434 JOUENAL OP THE DEPARTMENT OF AGRICULTURE. 



generally speaking its properties with reference fo the moisture condi- 

 tions of the soil are similar to those of clay, excepting that it does 

 not usually hinder the movement of water to anything like the same 

 degree; indeed, humus, by causing the clay i)articles to aggregate, 

 assists the movement of water in clay soils. A good humus content 

 is invaluable in sandy soils, as it tends to bind the particles together 

 as well as to increase the moisture-holding capacity of the soil. 



Reference to the mechanical analyses Vvill show that in some 

 cases figures relating to the permeability of the soil to water are 

 given. By permeability is meant the rate at which water passes 

 downwards through the soil. These figures are to be regarded as 

 ■complementary to the figures which give the proportions of various 

 sizes of particles, but which do not indicate to what extent the 

 particles are aggregated. 



Plate V. 



Showing the effect of a deficient food supply on the crop and the effect produced 

 by applying the deficient food substance to the soil. 



The mealies are growing on a virgin soil derived from the Molteno Beds. 

 Where the growth is poor no manure was applied ; but where the growth 

 is fair a phosphatic manure had been sown with the seed. 



The importance of water in plant growth will be manifest from 

 the following four points: — 



1. All plants contain considerably more water than any other 



substance. For example, about 90 per cent, of a mangel 

 consists of water. 



2. The presence of water in its tissues prevents the sun from 



drying it up and thereby killing it. 



3. It forms the vehicle by which the plant obtains its food 



from the soil and thereafter distributes it throughout its 

 tissues. 



4. It plays a very great part, directly and indirectly, in the 



preparation of the food substances which the plant obtains 

 from the soil. 



