poster - International Conference of Agricultural Engineering

After applying equation 2 to values of soil moisture and bulk density measured, as explained in
subsection 2.3, it was checked that the expression did not adjust well to the SR values obtained. This
deviation can be found in certain soil characteristics not taken into account in equation 2. Specifically, it can
be attributed to the content of calcium carbonate. The presence of calcium carbonate during drying causes
a greater cohesion of the soil, with the consequent effect on increasing the penetration resistance. Soil
samples tested had high lime content 615 g/kg. By contrast, Silva and Kay, and others like (Ross et al.,
1991; Rasmussen, 1999; Castro-Filho et al., 2002) Brazil, (Zhang et al., 2007) who have worked on this
topic, operate in soils ranging from neutral to acidic, carbonates.
Therefore, a new function for SR was obtained by linear regression according to the expression of
equation 1. The adjustment was accomplished using software ®Statgraphics Centution XVI, coefficients
were determined following least squares criteria and the quality of the adjustment is based on the best
value of the correlation multiple coefficient R 2 . Table 2 shows the new parameters obtained. Equation 3
depicts the new function for SR.
Table 2: Multiple regression results for the soil resistance curve model.
Parameter
Standard
Parameter Estímate Error
ln c -5,11865 0,269829
d 6,20902 0,492792
e -2,14001 0,104534
*F-Ratio = 211.31, Adjusted R 2 =0.865, Standard Error of Est. =0.35,
**SR=soil resistance (MPa), θ=volumetric water content (cm 3 cm -3 ) D b = bulk density (g cm -3 ).
ln (SR) = -5,12 - 2,14• ln(θ) + 6,21• ln (Db) [3]
Table 3 shows values of the variables measured in the tests: Db, Ѳ and SR 0 ; together with the values
estimated by the functions of equation 2 and equation 3 (SR [2] and SR [3]):
Table 3. Results and estimates of SR.
Db
(g/cm 3 )
1.46
Ѳ
(cm 3 ·
cm -3 )
SR 0
(Mpa)
SR [2]
(Mpa)
SR [3]
(Mpa)
Db
(g/cm 3 )
Ѳ
(cm 3 ·
cm -3 )
SR 0
(Mpa)
SR [2]
(Mpa)
SR [3]
(Mpa)
Db
(g/cm 3 )
Ѳ
(cm 3 ·
cm -3 )
SR 0
(Mpa)
SR [2]
(Mpa)
0.33 0.25 0.53 0.66 0.35 0.14 0.27 0.35 0.31 0.17 0.13 0.15
0.31 0.31 0.64 0.76 0.28 0.27 0.53 0.56 0.24 0.25 0.28 0.25
0.25 1.1 1.25 1.21 0.23 0.61 0.92 0.82 0.18 0.7 0.69 0.47
0.23 1.44 1.57 1.42 0.22 0.88 1.13 0.95 0.16 0.8 0.96 0.59
0.21 2 2.01 1.69 0.21 1.13 1.29 1.05 1.12 0.12 1.07 2.67 1.22
1.34
0.2 2.77 2.44 1.94 0.2 1.08 1.59 1.21 0.1 1.36 3.73 1.54
0.18 3.23 3.26 2.38 0.18 1.46 2.14 1.49 0.09 2.14 5 1.89
0.17 3.88 3.79 2.65 0.16 2.43 2.94 1.86 0.08 2.27 8.58 2.77
0.15 4 5.81 3.58 0.13 2.59 4.94 2.69
0.11 3.59 10.27 4.5
SR [3]
(Mpa)
0.07 4 14.74 4.05
Figures 9, 10 and 11 show the adjustment for three levels of bulk density and compaction,
representative of the eight samples tested. In general, SR [3] fits well and values obtained are close to those
measured in the laboratory, especially for soil moistures of 18-20%. It is also observed that on soil with a
bulk density higher than 1.4 g/cm3, the maximum acceptable resistance to normal root growth (of about 3
MPa) is achieved for moisture below 15%. While, in loose soils - density of about 1.2 g/cm3-, this
resistance does not appear until the moisture content falls below 10%. Even, a very loose soil, with 1.1
g/cm3 density may be in the wilting point -6 to 8% - but never reach high values of resistance to
penetration.