Soil penetration resistance mapping quality: effect of the number of subsamples

There is no consensus in the literature regarding how many subsamples are needed to perform accurate on-farm soil penetration resistance (SPR) mapping. Therefore, the objective of this study was to define the number of subsamples per sampling point needed to quantify the SPR. The experiment was performed in a 4.7 ha area and employed a 50 × 50 m grid system (18 sampling points). The SPR was evaluated using a digital penetrometer in two different years with 1, 2, 3, 4, 5, 6, 9, 12, and 15 subsamples per sampling point. The SPR maps produced with increasing numbers of subsamples were compared to the reference maps (15 subsamples) using the relative deviation coefficient and Pearson´s linear correlation. A reduction in the number of subsamples promoted an increase in the variability of the SPR data. Generally, the results from this study suggest the use of at least four subsamples per sampling point to achieve SPR maps with a coefficient of relative deviation less than 10% (30% maximum error per point around the mean) and significant correlation with the reference maps (15 subsamples).

Keywords:
soil compaction; sampling protocol; precision agriculture; microvariability

Authorship

Osmar Henrique de Castro Pias ^**Author for correspondence. E-mail: henriquepias@yahoo.com.br

Departamento de Ciência do Solo, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 7712, 91540-000, Porto Alegre, Rio Grande do Sul, Brazil. Universidade Federal do Rio Grande do SulBrazilPorto Alegre, Rio Grande do Sul, BrazilDepartamento de Ciência do Solo, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 7712, 91540-000, Porto Alegre, Rio Grande do Sul, Brazil.

Maurício Roberto Cherubin

Escola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, Piracicaba, São Paulo, Brazil. Universidade de São PauloBrazilPiracicaba, São Paulo, BrazilEscola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, Piracicaba, São Paulo, Brazil.

Claudir José Basso

Universidade Federal de Santa Maria, Frederico Westphalen, Rio grande do Sul, Brazil. Universidade Federal de Santa MariaBrazilFrederico Westphalen, Rio grande do Sul, BrazilUniversidade Federal de Santa Maria, Frederico Westphalen, Rio grande do Sul, Brazil.

Antônio Luis Santi

José Paulo Molin

Cimélio Bayer

*Author for correspondence. E-mail: henriquepias@yahoo.com.br

SCIMAGO INSTITUTIONS RANKINGS

Figures | Tables | Formulas

Figures (4)
Tables (3)
Formulas (2)

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Figure 1
Schematic representation of the sampling grid (50 x 50 m) used in this study area highlighting the distribution of the 15 subsamples of soil penetration resistance in each sampling point in Palmeira das Missões (RS), southern Brazil.

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Figure 2
Number of subsamples per sampling point required to obtain soil penetration resistance (SPR) values with maximum errors of 10, 20 and 30% around the mean, for different soil layers in two years in Palmeira das Missões (RS), southern Brazil.

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Figure 3
Relative deviation coefficient (RDC%) between soil penetration resistance maps (SPR, MPa) obtained with different numbers of subsamples (1-12) per sampling point and the reference maps (15 subsamples) for different soil layers in years I (a) and II (b) in Palmeira das Missões (RS), southern Brazil.

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Figure 4
Thematic maps of soil penetration resistance (SPR) obtained by considering different numbers of subsamples (1-12) per sampling point and the reference maps (15 subsamples) for the different soil layers in two years in Palmeira das Missões (RS), southern Brazil.

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Table 1
Descriptive statistics of soil penetration resistance (SPR, MPa) in the soil profiles obtained with different numbers of subsamples (1-15) per sampling point in year I in Palmeira das Missões (RS), southern Brazil.

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Table 2
Descriptive statistics of soil penetration resistance (SPR, MPa) in the soil profiles obtained with different numbers of subsamples (1-15) per sampling point in year II in Palmeira das Missões (RS), southern Brazil.

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Table 3
Correlation between the soil penetration resistance (SPR) maps obtained with different numbers of subsamples (1-12) per sampling point and the reference maps obtained with 15 subsamples in two years in Palmeira das Missões (RS), southern Brazil.

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Figure 1 Schematic representation of the sampling grid (50 x 50 m) used in this study area highlighting the distribution of the 15 subsamples of soil penetration resistance in each sampling point in Palmeira das Missões (RS), southern Brazil.

Figure 2 Number of subsamples per sampling point required to obtain soil penetration resistance (SPR) values with maximum errors of 10, 20 and 30% around the mean, for different soil layers in two years in Palmeira das Missões (RS), southern Brazil.

