Soil Physical Quality of Brazilian Crop Management Systems Evaluated with Aid of Penetrometer

Crop management affects soil attributes as well as its quality. We evaluated the following soil physical attributes: saturated hydraulic conductivity (K0), soil resistance (RP) and soil bulk density (BD), in Araras-SP, Brazil. Areas with sugarcane (Saccharum officinarum), soybean (Glycine max), physic nut (Jatropha curcas L.) and native forest presented an increase of soil compaction in the 0.10 m surface layer for the three attributes in a following order: native forest <physic nut < soybean < sugarcane. Significant regressions were obtained for RP × K0; BD × K0 and BD × RP. Penetrometer measurements were essential to indicate differences among areassugarcane, native forest, physic nut and soybean; but for the measurements of K0, only between sugarcane and native forest. RP measurements confirm anthropogenic changes in the soil profile up to the 0.3 m depth. In the “Canarache soil resistance classification” soils showed “low resistance” “without limitations to root development” for native forest and physic nut; “medium resistance” for soybean area with “some limitations to root development” and “high resistance” for sugarcane with “limitations to root development”. The use of penetrometers is discussed in relation to the readiness of field measurements.


Introduction
Intensive crop use can lead to soil degradation also affecting soil physical attributes.One of the soil parameters used as an indicative of physical soil quality is the saturated hydraulic conductivity (K 0 ).Considering that water and air flow better in pores of larger diameter, the parameters K 0 and the macroporosity (MP) can be taken as indicators of soil compaction.External pressures applied to soils, coming from management practices reduce preferentially larger pores.In this way, low values of MP imply low values of K 0 , low soil aeration and an increase of soil resistance to penetration (RP) of roots (Stolf, Thurler, Bacchi, & Reichardt, 2011;Mollinedo et al., 2015).Additionally to the importance of K 0 in the evaluation of soil compaction, RP has also been correlated with other soil physical parameters (Marques, Texeira, Reis, Junior, & Martins, 2008;Ramos et al., 2011;Mollinedo et al., 2016).K 0 is also essential in the description of water movement in soils, like the processes of infiltration, drainage, fertilizer losses, erosion, and leaching of chemicals (Warrick, 2002;Mesquita & Moraes, 2004).RP for the evaluation of the compaction status of a soil is one of the most employed practices because measurements are carried out directly in the field, without the need of a laboratory backup.Measurements are fast, approximately 1 min to sample a profile down to 0.50 m (Stolf, Murakami, Maniero, Silva, & Soares, 2012).Therefore, a large number of studies are found in the literature comparing native forest, orchards and other agro-forestry systems of low machinery impact, with agricultural systems (Martins et al., 2010;Portugal et al., 2010;Cardoso, Silva, Cury, Ferreira, & França, 2011;Iarema, Fonte, Fernandes, Shaefer, & Pereira, 2011;Ramos et al., 2011;Silva et al., 2011).Penetrometers have also been used in pastures (Ramos et al., 2010;Silva Filho, Cottas, & Marini, 2010;Cardoso et al., 2011;Castagnara et al., 2012;Moura, Marasca, Meneses, Pires, & Medeiros, 2012) and in crops with intense traffic of machinery as in areas of sugarcane and soybean (Machado et al., 2010;Debiasi & Franchini, 2012;Ecco, Carvalho, & Ferrari, 2012;Silva, Nunes, Caldeira, Arantes, & Souza, 2012).
In this study we look for relations between K 0 and RP in an area submitted to different forms of management, including a native forest which almost does not present environmental impacts.Four different scenarios were chosen: sugarcane production area (Saccharum officinarum); soybean production area (Glycine max), physic nut or jatrhopha production area (Jatrhopha curcas L.); and a native forest area.

Material and methods
The experimental sites used for the evaluation of K 0 , BD and RP are located in Araras, SP, Brazil, 22 o 18′52″S and 47 o 23′01″W, 560 m above sea level.The mean annual air temperature is 21.4 ºC and the annual rainfall is 1428 mm.According to the classification of Köppen adapted for Brazil, the local climate is of the type Cwa, mesothermic, with humid and warm summers and dry winters.Characteristics of the experimental areas are presented in Table 1.Note. a US taxonomy classification.
The farm presents four typical areas managed in different ways:  Native forest, with a seasonally semi-decidual cover, naturally growing on a clayey soil for very long time.
 Physic nut, an area cultivated for 16 years with Jatrhopha curcas L. at a spacing of 4 between rows and 2 m between plants in the row (Coelho & Gerald, 2013).Samplings were made in the plant row which was never exposed to machinery traffic.


Soybean, an area cultivated for one year with Glycine max L. at a spacing between rows of 0.45 m.The soybean crop is not truly soybean because it was rotated with sugarcane and the previous sugarcane crop had five harvests performed with heavy mechanized harvesters.For the establishment of the last soybean crop, the sugarcane rhizome was destroyed with two heavy diskings (disc of 0.81 m diameter), and the soil prepared with an intermediate harrow (disc of 0.71 m diameter) and one levelling disking (disc of 0.51 m diameter). Sugarcane, area cultivated for 20 years with Saccharum officinarum L. at spacing between rows of 1.4m.The crop was grown in cycles of five years and had five harvests performed with heavy mechanized harvesters.For the establishment of each new crop the previous sugarcane ratoon was destroyed with two heavy disks (harrow disc 0.81 m diameter) and the soil prepared with an intermediate disk (0.71 m) and one leveling disking (harrow disc 0.51 diameter).Samplings were made in ratoon after second harvest For K 0 and BD measurements, non-disturbed samples were extracted (February, 2013) from each plot with metallic cylinders of 0.070 m diameter and 0.072 m height, in the 0-0.1 m layer, with eight replicates.The constant head methodology was employed for obtaining K 0 (EMBRAPA, 1979;Reynolds et al., 2002).Soil samples were dried at 105 ºC during 24 h in the stove and weighted for obtaining soil BD.Penetrometer measurements were made close to K 0 and BD evaluations at eight sampling points, not only for 0-0.10 m, but for the 0-0.50 m soil layer, in 0.05 m increments.For measurements of RP the impact penetrometer described in Stolf, Fernandes and Furlani Neto (1983) was used as schematically shown in Figure 1.Field data were transformed into MPa according to (Stolf, 1991), using the software described in Stolf, Murakami, Brugnaro, Silva and Margarido (2014).Measurements consisted of eight replicates, and for the four treatments (32 profiles) result 320 RP data points.The speed of the penetrometer measurements, dispensing laboratory aid, combined with a computer program (Stolf et al., 2014), were some of the reasons for the adoption of this methodology.These instruments have been very useful in RP mapping, compaction and traffic control, soil spatial variability studies, and also as an indicator of soil quality (Stolf et al., 2012).

Results
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Table 1 .
Characteristics of the experimental areas