Soil Testing: What to Request - How to Interpret Results Dr. Robert Flynn, NMSU Extension Plant Sciences 4 Components of Soil Testing Sampling Laboratory Analysis Interpretation Recommendations Proper Sampling What to Do Interpretation Lab Results Avoid Garbage in Garbage out! A Good Sample is a Must Use Clean Equipment No Rusty or Galvanized Containers No Rusty Shovels or Probes Greatest Error: Sampling Methods 1
Different Tools TIMING Sample Prior to Fertilization Must be representative Avoid abnormal areas Submit air dry soil Reduces Microbial Activity Consistent Depth 6-12 inches Shoot for 20 subsamples to make 1 sample to analyze be Representative Consider Soil Texture Changes in Fertilization Traffic Irrigation patterns 2
Grid sampling At each point: 5-10ft apart Or composite all for one sample Address Irrigation Issues! Smart Sampling Plan Ahead Know Texture Differences (Web Soil Survey) http://websoilsurvey.nrcs.usda.gov/app/ Irrigation Variability Differences in Growth / Color 3
Web Soil Survey Example Textural Differences 4
PAP Performance Assessment Program Provide Information to Lab of Choice ID Depth Crop Yield Goal Test Desired Test Desired ID Depth Crop Yield Goal 5
Choose tests for high ph soils Test Packages Routine Analyses Need Tuning for high ph soils: ph, Soluble Salts, Phosphorus Specific Tests!! Saturated Paste for Salinity OLSEN P!! Olsen P! 6
Choose Saturated Paste for ph and EC & SAR ph (saturated paste) Salinity (saturated paste) SAR (saturated paste) Why Saturated Paste (SP) SP water content mimics soil water at irrigation. ph & salinity from SP most closely resembles what roots would experience. Saturated Paste Reference Point for Crop Response 7
Test Desired? CSU Individual Analyses Desired Test! CSU Results 8
CSU Results ph & Salinity Methods not specified on results! Nutrient methods shown. Ward Results Ward Results 1:1 soil ph = 7.4 1:1 Salinity (EC 1:1 ) = 0.57 Sat Paste ph = 6.9 Sat Paste Salinity (EC e ) = 0.75 9
1:1 ph extracts generally greater than saturated paste Can estimate SP from 1:1 EC e = -0.133 + (2.485 * EC 1:1 ) R 2 =0.85 Excess Lime Content 10
Soil Lime Controls Soil ph Soils with lime content >3% will most likely have a soil ph >7.5 Salinity Assessment is Important 1:1 salinity readings are not the same as SP 11
EC e can be estimated from 1:1 extracts EC e = -0.133 + (2.485 * EC 1:1 ) R 2 = 0.85 Choose the Saturate Paste Method Better Represents Soil ph & Salinity Better crop selection Better Management Recommendations g plant -1 800 600 400 200 0 Y = 840.7-87.92 EC e R 2 = 0.93 Threshold 0 5 10 EC e (mmhos cm -1 ) Yield/Performance as a function of EC e 12
g plant -1 800 700 600 500 400 300 200 100 0 Y = 840.7-87.92 EC e R 2 = 0.93 Threshold 0 5 10 EC (mmhos cm -1 ) Yield/Performance as a function of EC e g plant -1 800 700 600 500 400 300 200 100 0 Y = 840.7-87.92 EC e R 2 = 0.93 Threshold 0 5 10 EC (mmhos cm -1 ) Yield/Performance as a function of EC e g plant -1 800 700 600 500 400 300 200 100 0 Y = 840.7-87.92 EC e R 2 = 0.93 Threshold 0 5 10 EC (mmhos cm -1 ) Yield/Performance as a function of EC e 13
Sodium Adsorption Ratio (SAR) also an important soil test measure 14
SAR Sodium Adsorption Ratio Threshold for gypsum = 6 Exchangeable Ca, Mg, and Na Soil does not have to be saline! Gypsum Gypsum is Calcium Sulfate CaSO 4 Reclamation Rates could be missed or underestimated 15
Liquid Gypsum No undissolved solids TIME = 0 TIME = 30 min 16
TIME = 60 min TIME = 120 min TIME = 24 hours 17
Is Ag Lime a Source of Calcium? CaCO 3 is not soluble in water Needs to be destroyed in acid to release Ca Soil Nutrients What Soil Level = Best Yield? 18
What Extract Correlates to Best Yield? LOW PPM of Nutrient Sufficient Determine relationship between plant nutrient uptake & nutrient amount in soil Correlating Soil Tests Determine the relationship between plant yield and the amount of nutrient extracted by a particular soil test method. 19
% Rel. Yield % Rel. Yield % Rel. Yield 2/17/2016 Calibrating Soil Tests to Crop Nutrient Requirement Low Soil P Sufficient Soil P Medium Soil P Extracts for nutrition are not all the same Especially Phosphorus Bray P1 or P2 Mehlich P3 Olsen P (best for soils 6.5 and up) Ward CSU AB-DTPA usual default at CSU Specify Olsen or NaHCO 3 20
Olsen Phosphorus is meant for alkaline soils Minerals binding phosphorus in soil are: Calcium (Ca): high ph soils Iron and Aluminum (Fe & Al): low ph soils Western soils: most phosphorus bound with Calcium. How much P is available to plants best determined by the method that works in alkaline soils. Olsen Phosphorus (P) Ca P Fe/Al Lab Extract (Sodium Bicarbonate) IS Best Correlation with Plant Response to P in high ph soil Other Olsen 21
Organic Matter = N Credit Improved micronutrient availability & water holding capacity Methods for Soil Organic Matter Walkley Black (preferred) Loss on Ignition (most common) DTPA for Micronutrients Standard method Presence of lime can induce deficiencies WARD (DTPA) CSU (AB-DTPA) 22
Summary for Best Interpretation and What to Do Saturated Paste is Best for Salinity and Sodium Issues Olsen Procedure for Phosphorus DTPA best for New Mexico for Interpretation Pay attention to interpretation: Low, Medium, Adequate NMSU GUIDES Guide A-146: Appropriate Analyses for New Mexico Soils Guide A-114: Test Your Garden Soil 23
Questions? rflynn@nmsu.edu 575-748-1228 24