Martín Díaz-Zorita

Disruptive methods for assessing soil structure

The description and quantification of soil structure is very important because of the many agronomic and environmental processes related to the arrangement of secondary soil units (aggregates, peds or clods) and their stability. The purpose of this review is to present and discuss methods and indices used to characterize soil structure based on the size distribution and stability of fragments produced by breaking apart the soil matrix. The size of fragments is inversely related to the mechanical stress applied. Thus, the selection of an appropriate fragmentation procedure is critical if information on soil structure is to be recovered, and often depends upon the soil process of interest. Soil fragmentation starts at sampling in the field and continues during laboratory separation of soil units by sieving. It is useful to characterize the fragment mass-size distribution with parameters from a model, such as the log-normal distribution function. Fractal theory provides a physically based link between the size distribution and stability of fragments. Structural stability is based on the ratio of fragment mass-sizes measured before and after low and high mechanical stresses, respectively. Thus, an adequate description of the applied stress conditions is essential for the parameterization of structural stability as well as the fragment mass-size distribution.

Soil organic carbon recovery by the Walkley‐Black method in a typic hapludoll

Abstract Organic carbon (C) levels in a Typic Hapludoll under four management systems [moldboard plowing, chisel plowing, no‐tillage, and perennial fescue (Festuca arundinacea) pasture] were determined by two methodologies: wet oxidation with potassium dichromate (K2Cr2O7) and dry combustion with a carbon analyzer. The correlation between organic C levels obtained by both methodologies (r=0.87, n=48, p<0.01) rose when agricultural and pasture systems were considered separately. Carbon recovered by Walkley‐Black method under pasture was 15% lower than that under the agricultural systems. This difference was attributed to greater amounts of chemical stable organic compounds in pasture residues than in crop residues.

Safflower productivity as related to soil water storage and management practices in semiarid regions

In semiarid regions the availability of water has an important influence on dryland crop productivity. Water availability is closely related to soil organic matter (SOM) content, texture and soil thickness. Safflower (Carthamus tinctorius L.) is an important winter crop in some semiarid regions due to its deep roots and drought tolerance. However, its adaptation to different soil conditions is still not well known. The objective of this study was the evaluation of safflower productivity, in relation to soil properties, across 30 grower fields of the semiarid Pampas region of Argentina. The soils were Entic and Aridic Haplustolls under continuous row-cropping (CC) or pasture–row crop rotations (PC). Grain yields varied between 0 and 1600 kg ha−1, and were positively correlated with the maximum soil water retention (SWR) of the top layer (0–20 cm) and the soil use management (SUM, r=0.93, p<0.01). Oil yield and plant growth was also positively correlated with SWR and SUM. This trend was explained on the basis of better water and nutrient supply to the plants in finer textured soils during the pronounced moisture deficiency that occurred at the flowering stage of the crop. In soils with similar textures, safflower production was highest in sites that included PC, had high SOM levels, and in which indurate sub-surface layers were absent. Highly productive dryland safflower crops in coarse textured soils from semiarid regions can be achieved by using cultural practices that increase SOM levels (pasture–arable crop rotation) and loosen compacted layers.

CORN AND SOYBEAN RESIDUE COVERS EFFECTS ON WHEAT PRODUCTIVITY UNDER NO-TILLAGE PRACTICES

Wheat (Triticum aestivumL.) grain yields under no-till production systems have been shown to be reduced in the presence of maize (Zea maysL.) residues. It has been suggested that sowing a greater density of wheat seeds or removing maize residues from the planting rows contributes to avoid this problem. However, the causal factors and the mechanism that produce reductions in wheat yields are no clearly defined. Our objective was to determine the effects of different volumes of maize or soybean [Glycine max(L.) Merrill] residues on no-tillage wheat establishment and production under field conditions on a Typic Hapludoll from the Pampas region of Argentina. The study was performed during the 2002, the 2003 and the 2004 growing seasons. Two treatments [residue volume (0, 4, 8 and 16 Mg ha-1) and crop residue (maize and soybean)] were imposed after sowing wheat at low and high plant densities, 301 and 396 seed ha-1, respectively). The previous crop was sunflower (Helianthus annusL.) and the residues were applied on the soil surface immediately after planting and fertilizing with 125 kg ha-1of Nitrogen. Independently of the quality of the residues and the sowing density, wheat plants m-2, spikes m-2and grain yields ha-1decreased when residue volume increased. In general, lower soil temperatures values and variability were observed when increasing the volume of residues. The presence of large amounts of maize or soybean residues causes the reduction in no-tillage wheat productivity (plant stand and numbers of spikes). However, only maize residues causes significant reductions in grain yields, independently of the seeding rate. The absence of significant differences in soil temperature measurements between residues allows us to think that the effects on surface soil temperature are not the main factor explaining the reduction in wheat grain yields in the presence of maize residues. Increasing the seeding rate can contribute to ameliorate the grain yield reduction in the presence of maize residues but further research is required for explaining the reasons for the behavior of the crop

Impact of Soil Water Content and Core Sampler Diameter at Sampling for Dry Soil Fragment‐Size Distributions

Soil conditions at sampling and the dimensions of the sample are critical factors when soil aggregation is indirectly characterized by determining the distribution of soil fragments. Our objective was to determine the effects of gravimetric soil water content and core sampler diameter (16, 54, and 84 mm) at sampling on the dry‐fragment‐size distribution of two soils (Typic Paleudalf and Typic Hapludalf) under undisturbed Festuca arundinacea L. sod and recently rototilled treatments. The 16‐mm core diameter sampler compressed the rototilled soil, and it was not appropriate for soil aggregation studies. The fragmentation of samples taken with core diameters greater than 54 mm decreased with decreasing soil water content. A greater probability of discriminating between undisturbed and fragmented silt loam or clay loam soils was observed when sampling with a 54‐mm‐diameter probe and when the soil had a mean soil water content of 237g kg−1 or at a potential of −0.61MPa.