Figure 3 Relative deviation coefficient (RDC%) between soil penetration resistance maps (SPR, MPa) obtained with different numbers of subsamples (1-12) per sampling point and the reference maps (15 subsamples) for different soil layers in years I (a) and II (b) in Palmeira das Missões (RS), southern Brazil.

Figure 4 Thematic maps of soil penetration resistance (SPR) obtained by considering different numbers of subsamples (1-12) per sampling point and the reference maps (15 subsamples) for the different soil layers in two years in Palmeira das Missões (RS), southern Brazil.

Table 1 Descriptive statistics of soil penetration resistance (SPR, MPa) in the soil profiles obtained with different numbers of subsamples (1-15) per sampling point in year I in Palmeira das Missões (RS), southern Brazil.

Statistical	Number of subsamples
parameters	1	2	3	4	5	6	9	12	15
0.00-0.05 (m) (year I)
Minimum	1.19	1.27	1.57	1.58	1.62	1.55	1.74	1.78	1.81
Mean	2.05	2.09	2.14	2.11	2.08	2.10	2.18	2.20	2.21
Maximum	3.36	3.06	2.83	2.76	2.69	2.79	2.87	2.84	2.78
CV (%)^§	28.66	23.19	15.05	14.18	14.70	15.08	13.43	13.07	13.60
0.05-0.10 (m) (year I)
Minimum	1.62	1.68	1.85	1.83	2.02	2.17	2.11	2.22	2.28
Mean	2.66	2.55	2.56	2.47	2.52	2.58	2.58	2.62	2.64
Maximum	3.58	3.55	3.22	3.04	2.95	2.92	2.98	3.06	3.05
CV (%)	21.56	19.42	14.25	13.96	12.08	9.91	10.15	9.64	8.69
0.10-0.15 (m) (year I)
Minimum	2.03	2.05	2.24	2.18	2.25	2.09	2.29	2.41	2.44
Mean	2.79	2.79	2.78	2.74	2.76	2.77	2.77	2.81	2.83
Maximum	4.19	3.96	3.79	3.52	3.36	3.32	3.12	3.10	3.27
CV (%)	22.06	16.31	14.78	12.85	11.12	10.93	8.12	7.23	7.07
0.15-0.20 (m) (year I)
Minimum	1.70	2.05	2.22	2.27	2.28	2.31	2.43	2.63	2.65
Mean	2.93	2.87	2.92	2.93	2.94	2.93	2.97	3.02	3.04
Maximum	4.76	3.84	3.68	3.77	3.55	3.53	3.30	3.38	3.40
CV (%)	27.86	17.44	14.95	13.51	12.67	12.05	9.25	7.85	7.48
0.20-0.25 (m) (year I)
Minimum	1.57	1.59	1.86	2.35	2.43	2.52	2.65	2.65	2.66
Mean	3.02	3.09	3.09	3.09	3.09	3.10	3.14	3.15	3.17
Maximum	4.37	4.30	3.80	3.74	3.71	3.71	3.82	3.61	3.64
CV (%)	25.36	20.38	16.19	13.05	12.14	10.9	9.23	8.29	8.47
0.25-0.30 (m) (year I)
Minimum	1.40	1.94	2.14	2.31	2.34	2.43	2.61	2.58	2.60
Mean	3.12	3.10	3.05	3.12	3.13	3.13	3.17	3.18	3.17
Maximum	4.86	4.61	3.92	4.09	3.89	3.77	3.77	3.81	3.78
CV (%)	29.04	21.77	17.34	16.2	16.04	14.27	11.38	10.01	9.96

^§
CV = coefficient of variation.

Table 2 Descriptive statistics of soil penetration resistance (SPR, MPa) in the soil profiles obtained with different numbers of subsamples (1-15) per sampling point in year II in Palmeira das Missões (RS), southern Brazil.

