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Rh volatility and stability against the attack of bacteria. In green- houses, however, where the soil soon becomes " sick " through the excessive development of protozoa under the favourable con- ditions of moisture, temperature and manurial enrichment, the sterilization of the soil by heat has been worked out as a com- mercial process and is now part of the routine of all progressive cultivators under glass.

Microfungi. Great as is the attention that is now being" given to the soil organisms in all agricultural laboratories there would appear to be room for more work upon one group the micro- fungi, of which there is a large flora in the soil.

It has been shown that when from one cause or another a soil becomes acid, many bacteria concerned in the decay of vegetable matter are entirely inhibited and may disappear. Fungi instead take up the work, but the broad character of the process thereby changes, the vegetable matter is not burnt away as carbon dioxide but in part accumulates in the form of peat. The formation of a peaty material is in fact a concomitant of an acid reaction in the soil and the activity of microfungi rather than of bacteria, and this generalization fits in with many observations of the character of peat deposits.

Often trees are found at the base of these beds where trees no longer grow; and it may be surmised that the trees grew on the original neutral land surface when it became fit for vegetation after the close of the glacial epoch. That soil being of a non-calcareous nature gradually accumulates acids arising from the decay of the vegetation falling upon it, whereupon under the prevailing climatic conditions the further vegetable debris reaching the soil began to form peat. This accumulation of peat in its turn brought about the death of the forest.

Nitrogen. During 1910-20 agriculture received great benefit from the working out of processes on a large scale for bringing nitrogen into combination, processes which thus supplement the comparatively limited sources of nitrogen compounds afforded by the Chile deposits of nitrate of soda and the ammonia which is recovered as a by-product from the distillation or combustion of coal.

Prior to the World War two processes had been established com- mercially. At Notodden in Norway air is driven into a specially formed electric arc which results in the combination of nitrogen and oxygen so that the issuing gases contain about I -25 % of oxides of nitrogen which are then absorbed by passing up towers where they meet an absorbing stream of water or milk of lime. The product, nitrate of lime, contains about 13-5% of nitrogen, and is a most valuable fertilizer, quite as effective as nitrate of soda and on some soils more suitable.

At about the same time as synthetic nitrate of lime was in- troduced, another nitrogenous fertilizer began to be manufactured on a large scale, calcium cyanamide or nitrolim. The body arises from the combination which ensues at a temperature of about 600" C. between calcium carbide and pure nitrogen gas under slight pressure, with the resulting formation of a compound which in the soil decomposes mainly into ammonia and calcium carbonate. Cyanamide as a fertilizer requires a certain amount of care in use and on the majority of soils has not proved so effective as nitrate of soda or sulphate of ammonia. Its manufacture, however, received an immense impetus during the World War, as it was the simplest and most readily available process for bringing nitrogen into combination, from which by further steps ammonia and then the nitrates and nitric acid required in explosives could be obtained. The United States and many European countries have immensely developed the manufacture of cyanamide, which must in future be available as fertiliser either used directly or after prior conversion into some convenient compound of ammonia.

The war period was also marked by the development on a gigantic scale of a new process, which had only been finally worked out to the manufacturing stage in Germany in 1913 the Haber process of bringing nitrogen and hydrogen into combination as ammonia. In the presence of a suitable catalyst of activated iron these elements will unite at pressures of 250-300 atmospheres and a temperature approaching 600 C. to the extent of 8 % or so of the mixed gases. The ammonia can be removed and the remaining gases passed round again into the catalyser. Great as are the difficulties of work- ing at these temperatures and pressures the Haber process is cheap in power and materials. It was the mainstay of the supply of com- bined nitrogen for explosives to Germany during the war, and should become a most important future source of fertilizer to the agricul- turist.

