Protocols for Mapping Soil Salinity at Field Scale

Protocols for Mapping Soil Salinity at Field Scale: ECa Survey Considerations D. L. Corwin 1 and S. M. Lesch 2 1 USDA-ARS, U. S. Salinity Laboratory Riverside, CA 2 Department of Environmental Sciences University of California Riverside, CA

Dynamic Nature of Soil Salinity • Soil salinity is dynamic, varying both spatially and temporally. • Exhibits complex spatial patterns. • Mapping and monitoring at field scales requires a rapid, reliable, and easy means of taking geospatial measurements. 2

Apparent Soil Electrical Conductivity (ECa) • ECa is a fast, reliable, easy-to-take measurement that is easy to mobilize • ECa is a complex measurement, influenced by several soil properties: § salinity § texture § water content § bulk density § organic matter § clay mineralogy § temperature 3

Mapping Field-scale Soil Salinity • Apparent soil electrical conductivity (ECa): § electrical resistivity (ER) Veris 3100 Electrodes 4

Mapping Field-scale Soil Salinity • Apparent soil electrical conductivity (ECa): § electrical resistivity (ER) § electromagnetic induction (EMI) EM 38 EMv EMh 5

Approach to Mapping Salinity • Use geospatial ECa measurements as a • • surrogate to characterize soil spatial variability, particularly of soil salinity. Use spatial ECa information to develop a soil sampling plan that identifies sites reflecting the range and variability of soil salinity and/or other soil properties correlated with ECa. Referred to as “ECa-directed soil sampling” 6

Integrated System: Protocols, Mobile ECa Equipment, and Sample Design Software Sample design software Perform ECa survey Maps of salinity Identify Sites Lab analyses & GIS Conduct ECa-directed soil sampling 7

ECa-directed Soil Sampling Protocols • Seven-step protocols*: ◄ Site description and ECa survey design ◄ ECa data collection with GPS-based ECa equipment ◄ Soil sample design directed by spatial ECa data ◄ Soil core sampling at specified sites ◄ Laboratory analysis of soil physical and chemical properties as defined by project objectives ◄ Spatial statistical analysis to determine the soil properties influencing ECa ◄ GIS database development and graphic display of spatial distribution of soil properties *Corwin, D. L. , and S. M. Lesch. 2005. Characterizing soil spatial variability with apparent soil electrical conductivity: I. Survey protocols. Comput. Electron. Agric. 46(1 -3): 103 -134. 8

ECa-directed Soil Sampling Protocols • Seven-step protocols*: ◄ Site description and ECa survey design ◄ ECa data collection with GPS-based ECa equipment ◄ Soil sample design directed by spatial ECa data ◄ Soil core sampling at specified sites ◄ Laboratory analysis of soil physical and chemical properties as defined by project objectives ◄ Spatial statistical analysis to determine the soil properties influencing ECa ◄ GIS database development and graphic display of spatial distribution of soil properties *Corwin, D. L. , and S. M. Lesch. 2005. Characterizing soil spatial variability with apparent soil electrical conductivity: I. Survey protocols. Comput. Electron. Agric. 46(1 -3): 103 -134. 9

Conceptual Path Diagram: Factors Influencing ECa Survey of Soil Salinity Soil property effects: water content, bulk density, temperature, texture ECa Survey Primary Influences Secondary Influences Target Soil Property: soil salinity Soil condition effects: surface roughness, bedfurrows, etc. Goals: 1. Minimize soil property effects not related to the target property and 2. Avoid confounding influence of soil condition effects 10

Primary Soil Properties to Consider for an ECa Survey of Salinity • Soil moisture § § ü ü Ideal moisture condition is field capacity (FC) General rule: - ECa minimal influence when FC > 70% - ECa substantially influenced when FC < 50% Two EMI surveys (7 -ha sub-sections) 7 E-W traverses/survey 6 sites/sub-section at 4 depths Soils analyzed: ECe, θg, SP h Nort h Sout 11

Primary Soil Properties to Consider for an ECa Survey of Salinity • Soil moisture § § Ideal moisture condition is field capacity (FC) General rule: - ECa minimal influence when FC > 70% - ECa substantially influenced when FC < 50% Ave. ECa (d. S/m) North South EMv 1. 55 1. 70 0. 06 0. 41 Depth (cm) ECe (d. S/m) N S Saturation Water content Percentage (% of FC) N S 0 -30 30 -60 60 -90 90 -120 13. 1 13. 4 14. 5 12. 6 47. 5 43. 1 43. 5 41. 5 13. 8 14. 8 13. 9 48. 2 42. 8 39. 9 41. 1 75. 4 84. 6 82. 0 79. 1 40. 2 40. 3 44. 4 43. 2 h Nort h Sout 12

Primary Soil Properties to Consider for an ECa Survey of Salinity • Soil moisture § § Ideal moisture condition is field capacity (FC) General rule: - ECa minimal influence when FC > 70% - ECa substantially influenced when FC < 50% • Soil and ambient temperature § § § 1. 9% increase in ECa per 1ºC increase in soil temperature, which causes temporal effects (e. g. , seasonal) Take soil temperature measurements or conduct survey under same temperature EM 38: protect unit from heat buildup (Robinson et al. , 2004, SSSAJ 68: 339 -345) < shade the EM 38 • Texture § Be aware of sharp vertical and horizontal textural discontinuities 13

Secondary Soil Conditions to Consider for an ECa Survey of Salinity • Soil surface condition § Bed-furrows § Surface roughness (e. g. , smooth or disked) § Farm equipment traffic patterns (compaction) • Metal (e. g. , vineyards): concern for EMI units • Edge effects 14

Bed-Furrow Effect disked flat bed-furrow EMh ECa (m. S/m) < 34 34 -37. 5 -41 > 41 100 0 100 Meters N 200 15

Effect on ECa Due to High Traffic Compacted Furrows 2. 0 ECa (d. S/m) 1. 8 1. 6 1. 4 1. 2 1. 0 0. 8 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Furrow Number EMh reading Tractor wheel furrow 16

What can happen to an ECa survey when factors are ignored? EMI Survey Eleven separate surveys over a 6 -month period: • Seasonal effects (temperature effects) • Soil was at other than field capacity (water content effects) • Soil surface of varying roughness and presence/absence of bedfurrows (surface condition effects) 17

Thank You 18