SOILS. 



131 



disease has previously existed. In spraying 

 fruit-trees, Vines, &c, the trunk and bark should 

 be thoroughly drenched, so that the solution 

 may reach spores located in crevices of the bark; 

 walls, the ground under fruit-trees, &c, should 

 also be drenched. This work must be done 

 during the winter, before the buds begin to 

 swell, and must not on any account be used 

 when leaves are present, as these are at once 

 completely destroyed by the solution. 



Finally, although fungicides made in the 

 proportions given above have been repeatedly 

 proved not to injure mature foliage, neverthe- 

 less it is always advisable to commence oper- 

 ations with a more diluted solution, as tender 

 young leaves, or foliage grown under glass, not 

 unfrequently have the cuticle insufficiently de- 

 veloped to resist the action of the fungicide, 

 when spotting or scorching may result. 



LITERATURE. 



Diseases of Plants induced by Cryptogamic Parasites, Tubeuf & 

 Smith. Longmans & Co. 18s. Contains an account of nearly 

 all known parasites attacking both wild and cultivated plants. 

 Also a general introduction to the subject, preparation and 

 use of fungicides. Numerous illustrations. 



A Text-book of Plant Diseases caused by Cryptogamic Parasites. 

 G. Massee. Duckworth & Co. 5s. Describes the diseases of 

 plants cultivated in Britain, India, and the British colonies. 

 General introduction to the study, preparation, and use of 

 fungicides, with preventive and curative methods given under 

 each disease. Numerous illustrations. ["q. jyj ~| 



CHAPTEE XIV. 



SOILS. 



The Soil and its Pkoperties — Tillage of Soils — 

 Different Soils employed in Horticulture — 

 Ingredients of Soils — Sources of Nitrogen — 

 Do Plants assimilate Free Nitrogen? — Azalea 

 Culture. 



Formation of the Soil and its Properties. — Soils 

 are formed by the decomposition of rocks, 

 under the influence of air, rain, sun, frost, and 

 the lower plants. If the products of the de- 

 composition of the rocks remain on the place 

 where they have been formed, the composition 

 of the rock in the subsoil will determine the 

 character of the soil resting upon it. In most 

 instances the products of decomposition are 

 partly washed away into streams and creeks, 

 and thence carried into the rivers and into the 

 sea. It is just these, the soluble and more 

 finely pulverized parts, which contain most of 

 the available plant-food. As the current of the 

 rivers becomes slower a great deal of the sus- 

 pended matter sinks to the bottom: first the 



larger stones and pebbles, then sand and fine 

 clay. 



The matter held in solution in the waters of 

 the rivers, and so deposited, is taken up by the 

 soils with which they come into contact. In 

 this way the valleys receive a large part of the 

 plant-food which is formed by the decompo- 

 sition of the rocks at higher levels. 



A part, however, of the dissolved and sus- 

 pended matter is discharged with the river 

 water into the sea. The sea often deposits the 

 material thus obtained in the form of sand and 

 clay, the latter forming a soil of superior fertility. 

 The soils obtained from rocks alone, without 

 any admixture of vegetable matter, are not 

 fertile. In order that they may become so, the 

 products of plant-growth must be mingled with 

 them. When plants grow on a soil they deposit 

 in their dead leaves and stems the organic- 

 matter which they have formed by means of 

 their green leaves. The substances formed by 

 the decomposition of this organic matter is 

 called humus. To this substance the black 

 colour of garden soils is due. Humus is an 

 organic substance that can be destroyed by 

 fire. 



Many of the most important properties of the 

 soil are partly, or almost entirely, due to humus. 

 Plant-growth, however, deposits nitrogen com- 

 pounds as a part of the humus. These com- 

 pounds are not contained in rocks, but without 

 them no soil can be fertile. By the decom- 

 position of the organic matter of the plants and 

 of the humus other substances are formed, par- 

 ticularly carbonic acid, that help to decompose 

 the original rocks. Plant roots also, by means 

 of their acidity, actively take part in the de- 

 composition of rocks. At the same time they 

 effectively prevent losses of plant-food by sur- 

 face drainage. 



It is evident to anyone who has observed the 

 nature and disposition of soils, that vast quan- 

 tities of them have been removed from the 

 localities in which they were originally formed. 

 It is certain that violent eruptions of nature 

 have disturbed the solid masses, scattering at 

 the same time the disintegrated portions that 

 may have been previously reduced to the con- 

 dition of soils. Light sandy soils in some 

 localities are even now being drifted from place 

 to place by the winds. But the effects of water 

 with regard to the transportation of soils are 

 much more powerful and universal than those 

 resulting from the action of wind. The mechani- 

 cal effect of water can be traced almost every- 

 where, in a greater or less degree. 



