50 DISEASES OF GLASSHOUSE PLANTS 
contaminated and the infection spreads so rapidly that 
susceptible plants cannot be grown. The disease-causing 
organisms live upon the decaying organic matter of the 
soil and attack the young plants at the most suitable 
opportunity. 
It is thus apparent that the soil cannot be regarded 
as consisting of dead particles of rock, but rather as 
teeming with life, having a determining effect upon soil 
fertility and freedom from disease. Science has shown 
that fertility remains so long as the beneficial organisms 
are not swamped by those which are harmful. In the 
latter case, especially if parasitic organisms are abundant, 
the soil is said to be “sick” and many attempts have 
been made to devise methods for bringing the soil back 
to a healthy condition. 
Malnutrition 
Plants suffering from malnutrition have a stunted 
appearance and a light yellowish-green foliage, which is 
especially noticeable between the veins, which are 
yellowish or brown in colour. The flowers and fruits 
are small and misshapen ; the roots are ill developed and 
the secondary ones die prematurely. Investigation has 
shown that the disease is not attributable to parasitic 
organisms, but is the result of unsuitable soil conditions, 
generally set up by incorrect manuring. 
Fresh cow or pig manure has been shown to cause 
malnutrition of the tomato, and excess of acid fertilizers 
has a similar effect. An excess of readily available 
nitrogen in cucumber beds produces an incurling of the 
leaves, the margins of which turn brown and wither. 
On the other hand, cucumbers grown in frames and 
supplied with an insufficiency of available nitrogen 
become light in colour. Such fruits turn darker when 
the plants are given a soluble nitrogenous fertilizer, 
Growers of roses are familiar with the bronzing of 
the leaves of certain varieties. This begins as a bronze 
