Soil characteristics depend on the relationship among five things:
- composition of parent material—limestone, sandstone, etc.
- climate—arid, temperate, arctic, etc.
- organisms in and on the soil—earthworms, bacteria, fungi, as well as plants in a forest, native grassland, cropland, etc.
- topography—mountainous, flat, rolling hills, etc.
- time–soil ages can range from decades to centuries old, and may lose (e.g., erosion) or gain (e.g., flood plain deposition) material over time
Many components determine the composition and agricultural productivity of soil. Soils change over time, but it can take decades to centuries for inherent properties, like texture, to change even slightly. In contrast, man-made actions can change the dynamic properties of soil, such as its organic matter, in shorter time spans of months to years.
Soils are composed of:
- minerals—45%–59% of soil volume
- these influence the soil’s texture (sand, silt, clay, or loam)
- also determine the ability of a soil to hold onto nutrients—aka “potential for productivity”
- water—2%–50% of soil volume
- water-holding capacity is directly related to soil texture
- clay > silt > sand
- water-holding capacity is directly related to soil texture
- organic matter—1%–5% of soil volume
- ‘dead’ materials (plant, insect, or animal)
- can improve the ability of coarser-textured soils to hold onto water and nutrients
- air—2%–50% of soil volume
- oxygen, carbon dioxide, nitrogen
- occupies same spaces between soil particles as water
- allows soil microbes to respire (anaerobic or aerobic)
- microorganisms—less than 1% of soil volume
- one thimble of topsoil may hold more than 20,000 microorganisms
- soil is alive!
- microorganisms can form mutualistic relationships with roots, making nutrients more available or accessible to plants
Functions of soils:
- medium for plant growth—supplies water, air, and nutrients to plant roots
- regulator of water supplies—filters chemicals and sediments from water
- recycler of raw materials—allows for decomposition of organic materials such as leaves
- habitat for soil organisms—provides a home for beneficial microbes, bacteria, fungi, etc.
- landscaping/engineering medium—allows our built environment to be created
Seventeen elements essential to plant growth can be found in differing quantities in the soil:
- Boron (B)
- Calcium (Ca)
- Carbon (C)
- Chlorine (Cl)
- Copper (Cu)
- Hydrogen (H)
- Iron (Fe)
- Magnesium (Mg)
- Manganese (Mn)
- Molybdenum (Mo)
- Nickel (Ni)
- Nitrogen (N)
- Oxygen (O)
- Phosphorus (P)
- Potassium (K)
- Sulfur (S)
- Zinc (Zn)
These nutrients will be present in varying quantities. Their bioavailability depends on many soil factors—parent material, soil pH, previous soil and crop management, and soil age.
Though all nutrients are essential for plant health, some are required in larger quantities than others:
- Macronutrients: included in most all-purpose fertilizers, required in large quantities—e.g., 40–200 lb. per acre
- nitrogen—elemental N in most fertilizers
- essential to the creation of protein, mitochondria, chloroplasts, and other cell structures in plants; without it, plants cannot create new cells
- phosphorus—measured as units of P2O5 in fertilizer
- essential to energy storage and transfer in plants; without it, plants cannot grow or create reproductive structures
- potassium—measured as K2O in fertilizer
- required for the plant to transfer photosynthates to other parts of the plant; without it, fruits and flowers don’t have the energy to develop properly
- nitrogen—elemental N in most fertilizers
- Secondary elements: necessary for plant growth, but required in smaller quantities
- calcium—essential to cell membrane structure and permeability, which allows the plant to properly take in and maintain nutrients within plant cells
- magnesium—necessary for the plant to perform photosynthesis and produce energy for growth and development
- sulfur— required for chloroplast production and proper leaf growth
- Micronutrients: necessary for plant growth, but required in very small quantities because most plants get sufficient levels from the soil
- includes remaining B, Cl, Cu, Fe, Mn, Mo, Ni, and Zn nutrients from the “soil nutrient” list while excluding carbon, hydrogen, and oxygen
Carbon, hydrogen, and oxygen are supplied by air and water and make up 94% of plant biomass. The other 14 essential elements make up the remaining 6%.
- Carbon can be slowly added to the soil over time by increasing organic deposits such as leaves, manure, etc.
- Hydrogen and oxygen cannot really be managed except through water management, which will affect the content of soil pore space, or the space between soil particles.
Liebig’s Law of the Minimum states that the yield achievable by a crop is dictated by the nutrient (or resource) that is most limiting (Figure 1).
Soil pH is a critical aspect to both nutrient availability and soil health.
- The recommendation for most agricultural soils is a pH between 6.0 and 7.0, which is slightly acidic to neutral.
- Some crops or plants require more acidic soils, and some prefer more basic soils (pH greater than 7).
