68 THE PHYSIOLOGY OF PLANTS 



further enricli a poor soil. Spurrey and buckwheat are often 

 used, or on good soil white mustard may be grown for this 

 purpose. 



Very often a field may be greatly improved by the addition 

 of lime, marl, or gypsum. 



Heavy and caking clay soils which are poor in humu9 

 often stick to the plough and other implements when wet, and 

 when dry form large impracticable clods. Such a soil may 

 be improved by ploughing it up in the autumn and allowing 

 it to lie in its rough condition during the winter ; but the more 

 friable condition which it then assumes is soon lost during the 

 summer after heavy rains. If the addition of stable manure, 

 humus, or peaty soil is impossible, then lime or marls should 

 be added. 



The loosening of the soil which is thus brought about is 

 probably due to the chemical action of the lime. If it has to 

 be employed rapidly during a dry season, so that the natural 

 breaking up of the limestone cannot be awaited, and if no 

 powdered form is available, the following procedure can be 

 recommended : — The burnt limestone should be filled into 

 baskets and these plunged into water until no further air 

 bubbles appear (3 to 4 minutes). Then it should be piled 

 in heaps. The pieces break up of themselves, and the lime- 

 stone (slaked), which had given off its carbonic acid when 

 heated, now forms a white powder of calcium hydroxide 

 (OaHgOg) or so-called slacked lime, which is soluble in 730 

 parts of cold, or I 300 parts of boiling water (lime-water). 



The lime attacks the silicates of the clay, decomposes them 

 and liberates soluble potassium salts. It also accelerates the 

 decomposition of organic substances. This is the reason why 

 a field treated in this way uses up its organic manure more 

 rapidly than a field to which no lime is added. A lime soil 

 may therefore be said to be of a devouring nature. 



With regard to the manipulation of the lime, it should be 

 strewn very evenly over the ground, either by hand or with a 

 spade, and this operation should not be carried on during 

 windy weather. It will be found advantageous to lay the 

 lime on the stubbles in the autumn and then to plough it in. 

 If forced to wait until the spring, it is best to strew the lime 



