SOILS. 



149 



standing or by being rained or tramped upon, 

 nor is it loosened by ploughing, digging, or any 

 kind of tillage, for its particles, like the grains 

 in a vessel of shot, readily and inevitably take 

 on their closest contact when released from any 

 disturbance. A sandy loam containing particles 

 of all shapes and of very many sizes would, on 

 the other hand, evidently admit of still closer 

 fitting and packing, and would undergo the most 

 alteration of bulk between tillage and rest. In 

 case of clay consisting mainly of very fine par- 

 ticles, the compactness would be greater in virtue 

 of special properties that do not belong to sand. 

 Grains of sand stick together with slight energy 

 if moist, for the water which adheres to them 

 acts as a weak cement. When the water dries 

 out the grains fall apart. 



Particles of clay, when moist, also adhere 

 together, but much more closely and firmly than 

 those of sand ; so firmly, indeed, that the clay is 

 plastic, and may be moulded to shape by the 

 hand or on the potter's wheel. Clay mixed with 

 sand renders the mixture plastic ; in fact, heavy 

 clays, almost too tenacious for tillage, consist 

 very largely of sand and silt, and rarely contain 

 more than 25 per cent of real clay. When moist 

 clay dries, the particles stick to each other with 

 greater tenacity than before, and as the water 

 evaporates the clay shrinks to a hard mass that 

 requires pounding in a mortar to reduce it to 

 powder. Here is an actual cohesion of particle 

 to particle which water diminishes. 



A full consideration of these facts makes it 

 quite evident that in most soils there are tenden- 

 cies constantly exerted, with more or less vigour, 

 towards mechanical compacting, and one of the 

 important offices of tillage is to counteract these 

 tendencies. 



After this discussion of the points involved 

 in the natural compacting of soils, and in over- 

 coming this tendency by tillage so as to prepare 

 a suitable seed-bed and rooting-place for a young 

 crop, let us turn to some notice of how tillage 

 may affect the crop itself, which in course of 

 time comes to develop upon the land. 



That crops require water, and that they 

 demand a large amount of it, are statements 

 made evident to us by our common experience. 

 That too much water damages plants and crops 

 is also a fact which requires no illustration. 

 How much water plants require is a question 

 that is not easy to answer, and the share which 

 is taken in the production of garden crops by 

 water-supply — by the excess or by the deficiency, 

 and by the distribution of water — is not easy to 

 estimate. 



It is a fairly-well settled fact that water, to be 

 of any use to plants, must become a part of the 

 soil, and although without a suitable climate and 

 good weather, giving alternations of rainfall and 

 sunshine, gardening, as well as agriculture, would 

 ; shortly be brought to a stand-still, it is also a 

 fact that the actual coming together of plants 

 ; and water into mutual relations takes place 

 mostly in the soil, and the really critical passages 

 in these relations are largely determined by the 

 soil and by those qualities of it which are amen- 

 able to the influence of tillage. 



Fertility of Soils. — It is frequently a matter of 

 surprise, especially to farmers, who are apt to 

 lament the exhaustion of their soils, that gardens 

 that have been cropped for years are so much 

 more fertile as a rule than are their fields. There 

 are many causes for this, such as better drainage, 

 more thorough tillage, a more varied rotation of 

 crops, to say nothing of shelter. 



In regard to cultivation it may be stated that 

 agricultural tilths range from 3 inches to 9 inches, 

 with an average of about 5 inches ; whilst horti- 

 cultural tilths range from 6 inches to 36 inches 

 or more, and average probably about 18 inches 

 in depth. 



But the main reason why the soil of old 

 gardens is so fertile is, that there is a vast 

 accumulation of animal and vegetable matter 

 containing organic nitrogen, that is to say, 

 nitrogen in combination with carbon. 



The manner in which soil supports the life of 

 plants and animals is still mysterious, in the 

 sense that we are yet in the dark as to the 

 nature of many of the substances contained in 

 the soil, of the changes which they undergo, and 

 of the part which they take in plant nutrition. 

 This is specially true with regard to the organic 

 matters, consisting of carbon, nitrogen, hydrogen, 

 and oxygen, which the soil contains. The nitro- 

 genous organic matter of a soil, as we have before 

 mentioned, has been derived principally from the 

 decay of vegetable matter left in the land by 

 succeeding generations of plants. And the nitro- 

 genous capital or fertility of a soil depends, as a 

 rule, on the bulk and composition of the previous 

 crop residues, and on the extent to which these 

 have been subsequently destroyed. 



Evidently, therefore, the crop which leaves 

 behind the largest amount of roots, stubble, and 

 foliage will best maintain or increase the nitro- 

 genous and organic capital of the soil, while the 

 crop leaving the smallest residue in the soil will 

 be most exhausting in its effect. 



Permanent pastures, prairie lands, garden soils, 

 and forests will thus stand at the head of the list 



