materials in the manure, as, for example, the nitrogen, ammonia, lime, potash, &c., which most of the above manures also contain. To limit the manuring to the quantities of manure named above would, however, be a very hand-to-mouth kind of treatment of the soil. It does not leave any margin of phosphorus to improve the soil permanently, or with which to grow future crops, or for laying down in grass. It does no more than leave the soil where it was before the produce mentioned above was raised from it. A farmer who owns the land he is cropping, or who has a long lease of it, would deal generously with his fields and put in not a hundredweight of these manures per acre, but three or four hundred-weight per acre; and he would thereby be laying in material for big crops in the future. His land to the farmer is, in one thing, like his bank to the merchant. He cannot take out of it more than he has got in it.

The land contains a certain ascertainable amount of phosphorus to begin with. Let that represent the capital or principal or sum to his credit in the bank. He raises a crop of, say, 50 bushels of wheat to the acre, and sends that away to London. By doing so he draws 10lb. of phosphorus from every acre of that paddock, which of course is then 10lb. of phosphorus poorer. This is like drawing a cheque on his account with the bank. The manure he puts on then to restore this 10lb. of phosphorus is like paying a sum into the bank to keep his account healthy.

If the farmer keeps cropping away it will be only a question of time when the original stock of phosphorus will be so far reduced that the page 18 ground will not be able for lack of it to raise crops worth growing-diminishing year after year, in step with the diminution of the phosphorus. The farmer should farm in such a way as to improve the value of his land by increasing the phosphates in the soil. This he will do by putting into it more phosphates, &c., by manuring than he takes out of it by cropping.

We must not, however, overlook two other sources of available phosphorus in the ground itself. (1.) All soil consists of solid particles of greater or smaller size, and all more or less porous in texture; and mixed with these particles there are stones of different sizes, which are not porous in any effective sense. Now, a growing crop—by the root-hairs and root terminal spongioles—draws phosphates from the loose, porous particles, but not from the interior of the stony matter. Gradually, however, assisted by good cultivation and free exposure to the weather, these stony particles get decomposed and broken up and converted into soil, bringing into use now any phosphates they may have contained. A soil, therefore, nearly exhausted of available phosphates may, by a season of active fallow—ploughing and harrowing and free exposure to the air—be restored to a fertile state for a time by this bringing-forward of the phosphorus previously locked up in these hard stony particles. (2.) Long-rooted plants (old clovers, &c.), given time enough, will gather in from below (far under the depth to which the roots of a grain crop could reach) the phosphates, lime, and potash down there, and send them up into the stem and leaves above ground. These stems, then, eaten off or decaying or ploughed in, will thus bring into the surface soil and within the reach of the subsequent grain crop this additional contribution of fertilising materials. Hence another explanation of the enriching effects of old clover pastures.

The lecturer pointed out that it would be quite possible to impoverish a field by pasturing it, as in cattle-raising or dairying. That dairying would do so is self-evident from the fact that 1,400 gallons of milk contain 10lb. of phosphorus, or just as much of that precious element as is contained in 50 bushels of wheat. The phosphorus in the milk comes from the grass, which, in its turn, comes from the soil. It might easily happen, however, that if the pasture contained much clover (old and therefore long-rooted) and other long, wandering-rooted and enterprising fodder plants, these might for a long time take up from below in the manner already explained as much phosphorus as is removed by the dairy cows.

That cattle-raising would impoverish the land in the long-run is evident from similar considerations. Given, for comparison, the practice of two contiguous farmers—both feeding cattle for the butcher. One of them (call him A) raises his own bullocks from calves calved on his own ground from his own cows, and reared in his own paddocks to the size of three or four-year-old big bullocks. They are sold away with all their bones in them. I do not know what the weight of each bullock would be, but, whatever it is, something like one-tenth or one-twelfth of it would be bones, containing, as all bones do, about 55 per cent, of phosphate of lime, all of which has thus been produced and removed from the soil. The other farmer (call him B) also fattens cattle for the market, but he buys them as store cattle off the runs, puts them on his paddocks as big, lean, hungry beasts, but with all their bones already full grown. On his pastures they put on fat, which contains neither phosphorus nor nitrogen, and the lean of beef, which does not contain much phosphorus, but a good deal of nitrogen. And when he sells them away he has the satisfaction of knowing that the bones they take with them did not come from his paddock. A's paddocks would then be slowly but surely impoverished in the matter of phosphorus, while B's would maintain its fertility.

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There are about 1,800,000 sheep and lambs—not to speak of the frozen beef—exported frozen from New Zealand every year. There is also a large quantity of wheat and oats. The phosphorus sent away in this way is very considerable, but there is no help for it. It would not, however, be a bad rule for the farmer to follow if he were to replace on his land under cultivation every year a little more phosphorus, lime, and potash than he knows he is removing from it in his grain crops and potatoes, and sheep and cattle, milk and cheese. Of course this could not be done on the uncultivated lands on the runs; but these do not need it much, as they are not being impoverished by cropping, and it will be time enough to deal with them when they come into cultivation. A little bonedust scattered here and there from year to year would not, however, hurt them much.

The price of 100lb. of bonedust or good guano, or of 1cwt. of good superphosphate, is not prohibitive. With a crop of 50 bushels of wheat, the price of 3 bushels, in a bad season like this, would restore the phosphorus taken away. The price of 3 or 4 bushels of oats out of a crop of 80 bushels would not hurt much. And, of course, with smaller yields of wheat or oats less phosphorus will be removed, and therefore a less quantity of phosphatic manures would be required to replace it.