In this fourth installment in the Everyday Soil Science series, we move our discussion to two important indicators of soil health that are often overlooked by gardeners and landscape professionals — bulk density and porosity.

The weighty subject of bulk density relates simply to the ratio of solids (i.e., sand, silt, and clay particles) to pore space (porosity) in a given sample of soil. Bulk density typically increases as the ratio of solids in a soil increases, and conversely decreases as the ratio of solids decreases. Looked at from the porosity perspective, bulk density increases as pore space decreases. The relationship of bulk density and porosity is reciprocal. As one increases, the other decreases.

Picture a brick. The brick is nearly 100% solid, so we’d say it has a “high” bulk density. Now picture a big sponge with lots of nooks and crannies of different sizes. The sponge is approximately 50% pore space, so we’d say it has a “low” bulk density. I could get mathematical on you at this point to quantify bulk density, but let’s keep it simple and focus on the relative difference. Brick = high bulk density. Sponge = low bulk density.

Generally speaking, loose, rich in organic matter, well aggregated and porous soils have a low bulk density. Sandy soils on the other hand have relatively high bulk since the sand particles can interlock to form a more solid mass with little pore space. This translates into sandy soils being easily compacted.

Bulk density then is an indicator of a common soil health problem that we’ve all encountered at some point – compaction. Compaction affects infiltration, rooting depth, water holding capacity, soil porosity, the availability of plant nutrients, and the activity of soil microbes.

Compacted soils have high bulk density.

Compacted soil with restricted root depth

Improving bulk density, alleviating compaction, and increasing soil porosity can be achieved through several easy-to-do management practices.

1. Minimize soil disturbance by working the smallest area of soil possible with the least intrusive tool possible. Avoid rototilling, plowing and other forms of mechanical soil disturbance.
2. When transplanting plants from large-sized containers, create lateral openings in the planting hole to facilitate air, water, and root movement.
3. Never till or operate equipment when soils are wet.
4. Utilize mulch, plant stubble, compost, and green manure (cover) crops to displace the weight of people and equipment on the soil surface.
5. Always keep soil covered, even during short-term work activities.
6. Use a diversity of plant materials (deep rooted as well as shallow rooted) to break up compacted soil layers.
7. Aerate turf and planted areas twice annually using a tine-type aeration device.
8. Apply a top-dressing of ¼” compost and/or compost tea every three months to all turf and planted areas to maintain active soil biology.

Implementing the practices above will go a long way to keeping the bulk density of your soil more like a sponge rather than a brick. And who plants in a brick…?

For more information about bulk density and soil porosity contact your local Master Gardener office, or click here to download an educational factsheet from the USDA, Natural Resources Conservation Service.

Remember, soil sustains life!

~The Soil Sommelier

In this third installment in the Everyday Soil Science series, we move our discussion from soil as a “living” ecosystem “fed” with life-giving organic matter to the abiotic “mineral fraction” comprised of sand, silt, and clay particles.

In the soil world, being small rules. The diameter of sand particles ranges from 2mm to 0.05mm, silt particles from 0.05mm to 0.002mm, and clay less than two microns (about the size of a bacterium). As the particles get smaller, they get more difficult to see, and in fact, clay particles are invisible to the naked eye. Unable to actually see the proportion of sand, silt, and clay in a soil, soil scientists have cleverly utilized the difference in how each particle “feels” to determine the relative proportion in a given sample of soil.

To understand how each particle feels, let’s think about clothing. Imagine a burlap jacket or wool sweater against your skin. How would that feel? How about rayon, silk, or satin? And Velcro? We can associate the feel of the clothing examples with the feel of sand, silt, and clay. Sand is rough or “gritty” like burlap. Silt is soft and “smooth” like silk, and clay is “sticky” like Velcro.

Why does knowing the feel of soil particles and their relative proportion matter? Well, as you might imagine, sand, silt, and clay in addition to feeling different, have different physical properties that influence how a soil functions. These include water holding capacity, porosity, bulk density, and other physical characteristics that will be addressed in future issues of Everyday Soil Science.

The arrangement of various combinations of sand, silt, and clay are represented in the Soil Textural Triangle (shown below). The triangle features an axis for each soil separate (sand, silt, and clay), and where the three intersect is the textural class of the soil.

 

Looking at the triangle you’ll notice a textural class called loam. Now think about how sand, silt, and clay feel. Loam is gritty, smooth, and sticky all in one soil! Most gardeners, landscape professionals, and farmers consider loam to be the soil sweet spot. Why? Quite simply, loam represents the ideal blend of sand, silt, and clay. In round numbers loam is 40% sand, 40% silt, and 20% clay. This combination is easy to work (tilth), holds moisture (water holding capacity), drains well, and can be penetrated by roots and organisms with relative ease.

You can determine the feel of your home or worksite soil by utilizing the soil texture flowchart illustrated below. Go ahead, get the “feel” of your soil, and record the textural class. We’ll use this information next month when we discuss soil structure.

In the meantime, if you have a question about soil texture, email me at sandrews@berkeley.edu.

Get out there and put your hands to work feeling (texturing) some soil.

Stephen Andrews
~The Soil Sommelier

 

 

RESOURCES:
Flowchart for Determining Soil Texture by Feel
Modified from S.J. Thien, 1979. A flow diagram for teaching texture by feel analysis. Journal of Agronomic Education. 8:54-55.

http://soils.usda.gov/education/resources/lessons/texture/