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4.4.3: Field Indicators

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    Field Indicators

    The next approach involves addressing the same kinds of questions used to make field observations but in a more detailed manner. In several states, farmers and researchers have developed “soil health scorecards” based on observations made in the field. The Natural Resources Conservation Service (NRCS) has developed a somewhat different visual evaluation system, the "Cropland In-Field Soil Health Assessment Worksheet" (Table \(\PageIndex{1}\) is based on this worksheet). The goal of this type of assessment is to help you understand your soil’s health and improve it over time by identifying key limitations or problems. Most soil health issues are a result of low soil organic matter. That is why we are discussing soil health in the 'organic matter' chapter.

    Soil pentrometer being pushed into the ground by a man in jeans a button down and a hat.
    Figure \(\PageIndex{1}\): A soil penetrometer is a useful tool to assess soil compaction. Measurements are best made when soils are moist and friable (around field capacity).

    Whenever you try to become more quantitative, you should be aware that measurements naturally vary within a field or may change over the course of a year. For example, if you decide to evaluate soil hardness with a penetrometer (Figure \(\PageIndex{1}\)) or metal rod, you should perform at least 10 penetrations in different parts of the field and be aware that your results also depend on the soil moisture conditions at the time of measurement. If you do this after a dry spring, you may find the soil quite hard. If you go back the next year following a wet spring, the soil may be much softer. You shouldn’t then conclude that your soil’s health has dramatically improved because what you mostly measured was the effect of variable soil moisture on soil strength.

    Similarly, earthworms will be abundant in the surface 6–9 inch layer when it’s moist but tends to go deeper into the soil during dry periods, although you may still observe the wormholes and casts (Figure \(\PageIndex{2}\)). Make sure you select your locations well. Avoid unusual areas (e.g., where machinery turns) and aim to include areas with higher and lower yields.

    Holes in brown soil made by worms
    Figure \(\PageIndex{2}\): A soil with many wormholes suggests biological activity and improved potential for aeration and water movement.

    This type of variability with time of year or climatic conditions should not discourage you from starting to evaluate your soil’s health—just keep in mind the limitations of certain measurements. Generally, soil health is best measured in the early spring and late fall under moist (but not too wet) soil conditions. However, soil health problems are better observed during wet or dry periods when you might see runoff or crop drought stress symptoms.

    Table \(\PageIndex{1}\). Cropland Soil Health Assessment Worksheet.
    Indicator Soil Health Concern Best Time and Use Observation Benchmarks
    Soil cover Organic matter, organism habitat Anytime Greater than 75% surface cover from plants, residue or mulch
    Residue breakdown Organic matter, organism habitat Anytime; mostly no-till; farmer interview Natural decomposition of crop residues as expected; previous year residues partially decomposed and disappearing
    Surface crusts Aggregation Before tillage; before or early in the growing season Crusting in no more than 5% of the field
    Ponding Compaction, aggregation After rain or irrigation (not when frozen); farmer interview No ponding within 24 hours after major rainfall or irrigation
    Penetration resistance Compaction With adequate soil moisture; before tillage; before, early in or after the growing season Penetrometer rating <150 psi in surface layer and <300 psi in subsoil layer, OR slight or no resistance with wire flag inserted
    Water stable aggregates Aggregation, organism habitat Anytime Water-submerged in a glass jar: at least 80% remains intact after 5 minutes with little cloudy water
    Soil structure Compaction, soil organic matter, aggregation, organism habitat Anytime Granular structure in surface horizon and no platy structure in surface or subsoil horizons
    Soil color Organic matter With adequate soil moisture No color difference between field and fence row samples, OR value is in the darker range using a color chart
    Plant roots Compaction, organic matter, organism habitat During growing season Roots covered in a soil film or part of soil aggregates, OR living roots are healthy (no black/dry roots or lesions), fully branched, and extended into subsoil
    Biological diversity Organic matter, organism habitat With adequate moisture; before tillage More than three different types of organisms were observed without magnification
    Biopores Organic matter, compaction, aggregation, organism habitat Before tillage; mostly no-till Presence of root or earthworm channels that extend vertically through the soil, with some connecting to the surface
    Source: Modified from USDA (2021)

    Table \(\PageIndex{1}\) provides guidance on good soil health indicators, sampling times, and how to interpret measurements, and in the following paragraphs, we further clarify the practical considerations.

