When approved methods of soil management are followed, practically no danger attends the use of large amounts of commercial fertilizers. If the amount of vegetable matter decreases and the soil acidity increases, several difficulties may develop. The soil may become hard and unproductive, not because of large applications of fertilizers, but because of small additions of humus. With the continued application of large quantities of mineral fertilizers the soil may become extremely acid, when leguminous cover or green manurial crops cannot be grown successfully. Malnutrition diseases sometimes develop in strongly acid soils, especially if humus is deficient.
The causes and the control of malnutrition disease have been studied by the Virginia Truck Experiment Station. When the disease is present the following symptoms may be observed (Va. Truck Expt. Sta. Bul. i, p. 5): (1) "The plants stop growing when they should be making their most rapid development. In many cases they slowly weaken and die, while in others growth is resumed later in the season after rains have occurred. (2) There is a change of leaf color to a lighter green, especially in the spaces between the veins, which turn yellowish-green or even brown. In cabbage the margins of the leaves are frequently of a uniform yellow color. (3) The roots of the affected plants are poorly developed. Many of the lateral feeders are killed back repeatedly, until the root system becomes stubby. (4) No fungi or bacteria can be connected with the disease. In most cases none is present".
In the Norfolk region 3,000 pounds of fertilizer to the acre are often applied during the season's operations and this amount is used year after year, resulting in a strongly acid soil. The Virginia Truck Experiment Station states also (Va. Truck Expt. Sta. Bul. 1, p. 5) that "acid soils are less favorable for the production of most truck crops than neutral soils. A slight amount of acidity is not ordinarily injurious, but examinations made at our request by the Bureau of Soils of samples from fields where cabbage suffered from malnutrition, showed these soils to be abnormally acid, so much so that 3,500 to 6,300 pounds of lime would be required to neutralize an acre to a depth of 1 foot. This condition is apparently the result of many years of intensive trucking, involving the use of repeated heavy applications of commercial fertilizers made up in large part of chemicals which leave the soil more acid.
"Only a portion of the fertilizer applied is actually taken up by the plants, the remainder being left in a different form, which will have an influence on the soil reaction. For example, sulphate of ammonia, muriate and sulphate of potash, and acid phosphate tend to leave the soil more acid, while nitrate of soda, carbonate of potash and Thomas phosphate tend to make the soil alkaline. In the brands of fertilizers most used in this section, the acid-forming ingredients largely predominate.
"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 organic matter, while normally 3 to 5 per cent should be present.
"This deficiency is to be expected from such complete dependence on commercial fertilizers, which cannot take the place of stable manures and green manures in a permanent system of agriculture.
"Remedial and preventive measures recommended for malnutrition diseases are: (1) Limitation of the amounts of fertilizer 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".