Bulk Density

To dig or not to dig, that is the question…at least once a year for farmers and gardeners around the world. Even the no-till ones wonder about this sometimes, and rightly so. For a soil to be healthy, it must have sufficient air in it, because the soil breathes. It breathes in air from the atmosphere, through the physical forces of mass flow and diffusion. Mass flow and diffusion drive the transfer and exchange of atmospheric air and soil air, with mass flow caused by differences in air pressure, and diffusion cuased by gases in higher concentration wanting to move to areas where they are lower in concentration. Oxygen in atmospheric air diffuses into soils because there is typically a higher concentration of oxygen in atmospheric air compared to soil air. Similarly, carbon dioxide moves from the soil air to atmospheric air because there is typically a higher concentration of carbon dioxide in the soil than there is in atmospheric air. So with this air movement, it can be thought that the soil is breathing. In fact, the life in the soil is respiring, taking in oxygen and converting energy-containing carbon molecules (soil organic matter) to carbon dioxide (and other substances), which is the source of the lower concentration of oxygen and greater concentration of carbon dioxide in the soil air compared to atmospheric air. So how do we know the soil is breathing and not compacting and lacking air flow? Determining a soil’s bulk density is an excellent way to answer this question.

In general, if a soil’s bulk density is greater than 1.6 g/cm³, there is not enough air in the soil and roots will not be able to grow uninterruptedly into the soil, and the soil will not be breathing to the extent it should. And a soil that is not breathing is a soil that is dead, infertile, unable to support good biological activity, and prone to rapid degradation. And a soil that is not breathing is a soil that disallows crop roots from growing uninterruptedly, and causes crops to be vulernable to droughts, nutrient deficiencies and reduced yields. At the same time, we don’t want to till the soil if it already has plenty of air, as we will increase the decomposition rate and loss of soil organic matter (see the Soil Science Spotlight on Organic Matter). So, it is important to know if a soil needs more air by determining its bulk density.

Measuring a soil’s bulk density

Tools Needed:

• Metal Ring (3 in or 7.62 cm in diameter and height)
• Hammer or hand sledge
• Method to dry soil (air dry protected from rain, weigh repeatedly until it no longer loses weight)
• Scale to measure weight of dry soil
• (Optional) Block of wood optional to avoid denting the rim of the ring

Procedure:

1. Gently hammer the ring into the soil to the depth of 3 inches or 7.62 cm
2. Dig around the ring to remove it.
3. Clean off the top and bottom of the ring so the soil is flush with the ends of the ring. Avoid pushing the soil into the ring. You may need to slide a plate under the ring to avoid losing the sample..
4. Dry the soil and measure the weight (in grams).
5. Divide the dry weight (g) by the volume of the ring (in cubic centimeters) since bulk density is expressed in grams per cubic centimeter.

Note: since your ring is only 3 inches in height, then we have only determined the bulk density in the top 3 inches of soil. If we want to determine the bulk density of the top 6 inches of soil, then we need to repeat this process starting at the depth of 3 inches below the surface. We can repeat this process to whatever depth we desire. To determine the overall bulk density, simply average the bulk densities of each 3-inch depth.

Calculating the volume of the ring

Volume of ring = π  x  r²  x  h
π = 3.14
r = diameter of the ring (in centimeters) / 2
h = height of the ring (in centimeters)

Refining the answer of whether to till or not

Rather than using 1.6 g/cm³ as the limit for all soil types, a better way to gauge whether a soil needs more air is to know the soil’s texture, which has a large effect on how much air can enter and exit a soil. Once you know the soil’s texture, you can use the table below to determine if more air is needed.