Statistical	Number of subsamples
parameters	1	2	3	4	5	6	9	12	15
0.00-0.05 (m) (year II)
Minimum	1.25	1.25	1.41	1.35	1.46	1.42	1.42	1.61	1.67
Mean	2.02	2.04	1.99	1.97	2.00	2.07	2.12	2.11	2.12
Maximum	3.08	3.88	2.47	2.46	2.55	2.55	2.48	2.53	2.53
CV (%)^§	22.48	22.66	16.68	15.72	15.74	15.81	14.27	13.66	12.09
0.05-0.10 (m) (year II)
Minimum	1.51	1.76	1.93	2.21	2.10	2.27	2.38	2.38	2.46
Mean	2.60	2.63	2.70	2.69	2.71	2.75	2.75	2.72	2.74
Maximum	3.65	3.46	3.29	3.15	3.29	3.25	3.13	3.05	3.03
CV (%)	24.13	20.91	15.30	11.90	12.39	10.97	7.74	7.31	5.79
0.10-0.15 (m) (year II)
Minimum	1.77	2.14	2.38	2.40	2.42	2.38	2.49	2.43	2.48
Mean	2.82	2.88	2.91	2.90	2.91	2.92	2.92	2.92	2.92
Maximum	3.93	4.20	3.49	3.47	3.53	3.46	3.30	3.22	3.17
CV (%)	22.73	18.47	11.47	11.79	10.96	9.82	8.08	7.75	6.60
0.15-0.20 (m) (year II)
Minimum	1.77	2.14	2.38	2.40	2.42	2.38	2.49	2.43	2.48
Mean	2.82	2.88	2.91	2.90	2.91	2.92	2.92	2.92	2.92
Maximum	3.93	4.20	3.49	3.47	3.53	3.46	3.30	3.22	3.17
CV (%)	16.52	11.73	8.55	7.95	8.75	8.18	8.40	7.59	7.16
0.20-0.25 (m) (year II)
Minimum	1.83	2.09	2.20	2.17	2.18	2.25	2.25	2.24	2.26
Mean	2.83	2.75	2.63	2.63	2.63	2.65	2.65	2.65	2.68
Maximum	3.80	3.38	3.07	3.06	3.11	3.08	3.05	3.11	3.18
CV (%)	16.9	12.63	11.14	10.45	11.24	10.69	9.48	9.31	8.65
0.25-0.30 (m) (year II)
Minimum	1.53	1.99	1.98	2.02	2.03	2.16	2.21	2.16	2.21
Mean	2.69	2.69	2.64	2.64	2.64	2.69	2.68	2.67	2.68
Maximum	3.68	3.53	3.39	3.42	3.67	3.53	3.29	3.28	3.19
CV (%)	21.96	17.87	16.74	15.51	15.73	13.80	10.23	10.76	9.76

^§
CV = coefficient of variation.

Table 3 Correlation between the soil penetration resistance (SPR) maps obtained with different numbers of subsamples (1-12) per sampling point and the reference maps obtained with 15 subsamples in two years in Palmeira das Missões (RS), southern Brazil.

Number of subsamples	Soil layer (m)						Mean
Number of subsamples	0.00-0.05	0.05-0.10	0.10-0.15	0.15-0.20	0.20-0.25	0.25-0.30	Mean
	-------------------------------------Year I -------------------------------
1	0.42^ns	0.63*	0.42 ^ns	0.62*	0.30 ^ns	0.43 ^ns	0.47
2	0.44 ^ns	0.49*	0.49*	0.59*	0.41 ^ns	0.43 ^ns	0.49
3	0.46 ^ns	0.58*	0.58*	0.65*	0.54*	0.68*	0.58
4	0.46 ^ns	0.70*	0.64*	0.68*	0.67*	0.72*	0.65
5	0.44 ^ns	0.74*	0.77*	0.79*	0.78*	0.81*	0.72
6	0.61*	0.80*	0.75*	0.85*	0.86*	0.89*	0.79
9	0.87*	0.95*	0.86*	0.94*	0.92*	0.96*	0.92
12	0.95*	0.95*	0.93*	0.96*	0.97*	0.98*	0.96
	------------------------------------Year II -------------------------------
1	0.16 ^ns	0.16 ^ns	0.26 ^ns	0.46 ^ns	0.16 ^ns	0.46 ^ns	0.29
2	0.28 ^ns	0.17 ^ns	0.30 ^ns	0.42 ^ns	0.43 ^ns	0.37 ^ns	0.33
3	0.45 ^ns	0.39 ^ns	0.45 ^ns	0.50*	0.51*	0.54*	0.47
4	0.64*	0.51*	0.52*	0.65*	0.63*	0.63*	0.60
5	0.69*	0.56*	0.57*	0.83*	0.74*	0.76*	0.69
6	0.77*	0.64*	0.69*	0.86*	0.86*	0.83*	0.78
9	0.85*	0.89*	0.80*	0.85*	0.94*	0.90*	0.87
12	0.97*	0.95*	0.91*	0.92*	0.97*	0.97*	0.95

*and ns= significant and non-significant Pearson’s correlation coefficients (p < 0.05), respectively; n= 18 points.

n = \frac{t_{α}^{2} x S^{2}}{D^{2}}

RDC = ∑ [(SPRj - SPRref) / SPRref] x (100 / n)

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Soil penetration resistance mapping quality: effect of the number of subsamples

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[1] *Author for correspondence. E-mail: henriquepias@yahoo.com.br