poster - International Conference of Agricultural Engineering

the soil mechanical resistance to penetration, with these variables applying it to a horticultural soil in the
Mediterranean area.
Several laboratory and field studies have been linked empirically, adjusting multiple SR regression
functions (Busscher, 1990, da Silva et al. 1994 and 1997, Smith et al. 1997) to texture, porosity (Ф), organic
matter content (Spivey et al., 1986), water content (θ) (Ayers and Perumperal, 1982, Busscher et al, 1997
and Lehrsch et al, 1982), bulk density (Db) (Taylor and Gardner, 1963, Camp and Lund, 1968; Mirreh and
Ketcheson, 1972) and matric potential (ψ) and calcium carbonate content (Poch and Verplancke, 1997).
(Busscher, 1990) used up to ten different adjustments to estimate SR as a dependant variable of matric
potential and bulk density. Sample soils sieved to 2 mm, with textures ranging from sandy to sandy loam.
The function that best fit had the following structure: SR = a (ψ b ) • (D b ) c where ψ is the absolute magnitude
of the matric potential, and a, b, c are constant adjustments.
Da Silva and Kay (1997) also, obtained a fit function for SR based on the volumetric water content θ (SR
= c θ d • (D b ) e ) from samples with clay contents between 6% and 37% and organic matter between 9 and 39
g / kg. Coefficients d and e depend on the content of clay and organic matter in soil sampled. They
concluded that compaction has a greater impact on SR than it will on bulk density levels.
The objective of this study is the indirect evaluation of SR as a determinant of root growth and crop
development (Campbell et al., 1988, Cassel and Nelson, 1979; Grecu et al. 1988; Perumperal, 1987, Sojka
et al., 1991) and movement of water and air (Rivas et al., 1998).
2. Materials and methods
2.1 Soil studied
The study was carried out from soil samples collected in a horticultural farm in Eastern Spain (Liria,
Valencia), altitude 190 m. In this farm, a sprinkler with fixed installation is used as an irrigation system and
cropping intensity allows two annual crops in rotation with conventional tilled practices. There is a high level
of mechanization in agricultural tasks, in addition to the presence of large harvesters and crop transport
vehicles which have been stood for the last 20 years. This area has a semiarid Mediterranean climate type
with dry summers and rainy autumns and winters. The annual rainfall historical average is of 398 mm with a
evapotranspiration potential of 1091 mm (Montheih Pennman), rain incidence is higher in autumn season.
The soil is characterized as silty loam Haplic Calcisol (Word Reference Base, FAO 2003) with high
calcium carbonate content. The organic matter content is low and decreases with depth (Table 1).
Table 1. Soil properties (mean values of eight samples)
Particle size distribution:
Sand (50 -2000 μm), % 53.34
Silt (2-50 μm), % 24.71
Clay (< 2 μm), % 21.94
Chemical properties:
Organic carbon content, g kg -1 8.13
CaCO3 , g kg -1 615.8
pH (water, 1:2.5) 8.4
Electrical Conductivity ( 1:5) (mmhos cm -1 ) 0.2
2.2 Samples preparation
Ten samples (15 kg/each) were taken from random locations in the farm by digging to a depth of 30 cm
and mixing the extracted material. Samples collected from the field were taken to the laboratory. Coarse
elements such as stones or crop residues were manually separated from soil. The remaining material was
sieved with a 4 mm mesh to obtain a fine working soil for laboratory. Soil was dried in an oven at 105°C
until a moisture value near zero (Figure 1). Subsequently it was wetted up to a 15% of RH. This RH value
showed in a previous experience that achieves good levels of compaction. Wetting was conducting on dry
soil by using a spray. In order to get a uniform distribution of moisture, soil was spread in thin layers over
large trays. To warrant control weights of soil and water inputs a high precision digital scale was used
(Figure 2). The uniform water distribution process is completed by gentle stirring with a spatula (Figure 3).