An Experiment to Demonstrate Density and Water-Holding Capacity of Organic Matter

Abstract

An experiment is described that allows students to estimate particle and bulk density and water-holding capacity of organic matter. Using compressed disks that are readily available from nurseries, the particle density of organic matter can be approximated; and, after water absorption, the bulk density and water-holding capacity can be calculated. How students conducting this experiment can increase their understanding of the physical properties of organic matter is discussed in the paper. T HE physical properties of soil organic matter is an important topic in an introductory soil science course. The addition of organic matter to soils can increase their waterholding capacity and lower their bulk density. Determination of soil bulk density can be achieved in an introductory soil science lab using a core sampler in a straightforward manner (Blake and Hartge, 1986). However, organic matter does not lend itself to easy manipulation for bulk density calculations. To quantitatively appreciate the water-holding capacity of soil, students commonly calculate gravimetric water content. Gravimetric water content is commonly 1.0 Mg m -3 for organic soils. Mineral soils are readily available for lab experiments, but organic soils (Histosols) are more difficult to obtain in most parts of the world. Particle density of mineral soils is commonly determined through displacement in a water medium. Calculation of the particle density of organic matter is less common due to the difficulty of separating out the soil organic matter fraction and determining the volume of the extracted fraction. Organic matter disks are used as a medium for producing seedlings from seed. These disks are commonly composed of compressed sphaghnum peat moss. Upon the addition of water, these disks swell, predominately in the vertical dimension, to approximately 6 to 7 times their original height. A simple experiment using compressed organic matter disks was designed to help students: (i) appreciate the contribution of organic matter to the water-holding capacity and bulk density of soils, and (ii) estimate particle density of organic matter.