    Types of Field Indicators

    Soil Color

    Soil color is the result of a combination of the soil’s mineralogy, oxidation status, and organic matter content. Some soils are naturally redder (highly oxidized iron), brown (less oxidized iron), grey (poor drainage), or whitish (high lime content), but organic matter makes them dark. We therefore associate black soils with high quality, and within the same soil type and texture class you can reasonably conclude that the darker the soil, the better. However, don’t expect a dramatic color change when you add organic matter; it may take years to notice a difference.

    Crusting, Ponding, Runoff, and Erosion

    Crusting, ponding, runoff, and erosion can be observed from the soil surface. However, their extent depends on whether an intense rainstorm has occurred, or whether a crop canopy or mulch protected the soil. These symptoms are a sign of poor soil health, but the lack of visible signs doesn’t necessarily mean that the soil is in good health: it must rain hard for these signs to occur. Try to get out into the field sometime after a heavy rainstorm, especially in the early growing season. Crusting can be recognized by a dense layer at the surface that becomes hard after it dries. Ponding can be recognized either directly when the water is still in a field depression, or afterward in small areas where the soil has slaked (that is, aggregates have disintegrated). Areas that were ponded often show cracks after drying. Slaked areas going down the slope are an indication that runoff and early erosion have occurred. When rills and gullies are present, a severe erosion problem is at hand. Another idea: Put on your rain gear and actually go out during a rainstorm (not during lightning, of course), and you may actually see runoff and erosion in action. You might notice that most of the runoff and erosion that occurs comes from a relatively small portion of the field, and this may help in remedying the problem. Compare fields with different crops, management, and soil types. This might give you ideas about changes you can make to reduce runoff and erosion.

    Stability of Soil Aggregates

    You also can easily get an idea about the stability of soil aggregates, especially those near the surface. If the soil seals readily, the aggregates are not very stable and break down completely when wet. If the soil doesn’t usually form a crust, you might take a sample of aggregates from the top 3–4 inches of soil from fields that seem to have different soil quality (or from a field and an adjacent fencerow area). Gently drop a number of aggregates from each field into separate glass jars that are half-filled with water (the aggregates should be completely submerged in water). See whether they hold up or break apart (slake). You can swirl the water in the jars to see if that breaks up the aggregates. If the broken-up aggregates also disperse and stay in suspension, you may have an additional problem with high sodium content (a problem that usually occurs only in arid and semiarid regions).

    Soil Tilth and Hardness

    Soil tilth and hardness can be assessed with an inexpensive penetrometer (the best tool), a tile finder, a spade, or a stiff wire (like those that come with wire flags). Tilth characteristics vary greatly during the growing season due to tillage, packing, settling (dependent on rainfall), crop canopy closure, and field traffic. It is therefore best to assess soil hardness several times during the growing season. If you do it only once, the best time is when the soil is moist but not too wet (it should be in a friable state). Make sure the penetrometer is pushed slowly into the soil. Also, keep in mind that stony soils may give you inaccurate results: the soil may appear hard, but in fact your tool may be hitting a rock fragment.

    Soil is generally considered too tough for root growth if penetrometer resistance is greater than 300 psi, but fully unrestricted rooting in the surface layer generally requires soil resistance less than 150 psi. The soil is often harder in the deeper soil layers, and it is common to measure a dramatic increase in resistance when the bottom of the plow layer is reached, typically 6–8 inches into the soil. This indicates subsoil compaction, or a plow pan, which may limit deep root growth. It’s difficult to be quantitative with tile finders and wire, but the soil is generally too hard when you cannot push them in. If you use a spade when the soil is not too wet, evaluate how hard the soil is and also pay attention to the structure of the soil. Is the plow layer fluffy, and does it mostly consist of granules of about a quarter inch in size? Or does the soil dig up in large clumps? A good way to evaluate that is by lifting a spade full of soil and dropping it from about waist height. Does the soil break apart into granules, or does it fall into large clumps? When you dig below the plow layer, take a spade full of soil and pull the soil clumps apart. They should generally come apart easily in well-defined aggregates of several inches in size. If the soil is compacted, it does not easily come apart in distinct units.

    Healthy corn root system.
    Figure \(\PageIndex{3}\): A healthy corn root system with many fine laterals (roots shaken to remove aggregates and make them more visible).

