DISEASES OF CULTIVATED PLANTS 575 



One of the most important factors contributing to malnutrition 

 is the exhaustion of the organic material in the soil. Fields where 

 this disease occurs are found to contain only 1.65 per cent of organic 

 matter, while normally 3 to 5 per cent should be present. This de- 

 ficiency is to be expected from such complete dependence on com- 

 mercial fertilizers, which cannot take the place of stable manure and 

 green manures in a permanent system of agriculture. 



The fields used for the experiments quoted were given a bac- 

 teriological examination by the Office of Soil Bacteriology of the 

 Bureau of Plant Industry, which reports that the nitrifying organ- 

 isms which should be present in a fertile soil were deficient in num- 

 bers and nitrifying powei. This is to be attributed to the acidity 

 and deficiency in humus already mentioned. 



Remedial and Preventive Measures. The course to be adopted 

 for the correction of these difficulties should be a matter for the 

 earnest consideration of everyone, not merely of those whose fields 

 are already injured. 



The practices that have developed these nutrition disorders in 

 the cases reported are still followed by a majority of our truck 

 growers, who may look forward to a further deterioration in the 

 crop-producing power of their soils unless they profit by the errors 

 of their neighbors. 



Briefly stated, the points needing most attention are: 1. Limi- 

 tation of the amounts of fertilizers used. 2. Adjustment of the 

 composition of the fertilizer to suit the crop requirements. 3. The 

 rational use of lime. 4. The maintenance of the organic matter of 

 the soil. 



Amount of Fertilizer Most Profitable. The indications are that 

 the limit of profit is often exceeded by truck growers and that in 

 many cases actual injury results from the use of too large amounts 

 of commercial fertilizers. In the field experiments conducted on 

 land subject to malnutrition, larger yields of cabbage were obtained 

 from 1,000 pounds per acre than from any higher application. 



Doubtless other fields richer in humus would repay higher 

 fertilizing. The most profitable amount of fertilizer will have to be 

 determined for each field and for each crop, which varies from 

 season to season. Three points are borne in mind. 1. That the use 

 of large amounts of quickly soluble materials may result in direct 

 injury, "burning" the plants. 2. The large application may not 

 be utilized by the crop because of the lack of some necessary ingre- 

 dient. Thus, if lime is lacking the plants can assimilate only a lim- 

 ited amount of potassium and phosphorus, even though they may bo 

 E resent in liberal quantities. Absence of lime frequently appears to 

 e the limiting factor in this section, and much fertilizer is wasted 

 because the crops cannot use it until lime is supplied. 3. Our 

 experiments leave no doubt that the malnutrition disease described 

 may be induced by too heavy application of fertilizers. 



Composition of Fertilizers. Hitherto attention has been directed 

 chiefly to the proportions of nitrogen phosphoric acid and potash 