Soil Texture	Ideal bulk densities (g/cm3)	Bulk densities that may affect root growth (g/cm3)	Bulk densities that restrict root growth (g/cm3)
sands, loamy sands	< 1.60	1.69	> 1.80
sandy loams, loams	< 1.40	1.63	> 1.80
sandy clay loams, loams, clay loams	< 1.40	1.60	> 1.75
silts, silt loams	< 1.30	1.60	> 1.75
silt loams, silty clay loams	< 1.40	1.55	> 1.65
sandy clays, silty clays, some clay loams (35-45% clay)	< 1.10	1.49	> 1.58
clays (> 45% clay)	< 1.10	1.39	> 1.47

Soil texture is based on the amount of sand, silt and clay there is in the soil. Sand particles are quite large and visible to the eye. Silt particles are much smaller and are not visible to the naked air. And clay particles are much, much smaller than silt particles! While all of these particle can exist in a compacted state, when sand is compacted, there is still quite a lot of space between the compacted sand particles through which air and water can travel. When silt particles are compacted, and especially when clay particles are compacted, there can be very little space between the particles, making passage of air and water very slow and difficult. 

Soil texture is classified by the percentage (by weight) of sand, silt and clay particles in the soil. There are 12 different texture types (sandy, sandy loam, silty clay loam, clay, etc.). Determining a soil’s texture can be done in one of two ways. The first method is known as the jar method and while there are different procedures published to perform the jar method, this one works well.

How to determine soil texture with the Jar Method

How to determine soil texture with the Ribbon Method

The second method to determine soil texture is faster, requires no equipment, and can be done in the field, but does take a little more experience to do accurately, and it is called the ribbon method.

Knowing the texture of a soil is a very useful first step in understanding that soil. A sandier soil typically has larger pore space than a clayey soil, so water and air can pass through it more easily. At the same time, a sandier soil is more prone to leaching and holds less water after a rain. As a result, a sandier soil tends to have fewer nutrients and can hold fewer nutrients (has a lower cation exchange capacity) than a clayey soil. 

Understanding your soil’s unique characteristics is fundamental to knowing how to improve it. However, there are faster methods to determine if your soil needs more air. Rather than measuring bulk density, a penetrometer can be used to determine if your soil is compacted and in need of air. A penetrometer is easy and fast to use, but is more costly (US$200-300) compared to relatively simple bulk density equipment. A penetrometer measures pressure in PSI (pounds per square inch) or kPA (kilopascals) rather than density in g/cm³. The guidance with penetrometer measurements is that a soil needs more air if it requires greater than 150 PSI to penetrate. Penetrability is highly dependent on soil moisture content and PSI readings can vary 5-10x between a soil being dry and the same soil after irrigation. This variability can make a soil’s compaction level difficult to interpret especially in the upper 15-30 cm (6-12 inches) of soil due to moisture variability. 

With experience, you can also use a metal rod to determine if a soil is compacted at a level that could impede roots. Use the metal rod (18 inches long, 1/8 inch diameter, or 40-50 cm and 2-3 mm in diameter) to probe the soil and note changes in resistance that would indicate compactness and restrictions in water movement and root penetration. It is best to do this when the soil is near field capacity since, like the penetrometer, resistance depends strongly on the soil water content: the dryer the soil, the greater the resistance. Since this method is entirely subjective and does not provide any quantitative data, you would need to use this method in conjunction with the bulk density and/or penetrometer method until you gain sufficient experience to rely on it by itself.

When your soil is compacted, the immediate solution is to till the soil to add air. However, tillage encourages the loss of soil organic matter, since till adds air and exposed previously hidden organic matter to soil microbial decomposition. A better solution is to till if more air is needed in the soil and to also add compost to replenish soil organic matter that will be lost as a result of tilling. An even better solution is to also grow compost crops to produce your one compost that will be used to maintain and improve your soil’s organic matter in the future. Adding cured compost to the soil feeds the soil microbes which create macro- and microaggregates in your soil. Microbial exudates, fungal hyphae and root hairs all collaborate to bunch soil particles together into aggregates or “peds”, which are the vey small balls of soil that you can see with your naked eye when you pick up a healthy soil, like crumbs from a chocolate cake. Those aggregates create space between them through which air and water can flow freely into the soil, and having good aggregation, good soil structure, is an important goal for a sustainable farmer wanting to minimize tillage, retain soil organic matter, encourage carbon sequestration, allow roots to grow uninterruptedly, improve water infiltration and storage, and provide a good environment to support a wide range of soil biology.

For more information on soil bulk density and its impact on soil water, click here