The comparative effects of concentrated nitrogenous fertilizers on permanent soil acidity

1. This paper presents a theoretical discussion of the comparative effects of the addition of different concentrated nitrogenous fertilizers on permanent soil acidity, assuming equal yields for cropped soils. A definite distinction is made between the difference in the effect of two materials on acidity in contrast to the absolute effect of a single material. This discussion deals only with the comparative effect. Certain experimental data, previously reported, are discussed in the light of the facts and theories advanced. Particular emphasis is placed upon the importance of crops in relation to soil acidity. 2. In the absence of a crop, where the added nitrogen is either all leached out as nitrates or is left in the soil and accumulates as nitrates, the acidity which develops is theoretically the sum of the acidity due to the mineral acids (if any) added combined as nitrogenous fertilizer salts plus that caused by the nitric acid formed in the soil. These mineral acids ordinarily include only sulfuric, hydrochloric, and phosphoric. Carbonic, nitric, and nitrous acids would usually have no appreciable effect. The nitric acid of ammonium nitrate, however, would produce acidity when the NH4 nitrifies. 3. In the presence of a crop, assuming complete absorption of the nitrogen by the plants, the only appreciable direct source of permanent acidity that develops under ordinary conditions that can be attributed to the nitrogenous fertilizer is the acidity caused by any of the three mineral acids mentioned, added as nitrogenous fertilizer salts. Where nitrate salts of such bases as sodium, potassium, calcium, and magnesium are added, the soil is made more basic in proportion to the quantity of the basic elements added. 4. Under actual field conditions, both plant absorption of nitrogen and leaching take place simultaneously and the final acidity (if any) due to the fertilizer is a weighted average of that resulting from the processes as outlined above under 2 and 3 Evidence is presented to show that under actual experimental conditions the comparative acidity or alkalinity produced by nitrogenous fertilizers does correspond rather closely to the theoretical, if allowance is made for the limitations of our methods. 5. Data are presented which show that regardless of whether a given crop is fertilized with acid-producing or base-producing nitrogenous fertilizers the proportion of bases to acids in the ash does not vary greatly. 6. The available evidence indicates that whether nitrogen enters the plant as ammonia or nitrate the final effect is as though the free ions enter. Hence, it makes no appreciable difference in final soil acidity whether the nitrogen of a salt, such as ammonium sulfate, enters the plant as ammonia or whether it is first converted into nitric acid in the soil and is then absorbed by the plant. In neither case does the nitrogen, apart from the SO4, appreciably affect permanent soil acidity. Any increase in acidity is due to the larger crop with larger demands for basic elements as explained below. 7. Emphasis is also placed on the fact that in order to interpret accurately any soil acidity experiment, where a crop is grown and where titratable acidity determinations of the soil are made, it is necessary to know the total nitrogen content of the plant, total weight, and analysis of the ash showing the basic and acidic elements. Such data are necessary because, neglecting N and SiO2, most plants remove more bases than acids from the soil. The acid-base ratio varies for different crops and for the same plant species at different stages of maturity. If a correction is not made for this removal of excess base, then comparative acidity calculations for different nitrogenous fertilizers will be too high. Data presented show that in the experiments discussed the excess base for several crops expressed in terms of pounds of CaO per pound of nitrogen in the crop was as follows: Barley, 0.25 to 0.47; wheat, 0.29; swedes, 0