Volumen 22, Número 2 (Diciembre,2004)

Soil test interpretation in order to estimate P fertilizer requirements in wheat under no-tillage (SD) is based on the integration of the analytic result of available P Bray1 (Pd) and crop P requirements. The concentration of Pd depends on the position tubes during the extraction phase. There are, however, no published reports about their influence in the diagnosis of the fertilization requirements. The objec- tives of this work were to compare the effect of the position of the tubes during the analytic Pd determination in: I) the quantity of Pd obtained, and II) the relationship between response of wheat and Pd at sowing. The study was conduced in 17 farm fields with wheat ( Triticum aestivum L.) under SD in the southeast of Buenos Aires province. Pd was determined in samples taken to a soil depth of 20 cm and segmented into 0-5, 5-10 and 10-20 cm and using the tubes in horizontal or vertical position during extraction phase. Wheat yield response was expressed as relative yield (RR) and related to Pd by using linear-plateau model , for the depths of 0-5, 0-10 and 0-20 cm. Pd obtained using different tubes positions were positively correlated (r=0,85 , p=0,0001), but more Pd was extracted by the horizontal position than by the vertical placement (27,5 and 15,0 mg P kg-1; respectively). Soil samples taken from less than 20 cm deep did not improve the R2 values in the model used. Although the use of both positions tubes resulted in similar R2 values when 0-10 or 0-20 soil sampling depth was used (R2: 0,55-0,58 and 0,67-0,70 for 0-10 and 0-20 cm, respectively), the regression lines parameters were different and, as consequence, the critical value of Pd was higher when the tubes were placed horizontally.

This paper focused on the effect the of years under agriculture and soil moisture on soil nitrogen potential mineralization in a Typic Haplustoll located at Córdoba (Argentina). Plots of 2, 20 and 50 years under agriculture were selected and a cuasi-pristine soil was used as control. Samples from each layer were incubated at 30 °C for 32 weeks to quantify nitrogen mineralization in optimal environmental conditions. The relationship between N mineralization and water content was found by measuring N mineralized (Nmin) during 14 days at -10, -30, -100, -200, -500, -1000 y -1500 kPa of soil water potential. A simple exponential model (first order kinetics) was the one which best described “in vitro” N mineralization. Potentially mineralizable nitrogen (No) from the A horizons progressively decrease withyearsoffarmingfrom500to258mgkg-1.ItwasshownthatN theinitialmineralizationrate of the studied soil indicate adequately soil degradation induced by years of continuous farming. Under these experimental conditions Nmin was linearly associated to log of soil water potential in the solum. It may be inferred that the relation Nmin versus soil water content can be achieved using normalized variables. The interactive effect of mineralizable substrate with soil moisture (W) expressed in relative terms, on Nmin can be explained in a 90 % by the equation Nmin = a + bNo + c(W/Wmax). Besides, the mineralized N can be estimated from the expression Nmin/Nt = 0,022 – 0,49 (W/W )-1 which is max based on standard soil properties (Nt and W) and brings useful information for soil fertility management and input data for soil nitrogen budget simulations.

An investigation was conducted to study the relationships between soil organic carbon and those properties that may influence carbon accumulation in soils derived from volcanic ash through a range of bio-climatic conditions at the Andinopatagonian region in SW Neuquén, Argentina. Pedon and climatic data of nineteen plots were used and grouped, through principal component analysis, by soil hydric regime –udic group (U) and xeric group (X)-. Within each group, a simple and a multiple regression analysis was carried out between organic C -content (t ha-1) or concentration (g kg-1)- and mean annual precipitation, bulk density, pH(H2O) and pH(NaF), oxalate extractable Al (Alo), pyro- phosphate-extractable Al (Alp), Alp/Alo, and Alo-Alp –the last four so as content or as concentra- tion. Mean annual precipitation, explained little variation in soil organic carbon in both groups (R2 = 20), and bulk density was unrelated to soil organic carbon. Alp correlated strongly with organic C content in both groups (U: R2 = 0.67, p menor que 0.00001; X: R2 = 0.90, p menor que 0.00001), but when expressed as concentrationAlpexplanationwasconsiderablylower(U:R2 =0.10,pmenor que 0.02;X:R2 =0.46,p menor que 0.0006). When all factors were combined in a multiple regression analysis, the combination of Alp contents and mean annual precipitation, explained the greatest amount of variation in soil organic C content within U group (R2u= 0,69 p menor que 0,00001. According with previous studies relationships between Alp/Alo or (Alp-Alo) and soil organic C were very poor, this confirms that organically bound Al in allophane is not dominant. The fact that the better explanations were found when variables were expressed in content rather than in concentration was probably due to the influence of specific area on humic matter adsorption on volcanic ash soils. Our results suggested that in volcanic ash soils in SW Neuquén, Argentina, the chemical stabilization is the key process controlling soil organic C accumu- lation in some way independent of vegetation types.

