SOILS AND MANURES. 47 



is always imperfectly aerated, indeed this is the only condition 

 which excludes air from the soil. Rain water is an important 

 factor in soil aeration, for as it sinks into the ground it displaces 

 an equal volume of air, and as the water drains away air is 

 drawn in after it. In undrained land air is displaced very 

 slowly, and consequently the beneficial effects of rain water are 

 lost. Although it has not yet been decided whether roots 

 like leaves, breathe, still the fact remains that air is as necessary 

 to roots as it is to leaves. In employing drained swamps or 

 water-logged land for a plantation, a simple method, and one 

 which may be generally relied on for testing whether the soil is 

 sour, is to take samples of moist soil from different portions ot 

 the suspected region. Mix them all together, add sufficient 

 water to enable them to be stirred into a paste and insert a 

 piece of blue litmus paper. If after an hour has elapsed the 

 paper in contact with the soil turns red, the soil is probably 

 sour. To neutralise the acidity, broad-cast water-slaked lime at 

 the rate of about 2,000 Ibs. per acre. Stone-lime may be 

 expeditiously slaked by covering it with moist soil until it is 

 converted into powder, when it is ready for use. Applications 

 of lime also materially assist the work of nitrification bacteria. 



The temperature of water-logged soils is always lower than 

 that of more porous soils, hence the growth of the trees planted 

 therein would be checked and the amount of available plant 

 foods reduced owing to the retarded operations of soil bacteria. 

 After drainage, such land should be deeply cultivated and the 

 soil left in a rough condition to facilitate aeration before plant- 

 ing commences. 



The moisture content and physical condition of very light 

 porous soils may be increased and improved by ploughing in 

 animal or green manures. 



De Candolle's theory that a crop continuously cultivated on 

 the same soil is able to render it noxious to that particular crop 

 by its excretions has recently been confirmed by Schreiner and 

 Reed. It is claimed that the diminished yields of a crop culti- 

 vated continuously on the same land is not altogether due to 

 loss of soil fertility but largely to these toxic excretions. It has, 

 however, been proved that when the organic excretions of plants 

 are acted upon by air or micro-organisms the condition of the 

 soil is improved. It will be apparent that their poisonous 



