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particle does not return to the identical clod from which it was derived, for wind 

 and water displace many a leaf and twig, a storm may remove an entire tree, 

 and even an entire wood with its wealth of mineral matter may be transported 

 far away by an avalanche. Though such removals are the exception in nature, 

 in husbandry they are the rule. When the ripe plants are harvested, fruits, 

 leaves, and often stems as well are removed from the field ; the roots alone 

 remain where the plant grew. Although each plant contains only a small 

 quantity of ash, the total amount is very considerable when an entire field is 

 taken into account, and, according to EBERMAYER (1882) 200-300 kilg. per 

 hectare of mineral substances are withdrawn when the harvest is reaped, about 

 half of which is nitrogen, the remainder consisting of the other constituents 

 of the ash. When we remember that this goes on year after year, we can 

 easily understand how in a short time all the nutrients in the soil that are 

 capable of absorption must disappear, and so in the end plant-life is pos- 

 sible only if the soil still contains constituents capable of decomposition. But 

 decomposition of soil never takes place rapidly enough for cultivated plants 

 to obtain the amount of inorganic nutrients they require. The soil, in other 

 words, becomes exhausted by continuous cultivation, but is not on that account 

 lost to the agriculturalist for all time, for it can be rendered fertile once more, and 

 even improved by artificial means and by employing appropriate applications. 

 Since all cultivated plants do not impoverish the soil in the same way, one type de- 

 manding, for example, more potassium, another lime, one may economize the 

 soil by rotation of crops. During the period, for example, a calciphilous plant 

 grows in a field the soil has time to enrich itself in potash by weathering, so that 

 in the following year the conditions are favourable for the development of 

 a plant which makes a demand on potash. Since, however, many substances, 

 e. g. phosphates, always occur in the soil only in small quantities, and since all 

 plants require a good deal of them, one cannot employ this method of rotation ex- 

 clusively. A second method consists in leaving land to lie fallow. In this 

 method fields are not made to support a crop continuously, but the land is 

 allowed to be idle, weeds are permitted to grow on it, and these are subse- 

 quently ploughed in, thus adding to the humus constituents of the soil. By 

 far the most important method is, however, the addition of nutritive salts to 

 the soil, directly replacing what has been lost. This method is termed manuring, 

 and was practised in husbandry long before people had learned its inward 

 significance. The addition to the soil of the dung of animals mixed with the 

 straw, as well as the general sewage of a town, naturally replaces in the soil 

 a part at least of the mineral substances taken from it. LIEBIG showed 'that 

 the chief value of these bodies lay in their inorganic constituents, so that we 

 can understand how it is possible to improve or supplement such natural 

 manures by artificial combinations. 



Artificial manuring plays a great part in modern agriculture, and consists 

 in the addition to the soil of potash, lime, and phosphoric acid, apart from nitric 

 acid, of which we shall speak later. A sentence on this subject must suffice 

 at this point. Lime occurs so abundantly in nature that one need never fear 

 a deficiency of that mineral in agriculture. It is otherwise, however, with 

 potash and phosphoric acid. The former is manufactured in great quantities 

 at the potash works of Stassfurt and Leopoldshall in the form of karnallite, 

 cainite, and sylvinite, and of these cainite (KG . MgSo 4 . 3H 2 O) is specially 

 important, as it contains not only potassium but also magnesium in the form 

 of sulphate, also a plant nutrient. As a source of phosphoric acid, ' Thomas- 

 slag,' a by-product in the smelting of ores containing phosphorus, holds a fore- 

 most place. It is true that the phosphoric acid occurs in the form of a tricalcic 

 salt [Ca s (P0 4 ) 2 ], and that this is insoluble in water. Since, however, the manure 

 is laid on the field in an exceedingly fine state of subdivision in the form of the 



