Soil density, a term all anthophiles are accustomed to, is a very crucial part of growing a plant. Literally, soil density is the relation between the mass and the volume of a dry soil sample. It is expressed as gram per cubic centimetre or kilogram per cubic metre.
Density (D) = Mass (M)/Volume (V) gm./cc or lb/cft
It is critical for the soil to have access to air, water and nutrients, and soil density varies from plant to plant. It determines a number of factors like the plant’s growth rate, looks, height and width, colour, etc.
Source: Pinterest
See also: Gardening soil: A guide on different types of soil, price and gardening tips
Soil density: Constituents
In the world of botany, soil density is an umbrella term consisting of two separate terms.
-
Particle density
The weight per unit volume of the solid portion of the soil is known as particle density. It is expressed as Gm/cc or lb/cft. It is determined by the accumulative densities of the individual inorganic and organic constituents of the soil. It is also known as true density.
Usually, in normal soils, the particle density is 2.65 gm/cc.
It is also determined by the amount of heavy minerals in the soil like magnetite, limonite, hematite, zircon, etc. if the amount of organic matter in the soil increases, the particle density decreases. When particle density is divided by the density of water, a relative weight number is obtained, which is known as specific gravity.
-
Bulk density
Bulk density is defined as mass per unit volume of dry soil, including volume of pores spaces and solids. It is expressed in gm./cc, lb/cft, or megagram per cubic metre. The bulk de
nsity of soil is always lesser than the particle density of said soil. Loose and porous soils have a higher lower weight per volume than that of compact soil.
Usually, soils having low and high bulk densities display favourable and poor physical conditions, respectively. In normal soils, bulking density usually ranges from 1-1.6 gm/cc, with the exception of very compact sub-soils having a bulking density of 2gm/cc.
The bulking density of soils with large amounts of sand is about 1.7 gm/cc, whereas, in organic peat soils, the bulk density is 0.5 gm/cc. The bulk density decreases as minerals become finer in texture, and it is of more importance than physical density when it comes to understanding the importance of soil in plants.
Factors that affect bulk density
- Amount of pore space.
- The rigidity of the soil.
- The texture of the soil.
- Organic matter content.
- Soil structure.
Moreover, porosity is also a term that is commonly used. It is the ratio of the volume of pores in a collected soil sample to the total volume of the sample.
Mathematically;
Porosity, φ=volume of pores/total soil volume
To determine these quantities, something called an Uhland sampler is used to collect and examine specimens. It has a cylindrical structure that is inserted in the soil at a medium depth of the soil layer where the roots can be found. It is then removed with a collected sample of the soil being examined.
After the removal of the cylinder, the soil is allowed to dry and then is placed in the oven at 105 degrees celsius to determine its mass. The diameter and the height is used to calculate the volume of the sample.
see also about: bulking of sand
Soil density: Soil compaction
Source: Pinterest
Another botanical term that is related to soil density is soil compaction. Usually, the pore space and the pore density of the soil are reduced as the soil becomes more compact. This allows less space for water and air to get through.
Soil compaction is indicated by bulk density. The way soil is managed influences its soil density, but the effects still depend primarily on the texture.
Soil density: Tillage
The influence soil management can have on its density is fairly limited. One factor that has the most direct influence on soil density is tillage. Tillage is nothing but the term given to the process of tilting the soil.
Physically turning the soil breaks it up and allows more room for airflow and water. It can also help in removing crop residues and makes it easier to plant. Moreover, tillage makes the soil warm quicker in the spring, which may cause the plant to grow faster.
Source: Pinterest
One downside to tillage is that it gravely affects the soil structure and may hinder the plant’s natural growth. Weight from tillage equipment can cause trauma to the soil and even the plant’s roots, and compacts the soil even further, which can be detrimental to the plant’s health.
Another con regarding tillage is tilled soil with little to no residue cover is prone to wind and water erosion. It also affects carbon sequestration indirectly by breaking down organic soil matter, which can slow down carbon loss.
FAQs
What are proctor tests?
Proctor tests are tests that help find out the relative values of the moisture of the soil, the dry density of the soil and soil compaction.
What is the most common way to test soil compaction?
A standard cone test is a very old and reliable method to test soil compaction. It can be filled with errors, though.
