Introduction

Many garden retailers offer soil test kits. These kits offer fast results. However, the soil testing kits have not undergone the scrutiny that certified lab tests have.

Objective

Comparison of soil test results from three different soil test kits with a certified soil testing lab.

Methodology

Soil test results from the North Dakota State University Soil Testing Lab (NDSU STL), a certified commercial soil testing lab, were compared with the LaMotte¹ complete soil test kit (LaMotte), Luster leaf rapitest soil test kit² (Rapitest), and Luster leaf rapitest 4-way Analyzer³ (4-way Analyzer). 

Fourteen different composite soil samples were collected from the top 6 inches from 14 locations in the north, south, east, west, and center of North Dakota. These 14 soils plus store-bought compost and potting soils were all tested. The soils were dried, ground, mixed, and each divided into six separate bags. Three bags were randomly selected and analyzed by the NDSU STL. The other soil bags were kept for garden soil test kit analysis.

Soil pH, nitrate, phosphorus (P), and potassium (K) levels were determined following instructions by the individual soil test kits (Figure 1) and compared to the NDSU STL results (Table 1).

Results

The LaMotte pH, nitrate, and P soil tests were similar to the NDSU STL. The Rapitest soil pH and nitrate were similar to the NDSU STL results. The 4-way Analyzer soil tests were different from all NDSU STL soil tests.

Conclusions and Implications

• In some instances, the soil garden test kits provided similar soil test results to the NDSU STL. However, garden soil test kits tended to measure less nutrients than the certified soil testing lab
• North Dakota soils tend to have high levels of K. All garden soil test kits underestimated soil K levels.
• Using the garden soil test kits instead of a certified soil test lab may lead to over application of fertilizers, unless fertilizer recommendations are calibrated for the specific soil garden test kit.
• Several other garden soil test kits are available. Many of the untested kits have similar extraction solutions as the ones tested. If using an untested kit, the extraction solutions should be noted by the user to determine if the kit is comparable to a certified testing lab.  

References

¹LaMotte COMPANY. 2019. and LaMotte complete soil test kit. Chestertown, MD.

²Luster Leaf Products, Inc. 2019b. rapitest Soil Test Kit. Woodstock, IL.

³Luster Leaf Products, Inc. 2019a. rapitest 4-Way Analyzer. Woodstock, IL.

Jasper Teboh, Ph. D.
Jasper.Teboh@ndsu.edu
Research Soil Scientist

Acknowledgement
Authors: Chris Augustin, Jim Staricka, Ryan Buetow, Alicia Harstad^, Jasper Teboh, and Beth Burdolski