46 DISEASES OF GLASSHOUSE PLANTS 
in this country most soils are neutral or approximately 
so; strongly acid soils are occasionally found, while 
alkaline soils are comparatively rare. Acid soils result 
from the decomposition of large masses of luxuriant 
vegetation in the presence of water, and are found 
chiefly in low marsh lands and on some upland moors. 
In such soils most crops never reach their maximum 
development, but are stunted in growth, turning yellow 
and chlorotic, and their roots are slowly destroyed. In 
alkaline soils the effect is much the same. Such ab- 
normal conditions assist the incidence of disease by 
reducing the vitality of the plant, by producing suitable 
places of entrance for fungal or bacterial parasites 
through injured roots, or by providing increasingly 
favourable conditions for fungal or bacterial growth. 
On the other hand, some diseases, such as potato 
“scab,” may be prevented by making the soil slightly 
acid. 
(b) Balance of Plant Foods.—The amount and balance 
of the three most important plant foods—nitrogen, 
phosphates, and potash—bears an intimate relation to 
crop production and disease resistance. An excess of 
nitrogen induces a rapidly growing, soft, and sappy 
plant, highly susceptible to disease, which condition may 
be counteracted by suitable additions of phosphates and 
potash. Potash is especially valuable by reason of its 
hardening effect, which produces an increased resistance 
to disease. This may be illustrated by results obtained 
at Cheshunt in connexion with “ Stripe” disease of the 
tomato, caused by Bacillus lathyri (36). The number 
of striped tomato plants were counted on each of the 
plots in the experimental houses. These plots were laid 
down in 1915 and have received the same treatment 
each year since that time. 
Table 9, page 130, indicates that manurial treatment 
has an important effect upon the disease, for plots 
receiving no added potash yielded the greatest number 
of striped plants, while on those receiving no added 
