Learning Series: Understanding Soil Monitoring

Watch “Understanding Soil Monitoring” with Ruth Knight here:

Here is an audio version of the session for listening on the go.

This slide deck is an updated version of the slides used in Ruth Knight’s presentation on “Understanding Soil Monitoring”.

Reflection: Incorporating Monitoring Into Management

How could you incorporate monitoring into your farm management? What benefit could this have?

Monitoring Overview

Being present during the monitoring will give you the opportunity to instill a bit better understanding regarding what that monitoring is about and you can link the numbers from the lab to the actual outcomes you are seeing in the field. As there are many of you in the same ecoregion, you can develop an understanding of what the potential could be in your ecoregion. This also gives you a language in which to discuss and start using those monitoring outcomes in your language.

Physical Properties of Soil and Observations

Soil Texture
Texture is an inherent characteristic of soil and is reported as percent sand, silt and clay. Based on the relative amount of sand to silt to clay, soils are given descriptive names (e.g., sandy loam). When comparing fields, either within the farm or between farms, it is important to consider soil texture first. Differences in texture rather than management may be responsible for many of the differences between the sites.

Bulk Density
Bulk density is a weight of soil in a given volume of space. Higher bulk density means less pore space and more compacted soil particles. Bulk density is dependent on soil texture but is also influenced by management. It can be increased when heavy equipment repeatedly passes over an area or by repeated tillage that breaks down soil aggregates, causing soil structure to collapse. Bulk density can be alleviated through the addition of organic matter and by growing cover crops with diverse root structures. In soils with higher bulk density, plant growth is restricted as is the movement of water and air into the soil. Carbon assessments must be corrected to account for bulk density.

Soil Compaction
Soil compaction is the result of loss of pore space within the soil. This happens when soil aggregates are broken down. Moisture tends to reduce compaction as does the presence of living plants and active rhizosphere.

Soil Crusting and Water Ponding
Bare, exposed soil is susceptible to crusting and can lead to erosion and poor infiltration. Soil crusts can form when high velocity raindrops impact bare ground, breaking down soil structure and creating a surface layer of compacted soil (i.e. a crust). This crust is impenetrable by water and can result in surface ponding after a rain. Crusting on the surface can affect small seedlings as they are unable to push through these layers.

Water Infiltration
When there hasn’t been rain for a while, soil pores are held more closely together, reducing infiltration rate in dry areas/times. To account for the variability that occurs in fields, we suggest using at least three infiltration rings in any monitoring effort. After completing an infiltration test, dig up the rings and observe the texture of the soil within them. We want to see soil texture in the rings that is more like chocolate cake (indicative of good structure) than chocolate pudding.

Wet Aggregate Stability
Wet aggregate stability is an indication of whether soil can keep the pore space open when submerged in water. It is the carbon (organic matter) and microbes that hold the pore space in place in the soil. The microbes create “snots, slimes and glues” that stick the pieces of soil together, creating structure.

Root Growth
The “dreadlocks” on the roots (AKA rhizosheaths) are micro-aggregates of soil particles that are stuck together by the “snots, slimes and glues” created by microbes. They are created as the plant pumps carbon and other products into the soil for the microbes to feed on, and subsequently the microbes create the glues that bind the aggregates together to create habitat for themselves and structure in the soil. This is the process that creates long-term carbon storage in the soil as well as short- and medium-term carbon storage that becomes food for microbes.

Plant Health and Brix
Brix measures the dissolved solids (including sugars) that are present in a plant. Higher Brix means more dissolved solids and is dependent on the time of day and the weather. It is indicative of plant health. At the Schill field day, it was observed that the grasshoppers that were prevalent around the perimeter of the oat field were not present in the field itself . A Brix reading of 19 was measured in the oats. Grasshoppers cannot digest plant material with Brix above 12 and therefore the crop became inedible. Brix also changes between plant parts as the plant moves through its life cycle. By testing different plant parts at different times during their growth cycle, you can see how plants partition nutrients differently to achieve different outcomes (i.e., growth, photosynthesis, reproduction, etc.).

Soil Color, Feel and Smell
When looking at soils, consider depth. Erosion moves topsoil from high to low areas and from upstream to downstream areas.

Soil Macro Life
Use your observations as a compare and contrast – why are certain organisms present in one field (or the unmanaged site) but not in another?

