Nitrogen—Inorganic Forms

Most soils contain inorganic nitrogen (N) in the form of ammonium (NHt) and nitrate (NO)"). Nitrite (NOz) also may be present, but the amount is usually too small to warrant its determination, except in cases where NHt or NHt-forming fertilizers are applied to neutral or alkaline soils. Several other forms of inorganic N have been proposed as intermediates during microbial transformations of N in soils, including hydroxylamine (NH20H), hyponitrous acid (H2N20 2), and nitramide (NH2N02), but these compounds are thermodynamically unstable and have not been detected in soil. Until the 1950s, inorganic N was believed to account for <2% of total soil N, on the assumption that NHt and NO)" are completely recovered by extracting soil with a neutral salt solution. The validity of this assumption was challenged by the finding that some soils contain NHt in a form that is not extracted by exchange with other cations (e.g., Rodrigues, 1954; Dhariwal & Stevenson, 1958; Stevenson & Dhariwal, 1959; Bremner & Harada, 1959; Bremner, 1959; Schachtschabel, 1960, 1961; Young, 1962), and by estimates that the proportion of soil N in this form can exceed 50% for some subsurface soils (Stevenson & Dhariwal, 1959; Young, 1962). In such cases, NHt is said to be fixed, and fixed NHt has subsequently been defined as the NHt in soil that cannot be replaced by a neutral potassium salt solution (SSSA, 1987), such as 1 or 2 M KCI or 0.5 M K2S04, in contrast to exchangeable NHt, which is extractable at room temperature with such a solution. Existing information indicates that fixed NHt occurs largely, if not entirely, between the layers of 2: I-type clay minerals, particularly vermiculite and illite (hydrous mica), and that fixation results from entrapment of NHt in ditrigonal voids in the exposed surfaces upon contraction of the clay lattice (Nommik & Vahtras, 1982). The term, nonexchangeable NHt, has been used by Bremner (1965) and Keeney and Nelson (1982) in previous editions of this publication as a more precise alternative to fixed NHt. The same term is used in the present treatment, with specific reference to NHt determined by the method described in "Determination of Nonexchangeable Ammonium," which involves digestion with an HF-HCI solution following treatment of the soil with alkaline KOBr to remove exchangeable NHt and labile organic-N compounds.

Nitrogen - inorganic forms.

Inorganic Forms of Nitrogen

Steam distillation methods for determination of ammonium, nitrate and nitrite

Nitrogen—Total 1

The use of boric acid for trapping ammonia in the micro-Kjeldahl determination is examined. A suitable procedure, together with a modified type of micro-distillation apparatus, is described.

The buffer capacity of boric acid solutions was examined in relation to the sensitivity in the titration of ammonia dissolved in them. Two grades of boric acid, ‘pure’ (technical) and AnalaR, were compared; the former showed appreciable buffering on the acid side.

The pH values of solutions of both grades decreased with increasing concentration. The sensitivity of the three indicators examined for the titration of ammonia diminished with increasing concentration of boric acid as a result of the increased buffering. With concentrated boric acid, all three indicators showed wide transitional periods at the end-point, but in solutions of concentration up to 1-0% of AnalaR grade the end-points shown by all three indicators were sharp. The methylene blue-methyl red indicator, however, was generally preferable.

The ammonia-fixing capacity of boric acid was studied in air-bubbling tests and by recovery of a known amount of ammonia by distillation. A volume of 10 ml. of 1-0% boric acid fixed 5 mg. of nitrogen as ammonia sufficiently firmly to afford accurate analyses. For macro-determinations, 100 ml. of 2-0% boric acid held firmly up to 90 mg. of nitrogen.

Determination of nitrogen in soil and plant materials: Use of boric acid in the micro-kjeldahl method

1. A modified Nessler reagent in which the ratio HgI2 : KI : NaOH is 1 : 2.2 : 20 is described. It is clear and ready for immediate use and is less susceptible to interference by various cations. Distillation with phosphoric acid was satisfactory for preparing ammonia-free water. The best range of ammonia-nitrogen for this determination was from 0 to 100 μg.

