Digging Deeper into Soil Tests

In 2022, the technologies available to growers are vast and varied, spanning from biological to digital, all promising to maximize yields. Inputs on the other hand are quite limited, with fertilizer supplies across the globe diminished, leading to high costs for the needed materials. At Sound Agriculture we are developing products and tools that work alongside current technologies and resources to effectively harness the nutrients in the soil, boosting yields or decreasing the amount of necessary fertilizer. Regardless of whether or not you are using Sound products, a key tool in determining your inputs and tools is a basic soil test. 

Using the results of a basic soil test, a grower can utilize our Performance Optimizer in combination with nitrogen application rates and yield goals to gauge the predicted outcome of SOURCE application in that specific field. SOURCE is a soil amendment that works to stimulate the bacteria that already live within the soil to unleash more plant-available phosphorus and nitrogen. The components of a soil test used by the Performance Optimizer are soil pH, cation exchange capacity (CEC), and % organic matter.

Soil pH:

Soil pH is an important measure regarding nutrient use efficiency as changes in soil pH change the nutrient form, and therefore the nutrient availability. Most nutrients, like nitrogen and phosphorus, are most available in a soil with a pH around 7. Soil pH also has an effect on the soil microbes, which can also alter nutrient availability to the plant, as microbial activity changes at different levels of soil pH. Acidic soils also can make elements like aluminum and manganese soluble, which could lead to toxicity in the plant. Through a soil pH measurement we can gain an understanding of the availability of critical nutrients, like nitrogen and phosphorus, in a specific field. 

Factors that can affect soil pH include: 

• Rainfall: Excessive amounts of rain can lead to a reduction in soil pH. The excess water can flush out basic cations, creating a more acidic environment. Sandy soils can become more acidic as water moves through them more rapidly than soils with more clay. However, this increase in acidity takes many years to develop.

• Soil parent material: A soil’s parent material can affect the pH of the soil. As the parent material breaks down, the chemical composition can lead to acidification of the soil. Soils with a granite parent material are more likely to be acidic compared to limestone-based soils.

• Organic matter decay: Organic matter in soil is important for plant and microbial life, but the process of organic matter decay or decomposition leads to an increase in carbon dioxide. This carbon dioxide can react with water in the soil to form carbonic acid, a weak acid. Normally organic matter decay is a minor contribution to soil acidification.

• High yielding crop production: While sustainable high yields are the goal, higher yielding crops can uptake more lime-like elements from a field. This can lead to a more acidic soil, as the basic materials that counteract acidity are removed.

• Nitrification of ammonium: Nitrogen fertilizers are also a factor that can affect a soil’s pH. Ammonium fertilizers being transformed into nitrates in the soil can lead to an increase in acidity. Increased usage of ammonium fertilizers can make a soil more acidic and should be monitored.

Cation Exchange Capacity:

The cation exchange capacity (CEC) of a soil is the relative ability of soil to store positively charged ions, or cations, and is related to the clay and organic matter components of a soil. Cations that are normally abundant in soils are calcium, magnesium, potassium, ammonium, hydrogen and sodium, essential nutrients for plant growth. The cation exchange capacity of a soil relates to the amount of cations that can be supplied — the higher the CEC, the more essential nutrients the soil can hold and therefore supply to a plant. 

Factors that affect CEC include: 

• Composition: As soils are made up of sand, silt, and clay, the percentage of each component can affect soil CEC. Clay and silt particles are negatively charged and can retain more positively charged molecules (cations). This ability means that soils with higher clay contents tend to have a higher CEC than sandier soils.

• Organic matter: The dissociation of organic acids in organic matter leads to negative charges within the soil, ultimately leading to a better ability to retain cations. Therefore, increasing organic matter within a soil can help to increase CEC. This organic acid dissociation is dependent on soil pH.

• pH: Soil pH can alter the organic acid dissociation of soil organic matter which can change the amount of negative charged molecules in the soil to hold plant-beneficial cations. As the soil pH becomes more acidic, the CEC would be decreased.

