Crop residues in the soil play a fundamental role in organic matter dynamics and have effects on numerous beneficial soil properties. The humification of residues depends on soil nutrient status and physical and meteorological conditions. The prediction of the humification rate and of the size and decomposition of fresh and humified soil organic matter has been attempted with equations and models which require several constants. To simplify the procedure, Janssen developed an equation which utilizes only one constant, allows to calculate the young (fresh) and old (humified) soil organic matter pools and predicts the production of available or mineralized nitrogen. The validity of Janssen's equation was tested in three production systems (a crop rotation and two tillage procedures) in a sandy-loam textured, thermic, entic Haplustoll soil in the pampean semi-arid region of Argentina. The model produced results compatible with actual field data, but information on soil available water was essential for fitting and interpreting the organic matter dynamics.
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