2. The amount of the reagent used, the time and temperature of colour development, and the acidity of the test solution were critical factors in the accuracy of the determination.

3. Many cations interfered with the test to various extents. Among anions tested only arsenite, chromate, cyanide, dichromate and permanganate caused serious interference. Addition of sodium potassium tartrate, which did not alter the effects of anions, generally lessened cation interference.

4. With the modified reagent and under the conditions described quantities of ammonia-nitrogen in the range 0–100 μg. may be determined with an accuracy of 2–3%.

The determination of nitrogen in agricultural materials by the nessler reagent. I.—preparation of the reagent

Modifications of the Fiske-Subbarow method are described by which susceptibility to interference by silicate, arsenate, and certain acids and salts was reduced. In view of the possibility that some soils may contain considerable amounts of extractable arsenate and ferric iron, a method for eliminating interference by these two ions, by reduction with metabisulphite, is described. By this procedure, arsenate and ferric iron up to 20 p.p.m. As2O5 and 80 p.p.m. Fe had no effects on the determination.

The applicability of the modified technique to the determination of phosphate in soil extracts and in plant ashes is examined.

The fiske-subbarow method for determining phosphate with special reference to soil extracts

A modified molybdenum-blue method is described, with stannous chloride as the reducing agent, for determining phosphate in soil solutions and soil extracts; ferric iron in amounts up to 15 p.p.m. had no ill effect.

The influence of varying concentrations of sulphuric acid, ammonium molybdate and stannous chloride on the formation of phosphomolybdenum-blue complex and the interference due to silicate, arsenate and ferric iron were examined.

A study of the effects of various ions showed that ions commonly occurring in soil solutions and some soil extracts did not affect the accuracy of direct determinations of phosphate, but arsenate should be absent from the test.

Deniges' method for determination of phosphate, with special reference to soil solutions and extracts

The effects of fertilizer treatments and of season on the constituents of the soil solution, with special reference to phosphate, have been examined, using soils tested as part of a long-term fertilizer-experiment.



The concentration of inorganic phosphate in the soil solution in phosphate-treated plots was higher than that of the control throughout the year, the increases ranging from 21-5 to 66.6%. The effect of superphosphate did not increase during cropping; the concentrations of inorganic phosphate in the soil solution were low in summer and varied very little through the year. Ammonium sulphate and lime reduced the inorganic-phosphate concentration. In plots receiving dung the concentrations of inorganic phosphate were unusually high, the increases ranging from 120 to 635% in the 0–8-in. soil layer. In the presence of lime the increases of phosphate due to dung were lower. There was no indication of the downward movement of phosphate through the soil.



The concentration of organic phosphorus in the soil solution decreased during the cropping season. Ammonium sulphate, superphosphate and potassium sulphate tended to increase the organic phosphorus in soil solutions but lime tended to reduce its accumulation in July. Dung did not increase the organic phosphate greatly, especially in the presence of lime.



Simple and partial correlations between inorganic phosphate and other constituents in the soil solution were studied. Inorganic phosphate examined in April was negatively correlated with both pH and calcium. The relationship with pH was most clearly observed in plots without phosphate treatment and that with calcium in the superphosphate-treated plots. The relationship was unchanged when the correlation coefficients were calculated between phosphate and calcium with pH or nitrogen constant, and between phosphate and pH with calcium or nitrogen constant. There was no significant correlation in the July and September samples.



Colloidal absorption of phosphate appears to dominate the fixation of phosphate in the soil although there is evidence of chemical precipitation by calcium.

S. H. Yuen

Papers

Determination of nitrogen in soil and plant materials: Use of boric acid in the micro-kjeldahl method

The determination of nitrogen in agricultural materials by the nessler reagent. I.—preparation of the reagent

The fiske-subbarow method for determining phosphate with special reference to soil extracts

Deniges' method for determination of phosphate, with special reference to soil solutions and extracts

The availability of soil phosphate. I.—Examination of the soil solution with special reference to the fixation of phosphate