23.3: Field Indicators

The next approach involves addressing the same kind of questions listed above, but in a more detailed manner. In several states, farmers and researchers have developed “soil health scorecards” that are based on observations made in the field. The NRCS has developed a somewhat different visual evaluation system, the Cropland In-Field Soil Health Assessment Worksheet (Table 23.1 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.

Figure 23.1. 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 23.1) 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 tend to go deeper into the soil during dry periods, although you may still observe the wormholes and casts (Figure 23.2). 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.

Figure 23.2. 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 condition. But soil health problems are better observed during wet or dry periods when you might see runoff or crop drought stress symptoms.

Table 23.1 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 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 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 darker range using 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 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 23.1 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.

Soil color is the result of a combination of the soil’s mineralogy, oxidation status and organic matter content. Some soils are naturally more red (highly oxidized iron), brown (less oxidized iron), grey (poor drainage) or whitish (high lime content), but organic matter makes them more dark (see Chapter 2). 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 can be observed from the soil surface, as we illustrated in Chapter 15. 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 (Figure 15.1). 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.

You also can easily get an idea about the stability of soil aggregates, especially those near the surface (see Figure 15.1). 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 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 the friable state). Make sure the penetrometer is pushed slowly into the soil (Figure 23.1). 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.

Figure 23.3. A healthy corn root system with many fine laterals (roots shaken to remove aggregates and make them more visible). Compare with Figure 15.3.

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 23.1). 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 (Figure 23.2), 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. With 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 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 23.3), 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, 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 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 with poor soils they may show problems when heavy rainfall, causing soil settling after tillage, is followed by a long drying period. Soils may temporarily hardset and stop crop growth altogether under these circumstances.

Nutrient deficiency symptoms can appear on plant leaves when soils are low in a particular nutrient (Table 23.2). (Note that crop nutrient deficiencies can sometimes result from compaction and poor aeration, even though enough nutrients are present in the soil). Many nutrient deficiency symptoms look similar, and they may also vary from crop to crop. In addition, typical symptoms may not occur if the plant is suffering from other stresses, including more than one nutrient deficiency. However, some symptoms on some crops are easy to pick out. For example, N-deficient plants are frequently a lighter shade of green than plants with sufficient N. Nitrogen deficiency on corn and other grasses appears on the lower leaves first as a yellowing around the central rib of the leaf. Later, the entire leaf yellows, and leaves farther up the stem may become yellow. However, yellowing of the lower leaves near maturity is common with some plants. If the lower leaves of your corn plant are all nice and green at the end of the season, there was more N than the plant needed. Potassium deficiencies on corn also show as yellowing on lower leaves, but in this case around the edges. Phosphorus deficiency is normally noted in young plants as stunted growth and reddish coloration. In corn, this may appear early in the season due to wet and cold weather. When the soil warms up, there may be plenty of phosphorus for the plants. For pictures of nutrient deficiencies on field crops, see Iowa State University’s publication Nutrient Deficiencies and Application Injuries in Field Crops (IPM 42).

Table 23.2 Examples of Nutrient Deficiency Symptoms

Nutrient	Deficiency symptoms
Calcium (Ca)	New leaves (at top of plant) are distorted or irregularly shaped. Causes blossom-end rot.
Nitrogen (N)	General yellowing of older leaves (at bottom of plant). The rest of the plant is often light green.
Magnesium (Mg)	Older leaves turn yellow at edge, leaving a green arrowhead shape in the center of the leaf.
Phosphorus (P)	Leaf tips look burnt, followed by older leaves turning a dark green or reddish purple.
Potassium (K)	Older leaves may wilt and look scorched. Loss of chlorophyll between veins begins at the base, scorching inward from leaf margins.
Sulfur (S)	Younger leaves turn yellow first, sometimes followed by older leaves.
Boron (B)	Terminal buds die; plant is stunted.
Copper (Cu)	Leaves are dark green; plant is stunted.
Iron (Fe)	Yellowing occurs between the veins of young leaves. Area between veins may also appear white.
Manganese (Mn)	Yellowing occurs between the veins of young leaves. These areas sometimes appear “puffy.” Pattern is not as distinct as with iron deficiency. Reduction in size of plant parts (leaves, shoots, fruit) generally. Dead spots or patches.
Molybdenum (Mo)	General yellowing of older leaves (at bottom of plant). The rest of the plant is often light green.
Zinc (Zn)	Terminal leaves may be rosetted, and yellowing occurs between the veins of the new leaves. Area between veins on corn leaves may appear white.
Source: Modified from Hosier and Bradley (1999)

Figure 23.4. 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.

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 23.4). 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.