If you perform soil testing to understand nutrient concentrations or to test your soil for important parameters like pH and electrical conductivity, the 2:1 dilution method is an option known for its precision and relative simplicity.
Here, we cover this method step-by-step and provide recommendations for soil sample collection.
Soil Testing Best Practices: how to collect the soil sample
To get the most accurate soil test, it is critical to collect representative samples of the field to be analyzed.
Since soil composition is highly variable, we recommend collecting different soil samples in a random pattern across the field to have a homogeneous representation.
Best Practices for Soil Sampling:
Make sure that all soil samples are collected within the same day
Sampling consistency: always collect your sample in the same manner
Collect at least 10 samples per acre at randomly selected locations
Use composite sample: mix samples collected into one composite sample
Collect your samples at crop roots' level.
Soil Sampling Protocol
Using a soil sampling probe, collect the sample at a depth of 6 inches (note that this is a typical root depth, but it can change depending on the crop type. Make sure you verify the roots' depth for your specific crop.).
For each sample, collect around 1 ounce (around 30g.)
Place all the samples collected in a plastic bag.
Once all the soil samples have been collected, mix the soil inside the plastic bag in order to obtain one composite sample.
Drying soil in the oven: who should do it and when it may be needed
When soil samples are sent for traditional laboratory analysis, the soil is dried out in an oven to remove the moisture prior to its analysis.
You may want to follow this protocol in case you want to compare your field data with lab analysis.
Before you dry the soil in the oven, keep in mind the following:
· Drying the soil sample is not mandatory, especially if you seek the convenience of immediate field testing.
· Drying the sample does not improve the precision. Remember that each method will provide you with different type of results that cannot be compared easily.
· Note that it’s usually not possible to measure dry soil with professional field testers (i.e. LAQUAtwin and Hanna meters).
How to dry soil in the oven:
· Take approximately 250 grams of soil sample
· Place it in an oven safe container (metal or glass) for 24 hours at 225°F / 105 °C.
· Once the soil is dry, select the soil dilution method 1:2 for further testing
After the analysis, the result displayed by the meters will need to be corrected to reflect the concentration in dry soil prior to the dilution.
Here is the dilution factor formula:
Concentration in dry soil = 1:2 soil dilution sample result multiplied by the dilution factor (x3).
For example: My LAQUAtwin potassium meter displayed 250 ppm of K in the soil sample that I prepared using the 1:2 dilution method. My potassium concentration in the dry soil is 250 x 3 = 750 ppm.
NB: If your selected soil preparation method is Saturated Media Extract (SME) DO NOT DRY the soil sample.
Since all moisture is removed from the soil before lab analysis, and the soil sample is measured dry, the laboratory results will always show higher values than wet soil solution sample analysis.
Soil sieving
Sieving the soil will help to remove large stones or pieces of organic matter. (Since it can interfere with weight and dilution factors of your soil solution).
Use the sieve from the Sampling Expert kit with aperture of 1.25 mm, pour all the soil inside the sieve and pass it all through the sieve. Collect the sieved soil sample inside another plastic bag.
1. Sieve the soil to separate stones
2. Ideally your soil should look as if it was finely milled like in the picture above
How to prepare soil slurry (1:2 dilution method)
To prepare a soil solution using 1:2 dilution method you will need:
- Scale, spatula, cleaning bottle, 2 x plastic beakers, soil extractor solution
(all available in our Sampling Expert kit)
1) Take a plastic beaker, place it on the scale and tare the scale (to zero out the weight of the beaker)
2) Using the spatula measure 20 g of soil
3) Add 40 g (ml) deionized water - making total weight of your sample (inside the beaker) 60 g for 1:2 dilution
4) Add 5 drops of the soil extractor solution (Ammonium Acetate, available in Sampling Expert kit)
5) Stir the soil paste obtained for 1 minute and let it rest for 5 minutes
6) Your soil solution is now ready to be analyzed.
Dilution 1:2 => dilution factor 3
Understanding the dilution factor:
With LAQUAtwin Nitrate, Potassium, Calcium, and Sodium meters, multiply the meter results by the dilution factor to reflect the actual nutrient concentration before dilution.
For example:
If your dilution factor is 3 and the potassium meter reads 60 ppm,
the actual soil potassium concentration is: meter reading x dilution factor -->
60 x 3 =180 ppm.
Soil paste filtration: when you may need to filter your sample
The soil paste samples can be measured directly on HORIBA LAQUAtwin meters.
For Hanna HI-706 Phosphorus Checker, sample filtration is necessary before measuring Phosphorus.
For more accurate analysis of ions (Nitrate, Potassium, Calcium and Sodium) using HORIBA LAQUAtwin meters, sample filtration is also recommended.
To perform filtration use a plastic beaker, funnel and the filter included in the Sampling Expert kit.
- Fold the filter into quarters, open it, and position it in the funnel.
- Place the soil paste onto the filter and allow the filtrated sample to form.
Now your sample is ready for testing!
Final Thoughts
The soil slurry (1:2 dilution method) is the most common soil preparation method. This technique is quick and easy, making it perfect for on-site analysis.
This method ensures accuracy by precise dilution measurements.
Soil slurry is the best method for open field crops and in scenarios involving compacted and/or dry soil, where other methods might pose challenges.
I'd recommend this technique to crop consultants and agronomists unable to utilize lysimeters for any reason.
The soil paste obtained can be directly measured with pH and Conductivity meters.
However, for nutrient measurement, it's advisable to filter the paste, as previously described.
If you are looking for an all-in-one, user-friendly field testing kit that tests your soil and plant sap samples for 8 parameters, you can find more information about our bestselling NutriCheck LAQUAtwin kit below.
-Seb