SOIL MANAGEMENT. 19 



Puddling. — This process is directly opposed to granula- 

 tion, and implies a complete breakdown of crumb structure. 

 In a puddled soil plasticity and cohesion are at a maximum. 



" Light " and " Heavy," when used in connection with 

 soils, are terms referring to their mechanical condition — 

 whether or not they are easily workable. Thus a sandy soil is 

 said to be light on account of the ease with which it is 

 ploughed. These terms have no reference to the specific 

 gravity of soils. 



Root-zone may be regarded as that layer of soil in which 

 the roots of plants are distributed and in which all the physical, 

 chemical and biological processes are most active. 



Pore-space is the total volume of the spaces between the 

 soil particles and usually amounts to about 30 to 50 per cent, 

 of the volume of a soil. Structure to a great extent determines 

 the amount of pore space. In puddled soils the granules are 

 deflocculated {i.e., broken down), the pore-space is at a mini- 

 mum and, consequently, the aeration is poor. It has been 

 estimated that in a clay soil of good granular condition 50 to 

 65 per cent, of its volume is pore-space, while in the same soil 

 when puddled this is reduced to 25 to 45 per cent. The 

 average size of the pores is more important than the total 

 volume, and soils of intermediate texture, and consequently 

 of intermediate size of pores, are suited to a large variety of 

 crops and command the highest prices. 



Hardpan is a layer of soil that is particularly dense and 

 difficult to penetrate. It may be due to a thin stratum of clay 

 or to the packing caused by continuously ploughing at the same 

 depth ; but in arid areas it is more frequently caused by the 

 cementing together of soil particles under the influence of such 

 substances as lime, iron compounds, carbonate of soda, etc. 

 Soil Tilth. — The promotion and maintenance of good tilth 

 are of fundamental importance in crop production. The state 

 of tilth is closely related to the condition of the soil in regard 

 to granulation. A puddled soil is necessarily in a bad state 

 of tilth , and the promotion of granulation is the first step to- 

 wards improving the tilth. 



The chief agencies which promote granulation, and con- 

 sequently oood tilth, are as follow : — 



(a) Wetting and Drying.— The changes in volume due to 

 alternate wetting and drying tend to reduce the soil mass into 

 a large number of small aggregates, i.e., granules. 



(b) Freezing and Thawing.— The soil moisture on freez- 

 ing exerts an enormous force, shattering the soil. On thawing 



