Guide to Interpreting Radicle Lab™ Soil Test Results – Rev 7.0 (9/13/2024)
Brad Joern Ph.D. – Radicle Soil Scientist
Overview
Precision Planting’s Radicle Lab™ device reports soil pH, buffer pH, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), cation exchange capacity (CEC), percent base saturation, and the percent of the CEC occupied by Ca, Mg, K, and hydrogen (H). The biggest difference between the Radicle Lab and traditional soil testing laboratories is that the Radicle Lab analyzes field-moist soil samples. Analyzing field-moist soils more accurately mimics the soil that plants encounter in the field.
This document describes how to interpret Radicle Lab results and explains some of the more nuanced advantages of the soil test methods used by the Radicle Lab.
Soil pH
The Radicle Lab reports field-moist 0.01 M CaCl2 pH values, which is often referred to as salt pH. Traditional soil testing laboratories measure soil pH in water, which can be affected by rainfall, fertilizer additions, and crop nutrient uptake. Measuring soil pH in 0.01 M CaCl2 is a superior method because it normalizes the background salt concentration in the soil and provides a more stable and repeatable soil pH measurement than pH in water. The Radicle Agronomics software can automatically convert Radicle Lab soil salt pH values to soil water pH values when you export your soil test results. You may see some differences between traditional soil testing laboratory water pH values after converting Radicle Lab soil salt pH to soil water pH due to the inherent variability in soil water pH. The converted salt pH values are more reflective of the true soil water pH as the factors that affect soil water pH values have been eliminated.
Soil Buffer pH
The Radicle Lab reports field-moist Sikora buffer pH values which are the same as dry soil Sikora buffer pH values. Most traditional soil testing laboratories in the Midwest US use the Sikora buffer pH method, so these results will be similar to what you normally see. No additional interpretations are needed.
Soil Test P
The Radicle Lab reports field-moist soil Mehlich 3 colorimetric P values, which are roughly equivalent to a field-moist soil Bray P1 P values for soils with a pH 7.4 or lower. The Radicle Lab uses a Mehlich 3 P extraction method with a turbidimetric analysis. Most crop response trials that underpin university P fertilizer recommendations are based on dry soil test P values with a colorimetric P analysis, which is equivalent to a turbidimetric analysis. Traditional soil testing laboratories typically use an inductively coupled argon plasma (ICAP or ICP) for P analysis. When P is analyzed with an ICP, all the P in the soil extract is analyzed, including forms of P that the plant can’t access. That is why ICP analyses are generally greater than colorimetric analysis in the crop response range, regardless of soil test method. The Radicle Lab measures phosphate P, which is the form of P taken up by plants. The Radicle Agronomics software can automatically convert Radicle Lab field-moist soil Mehlich 3/Bray P1 colorimetric P values to dry soil Mehlich 3/Bray P1 colorimetric P values or dry soil Mehlich 3 ICP P values when you export your soil test results. Because the amount of organic P extracted by the Mehlich 3 soil test varies among soils, you may see some differences between Radicle Lab dry soil Mehlich 3 ICP P results and your traditional soil testing laboratory dry soil Mehlich 3 ICP P soil test P results.
Soil Test K
The Radicle Lab reports field-moist soil Mehlich 3 ICP K values. Traditional K fertilizer recommendations in most states are based on dry soil test K values. Research in Iowa has shown that field-moist soil K analysis is superior to dry soil K analysis for estimating plant available K. This is especially true at lower soil test values where soil test K can increase substantially upon drying. The Radicle Agronomics software can automatically convert Radicle Lab field-moist soil Mehlich 3 ICP K values to dry soil Mehlich 3 ICP K values when you export your soil test results. The relationship between field-moist soil test K values and dry soil test K values is not always direct because changes in soil test K when soils are dried are affected by soil test level, clay content, and the type of clay minerals present in the soil. Therefore, you may see some differences between Radicle Lab dry soil Mehlich 3 ICP soil test K results and your traditional soil testing laboratory dry soil Mehlich 3 ICP soil test K results.
Soil Test Ca
The Radicle Lab reports field-moist soil Mehlich 3 ICP Ca values. Traditional soil testing laboratories report dry soil Mehlich 3 ICP soil test Ca values. Although Ca is not a nutrient that is deficient in most soils unless soil test values are below 200 ppm, field-moist soil Mehlich 3 ICP Ca values are slightly greater than dry soil Mehlich 3 ICP Ca values and may have a small impact on calculated CEC values. The Radicle Agronomics software can automatically convert Radicle Lab field-moist soil Mehlich 3 ICP Ca values to dry soil Mehlich 3 ICP Ca values when you export your soil test results.
Soil Test Mg
The Radicle Lab reports field-moist soil Mehlich 3 ICP Mg values. Traditional soil testing laboratories report dry soil Mehlich 3 ICP soil test Mg values. Although Mg is not a nutrient that is deficient in most soils unless the values are below 50 ppm, field-moist soil Mehlich 3 ICP Mg values are slightly greater than dry soil Mehlich 3 ICP Mg values and may have a small impact on calculated CEC values. The Radicle Agronomics software can automatically convert Radicle Lab field-moist soil Mehlich 3 ICP Mg values to dry soil Mehlich 3 ICP Mg values when you export your soil test results.
Soil CEC
The Radicle Lab reports field-moist soil CEC by summation in meq/100 grams of soil. Traditional soil testing laboratories report dry soil CEC by summation in meq/100 grams of soil. If you select dry soil Mehlich 3 ICP values for Ca, Mg, and K in the Radicle Agronomics software, the CEC will be converted to dry soil CEC automatically. Radicle Lab soil CEC by summation is calculated using the following equation:
Soil CEC = 12*(7.0 - BpH) + (ppm Ca/200.4) + (ppm Mg/121.5) + (ppm K/391)
Soil Percent Base Saturation
The Radicle Lab reports field-moist base saturation as a percent of total soil CEC. Traditional soil testing laboratories report dry soil base saturation as a percent of total soil CEC. If you select dry soil Mehlich 3 ICP values for Ca, Mg, and K in the Radicle Agronomics software, the percent base saturation will be converted to dry soil percent base saturation automatically. Soil percent base saturation is calculated using the following equation:
Soil percent base saturation = (CEC from Ca + Mg + K)/(total CEC)*100
Percent of Soil CEC Occupied by individual Cations
The Radicle Lab reports the percent of the soil CEC occupied by Ca, Mg, and K based on field-moist soil test values. Traditional soil testing laboratories report the percent of the soil CEC occupied by Ca, Mg, and K based on dry soil test values. The percent of the soil CEC occupied by H is determined from soil BpH and is not affected by soil moisture. If you select dry soil Mehlich 3 ICP values for Ca, Mg, and K in the Radicle Agronomics software, the percent of the CEC occupied by Ca, Mg, K, and H will be converted to a dry soil basis automatically. The percent of the CEC occupied by Ca, Mg, K, and H is calculated using the following equations:
Percent of soil CEC occupied by Ca = (CEC from Ca)/(total CEC)*100
Percent of soil CEC occupied by Mg = (CEC from Mg)/(total CEC)*100
Percent of soil CEC occupied by K = (CEC from K)/(total CEC)*100
Percent of soil CEC occupied by H = (CEC from H)/(total CEC)*100