. Plant physiology. Plant physiology. ASSIMILATION OF NITROGEN 67 Fertilizer Dry Yield Increase in Yield, Compared with Cul- ture WITHOUT Nitrogen Without nitrogen grams 6-3 94-4 grams 2 g. of nitrogen as ammonium Without nitrogen With lime 2 g. of nitrogen as ammonium Thus, ammonium salts have but little value as fertilizers for soils poor in lime. But soils rich in lime show almost as good yields with ammonium salts as with nitrates (Fig. 43). These experiments show that nitrate-fertilizer is suitable. Fig. 43.—Comparison of the effect of
. Plant physiology. Plant physiology. ASSIMILATION OF NITROGEN 67 Fertilizer Dry Yield Increase in Yield, Compared with Cul- ture WITHOUT Nitrogen Without nitrogen grams 6-3 94-4 grams 2 g. of nitrogen as ammonium Without nitrogen With lime 2 g. of nitrogen as ammonium Thus, ammonium salts have but little value as fertilizers for soils poor in lime. But soils rich in lime show almost as good yields with ammonium salts as with nitrates (Fig. 43). These experiments show that nitrate-fertilizer is suitable. Fig. 43.—Comparison of the effect of nitrate and ammonium salts on growth of plants in soil rich in lime. 0, no fertilizer; NOs, nitrate added; NHz, ammonium salts added. (,After P. Wagner.) for many different kinds of soils whereas ammonia-fertiUzer is suitable for only a limited number. There are two reasons for this: first, if we suppose that the ammonia is all oxidized to nitric acid before assimilation, then free nitric acid may be produced in the soil that lacks calcium (as in the first series of ex- periments just described), and this acid retards the growth of the plants as well as the nitrification process. Secondly, if we suppose that a part of the ammonia is assimilated unchanged, then free acid may again accumulate in the soil lack- ing calcium; for ammonium salts are physiologically acid, their basic radicals being absorbed by the plants to a greater extent than are their acid radicals.^ The presence of calcium carbonate prevents the accumulation of free acid in both cases. Such experiments with natural soils cannot answer the question regarding the direct assimilation of ammonia. Sterilized soils must be used, in which the 1 This is more fully considered in Chapter Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Palladin, V
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