Electrostatic Method to Remove Particulate Organic Matter from Soil

Summary

Removing recently deposited and incompletely decomposed plant material from soil samples reduces the influence of temporary seasonal inputs on soil organic carbon measurements. Attraction to an electrostatically charged surface can be used to quickly remove a substantial amount of particulate organic matter.

Abstract

Estimations of soil organic carbon are dependent on soil processing methods including removal of undecomposed plant material. Inadequate separation of roots and plant material from soil can result in highly variable carbon measurements. Methods to remove the plant material are often limited to the largest, most visible plant materials. In this manuscript we describe how electrostatic attraction can be used to remove plant material from a soil sample. An electrostatically charged surface passed close to dry soil naturally attracts both undecomposed and partially decomposed plant particles, along with a small quantity of mineral and aggregated soil. The soil sample is spread in a thin layer on a flat surface or a soil sieve. A plastic or glass Petri dish is electrostatically charged by rubbing with polystyrene foam or nylon or cotton cloth. The charged dish is passed repeatedly over the soil. The dish is then brushed clean and recharged. Re-spreading the soil and repeating the procedure eventually results in a diminishing yield of particulates. The process removes about 1 to 5% of the soil sample, and about 2 to 3 times that proportion in organic carbon. Like other particulate removal methods, the endpoint is arbitrary and not all free particulates are removed. The process takes approximately 5 min and does not require a chemical process as do density flotation methods. Electrostatic attraction consistently removes material with higher than average C concentration and C:N ratio, and much of the material can be visually identified as plant or faunal material under a microscope.

Introduction

Accurate estimates of soil organic carbon (SOC) are important in evaluating changes resulting from agricultural management or the environment. Particulate organic matter (POM) has important functions in the ecology and physics of a soil but it is often short lived and varies based on several factors including season, moisture conditions, aeration, sample collection techniques, recent soil management, vegetation life cycle, and others¹. These temporally unstable sources can confound estimates of long-term trends in stable and truly sequestered soil organic carbon².

Despite being well-defined, common, and important, POM is not easily separated from soil nor is it easy to measure quantitatively. Particulate organic matter has been measured as that which floats in liquids (light fraction, typically 1.4-2.2 g cm^-3), or as that which can be separated by size (e.g., > 53-250 µm or > 250 µm), or a combination of the two^3,4,5. Both size-based and density-based techniques can influence the quantitative and chemical outcomes of POM measurement⁴. A careful visual inspection of soil that has been size-fractionated using routine methods often reveals long, narrow structures like roots and slivers of leaf or stem that have passed through the screen. Simply removing these structures by hand has been shown to substantially reduce measurements of total SOC^2,6 but the method is notably subject to the diligence and visual acuity of the operator. POM separation from a soil sample as the light fraction during flotation in a dense liquid⁷ does not capture all POM, and excessive shaking during the flotation process can actually reduce the amount of light fraction recovered from a sample⁸. Flotation requires many steps and exposes the soil to chemical solutions which can change the chemical characteristics or dissolve and remove constituents that may be of interest⁴.

Alternative methods for removing POM have been used to avoid or augment the use of dense aqueous solutions. Kirkby, et al.⁶ compared light fraction removal using two flotation procedures to a dry sieving/winnowing method⁹. Winnowing was performed by passing a light current of air across a thin layer of soil to gently lift away the light from the heavy fraction. The dry sieving/winnowing performed similarly to the two flotation methods with regard to C, N, P, and S content; however, the authors suggest that dry sieving/winnowing produced "slightly cleaner" soils⁶. POM has also been separated from soil using electrostatic attraction^10,11 in which organic particles are isolated by passing an electrostatically charged surface above the soil. The electrostatic attraction method successfully recovered POM, referred to as course organic particles, from dried, sieved (> 0.315 mm) soils with statistical repeatability comparable to other methods of size and density fractionation¹⁰.