Organic matter:

Soil organic matter is critical to the nutrients available to crops. Microbes within the soil can break down organic matter into plant-available nutrient forms. This breakdown can increase nitrogen and phosphorus levels within a soil. In addition to being a source of nutrients, organic matter can improve other soil characteristics like water holding capacity and soil structure.

Factors that affect soil organic matter include:

• Temperature: Temperature is related to the rate of decomposition of plant residues; higher temperatures lead to more decomposition of organic matter. Soils in cooler climates tend to have more soil organic matter due to the slower rate of decomposition.

• Soil moisture: Organic matter tends to increase as soil moisture increases. Soil microbes need both water and air to survive. Optimal soil microbial activity occurs when around 60% of soil pore space is filled with water. However, if there is too much moisture, microbes can be starved of the necessary oxygen they need to function.

• Topography: Organic matter can accumulate at the bottom of hills. This accumulation can be due to wetter conditions at the base of a hill compared to the slope or the transport of organic matter down a slope via erosion and runoff.

• Soil Texture: As the clay content of a soil increases, the organic matter increases. This increase is created by two components. The first component is increased bonds between clay particles and organic matter, leading to decreased decomposition of organic matter. The second component is increased soil aggregation, which causes less surface area to be available for soil microbes to break down organic matter.

• Soil pH: pH can affect the activity of soil microbes. Acidic soils can reduce the growth of the microbial community, leading to reduced decomposition.

Haney Test:

While a basic soil test can provide a lot of information about the nutrient availability of your soil, a Haney test can provide a deeper understanding of soil health and the amount of nutrients for soil microbes. A Haney test measures many of the same metrics as a basic soil test, like soil pH, NPK and organic matter, but with different soil extracts to gain additional information about soil fertility. It also measures a number of other soil health parameters including soil respiration, water extractable organic carbon, water extractable organic nitrogen and provides a soil health indicator. A Haney soil test is another effective tool to help growers make sustainable decisions regarding management practices. 

• Soil respiration: This test measures the amount of carbon dioxide a soil generates in 24 hours after drying and rewetting. The more carbon dioxide produced by a soil, the more microbial biomass is present in the soil. These readings can be between 0 and 1000 ppm of carbon dioxide, but many soils measure under 200 ppm. Soils with low scores, and potentially less microbial biomass, may break down residue slower than soils with higher soil respiration scores. These scores can change between growing seasons and environmental conditions. 

• Water extractable organic carbon (WEOC) and water extractable organic nitrogen (WEON): Water extractable organic carbon (WEOC) is a measure of the amount of organic carbon available to feed the soil microbes. The higher the WEOC, the more energy available to sustain the microbial biomass. These readings can also be affected by environmental conditions and can fluctuate throughout the year. Soil temperatures can alter microbial activity, which can alter WEOC. Similar to the WEOC, the water extractable organic nitrogen (WEON) represents the amount of organic nitrogen that can be utilized by soil microbes as feed. The higher the WEON, the more quality N available for the microbes to use. WEOC and WEON are tied together and the ratio of the two can be used to describe how well soil microbes can use these nutrients. An ideal WEOC:WEON ratio would be between 10:1 – 12:1, suggesting a good balance of energy available for the microbes and the best potential for N mineralization from soil nitrogen sources. 

• Soil health indicator: This is a summary of the soil respiration, WEOC and WEON, and is an indicator of the health of a soil at that time, under its current management practice. The higher the soil health indicator score is the better; which can range from 0 to 50 with many fields scoring under 30. Management practices have a large impact on the soil health score, as such this measurement can help guide management decisions to build a healthy soil microbial system. 

Taken together, these soil parameters and the Performance Optimizer can allow growers to capitalize on their available resources and use technologies that efficiently use those resources. As SOURCE works with microbes already living in that soil environment, using soil test data allows growers to pinpoint fields that are expected to perform well. This prescriptive approach allows the grower to make data-driven decisions on which fields would benefit the most, maximizing return on investment.

There are many options out there today for growers looking to maximize their yields and push toward sustainable agriculture. With SOURCE and the Performance Optimizer, a simple soil test can help growers efficiently harness the available resources to achieve those goals.