Ammonium, Ammonia, and Urea Reactions in Soils.

Chapter 4 of the new Nitrogen Monograph has several new items that receive significant discussion. The chapter begins with a discussion of acid base reactions in soils as related to ammonia retention. Quantifying ammonia retention is covered in considerable detail. The discussion includes an evaluation of the importance of CEC in the retention of ammonia compared to the soil’s titratable acidity. A section is also provided on the modeling of ammonia retention when it is applied in bands for crop production. Another new section deals with factors affecting urea hydrolysis in soils. The effects of urea concentration, soil pH, soil water content and potential, and soil temperature are all treated in the discussion. Updated information is provided on ammonium fixation by clay minerals, including studies to distinguish between recently fixed ammonium and that of geologic age, the equilibria associated with recently fixed ammonium, and its bioavailability. Also covered are recent advances in methodology to study ammonium fixation, along with a discussion of the merits of the new methods compared to traditional approaches.