Sampling Size Requirements to Delineate Spatial Variability of Soil Properties for Site-Specific Nutrient Management in Rubber Tree Plantations

doi:10.5814/j.issn.1674-764X.2019.04.011

Abstract

Abstract:

Site-specific nutrient management is an important strategy to promote sustainable production of rubber trees in order to obtain high yields of natural rubber. Making effective nutrient management decisions for rubber trees depend on knowing the spatial variations of soil fertility properties in advance. In this study the Kriging geostatistical method was used to examine the spatial variability of soil total nitrogen (TN), organic matter (OM), available phosphorus (AP) and available potassium (AK) in a typical hilly rubber tree plantation in Hainan, China. The spatial variability of the soils was small for the TN and OM and had medium variability for the AP and AK variables. Anisotropic semivariograms of all soil properties revealed that elevation and building contour ledge can profoundly affect the spatial variability of soil properties in the plantation, except for the AK variable. Soil samples had to be collected in alignment with the direction of elevation and perpendicular to the direction of building contour ledges, which was needed to obtain more reliable information within the study area in the rubber tree plantation. In formulating a sample scheme for AK, the distribution features of the soil’s parent material should be considered as the influence factor in the study field. The Kriging method used to guide the soil sampling for spatial variability dertermination of soil properties was about 2-5 times more efficient than the classic statistical method.

Key words: hevea brasiliensis, soil sampling size, spatial soil variability, anisotropic semivariogram, site-specific nutrient management.

LIN Qinghuo,LI Hong,LI Baoguo,LUO Wei,LIN Zhaomu,CHA Zhengzao,GUO Pengtao. Sampling Size Requirements to Delineate Spatial Variability of Soil Properties for Site-Specific Nutrient Management in Rubber Tree Plantations[J]. Journal of Resources and Ecology, 2019, 10(4): 441-450.

Figures/Tables 10

Fig. 1 Sampling sites (100 sites) in the rubber tree plantation (4200 m2)Note: The dimension of each rectangle grid for sampling was 6 m × 7 m.

Fig. 2 Sampling points of different sampling strategiesNote: (a) sampling spacing 6 m × 14 m, 50 sampling numbers; (b) sampling spacing 12 m ×7 m, 50 sampling numbers; (c) sampling spacing 6 m × 21 m, 40 sampling numbers; (d) sampling spacing 18 m × 7 m, 40 sampling numbers.

Table 1 Statistical parameters for selected chemical properties of soil in a rubber plantation (100 samples in total)

Soil properties	Mean	Range	SD	CV(%)	Skew	Kurt	P_K-S^*
TN(g kg^-1)	0.79	0.48-1.13	0.12	14.64	0.35	0.58	0.43
OM(g kg^-1)	14.29	8.44-19.64	1.99	13.95	0.20	0.93	0.67
AP(mg kg^-1)	5.20	2.52-9.46	1.49	28.68	0.58	-0.08	0.54
AK(mg kg^-1)	34.68	20.01-62.39	10.67	30.77	0.93	0.17	0.10

Table 2 Classes of soil properties in a rubber plantation on Hainan Island (Lu, 1983)

Class	TN(g kg^-1)	OM(g kg^-1)	AP(mg kg^-1)	AK(mg kg^-1)
Abundant^*	>1.4	>25	>8	>60
Normal	0.8-1.4	20-25	5-8	40-60
Lack	<0.8	<20	<5	<40

Table 3 Semivariogram model of soil properties and their parameter in a rubber plantation

Variable	Best model	A₁ (m)	A₂(m)	κ	φ (°)	C₀	C₁	C₀/(C₀+C₁)	Cross-validation indices
Variable	Best model	A₁ (m)	A₂(m)	κ	φ (°)	C₀	C₁	C₀/(C₀+C₁)	ME	RMSE	RMSSE
TN	Spherical	52.93	16.93	3.13	20.5	4.18E-03	0.010	0.29	2.84E-03	0.087	0.975
OM	Spherical	46.19	16.57	2.79	8.8	1.579	2.694	0.37	2.41E-02	1.737	1.06
AP	Spherical	62.23	36.10	1.72	44.6	1.655	0.730	0.69	8.20E-04	1.439	1.028
AK	Spherical	52.86	23.98	2.20	286.2	72.663	47.192	0.61	-3.47E-02	9.528	0.985

Fig. 3 Anisotropic semivariogram models for the selected soil properties in a rubber plantation. Chemical properties of soil included total nitrogen (TN); organic matter (OM); available phosphorous (AP); and available potassium (AK).

Fig. 4 Anisotropic semivariogram models for the selected soil properties in a rubber plantation. Soil chemical properties included total nitrogen (TN); organic matter (OM); available phosphorous (AP); and available potassium (AK).

Table 4 Estimated sample sizes (numbers) to obtain mean values within ±5% and ±10% of classic and Kriging methods

Variable	±5%^*			±10%
Variable	SD	Classic	Kriging	SD	Classic	Kriging
TN	0.04	27	9	0.08	10	2
OM	0.72	25	9	1.44	10	2
AP	0.26	57	29	0.52	26	8
AK	1.73	60	32	3.47	29	9

Fig. 5 Kriging and classic statistical standard errors in relation to sample sizes for estimating chemical properties of soil at a rubber plantation. Soil chemical properties included: (a) TN, total nitrogen; (b) OM, organic matter; (c) AP, available phosphorous; (d) AK, available potassium.

Table 5 Test results of soil properties for various sampling strategies

Samplestrategy^*	Sample number	TN		OM		AP		AK
Samplestrategy^*	Sample number	RMSE	G	RMSE	G	RMSE	G	RMSE	G
A	50	0.07	61.09	1.27	58.78	1.08	46.71	6.75	59.58
B	50	0.07	64.61	1.19	63.98	1.03	52.30	7.57	49.19
C	40	0.08	47.51	1.47	44.80	1.18	36.47	7.98	43.57
D	40	0.08	48.37	1.44	47.09	1.21	33.58	9.30	23.27

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