Twelve traits can describe the physical properties of soil. Each of these physical properties of soil plays a vital role in agriculture and contribute to the overall health of plants. When looking at the soil horizons, and what they contain in a soil profile, knowledge of these physical properties is absolutely essential.
This article is discussing the physical properties of soil, click here If you were looking for information about soil horizons O, A, E, B, C, and R.
What Is Soil?
USDA Natural Resources Conservation Service Soils definition of soil:
(i) The unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants.
Soil uses for a particular type of soil depend on the physical properties of soil.
Physical Properties of Soil
All soil types are different in their physical, chemical, and biological properties. Among other factors, these differences also derive from the process of how the soil was formed.
Using this information on the physical properties of soil, you can conduct a physical property analysis, which can increase your knowledge of soil characteristics and how plants will respond to certain aspects when changed.
Soil PH is the measurement of acidity in soil. Soil can be acidic or alkaline, which depends on the pH level found.
When looking at the pH scale, a pH level of 6 or below is acidic, and a pH level of 8 or above is alkaline. The entirety of the scale has a range of 0 to 14, with 7 being neutral.
To first determine the pH of soil, it helps to collect a sample. Mix a few samples from more than one spot in the ground and then use it in a soil pH test kit, which is available in multiple forms online.
Be aware that some plants prefer a specific pH level. Most vegetables, herbs and crops enjoy a comfortable 6.5 level of pH, but some vary. Attention to research when starting helps the plants to grow big and strong.
You can use a pH test kit like Sonkir Soil pH Meter to determine the pH of soil – Click here for prices.
With the ideal soil temperature, plants absorb soil nutrients better and grow taller. Temperature is one of the most important physical properties of soil because of how it can make or break the growth of plants. Regulation of soil temperature is even more imperative during season changes because the plants become exposed to more severe weather. Soil temperature can also help with the land treatment of organic wastes.
A common misconception is that soil temperature is the same as the air around it. When, in fact, the soil is usually warmer. According to ScienceDirect (in relation to tropical soils), “mean annual soil temperatures are 2–4 °C warmer than mean annual air temperatures,” which is about a 36-39o difference in Fahrenheit.
Some plants can endure cold weather, but most will need repositioning over a protective covering when the soil temperature gets too low.
Here are some of the best soil temperatures for common garden plants.
Best Soil Temperatures:
- Tomatoes – 75 to 85o F
- Potatoes – 65 to 70o F
- Corn – at least 50o F
- Sunflowers – 70 to 85o F
- Apple trees – 64 to 77o F
- Basil – 60 to 85o F
- Pumpkin – 60 to 100o F
Learn how to measure subsurface soil temperature.
Soil resistivity refers to the measurement of the conductivity of soil. Resistivity tests determine how well electricity can move through soil.
Wine vineyards are one of the types of gardens that can benefit the most from soil resistivity testing because it can ensure that other types of testing will be successful. Vineyards need special testing, such as an electric resistivity tomography, which cannot happen if the resistivity is low.
To test the resistivity of soil, use one of these ground resistance testers.
The texture of soil comes from its contents of sand, silt, and clay. Each of these contents makes up a percentage of the soil, which can vary depending on the type. For example, clay soil has its name because it has at least 40% clay, less than 45% sand, and less than 40% silt. Desert soil is composed mostly of sand (90-95%)
Each type of soil has a soil triangle, which provides a visual representation of its texture elements. This triangle is called a soil textural triangle.
Some soil textural triangles are available online, but you can also create one based on soil testing. Try using this Soil Texture Calculator. It may provide a better understanding of the soil’s exact texture.
Use these resources to determine the texture of a soil:
To test ground resistance you can use a tool like Apera Instruments, a ground resistance tester – Click here for prices.
The structure of soil refers to the particles, minerals, organic matter, and living organisms that are in the soil. The size of a soil’s particles depends on the type of soil (its texture). Minerals depend on this as well, but the organic matter and living organisms are unique to climate-specific regions.
As we discussed, soil is made up of clay, sand, and silt. Each of these textures in the soil has particles of different sizes.
Sand particles are 2.0 to 0.05 mm in diameter, silt particles are 0.05 to 0.002 mm, and clay particles are usually less than 0.002 mm.
Clay soils are the smoothest because the clay particles have the smallest diameter compared to sand and silt.
Soil structure has a significant influence on agricultural soil aspects such as water movement and aeration
Soils that have large particles and a heavy texture will hold more water because there is more space available within; this promotes drainage.
The topsoil part of a soil structure will affect water drainage in agriculture, as well. Topsoil can be granular, blocky, prismatic, or platy.
Click here to find out how to check soil structure.
The bulk density of soil shows its level of compaction and health. Bulk density can affect the rooting depth, water capacity, soil porosity, nutrient availability, and microorganism activity of soil.
Bulk density is the weight of dry soil per unit of volume (grams/cm3).
Bulk density in soil can change with excessive agricultural preparation. When soil faces erosion from this tillage, the particles begin to fill pore space, leading to an increase in bulk density.
To find the bulk density of soil, take the dry weight divided by the volume. If the moisture content of the soil is known, it is subtracted from the weight of the soil to find the dry weight.
Here are the ideal bulk densities of different soil textures:
|Sands, loamy sands||< 1.60|
|Sandy loams, loams||< 1.40|
|Sandy clay loams, clay loams||< 1.40|
|Silts, silt loams||< 1.40|
|Silt loams, silty clay loams||< 1.40|
|Sandy clays, silty clays, clay loams||< 1.10|
Porosity is the space between the soil particles. The porosity of a soil is a direct relation of the pores to the total volume. Here is the equation for finding the soil porosity.
