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In the semiarid central region of Argentina the probability that rainfall meets crop requirements during growing season is less than 10%, therefore fallowing has been the most important practice to assure water availability during the growing season. Various site-specific and management factors have been identified as crucial for defining fallow efficiency (FE) and final available water contents (AW). The objective of the present study was to improve our knowledge about the interactions between residue cover, weed control, soil profile depth and water storage capacity (WSC) on FE. In 10 sites covering the environments of calcareous plains and sandy plains of the semiarid central region of Argentina and with different WSC, experiments with 3 different levels of residue cover (H, M, L) and with and without weed control (C and W respectively) during fallow were set up. A completely randomized block design with four repetitions and splits plots to consider weed control was used. Soil texture and organic matter were determined in samples of the A horizon (0.20 m). Bulk density, field capacity, permanent wilting point and soil water contents (monthly frequency) were measured at depth intervals of 0.20 m to the depth of the calcite layer or to 2.00 m depth. Soil temperature was taken in weekly intervals at 0.05 m depth and weed plants, separated by species, were counted at the end of fallow in 4 repetitions of 0.25 m2 in each treatment. An empirical model was developed to predict final AW under these experimental conditions. Model parameters were: Residue level, weed control, WSC, profile depth, and rainfall during fallow. Site-specific conditions (WSC and profile depth) affected water storage during fallow; soils with highest values for both parameters showed highest final AW. Weed density was the most important factor that controlled AW, with on average 35 mm less AW in W than in C treatments. Residue level had a positive effect on final AW in both C and W treatments, with a difference of 18.5 mm between H and L. An interaction between residue level and weed density was observed, indicating weed suppression in H treatments. This was also confirmed by correspondence analysis between residue level and weed species which revealed that different species were related to each level. High residue levels also decreased soil temperature, thus affecting germination of post-fallow crops. The empirical model had an overall average prediction error of 13.7% and the regression between measured and predicted values showed a determination coefficient of 0.77

The fulvic acid fraction is considered an important constituent of soil organic matter and is sensitive to management regimes and tillage impact. Both humic acids (HA) and fulvic acids (FA) are able to form metal complexes which bind appreciable amounts of phosphate or phosphorus (P) species. The aim of the present study is to evaluate the fulvic acid carbon content of agricultural soils in Argentina and to explore the possible relationship between acid carbon (FC) content and the level of available P in the soil. A total of 74 surface samples of Hapludolls and Haplustolls covering areas with decreasing levels of annual rainfall were analyzed. It was found that soils with FC content >5.5 g kg−1 exhibited available P levels >30 mg kg−1, whereas available P levels

Laboratory experiments were conducted to study organic matter (OM) loss from soil columns with different clay mineral composition and treated with high sodium (Na) and potassium (K) saline solutions (SAR and Removal of organic and inorganic colloids was evaluated by determining organic matter content in the soil at the end of the trial and measuring absorbency of the percolated solutions. Though clay dispersion after magnesium (Mg) treatment was marked in the case of illitic and smectitic soils, little effect was detected in kaolinitic soil. For treatments with solutions containing Na, the highest removal of organic colloids was observed in illitic and kaolinitic soils. In smectitic soil this effect was more pronounced with calcium (Ca) in combination with Na and K. These findings have significance to explain differences in OM mobilization in salt affected depressions of different composition.

The aim of the present study is to determine quantitatively the magnitude of the structural changes that occur in the soil during the degradation process affecting natural grassland in the semi-arid ecosystem of the southern area of the District of the Calden, Argentina. Topsoil horizons show a reduction in depth from 20 cm in ungrazed sites to 16 cm in grazed sites, mainly as a consequence of soil compression caused by animal hooves. Bulk density values in the first 5 cm of the profile in grazed sites are approximately 27% higher than those in ungrazed sites. Total porosity values in the top few centimeters are, on average, 17% lower in grazed sites, primarily due to the collapse of macropores (>50 μm) and larger mesopores (50-9 μm). Water retention curves for the three depths in the different patches reflect the changes in pore space distribution. Grazing causes a sharp increase in mechanical impedance-an average increment of 210% over ungrazed values in the first 10 cm of soil. There are marked differences in the degree of aggregation in grazed sites compared with ungrazed sites. The former show a lower quantity and density of roots in the top few centimeters of the profile. Changes are also evident in average root diameter as a result of vegetation substitution brought on by overgrazing. The lack of significant differences in soil physical properties in areas subject to different grazing intensities underscores the gravity of the problem caused by animal traffic, which poses a serious threat to the conservation and recovery of the grazed patches.

To obtain information about the cation exchange capacity (CEC) of montmorillonite as affected by the sorption of charged organic molecules, montmorillonite samples were loaded with different amounts of positively charged molecules of safranin, toluidine blue, and negatively charged alizarin red S. Dye sorption by the clay was carried out by equilibrating samples with different concentrations of the dye. The adsorbed amounts were determined from initial and final dye concentrations using spectrophotometric methods. The CEC of the clay-organic complexes were determined by the BaCl{sub 2} base exchange capacity method. The CEC was found to decrease linearly with the increase in the amount of safranin and toluidine in the complex, giving rise to a 1:1 charge neutralization ratio. For alizarin the CEC increased linearly but was not in proportion to the amounts of alizarin in the complex. This noncumulative phenomenon was attributed to the hindering of free diffusion of exchanging cations by the adsorbed molecules.