Fertilization at sowing or tillering is used to improve the efficiency of fertilizer nitrogen. The 15N enriched fertilizers help to accurately follow the fate of N from fertilizers and to find out the utilisa- tion efficiency by plants. A field experiment was conducted to investigate the dynamic of urea applied (labelled15N-urea) at sowing or at tillering to a wheat crop. Aboveground plant N and organic and mineral soil N were analyzed on tillering, stem extension, flowering and physiological maturity. The treatments included 80 kg ha-1 urea-N at sowing (80S), 80 kg ha-1 urea-N at tillering (80T) and no N (0N). Grain production was 2130, 3740 and 3970 kg ha-1 for 0N, 80S and 80T treatments, respec- tively. There were significant differences between fertilized and 0N treatments (p menor que 0.001). The pro- portion of plant-N derived from fertilizer had a decreasing trend toward the end of the growing cycle while total N did not decline, suggesting that plants lost labeled N from aerial parts and they simulta- neously took non labeled N from the soil. High NO- -N accumulation in soil was not found after fertilizations probably due to a fast uptake by plants sustained by high water contents at these samplings time. There was an important N immobilization from fertilizer N in the organic pool for 80S (30 kg Ndff ha-1) and 80T (43 kg Ndff ha-1) treatments and a subsequent drop, in synchrony with the take-off in the plant-N accumulation rate.

Most of the wheat crops are followed by double cropped soybean in the northern Pampas of Argentina. Low mobility nutrients applied in wheat could leave residual effects for the double cropped soybean. Some studies show soybean responses to residual phosphorus (P) or sulphur (S) when they are applied at rates in excess to wheat requirements, but there is no evidence showing if these effects on yield are the same than applying the nutrient level needed for each crop in the sequence and how nutrient use efficiency is affected. The objectives of this work were to: i) Evaluate the relationship between extractable P and sulphate-S content in soil and soybean yield response, ii) Evaluate P and S residual effects applied in wheat on soybean yield and iii) Compare response level between residual and fresh application of these nutrients on soybean yield and on nutrient use efficiency. Seven wheat/ soybean experiments were conducted in two seasons (2001/02 and 2002/03) under no-tillage in the northern pampas with soybean as previous crop. Treatments were established to analyze the effect of P and S application strategies: i) Fertilization (P and S) based on the demand of a wheat/soybean sequence (wheat plus soybean requirement) applied when wheat was planted, and ii) The same fertilization level but applied in each crop based on the demand of each crop. Soils had low content of organic carbon and Bray P (below 15 mg kg-1). P * S interaction was not significant in all the experiments. Wheat yields were low because of fungal diseases, and there was no response to S application. Soybean yield increased 15% and 17% in average because of P and S fertilization, respectively, and there were no differences in soybean yield and in nutrient use efficiency as a consequence of fertilizer application strategy. These results contribute to making P or S recommenda- tions in double cropped soybean depending on operative or economic convenience, under the condi- tions in which the experiments were performed.

