George Stanford

Bio: George Stanford is an academic researcher from United States Department of Agriculture. The author has contributed to research in topics: Potassium & Nitrogen deficiency. The author has an hindex of 14, co-authored 21 publications receiving 2369 citations.

Nitrogen Mineralization Potentials of Soils

Abstract: Net mineralization of N in 39 widely differing soils was determined over a 30-week period at 35C, using incubation intervals of 2, 2, 4, 4, 4, 6, and 8 weeks. Mineral N was leached from the soils before the first incubation and following each of seven incubations by means of 0.01M CaCl₂ and a minus-N nutrient solution. Soil water contents were adjusted by applying suction (60 cm Hg), and losses of water during incubation under aerobic conditions were negligible. With most soils, cumulative net N mineralized was linearly related to the square root of time, t½. The pH of soils changed very little in the course of 30 weeks' incubation. Because of the generally consistent results, the data were employed in calculating the N mineralization potential, Nₒ, of each soil, based on the hypothesis that rate of N mineralization was proportional to the quantity of N comprising the mineralizable substrate. Values of Nₒ ranged from about 20 to over 300 ppm of air-dry soil. The fraction of total N comprising Nₒ varied widely (5 to 40%) among soils. Mineralization rate constants did not differ significantly among most of the soils. The most reliable estimate of the rate constant, k was .054 ± .009 week⁻¹. The time required to mineralize one-half of Nₒ, t½, was estimated to be 12.8 ± 2.2 weeks. Results suggest that the forms of organic N contributing to Nₒ were similar for most of the soils.

Estimates of Potentially Mineralizable Soil Nitrogen Based on Short-term Incubations

Abstract: Nitrogen mineralization potentials were determined for a large number of soils by a method involving determination of N mineralized after several consecutive incubations at 35C under optimum soil water conditions. The determination of N mineralization potential, No, based on the first-order equation, log (No — Nt) = log No — kt/2.303, is laborious and usually requires incubation periods of 8 weeks or more. From the present study, involving soils from major agricultural areas throughout the United States, it was demonstrated that No could be estimated reliably from the amounts of N mineralized during 2-week incubations following preliminary incubations of 1 to 2 weeks. From the above first-order equation, No = Nt / (1-10–kt/2.303). Hence, for a 2-week incubation (t = 2), No = 9.77Nt (Nt = N mineralized in 2 weeks and k is the rate constant, weeks-1). Estimates of No from short-term incubations were similar to those derived after extensive periods of incubation. Preincubation of soils is required in order to decompose plant residues and for other possible reasons noted. Estimates of No from preliminary incubations are meaningless. The implications of No as a basis for predicting amounts of soil N mineralized under fluctuating temperature and soil water conditions are discussed.

A Method for Measuring Short-Term Nutrient Absorption by Plants: I. Phosphorus

Abstract: A method has been devised which permits measurement of P absorption by plants during relatively short periods of root-soil contact. Plants are grown without added P in sand during which time a mat of roots develops at the bottom of the container. The exposed root mat with plants intact is then placed in contact with the soil or soil-fertilizer system. Results obtained using oats as the principal crop and concentrated superphosphate applied to Hartsells fine sandy loam support the following conclusions: (1) P uptake by oats from applied fertilizer increased linearly with time between 1 and 7 days and increased linearly with rates of applied P ranging from 0 to 50 mg. or more per 200 g. soil. (2) Moisture equivalent was approximately the optimum soil moisture level for P absorption. (3) Recovery of applied P commonly ranged from 2 to 5% after absorption periods of 3 to 7 days. The method holds considerable promise as a means of studying the influence of various factors on P absorption by plants. These include environmental factors as well as soil and fertilizer characteristics.

Nitrogen - inorganic forms.

Abstract: 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.

A model of nitrous oxide evolution from soil driven by rainfall events: 1. Model structure and sensitivity

Abstract: Simulations of N2O and CO2 emissions from soils were conducted with a rain-event driven, process-oriented model (DNDC) of nitrogen and carbon cycling processes in soils. The magnitude and trends of simulated N2O (or N2O + N2) and CO2 emissions were consistent with the results obtained in field experiments. The successful simulation of these emissions from the range of soil types examined demonstrates that the DNDC will be a useful tool for the study of linkages among climate, soil-atmosphere interactions, land use, and trace gas fluxes.