Soil Chemistry

Soil tests are generally utilized by conventional farmers to determine amendment (fertilizer) requirements but are often overlooked in organic systems. Understanding soil chemistry can provide insight into how your soil (eco)system is working.

pH
pH provides a great overview of how your soil is functioning chemically. Soils with pH in the 5 range (acidic) should be monitored and consideration should be given to how this could be addressed/changed (potentially using an amendment, like calcium carbonate). Low pH changes the biological community and is a limiting factor for many nutrients that become less available to plants/crops. As you move away from a neutral pH (pH=7), the availability of all of the macro- and many of the micro-nutrients changes. Phosphorus is especially sensitive to changes in pH and can become unavailable very quickly when you get to either the acidic or alkaline zone. For many organic farmers, phosphorus is being made available to the plants/crops by the soil biology that is converting it to plant available forms and are also changing the pH in the areas immediately around the plant roots. If pH is 8-8.5 (i.e., alkaline/basic), one opportunity is to acidify the area immediately around the germinating seeds (using sulphur), and another opportunity is to add biology.
pH is dynamic in the soil. The buffer pH gives an idea of how dynamic the pH is in the soil. The closer the buffer pH is to 7 or your desired pH, the more it gives you an indication of less risk, but you should be monitoring it. If it’s closer to the actual pH and the pH is too low, you should be addressing it or at least monitoring it closely.
NOTE: many labs don’t report buffer pH or H cation levels when close to neutral (7). In the overall examination of soil health, we start by focusing on the macro-nutrients and leave the micro-nutrients.

Cation Exchange Capacity (CEC)
CEC is how the cations (positively-charged nutrients like Ca, Mg, K, H) are able to move on and off of the clay particles in the soil. Organic matter also has an influence on CEC and nutrient availability. Because of their charge, cations stick and bind to the particle, and when they break apart, that is the CEC. Roots don’t consume the soil as a solid. These minerals (that are not singular but complexed together) need to be in soil solution before they can be absorbed by the roots. Soil with a sand texture will likely have a lower CEC (though other factors also change the CEC, i.e., the type of clay). In exchange, the root hair is giving up hydrogen to the clay particle. This affects both the CEC and the acidity of the soil. When you add fertility (i.e., Ca), it will either go into solution or it will bump off another cation from the surface of the clay particle, making that one available in solution. The CEC in the lab analysis is a benchmark calculation calculated by adding together the values for all the Na, H, K, Ca, Mg (cations) in the soil sample.

Target Ranges for Nutrients by CEC
Be careful with phosphorus; too little and you don’t have enough productivity, too much and you will create a negative environmental impact (think nutrient loading and eutrophication of Lake Winnipeg and other waterways). Use Bray for P when you’re at a lower (<7) pH. ppm of nutrients gives you an idea of how much (absolute amount) is in the soil. The percentages give you an idea of the distribution of the nutrients in the soil relative to one another. A deficiency in minerals should not be our focus when we are observing water cycles or other ecological processes that are not working.

The Carbon Component of Soil

Soil Organic Matter
Organic matter is reported as a percent of the whole volume of soil. There are many types of organic matter in the soil:

• Roots
• Microbes
• Crop residues
• Manures
• Composts
• Living macro organisms (and their bodies)

Healthy soil that is clay-dominant will have higher levels of organic matter than equally healthy soil that is sand-dominant. That means that a score of 3 is “better” in sandy soil than in clay soil. The Cornell Soil Health Assessment sets organic targets for sand at 4%, silt at 5% and clay at 6%. Much of the organic matter in the soil is legacy organic matter that has built over a long period of time. The smaller portion is the more recent additions (both the dead organisms/plant material and the living organisms/plant roots, etc.). This means that in agriculture, we are starting to mine what was once the stabilized soil organic matter. However, adjustments in management can change this. Organic matter has a lot of functions in the soil and that’s what makes it so important. It affects the biology, chemistry and physical structure of the soil. It contributes to the overall resiliency of the soil (i.e., how much stress the soil can take). When balancing the inputs and the losses, we want to increase our additions and know that different qualities of those additions are important. We also want to minimize our losses (either taking away OM as residues or crops or losing them to oxidation/gassing off). When we add carbon to the soil, we are providing food for the living organisms that are then able to create nutrients for plants, build soil aggregation and change some of that carbon into longer term carbon. We need to increase soil organic matter and productivity at the same time.

Biology
Manure will bring you a lot of bacteria. When looking at the CARA reports, look at the red (poor performance) and blue (good performance) and use this to identify where there’s the most room for improvement.

Management Planning
Start with an area you’re comfortable with. Identify what is constraining your system and what your goal would be (e.g., compaction) so you have a goal to work towards.