The aim of this work was to prove a methodology to evaluate water erosion risk in La Colacha basin in the Southwest of Cordoba Province, Argentina. The susceptibility to soil erosion was estab- lished through the evaluation of parameters which are involved in water erosion such as geomorphologic, lithologic, previous erosion and soil parameters. In this area, changes in land use have produced an increment in water erosion processes due to the alteration of physical, chemical and biochemical soil conditions. This led us to consider these changes as the hazard. Erosion risk was considered as the product between susceptibility and hazard. Susceptibility, hazard and risk maps were obtained through a GIS. The results showed that La Colacha basin presented low water erosion risk in most of its area. Units with slopes lower than 1% under a beef cattle production system did not present water erosion risk. Areas with slopes higher than 3% under a conventional tillage crop production system showed a moderately water erosion risk. Low water erosion risk classes corresponded to sites with intermediate slopes. This methodology permits to obtain a rapid assessment of water erosion risk and can be used in a medium scale territorial and environmental planification. The method can be applied in other environments through assigning indexes according with the characteristics of the site and with a right evaluation of land use pressure. The advantage of this methodology is that it may be easily applied using only a soil map.

The aim of this work was to evaluate the influence of soil management on soil mite densities in agroecosystems of La Colacha basin (Córdoba, Argentina). Soil Acari community in four systems with different soil disturbance degree was studied. Soil samples were taken bimonthly, from August 1999 to June 2000, in an undisturbed natural system (NA), in a beef cattle production system (GA), in a rotational plot (MI) and in a conventional tillage crop production system (AG). Six samples by system were taken in each sampling date for mite extraction. Additionally, two composite soil samples were taken at random in each system to evaluate chemical, physical, and physicochemical soil prop- erties. Differences in soil moisture, temperature and organic matter content were observed between the two more intensively managed systems and the others. In one sampling date, mite density response agreed in part with the expected pattern of lower density in intensive managed systems: NA and CA plots showed the highest values, followed by MI and AG, but differences only were statisti- cally significant between NA and CA with MI. In two months, the highest density was found in the GA plot and in other two months, GA showed higher density than NA and MI. Although not definitive proof of the hypothesis, these data are consistent with the suggestions of others that intensive management systems tend to reduce soil mites density. However, the diminution of soil mites density as a result of intensive soil management could not be verified in all samplings. Some authors suggested that the effect of tillage on mites is different for different taxa. This can be the explanation of the variable response of overall Acari density to soil cultivation practices showed in this work.

The horizontal and vertical movement of water and particles in the upper 6 cm of a Vertic Argiudoll under continuous no tillage since 1999 was analyzed. For this purpose undisturbed soil cylindrical macro samples were taken from the field and used as runoff plots in the laboratory. Treatments consisted in bare and covered plots using soybean crop residues as surface cover. After the simulated rainfall application, 4 cylinders were extracted from each runoff plot for measuring satu- rated hydraulic conductivity. The coefficient of variation of this soil property for each plot varied between 36% and 204% indicating the existence of preferential flow paths. Runoff rates differed between treatments with rainfalls of more than 425 J m -2 . During the second rain total infiltration, final infiltration rate and saturated hydraulic conductivity were significantly higher for the covered treat- ment compared to the bare treatment. On the other hand, the bare treatment showed higher runoff rate and horizontal and vertical particle movement (P menor que 0,05) compared to the covered treatment. These results show that the bare treatment under no tillage can suffer significant soil physical degradation due to falling drop impact, although it has high aggregate stability and moderate organic carbon content. Particles moving downwards can obstruct water conducting macropores, reducing the satu- rated hydraulic conductivity and hence the infiltration rate.

The present report analyzed the sheet erosion process in soils located at the stream borders of the Rolling Pampa compared to soils of higher landscape positions. The sodic soils of stream borders are scarcely studied but highly erodible and can potentially pollute the permanent waterways in this region. The present experiment consisted in the simulation of sheet erosion in the laboratory employing runoff microplots with bare or surface covered soils belonging to the Matanza-Riachuelo basin. Surface cover was used to reduce drop kinetic energy and disturbed soil samples of sodic and non sodic soils were mixed with vermicompost or diammonium phosphate fertilizer. Results showed that surface cover was a management alternative for effectively controlling sheet erosion for both soils by reducing drop kinetic energy impact. The use of fertilizer did not promote any short term erosion control. The use of organic amendments reduced 60% the erosion in the sodic soil and 30% in the non sodic soil. These significant differences in the erosion control relative to the use of vermicompost might be ascribed to physical, chemical and biological processes developed in a short period of time (“aging”) in each of the studied soils related to changes in pH, EC and ESP.