It has long been recognised that in plants the susceptibility to disease is influenced to a considerable extent by the manuring practice followed. Ill-balanced fertiliser mixtures decrease the capacity for disease resistance; this has been frequently observed after the application of excessive amounts of nitrogenous manures, and has been demonstrated at Rothamsted and Woburn, where wheat and barley suffered most severely from rust and mildew in those plots which received excessive quantities of nitrogen; also at Cheshunt, where tomatoes heavily dressed with nitrogenous manures were readily attacked by disease-producing organisms. The effect of applying phosphoric acid has not, apparently, been clearly demonstrated, but it is generally assumed that, by favouring the development of a vigorous root system, its action is beneficial. Experiments at Cheshunt have shown that the use of manures from which potash has been omitted, produced a condition similar to that following the application of excessive amounts of nitrogenous fertiliser, and that the addition of potash tends to increase appreciably the ability of the plant to resist disease.