Soil porosity = ____________ X 100%
The Pore Volume is equivalent to the volume of water added to the soil. The Total Volume includes the volume of the sample plus the pore volume.
Determining the soil porosity can help to ensure that the plants in the soil can absorb enough water and oxygen. When testing for other physical properties of soil, referencing back to the soil porosity can be enlightening, and will also save a lot of time with the entire physical properties analysis.
Soils take on the color of the minerals they contain. Red earthy soils in Arizona contain high levels of iron, and black garden soils contain mostly organic matter, nutrients, moisture, and all the major minerals that plants need (Nitrogen, Phosphorus, & Potassium).
A soil’s color can tell a farmer or gardener a lot about its compositional makeup.
Darker colors indicate moisture in the soil, which can help with knowing how much water to add to the plants. Other indicative colors include different shades of brown. A light brown soil likely has very little organic matter and may need fertilizer, but a black soil that has a dark brown color has a high nutrient content.
To determine the exact soil color for comparing to other sources of information on minerals, use the Munsell color system.
You can also get the Munsell Soil Book of Color – Click here for prices.
A soil that can hold a large capacity of water will require less maintenance. You can learn a lot about soil by finding out all of the physical properties of soil and learning what a particular soil contains. Aspects such as the porosity and texture can greatly change the water-holding capacity.
With the information gained from finding the water-holding capacity of the soil, irrigation techniques will be more straightforward. More importantly, the water-holding capacity number obtained from making these calculations tells how much water can be added to the soil at one time.
Some soil types have an average water-holding capacity number that can provide a little guidance. Here are a few of the water-holding capacities for various soil textures:
Sand = 0.8”/ft
Loamy Sand = 1.2”/ft
Clay = 1.35”/ft
Silty Clay = 1.6”/ft
Fine Sandy Loam = 1.9”/ft
Silt Loam = 2.4”/ft
During a soil porosity test (mentioned above), dual testing of the water-holding capacity is possible. Water-holding capacity is simply the moisture difference between a dry and a wet sample of soil.
Soil aeration plays hand-in-hand with the bulk density. The density comes from moisture content and air content, and the air content is the aeration of the soil.
Soil aeration is vital for the success of home gardens, lawns, farms, and other types of landscaping. Plants and flowers have roots underground that need not only soil nutrients and water but air as well. Also, if soil becomes too compacted from frequent human intervention, it may need aeration to become a healthy soil again.
To determine whether a soil needs aerating, try pushing a garden rake into the soil to feel how dense it is, or test the moisture content of the soil.
Items you may need:
Soil Aeration Tools
|LawnStar Liquid Lawn Aerator||Click here for prices|
|Rittenhouse Soil Sampler Probe||Click here for prices|
|Sonkir Soil pH Meter – Moisture/Light/PH||Click here for prices|
|Agri-Fab 48-Inch Tow Plug Aerator||Click here for prices|
Soil needs water to grow plants, but too much water can cause the plant’s roots to decay and mold to form. Well-drained soil will thrive the best because it avoids these typical problems. Finding the drainage rate of soil can provide helpful information on what needs to change with a garden setup.
Loose soils drain the best because there is more air in the soil to provide space for the drainage. The ideal soil drainage is around 2” of water per hour. Any less than 2” and the landscaper should either aerate the soil or create an underlying drain.
The University of Tennessee provides some great information on how you can test for the drainage rate of the soil. It involves digging a hole, filling it with water, and measuring the water level difference every hour. For the complete instructions – click here.
Organic matter is the substance that forms from decomposing plants, animals, and organisms that have died on the surface of the soil. As they decay, the organic matter is formed and gets pushed down through the soil horizons.
Organic matter is what provides plants with extra nutrients and moisture. When using compost, it acts as organic matter, that is why it helps gardens thrive so much compared to artificial fertilizers.
The amount of organic matter in soil makes a visual difference. Light brown soils are not only dry but are also likely to have very little organic matter in its structure. Soils with the most organic matter have an appearance closer to black soil.
Soil Physical Properties of Clay, Sand, and Silt
Clay Soil Properties
The soil particles in clay soil are very small, making the soil thick and dense. Clay has more plasticity than normal dirt.
Soils that have a high percentage of clay will be malleable and can easily form into a ball without falling apart.
Clay loam soils are the best soils for gardening.
Sand Soil Properties
Sandy soils tend to be acidic and have high drainage rates. Aeration is seldom needed in areas of land that contain sandy soil, but it can become fairly dense in some cases.
Silt Soil Properties
Soils with high silt content have a slippery feeling because of the shape of its particles. Silt soils also hold a large capacity of water but can still drain well enough for the growth and production of plants.
Soil Chemical Properties
Cation Exchange Capacity
Soils contain nutrients and minerals that act as positively charged ions (cations). Some of these cations are hydrogen, aluminum, calcium, magnesium, and potassium, which are all present in most healthy soils.
The cation exchange capacity is important for retaining and supplying plant nutrients. Wastewater treatment in soils is the most likely situation where cation exchange capacity would be important.
Soil Reaction (PH)
When the acidity of the soil is measured, this is actually a measure of the active hydrogen ion concentration.
Soil Biological Properties
The microorganisms that are present in soils are necessary for plant and root health. Microorganisms shape the soil to help with water drainage, and they also contribute to the soil nitrogen cycle. A lack of microorganisms would be a devastation to the entire ecosystem on Earth.
The physical properties of soil have a direct relationship with the health of the plants that grow in it. Gardeners, farmers, and landscapers can all benefit from this type of information. Take all of the tests we discussed and apply them to your future soil endeavors.