Soil and water are two fundamental resources of our agricultural environment With the advent of new agricultural technologies, tremendous quantities of water and soluble chemicals are applied to soils each year This enhanced use, or in some cases abuse, has endangered our soil and water resources In addition, there are increased incidents of groundwater contamination, erosion, and contamination of surface waters from point and nonpoint sources In response to these environmental concerns there has been an increased effort to develop improved agricultural management practices

Estimating Soil Water Retention from Soil Physical Properties and Characteristics

Publisher Summary This chapter reviews the scientific literature related to the importance of the different humus pools present in soils regarding the stabilization of soil aggregates. It evaluates the potentials of various practices that utilize exogenous organic material to improve soil physical qualities and to reduce soil erosion. It points out that humified, humic materials are responsible for soil aggregation in natural and agricultural environments. The conservation and enhancement of humus in soils exert beneficial effects on plant nutrient supply, soil structure, compactability, and water-holding capacity. Research on organic matter turnover in the soil environment has shown that the microbially unstable organic matter fractions have a mean residence time hardly exceeding a few months, whereas the rest of the organic matter resides in soils from a few to many years. The hydrophobic concept represents a guide to interpret organic matter dynamics in soils. In the course of microbial degradation of fresh organic matter in soils, all compounds are rapidly mineralized except those that are either physically protected or chemically recalcitrant. No chemical can resist microbial or fungal degradation in the soil solution unless it is further protected by chemical or physical adsorption on the soil solid phase.

Humus and Soil Conservation

Summary
•Soil A horizons from a large area typical for beech (Fagus sylvatica) in northern Italy were investigated to understand the phyto-hormone-like activity of humic substances (HS) with respect to the vegetation. These soils had developed under thermophilous (Cephalonthero–Fagion with Carici albae–Fagetum), mesophilous (Fagion sylvaticae with Dentario pentaphylli–Fagetum and Galio-odourati–Fagetum) or acidophilous (Luzulo–Fagion with Luzulo niveae–Fagetum) beech forests.
•The development of organic and humic matter in the three forest types was assessed by quantifying chemical parameters. Phyto-hormone-like activity of HS was evaluated by measuring auxin-like (IAAlike) and gibberellin-like (GAlike) activity as well as IAA concentration, invertase and peroxidase activities.
•Soil pH differed between the forest types. Soil C: N distinguished the thermophilous and the mesophilous from the acidophilous types but did not distinguish thermophilous from the mesophilous types. Phyto-hormone-like activity of HS varied significantly across the beech forests. Acid conditions were essential for the release of IAAlike activity whereas neutral conditions promoted GAlike activity. Plant isoenzymatic polymorphism confirmed the different auxin activities of HS.
•The phyto-hormone-like activity of HS is highly suitable, and better than soil chemical parameters, for differentiating between these ecosystems.

Hormone-like activity of humic substances in Fagus sylvaticae forests.

To understand if the biological activity of humic substances may be related to their molecular weight or chemical structure, two humic substances, derived from an Ah horizon of uncultivated couch grass [Elytrigia repens (L.) Desv. ex Nevski] and an Ah horizon of forest soil, were extensively characterized by means of different spectroscopic techniques (diffuse reflectance infrared Fourier transform [DRIFT] and 1 H nuclear magnetic resonance [NMR]). The two humic substances, each separated in fractions with low ( 3500 Da) relative molecular mass were compared for their effects on Pinus nigra J.F. Arnold callus. Growth of callus, the soluble sugar content, free amino acid pool, and the activities of the key enzymes involved in C and N metabolism were investigated. Callus was grown for a subculture period (28 d) on basal Murashige and Skoog medium plus humic matters with or without different hormones: indole-3-acetic acid, 2,4-dichlorophenoxyacetic acid, or 6-benzylaminopurine. The results of 1 H-NMR spectra and the DRIFT spectroscopy showed significant differences in the chemical composition between forest and grass humic substances. A large amount of aliphatic and H-sugarlike component and an intense chemical shift of the β-CH 3 region in both grass humic fractions were observed, while high contents of betaine, organic acid, and COOH groups in both forest humic fractions were detected. A different biological activity between the grass and forest humic fractions was also observed. Thus, the different activity of the two humic substances used seems related to the diverse chemical composition rather than to different molecular weights.

http://dzumenvis.nic.in/microbes and soil fertility/pdf/biological activity of humic substances.pdf

Biological Activity of Humic Substances Is Related to Their Chemical Structure

An 11 year study conducted in the semiarid prairie of southwestern Saskatchewan, Canada, to determine the influence of fallow frequency and tillage on yields and economics of durum wheat (Triticum turgidum L.) production, was used to assess changes in soil organic C and N. The soil was a coarse-textured, Typic Haploboroll. Cropping frequency (fallow-wheat vs. continuous wheat) had no effect on organic C or N concentrations or amount in this study. Compared with conventional mechanical or minimum tillage, no-tillage increased organic C and N concentrations in the 0–7.5 cm soil depth during the final 4 years of the experiment when crop yields and residue production were above average. However, there was no effect of tillage on C or N concentration in the 7.5–15 cm depth, nor on the mass of organic C or N in either depth. The amount of organic C in the top 15 cm of soil increased by 1.6 t ha−1 over the 11 year period (average for all systems). Most of this increase occurred in the final 4 years and was generally related to the amount of crop residues returned to the soil. Because neither rotation nor tillage influenced crop production, these practices did not affect organic C or N. We concluded that there was limited opportunity to sequester additional C in coarse-textured soils located in semiarid climates if monoculture cereal systems are used under conventional (mechanical) or no-tillage.

Tillage and crop rotation effects on soil organic C and N in a coarse-textured Typic Haploboroll in southwestern Saskatchewan

Functional relationships between soil water content and water suction were examined and related to textural and organic carbon content data. Soil water retention curves between 5 and 10 000 kPa were determined on disturbed samples of 18 soils representing various soil Great Groups in the Canadian prairies. The best fit was obtained with a two-straight-line regression model. Correlation and regression analysis showed that texture was the main soil property influencing the shape and position of the water retention curve. Organic matter influenced primarily the water content at which a break in the curve occurred. Soil zone and cultivation history had little effect on water retention. Key words: Water retention, texture, organic matter, two-straight-line regression

W. Nicholaichuk

Papers

Water retention equations and their relationship to soil organic matter and particle size distribution for disturbed samples