During the war the demand for nitrogenous fertilizers greatly increased in all countries ; the United Kingdom for example increased her consumption of sulphate of ammonia from 60,000 tons to 269,000 tons per annum, part of this being of course substitution for the pre-war use of 80,000 tons of nitrate of soda, which was no longer available. Potentially, however, the establishment of so many war plants for the manufacture of synthetic nitrogen products

has increased the supply of nitrogen available as may be seen from the following table:

Metric Tons of Nitrogen.

Output 1912

Output 1917

Productive Capacity 1920

Chile Nitrate Ammonium sulphate (by-product). Cyanamide. Haber process. Arc process

4".329

272,007

22,435

9,9f>7

465,000

340,000 190,000 100,000 27,000

471,000

413,000 325,000 308,000 33,6oo

715,678

1,122,000

1,550,600

It should be noted, however, that the 1920 figures are not actual but only potential supply, if existing plants are worked up to their capacity.

Potash. As the only extensive potash deposits in the world that had been commercially developed Stassfurt and Alsace were in German hands, there was during the war a great shortage of potash fertilizers outside central Europe. Great efforts were made to develop processes for the extraction of potash from felspars and other natural sources, but without much success.

The only method which proved of value was the discovery made in the United States that the dust which accumulates in the flues through which the gases from blast furnaces are led contains a not inconsiderable amount of potash in a readily soluble form, one-half indeed consisting of sulphates and carbonates soluble in water. Different grades of flue clust can be collected : the finest is a cream- coloured material containing as much as 60% of potash. The dust was collected and used for agricultural purposes during the war though only some 15,000 tons per annum were obtainable in Great Britain. It is now worked up for industrial purposes, but the output of potash salts from this source cannot exceed a few thousand tons per annum in the United Kingdom. The supply of potash salts for agricultural purposes since the war has been .entirely changed by the transfer to France of the Alsatian deposits which occupy an area of some 77 sq. m. between Miilhausen and Colmar in Alsace. This deposit consists of two beds, the upper about 4 ft. thick, the lower about llj ft., which form practically unbroken strata at an approxi- mate depth of 1, 800 ft. and present no difficulties in mining. The material is very uniform in composition, consisting in the main of sylvinit, mixed chlorides of potassium and sodium, containing about 20 % of potash reckoned as K 2 O. It can be used for agriculture in its crude state and though the development of the field is still very in- complete the former German monopoly of potash supplies is thereby broken down. Another extensive deposit is known in Spain, but it has not reached the stage of commercial development and is generally considered to be controlled by the German company which works the Stassfurt deposits.

Superphosphates. During the war the manufacture of superphosphate in the United Kingdom was considerably re- stricted, on the one hand by the withdrawal of sulphuric acid for the manufacture of explosives, and on the other by the shortage of tonnage for the importation of phosphate rock. American supplies were completely cut off and receipts from the North African deposits fell to something like 500,000 tons per annum. In consequence British farmers were compelled to resort mainby to basic slag of which this country produced about 400,000 tons per annum, though prior to the war only some 280,000 tons had been consumed by British agriculturists. With the extended programme of arable farming the demand for phosphatic fertilizers was greatly increased and the whole of the basic slag produced at home was absorbed, though the output was in- creased to as much as 565,000 tons from the year ending May 1919. Unfortunately this increase in amount was accompanied by a decline in character, owing to changes in the processes generally adopted for making steel.

The Bessemer process has been almost displaced by the open- hearth process which produces a slag less rich in phosphoric acid. The practice has also been adopted of adding fluor-spar to the furnace in order to induce the formation of a more fusible slag, but thereby the solubility of the phosphoric acid of the slag in the weak citric acid generally used in testing its quality becomes impaired. The bulk of the basic slag now sold contains only about 10 % of phosphoric acid against 15 to 20% in the older types of slag and the phosphoric acid is no longer soluble in weak acids. The new type of basic slag proves, however, little less effective, unit for unit of phosphoric acid, as a fertilizer, but freight charges, always a large item in the cost of basic slag to the farmer, now become doubled for the amount of phosphoric acid that is carried, apart from the increase in these