Distribution and transformation of SOM in an Argentinian Hapludoll under arable land use and afforested with Pinus radiata was investigated by a combined approach using particle-size fractionation, wet-chemical analysis and 13C NMR spectroscopy. The soils showed thick mollic A horizons and had high organic carbon (OC) contents even in the subsoil, clay-sized separates having the highest OC concentrations. Under pine, a thick forest floor was built up. CuO oxidation data indicated low transformation of lignin in the forest floor, but advanced oxidative decomposition in the mineral soil horizon. In contrast, non-cellulosic carbohydrates, appeared to be stabilized in the mineral soil horizon against mineralization. Humic acids extracted from the mineral soil horizons showed an extremely high aromaticity. We assume that this was due to the production of pyrogenic aromatic moieties (black carbon) as a result of frequent fires in this ecosystem. No clear profile differentiation with respect to SOM quality was obtained. Composition of SOM in the mineral soil appeared not yet influenced from land use. Kennzeichnung der organischen Bodensubstanz argentinischer Schwarzerden Gegenstand dieser Studie war die Kennzeichnung der Verteilung und der Transformationsprozesse der organischen Bodensubstanz argentinischer Schwarzerden (Hapludolls) unter Acker und unter Pinus radiata mittels Korngrosenfraktionierung, naschemischer Analyseverfahren und 13C-NMR-Spektroskopie. Die Boden waren durch machtige Axh-Horizonte und hohe Gehalte an organischem Kohlenstoff (OC) auch im Unterboden charakterisiert, wobei Partikel der Tonfraktion die hochsten Konzentrationen an OC aufwiesen. Unter Kiefer kam es zur Ausbildung einer rohhumusartigen Auflage. Lignin war in den Auflagehorizonten nur schwach abgebaut, erfuhr jedoch einen intensiven Abbau im Mineralboden. Im Gegensatz hierzu erschienen nicht-cellulosische Kohlenhydrate im Mineralboden gegenuber Biodegradation stabilisiert. Die aus den Mineralbodenhorizonten extrahierten Huminsauren zeichneten sich durch eine extrem hohe Aromatizitat aus. Diese ist vermutlich das Ergebnis der Produktion signifikanter Mengen pyrogenen Kohlenstoffs durch die verbreiteten Feuer in diesem Okosystem. Es konnte keine ausreichende Profildifferenzierung mittels Charakterisierung der organischen Bodensubstanz nachgezeichent werden. Die unterschiedliche Landnutzung weist bisher keinen Einflus auf die strukturchemische Zusammensetzung der organischen Substanz im Mineralboden auf.

A simple method was developed for estimating the carbon concentration of fulvic acid from soil extracts at pH 2. Visible absorbance (410 nm) was measured in 20 samples of fulvic acids from different regions of Argentina. Linear regression analysis of absorbance as a function of carbon concentration yielded a high correlation coefficient (r = 0.983∗∗). The proposed method is expected to be useful in assessing changes in organic matter content as a result of different tillage systems, erosion, plant residues, etc.

Though it is commonly held that sodium (Na) rather than potassium (K) salts affect soil properties, in some treatmets the opposite has been found to be the case. The present paper describes soil trials carried out to determine changes in organic matter and colloidal content, pH, cation exchange capacity, and exchangeable cations as a consequence of leachings with Na and K salts. It was possible to classify salts into weakly and strongly alkaline according to their effect on soil properties. On the basis of the effect of K on soil properties, it is concluded that this salts should be included in soil salinity classifications.

The chemisorption of phosphate on aluminium hydroxide (bayerite) and its kinetics were studied. A rate equation was derived on the basis that each phosphate molecule replaces a surface hydroxyl group and binds directly to an aluminium atom. The rate constants were evaluated and the free energy change of the sorption reaction was calculated as 20.4 KJ/mol. Phosphat-Sorption durch Bayerit Die Chemosorption von Phosphat an Bayerit wurde einschlieslich ihrer Kinetik untersucht. Eine kinetische Gleichung wurde auf der Grundlage abgeleitet, das jedes Phosphatmolekul eine Oberflachen-Hydroxylgruppe ersetzt und direkt an ein Aluminium-Atom gebunden wird. Die Geschwindigkeits-Konstanten wurden bestimmt und die freie Energie der Sorptionsreaktion wurde zu 20,4 KJ/mol berechnet.

Water evaporation measurements in small soil columns saturated with different salt solutions show that the surface salty crusts affect the water content as well as the water movement in the soils in the order: NaCl

The cation exchange capacity (CEC) and specific surface properties were investigated in four particle-size fractions