Here we demonstrate how electrostatic attraction can be used to remove POM of sizes ranging from visible to microscopic. Unlike other reported methods, electrostatic attraction of fine soil also removes a small portion of mineral and aggregated soil which is visibly like the remaining soil. Given our results to date, it is reasonable to assume that the removal of a small portion of non-POM soil will have no substantial effect on the downstream analyses; however, this assumption should be verified for a specific soil if large proportions of the total soil sample are being removed electrostatically. The methods and examples provided here were performed on silt loam loess soils from a semi-arid environment.

This method may not be suitable for all soil types but has the advantages of being quick and efficient in removing particulate organic matter too small to remove manually or by an air current. Process speed is important in reducing fatigue, ensuring consistency, and encouraging greater replication for better accuracy of conclusions. Additionally, the ability to remove very small particulates is important in avoiding bias toward soils with larger rather than small particulate sizes.

Protocol

1. Soil preparation

1. Collect soil samples to the desired depth. Thoroughly dry the soil at 40 °C or following lab-specific standard protocols.
2. Sieve the soil through appropriate-sized soil sieves to obtain approximately 10-25 g of sieved soil. Many studies use a 1- or 2-mm sieve. The amount of soil is based on the mass required for the downstream analyses and will impact the number of times the electrostatic removal step will need to be repeated.
3. Place the soil in a clean, dry metal or glass flat-bottomed pan that is large enough for soil to be spread thin (at least 20 cm in diameter). Gently shake the pan horizontally to distribute the soil evenly in as thin a layer as possible.

2. Charge an electrostatic surface

1. Hold a 100 mm diameter glass or polystyrene Petri dish top or bottom in one hand and vigorously rub the outer surface with a clean piece of nylon cloth, cotton cloth, or polystyrene foam several times. Perform the surface charging away from the sample to prevent fabric fragments from being introduced into the sample.
2. Inspect the surface of the Petri dish to make sure it is clean.

3. Remove particulate organic matter

1. Lower the charged surface to within 0.5 cm to 2 cm above the soil and move it horizontally to pick up as much particulate material as possible. Attraction to the surface can be noted visually and audibly.
2. When the Petri dish no longer attracts additional particles, move the dish away from the sample.

4. Clean the electrostatic surface

1. Hold the charged surface over a collection dish and use a fine brush to transfer the electrostatically attracted material from the Petri dish surface into the collection dish. A camel hairbrush works well.

5. Repeat until the yield of particulates decreases

1. Repeat steps 2 through 4 until the number of organic matter particles being picked up decreases. Redistribute the soil sample by horizontal shaking of the soil pan to expose new material at the surface and continue electrostatic collection.
   NOTE: The endpoint is arbitrary and depends on the judgement of the researcher. Inspection of the charged surface after exposure to the soil gives a visual indication of whether a significant amount of organic particulates are still being removed from the soil. The final products are soil with reduced particulate content, and concentrated POM containing a small amount of electrostatically removed soil.

Results

The results presented here are based on the analysis of silt loam soils from agricultural sites in the Pacific Northwest (Table 1). Soils were collected to depths of 0-20 cm or 0-30 cm, dried at 40 °C, passed through a 2 mm sieve, and treated using a polystyrene surface charged with a nylon cloth.

The amount of soil electrostatically removed from a sample varied. About 1% to 6% of the total soil mass was removed...

Discussion

The electrostatic attraction method was effective in removing POM from the silt loam soils. The method described here is slightly different from Kaiser, et al.¹⁰ which used a combination of glass/cotton. We treated all but the finest soil fraction and used polystyrene rather than glass due to the triboelectric difference, which for polystyrene/nylon is 100 nC/J compared to glass/ cotton at 20 nC/J¹². Glass and polystyrene foam have proven effective and convenient in more re...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
brush, camel-hair
petri dish, glass or plastic
polystyrene foam, cotton or nylon cloth
soil
soil sieves