Written by Connor Ruen, SDSU Department of Agronomy, Horticulture and Plant Science Graduate Research Assistant, under the direction and review of Thandi Nleya and Kristine Lang.
Soil health is defined as the ability of soil to function in an ecosystem that can sustain plants, animals, and humans. Soil has a large impact on the water and air quality all around us, so it is important to properly manage soil in your garden and on your farm. The three pillars to maximize healthy soil are when the physical, chemical, and biological properties of soil are optimal (Figure 1).
Biological properties are measured by the biodiversity of the plants, animals, and microbes in and around the soil and also by pest pressure on all organisms. Biodiversity is very important in a balanced ecosystem to prevent one population from getting too large and damaging the system, like some invasive species, such as emerald ash borer and Japanese beetles.
To manage biological properties, the use of crop rotation by planting crops of many different families can prevent the buildup of pest populations. A minimum three-year crop rotation is recommended, but more is usually better. Another way to introduce biodiversity is by planting beneficial plants, which are most attractive to pests and can be used to save cash crops or attract predators of pests, such as lady beetles to feed on aphids, or parasitic wasps to control caterpillars.
Chemical properties are measured by determining pH and the presence of the major nutrients present in the soil. Both properties affect which crops will grow best in your garden. It is important to optimize soil nutrients and pH to have healthy plants that can absorb all available nutrients.
Nutrient management is very important to prevent soil from having too much or too little of any nutrient. The most-talked-about nutrient is nitrogen, which is very mobile and can easily be leached out of the soil, making it unavailable to plants, while an overabundance of it can damage plants as well. Soil pH has a large role in what nutrients are available to plants, since some nutrients change forms at different pH’s (Figure 2).
The ideal pH of soil for most plants is 6.0 to 7.0. To manage chemical properties, at-home pH tests can be used to determine if the soil is to alkaline or acidic, and soil tests to measure soil nutrients can be used to determine which nutrients are in short supply.
Physical properties are very important to carefully manage when it comes to maintaining healthy soil. Soil structure and texture have an influence on all other measures of the physical properties of soil, such as water infiltration, compaction, and bulk density. Soil structure that took generations to form can easily be destroyed in one afternoon.
Compaction is when the soil particles are pressed together so tight that plant roots, and sometimes even water, cannot pass through. This can cause plants to have shallow roots and be flooded from the lack of water infiltration.
To manage compaction and other physical properties, it is important to have roots in the ground as much as possible by having living pathways and cover crops when other plants are not growing. The reduction of tillage and use of heavy equipment also can reduce compaction, but, most importantly, it takes time. Soil structure takes many years to form, so it is important to work towards improving it sooner than later.
Tools to Measure Soil Properties
A penetrometer is a tool used to measure how much pressure it takes to penetrate the soil, hence soil compaction (Figure 3).
This is measured in pounds per square inch (abbreviated as PSI) and can be used to identify how deep plant roots can grow before they are stopped by a compaction layer, often referred to as a plowpan.
To use a penetrometer, the instrument is placed on the ground and slowly pushed into the soil at no more than one inch per second.
Once the desired depth is reached, the metal rod is pulled out of the soil and will read the highest PSI that was read when moving through the soil.
An infiltration ring is a tool that is used to measure the rate at which water can move through the soil (Figure 4).
The ring, which can be metal or plastic, is pounded into the ground one-to-two inches and has a known diameter to calculate the surface area. Once the ring is securely in the ground, it is filled with water to a known point, which is usually marked with a permanent marker. Then, at every minute mark, water is added to the mark using a 1000 milliliter (abbreviated as mL) graduated cylinder. The amount of water added can then be calculated by subtracting the amount of water left in the 1000 mL graduated cylinder from 1000. This is repeated every one-to-five minutes depending on the speed and size of the infiltration ring for at least 30 minutes.
Once the data is collected, the average is taken to measure how quickly water can infiltrate the soil in inches or millimeters per minute or hour. This measurement is helpful when measuring soil health and risk of erosion to topsoil, since low infiltration rates cause water to run across the ground.
Manage Soil Health for Long-term Impacts
Soil health is very important and influences every part of your life, from the air you breathe to the food you eat. Soil helps store carbon in the form of humus and organic matter, filter rainwater that will eventually refill our aquafers, and, most importantly, soil provides nutrients needed to produce the food we eat. Investing in soil health will benefit you and generations to come.
References and Resources
- Building Soils for Better Crops: Chapter 8 - Soil Health, Plant Health and Pests, Sustainable Agriculture Research and Education.
- The Effect of Soil pH on Nutrient Availability, ResearchGate.