AGRICULTURE. 



turnip, which must first receive a liberal supply of 

 these substances from the soil, in order to form 

 a surface capable of taking food from the aerial 

 medium. 



Plants like wheat and turnips are said, naturally 

 enough, to be great exhausters of the soil, because 

 they cannot be raised unless they are fed or 

 manured by materials yielding an abundant supply 

 of carbonic acid and ammonia. It is now gener- 

 ally admitted that a liberal supply of substances 

 yielding nitrogen is essential for raising a maxi- 

 mum produce of grain and root crops. At the 

 same time, it must be borne in mind that nitrogen- 

 ous manures have a beneficial influence on all our 

 cultivated crops, but some crops are more depend- 

 ent on a supply than others, and some can digest 

 or assimilate more in a given time. 



The first great physiological distinction which 

 may be drawn with respect to the capabilities of 

 plants for drawing a supply of food from the 

 atmosphere is, that annuals are much more de- 

 pendent on a supply of carbonic acid and ammonia 

 in the soil than perennials. Indeed, comparatively 

 speaking, annuals exhaust, but perennials amelior- 

 ate the land. 



Perennials have not everything to do in one 

 season, for every year strengthens their hold upon 

 the soil, and increases the surface of leaves which 

 they expose to the air. Perennial vegetation, there- 

 fore, often exhibits considerable luxuriance when 

 the soil is deficient in matters yielding carbonic 

 acid and ammonia. On the other hand, the larger 

 class of annuals are never found growing luxuri- 

 antly on soils deficient in substances yielding 

 carbonic acid and ammonia. In fact, annuals 

 must be so far regarded as a sort of parasites. 

 The thriftless annual casts away its roots and 

 stems, which, as they decay, are liable to be 

 washed out of the soil by the rains. A perennial 

 plant, however, may be said to be manured, and 

 well manured too, by its roots and branches, which 

 are kept in a vital state. The habits of growth in 

 perennials are thus of a very economical character. 



It is annual plants that principally supply man 

 with food in the temperate latitudes. Their grand 

 deficiency in having no perennial roots and 

 branches must be made up by manuring. Thus 

 all our fallow crops, such as turnips, mangel, 

 beans, and potatoes, are freely treated by sub- 

 stances yielding carbonic acid and ammonia. 

 These crops being all annuals, are particularly 

 dependent on a supply of such substances. 



On the other hand, when our fields become 

 exhausted by the growth of cereals, we restore 

 their fertility by abandoning their cultivation for 

 a time, and allowing them to accumulate vegetable 

 matter through the growth of perennials, such as 

 grasses or clovers. Annuals succeeding perennials, 

 perennials succeeding annuals, has been the 

 primitive rotation when a rotation was neces- 

 sary in all countries and in all ages. It was 

 Liebig who first demonstrated that the effect of 

 pasturing land was to accumulate ammonia from 

 the atmosphere. 



The more nearly that an annual approaches in 

 its habits to a perennial, the less dependent it is 

 upon a supply of carbonic acid and ammonia in 

 the soil So long as an annual plant puts forth 

 fresh leaves, it is capable of drawing upon the 

 food contained in the atmosphere, and therefore 

 less dependent on a supply furnished to its roots. 



Thus a pea, which puts forth fresh leaves and 

 blossoms, and eve.n ripens fruit at the same time, 

 can draw more largely from the atmosphere than 

 the wheat-plant, which puts forth no fresh leaves 

 after it flowers, when the most of the leaves lose 

 their vitality. The wheat-plant, being in a great 

 measure devoid of foliaceous surface, must be 

 liberally supplied by manure, and hence it has 

 the character of being an exhausting crop. 



Viewing the physiological aspects of the ques- 

 tion, the principle may be stated so as to embrace 

 annual and perennial vegetation in the following 

 terms : Plants are less dependent on a supply of 

 carbonic acid and ammonia in the soil when their 

 vegetative powers coexist, as they do in grasses and 

 clovers, with their flowering and seed-forming 

 processes. 



As was pointed out by Liebig, the atmosphere 

 contains a supply of carbonic acid and ammonia 

 sufficient for all the plants that grow upon the 

 surface of the earth, whether wild or cultivated. 

 This proposition is rendered evident when we 

 reflect that an acre of land under a forest assimi- 

 lates as much carbon, oxygen, hydrogen, and 

 nitrogen during the growing season, as an acre 

 under any of our crops, such as turnips or clover. 

 It is well to bear in mind, therefore, that the 

 necessity of manuring our fields with substances 

 yielding carbonic acid and ammonia, does not 

 arise so much from a deficiency of those sub- 

 stances in the natural source of supply, the atmos- 

 phere, as from an inherent deficiency in the plants 

 cultivated for drawing largely upon this source. 



Perennials not only possess greater facilities of 

 abstracting food from the atmosphere, but they 

 have greater facilities of abstracting it from the 

 soil By forgetting this very obvious principle, 

 great confusion has arisen among agricultural 

 writers respecting the action of some manures. 

 For example, much ingenious speculation has 

 been indulged in to account for the well-known 

 fact, that phosphates have, as a general rule, a 

 more beneficial action on the turnip-crop than on 

 any other. This, however, admits of a simple and 

 consistent explanation, when we reflect that the 

 size of the seed of the turnip is immensely less in 

 proportion to the space which the plant ultimately 

 occupies, than is the case with any other plant. 



The seeds of clover and grasses are small, but 

 as they are thickly sown over the ground, the 

 individual plants do not require to run far in 

 search of phosphates ; and they have also more 

 time to search for a supply. With the turnip, it 

 is different ; it is sown at a season of the year 

 when vegetation is stimulated by a high tempera- 

 ture. The small seed of the turnip has no phos- 

 phates within itself to produce a large growth of 

 leaves and rootlets ; and thus, unless it have a 

 liberal supply added as manure, it cannot grow 

 with vigour or rapidity. For this reason, phos- 

 phates must often be added to the soil to produce 

 a crop of turnips, when these substances are by 

 no means wanting, and not even deficient in 

 quantity for growing other crops, which must 

 absolutely obtain a supply as large, but whose 

 facilities for obtaining it are much greater. 



The practical application of these principles, 

 however, will be further illustrated when we come 

 to treat of manuring particular crops. The next 

 theoretical division of our subject is one which 

 has an intimate bearing on the physiology of 



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