Effect of crusting on the physical and hydraulic properties of a soil cropped with Castor beans (Ricinus communis L.) in the northeastern region of Brazil
Introduction
An important characteristic of soils in many tropical and subtropical areas is the presence of surface seals or crusts (Casenave and Valentin, 1989, Valentin and Bresson, 1992). Soil surface seals or crusts are thin layers characterized by greater density, higher shear strength, finer pores, and lower saturated hydraulic conductivity than the underlying soil (Assouline, 2004, Lado et al., 2005). They result from complex and dynamic processes where the soil particles are rearranged and then consolidated into a cohesive superficial structure, whose thickness varies from 0.1 to 50 mm (Valentin and Bresson, 1992).
Surface crusting is an important factor in land degradation, as they can lead to decreased infiltration rates and lower air permeability values (Le Bissonnais, 1990), increasing runoff and, thereby, accelerate sheet and rill erosion (Ries and Hirt, 2008). They can also substantially increase mechanical and hydraulic resistances, leading to several adverse effects (Bedaiwy, 2008); this resistance impedes seedling emergence (Panayiotopoulos et al., 1994, Beemster and Masle, 1996) and root penetration, and ultimately reduces crop yields (Martino and Shaykewich, 1994, Costantini et al., 1996, Samson et al., 1996).
The formation of surface crusts has been a prevalent problem in crop establishment and production in many tropical regions of the world (Wakindiki, 2002, Ndiaye et al., 2005, Bedaiwy, 2008, Materechera, 2009). This is aggravated by the heavy and intensive rainfall typical of these regions (Mbuvi et al., 1995, Ndiaye et al., 2005, Bedaiwy, 2008). Like many semi-arid regions in the world, the occurrence of soil crusting is of great concern, and has been used as an indicator of soil degradation of agricultural and natural land in northeast of Brazil (Galindo et al., 2008).
Studies on soil surface sealing and crusting have been conducted since the early of the 20th century, and the interest on both phenomena, not surprisingly, continues due their important consequences for the environment (e.g. desertification) and economical crop production. During recent years, several studies have been conducted in order to develop remote non-destructive thermography techniques to distinguish between different surface crusts under laboratory scale conditions (e.g. Soliman et al., 2010), and under field conditions to establish spectral response and remotely sensed mapping of soil surface crusts (e.g. de Jong et al., 2011), to propose new in situ methods to determine the hydraulic resistance (Touma et al., 2011), as well as to assess the influence of crust formation under natural rain on physical attributes of soils with different textures (Castilho et al., 2011). However, despite the importance of soil surface sealing and crusting on agricultural land, few studies consider the consequence of soil surface crusting on crop productivity.
The aim of the present study was to assess, under field and natural rainfall conditions, the effect of soil surface crusting on the mechanical and hydrodynamic properties of a cropped soil, and the consequences on infiltration rate and the productivity of castor beans in an experimental field located in the northeastern region of Brazil. Castor bean cultivation in this region is currently being promoted, by the Brazilian government, for the production of biodiesel. However, because plant spacings are greater than in the traditional corn-bean intercropping systems, the soils become more prone to crusting.
Section snippets
Experimental site
This experiment was performed, in 2007, on a field that had been cropped with castor beans since 2005. During infield tests to measure infiltration rates, it was observed that the soils were especially susceptible to crusting. The area is located on a farm in the municipality of Areia (6°58′12″ S and 35°42′15″ W), state of Paraíba, in northeastern Brazil, which belongs to the Center for Agrarian Sciences of the Federal University of Paraíba (CSA/UFPB). Local topography is smooth to slightly
Results and discussion
Average values of λm and Cλm, and their 90% confidence intervals, are presented in Fig. 2. The characteristic mean pore radius values (Eq. (5)) were not significantly different between the two soil conditions. It was observed that the 90% confidence interval for mean values of λm were between 0.11 and 0.18 mm in the non-crusted surfaces, and between 0.12 and 0.22 mm, in the crusted surfaces. These values are in agreement with the values mentioned in other works (Angulo-Jaramillo et al., 1996,
Conclusions
The yield of castor bean was found to be higher in the non-crusted soils (1.53 t ha−1) than in the crusted ones (0.62 t ha−1). Its negative correlation with the soil shear resistance (τ) and the positive correlation with hydraulic conductivity (Ks) indicate that mechanically and hydraulic resistant soils are less productive.
No clear significant relationship was found between yield and the values of Cλm, λm, ρd, θ0 and θs. However, highly significant relationships between soil shear strength,
Acknowledgements
This research was financially supported by FACEPE (process APQ – 1178-3.01/10 – PRONEN), CAPES (process no. 677/10 – CAPES/COFECUB) and CNPq (processes nos 305727/2009-5 and 482250/2010-0) which are gratefully acknowledged.
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