44 DISEASES OF GLASSHOUSE PLANTS 
THE PHYSICAL CONDITION OF THE SOIL 
The nature, size, and manner of arrangement of the 
particles of the soil very largely determine its texture, 
which is so intimately related to plant growth and 
resistance to disease. Upon the texture of the soil 
depends its water-holding capacity, food-holding capacity, 
degree of aeration, suitability as an anchorage ground for 
plant roots, and degree of compactness. 
(a) Water-holding Capacity—The soil particles vary 
in size from the very finest silt to coarse sand and stones. 
The smaller the particles of soil the smaller the spaces 
between them, and therefore the more retentive is the 
soil of moisture. Thus soils containing a large percentage 
of sand are more porous than those with a large proportion 
of clay or silt. The addition of straw manure to a soil, 
and the consequent increase in humus, increases its 
water-holding capacity. 
The quantity of water, the length of time it remains, 
and the ease with which it leaves the soil are important 
factors in glasshouse cultivation, with its specialized 
water requirements. Different crops have different 
requirements, but generally speaking the best soils 
possess efficient drainage and are neither too wet nor too 
dry. Perhaps the most important condition is that of 
constancy of water content. Soils which are alternately 
very wet and very dry produce unhealthy plants, which 
are more susceptible to disease than those in uniformly 
moist soils. Waterlogged soil conditions also induce an 
increased susceptibility to disease. 
(6) Food-holding Capacity.n—Porous soils are generally 
deficient in plant foods. They allow the rapid drainage 
of water through them, and soluble foods are washed 
away very quickly. Bacterial, fungal, and algal growth is 
at a low ebb, and consequently the rate at which these 
organisms manufacture plant foods is greatly reduced. 
Fertilizers, therefore, should be given in an organic form, 
for these are not easily washed out of the soil and yield 