- Soil nutrients become more or less available at different pH levels.
- For example, phosphorus becomes much more unavailable below a pH of 6.0, but iron becomes most available below 6.0.
Depending on the positive or negative charge of the nutrient, it will either be held on the soil particle or will be in the soil solution (soil water). Nutrients held on the soil particles are usually lost only by erosion. Nutrients that are in the soil solution can be lost much easier, by leaching or runoff. This is why understanding the dynamics of nutrients is necessary when deciding how much and when to apply.
Different fertilizer forms have a different nutrient charge and/or availability to plants. Some nutrient forms must be mineralized—converted from organic to inorganic forms—before the plant can take them up. It is important to consider this and when the plant will require the nutrient in question to determine when to apply fertilizer. In addition, placement of readily available inorganic nutrients below the soil surface and closer to the root zone can help reduce nutrient losses by allowing plants to take up nutrients faster.
Important considerations for nutrient applications:
- Application of a nutrient in a larger quantity than necessary will most likely result in nutrient losses.
- If the plant cannot use all of the nutrient, it will remain in the soil until the next growing crop can utilize it or it is lost via erosion, runoff, leaching, and/or volatilization.
- Application of a nutrient at the wrong time can result in nutrient losses.
- Fertilizer applications too close to a forecasted rain can result in loss of the fertilizers before the plant gets a chance to take them up.
- Fertilizer applications in the wrong season can result in nutrient losses because plants won’t readily absorb nutrients applied to a bare field or dormant crop in the fall or winter.
Soils with more exchange sites (places for the nutrient to adhere to the soil particle) will be able to better hold nutrient particles more strongly and in larger quantities.
- Clay soils have greater inherent ability to hold nutrients than sandy or silty soils, due to their larger surface area.
- Soils with higher organic matter (OM) contents have greater ability to hold nutrients because OM provides additional exchange sites and nutrients to the soil as it decomposes.
Management tactics, like the ones listed below, can affect the levels of OM in soil:
- tillage intensity or no-tillage
- organic amendments, such as manure
- cover crop usage for soil cover, grazing, forage, or green manure purposes
- crop rotation and residue management
Questions to consider about soil type:
- How do the plants or crops grown in a given location provide—or not provide—residue cover to the soil?
- How is any residue left behind managed? Is it tilled into the soil or left on the soil surface?
Soil health is an emerging topic within the field of agriculture, with researchers and farmers aiming to better understand and improve the ability of soils to function. Soil health refers to finding the right balance of living soil organisms, nutrient availability, and crop productivity by using sound management practices and principles. Soil health is determined by the interaction of biological, physical, and chemical factors, which come together to govern how soils function.
Healthier soils are more productive and provide increased ecosystem services:
- increased organic matter
- improved microbial activity
- greater ability to sequester carbon
- increased water infiltration, leading to less runoff and fewer nutrient loss events
- improved habitat for pollinators and wildlife
- healthier crops with higher yields by using fewer inputs, saving farmers time and money
- greater resilience to weather extremes
Many soil labs offer soil health testing services, but there is no standardized set of tests that universally defines soil health. Different labs offer various combinations of soil health tests that they believe work best. But they often don’t provide any baseline data representing good, bad, or mid-level soil test results to compare against. The Ohio State University has conducted a vast amount of research and continues to investigate the subject to determine what different soil test values mean to create a standard for soil health test results to be compared against.
For more information about the results of research from 2022, look in the 2022 eFields publication, pages 208–211. A full report was written to summarize six years of this data, titled “Baseline Assessment of Soil Health in Ohio.” View this report at soilhealth.osu.edu/sites/soilhealth/files/imce/WhitePapers/Baseline%20Ohio%20Soil%20Health.pdf.
In addition, the Ohio State University Extension Agronomic Crops Team offers winter programming on the subject of soil health. Recordings of webinars from this year and the previous two years can be found on the OSU Extension Agronomic Crops Team YouTube page. The 2023 Soil Health Webinar Series topics are as follows:
- Jan. 5, 2023, webinar: Precipitation & management and their effects on soil health by Dr. Peter Tomlinson, Kansas State University.
- Feb. 2, 2023, webinar: Know your biologicals and what they can (or cannot) do for you by Dr. Mark Licht, Iowa State University.
- March 2, 2023, webinar: Intercropping & soil health by no-till producer Lucas Criswell.
- March 30, 2023, webinar: Soil health & water quality by Dr. Vinayak Shedekar, The Ohio State University, and Dr. Will Osterholz, USDA Agricultural Research Service.
All previous soil health webinar recordings can be found on the OSU Agronomic Crops Team YouTube page, at youtube.com/channel/UCbqpb60QXN3UJIBa5is6kHw.