    Soil Organisms

    Soil organisms can be divided into six groups: bacteria, fungi, protozoa, nematodes, arthropods, and earthworms. Most are too small to see with the naked eye, but some larger ones like ants, termites, and earthworms are easily recognized. These larger soil organisms are also important “ecosystem engineers” that assist the initial organic matter breakdown that allows other, smaller species to thrive. Their general abundance is strongly affected by temperature and moisture levels in the soil. They are best assessed in mid-spring, after considerable soil warming, and in mid-fall during moist, but not excessively wet, conditions. Just take a full spade of soil from the surface layer and sift through it looking for bugs and worms. If the soil is teeming with life, this suggests that the soil is healthy. If few invertebrates are observed, the soil may be a poor environment for soil life, and organic matter processing is probably low. Earthworms are often used as an indicator species of soil biological activity (see Table \(\PageIndex{1}\)). The most common worm types, such as the garden worm and red worm, live in the surface layer when soils are warm and moist, and they feed on organic materials in the soil. The long nightcrawlers dig near-vertical holes that extend well into the subsoil, but they feed on residue at the surface. Look for the worms themselves as well as their casts (on the surface, for nightcrawlers), and holes are evidence of their presence, which are typically greatly enhanced in no-till systems. If you dig out a square foot of soil and find 10 worms, the soil has a lot of earthworm activity. After soaking rains, many worms will come to the surface as the channels and burrows become saturated.

    With a little more effort, nematodes, arthropods, and earthworms can be removed from a soil sample and observed. Since these soil organisms like their environment to be cool, dark, and moist, they will crawl away when you add heat and light. A simple desk lamp shining on soil in an inverted cut-off plastic soda bottle with a small piece of screen at the bottom (what was the lower part of the bottle top) to keep the soil from falling through (called a Berlese funnel), you will see the organisms escape down the funnel, where they can be captured on an alcohol-soaked paper towel (the alcohol keeps them from escaping). Descriptions of how to make and use a Berlese funnel are readily available on the internet.

    Root Development

    Root development can be evaluated by digging anytime after the crop has entered its rapid growth phase. Have the roots properly branched, and are they extending in all directions to their fullest potential for the particular crop? Do they show many fine laterals and mycorrhizal fungal filaments (Figure \(\PageIndex{3}\)), and will they hold on to the aggregates when you try to shake them off? Look for obvious signs of problems: short stubby roots, abrupt changes in direction when hitting hard layers, and signs of rot or other diseases (dark-colored roots, lesions; fewer fine roots). Make sure to dig deep enough to get a full picture of the rooting environment because many times there is a hardpan present.

    General Crop Performance

    General crop performance as affected by soil health is most obvious during extreme conditions. During prolonged wet periods, poor soils remain saturated for an extended time, and lack of aeration stunts crop growth. Leaf yellowing indicates loss of available nitrogen by denitrification. This may even happen with high-quality soils if the rainfall is excessive, but it is certainly aggravated by poor soil conditions. Dense, no-till soil may also show greater effects.

    Watch also for the onset of drought stress—leaf curling or sagging (depending on the crop type)—and for stunted crop growth during dry periods. Crops on soils that are in good health generally have delayed signs of drought stress. But poor soils may show problems when heavy rainfall, causing soil settling after tillage, is followed by a long drying period. Soils may temporarily hard set and stop crop growth altogether under these circumstances.

    Field Images

    Field images from satellites, aircraft, or drones help you see crop performance anomalies and whether certain areas in a field have soil health problems. On a conventional color image, compacted or poorly drained areas show less crop biomass during the early season, i.e., more soil and less crop reflectance in the image. In wet years, areas with poor drainage may exhibit nitrogen deficiencies and appear more yellowish. Vegetation indexes (like NDVI, normalized difference vegetation index) can also help gain insights by showing vegetation density (Figure \(\PageIndex{4}\)). It may not give you a direct cause for the apparent problem, but it will at the least allow you to identify the location and check it out at ground level.

    You can evaluate your soil’s health using the simple tools and observations suggested above. Scorecards or assessment worksheets provide a place to record field notes and assessment information to allow you to compare changes over the years.

    Normalized difference vegetation index (NDVI) map.
    Figure \(\PageIndex{4}\): A normalized difference vegetation index (NDVI) map of a field shows areas of lower and higher vegetation density (darker areas have more vegetation), and can guide soil health examinations. Source: Yara International.

    This page titled 4.4.3: Field Indicators is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Valerie Dantoin via source content that was edited to the style and standards of the LibreTexts platform.