Building Soils for Better Crops, Third Edition

Field Indicators

Overview
Figure 22.1. A soil penetrometer is a useful tool to assess soil compaction.
Figure 22.1. A soil penetrometer is a useful tool to assess soil compaction.

The next approach is more specific. In several states, farmers and researchers have developed “soil health scorecards.” The differences in soils and climates suggest that there is no uniform scorecard that can be used everywhere. Nor is there a magic number or index value for soil health. The goal of these scorecards is to help you make changes and improve your soil’s health over time by identifying key limitations or problems.

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 22.1) or metal rod, you should perform at least ten 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 in June 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 would have measured was the effect of variable soil moisture on soil strength. Similarly, earthworms will be abundant in the plow layer when it’s moist but tend to go deeper into the soil during dry periods. 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.

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. Also, you can take advantage of the fact that soil health problems tend to be more obvious during extreme conditions. It’s a good idea to spend some extra time in your fields during extended wet or dry periods.

Table 22.1: Qualitative Soil Health Indicators
IndicatorBest AssessedPoorMediumGood
EarthwormsSpring/fall. Good soil moisture.0–1 worms in shovelful of top foot of soil. No casts or holes.2–10 in shovelful. Few casts, holes, or worms.10+ in top foot of soil. Lots of casts and holes in tilled clods. Birds behind tillage.
Organic Matter ColorMoist soil.Topsoil color similar to subsoil color.Surface color closer to subsoil color.Topsoil clearly defined, darker than subsoil.
Organic Matter ResiduesAnytime.No visible residues.Some residues.Noticeable residues.
Root HealthLate spring (rapid growth stage).Few, thick roots. No subsoil penetration. Off color (staining) inside root.Roots well branched. A few roots grow through cracks and reach into subsoil.Roots fully branched and extended, reaching into subsoil. Root exterior and interior are white.
Subsurface CompactionBest pre-tillage or post harvest. Good soil moisture.Wire breaks or bends when inserting flag.Have to push hard, need fist to pish flag in.Flag goes in easily with fingers to twice the depth of plow layer.
Soil Tilth Mellowness FriabilityGood soil moisture.Looks dead. Like brick or concrete, cloddy. Either blows apart or is hard to pull drill through.Somewhat cloddy, balls up, requires multiple secondary tillage passes for good seedbed.Soil crumbles well, can slice through, like cutting butter. Spongy when you walk on it.
ErosionAfter heavy rainfall.Large gullies over 2 inches deep joined to others, thin or color of soil.Few rills or gullies, gullies up to 2 inches deep. Some swift runoff, colored water.No gullies or rills, clear or no runoff.
Water-Holding CapacityAfter rainfall.   During growing season.Plant stress two days after a good rain.Water runs out after a week or so.Holds water for a long period of time without puddling.
Drainage, InfiltrationAfter rainfall.Water sits for a long time, evaporates more than drains, always very wet ground.Water sits for short period of time, eventually drains.No ponding, no runoff, water moves through soil steadily. Soil not too wet, not too dry.
Crop Condition (how well it grows)Growing season. Good soil moisture.Problem growing throughout season, poor growth, yellow or purple color.Fair growth, spots in field different, medium green colorNormal healthy dark green color, excellent growth all season, across field.
pHAnytime, but at same time of year each time.Hard to correct for desired crop.Easily corrected.Proper pH for crop.
Nutrient-Holding CapacityOver a five-year period, always at same time of year.Soil tests dropping with more fertilizer applied than crops used.Little change or slow downward trend.Soil tests trending up in relation to fertilizer applied and crop harvested.
Source: Modified from USDA (1997).

The following paragraphs present some soil health indicators developed for scorecards in Maryland, Oregon, and Wisconsin. They are not discussed in any special order—all are important to help you assess soil health as it relates to growing crops. Table 22.1 provides further guidance on good sampling times and interpretation of the measurements.

Soil color is an indicator of soil organic matter content, especially within the same general textural class. Darkness indicates the amount of organic matter (see chapter 2) in the soil. We generally associate black soils with high quality. 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. However, their extent depends on whether an intense rainstorm has occurred, and whether a crop canopy or mulch protects 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 after heavy rainstorms, 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 by 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 raingear and go out during a rainstorm (not during lightning, of course), and you may actually see runoff and erosion in action. 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. If the soil crusts 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. Gently drop a number of aggregates from each field into separate jars that are half filled with water—the aggregates should be covered with water. See whether they hold up or break apart (slake). You can swirl the water in the cups 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 very slowly into the soil (figure 22.1). 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.

Soil is generally considered too hard for root growth if penetrometer resistance is greater than 300 psi. Note also whether the soil is harder beneath the plow layer. It is common to measure a dramatic increase in resistance when the bottom of the plow layer is reached. 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 easily 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 slowly dropping it from about waist height. Does the soil break apart into granules, or does it drop in 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.

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. They are also important “ecosystem engineers” that assist the initial organic matter breakdown that allows other species to thrive, but their general abundance is strongly affected by temperature and moisture levels in the soil. Their presence is 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 22.1). The most common worm types, such as the garden and red worms, live in the surface layer when soils are warm and moist and 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 to assess their presence, which is typically greatly enhanced in no-till systems. If you dig out a square foot of soil and find ten worms, the soil has a lot of earthworm activity.

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 we add heat and light. With a simple desk lamp shining on soil in an inverted cut-off plastic soda bottle (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; see a description of the procedure at https://pnwsteep.wsu.edu/edsteep/SoilInvertebrates/Berlese.doc).

Figure 22.2. A healthy corn root system with many fine laterals.
Figure 22.2. A healthy corn root system with many fine laterals. 

Root development can be evaluated by digging anytime after the crop has entered its rapid phase of growth. 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, and will they hold on to the aggregates when you try to shake them off (figure 22.2)? 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, fewer fine roots). Make sure to dig deep enough to get a full picture of the rooting environment, because many times there is a hard pan present.

The effects of soil health problems on general crop performance are most obvious during extreme conditions. That’s why it is worthwhile to occasionally walk your fields during a wet period (when a number of rains have fallen or just after a long, heavy rain) or during an extended drought. During prolonged wet periods, poor soils often remain saturated for an extended time. The lack of aeration stunts the growth of the crop, and leaf yellowing indicates loss of available N 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-tilled soil may also show greater effects. Purple leaves indicate a phosphorus deficiency and are also often an indirect sign of stress on the crop. This may be related to soil health but also can be brought on by other causes, such as cold temperatures.

Watch for stunted crop growth during dry periods and also look for the onset of drought stress—leaf curling or sagging (depending on the crop type). Crops on soils that are in good health generally have delayed occurrence of drought stress. Poor soils, especially, may show problems when heavy rainfall, causing soil settling after tillage, is followed by a long drying period. Soils may hardset and completely stop crop growth under these circumstances. Extreme conditions are good times to look at crop performance and, at the same time, evaluate soil hardness and root growth.

Nutrient deficiency symptoms can appear on plant leaves when soils are low in a particular nutrient (table 22.2). However, many nutrient deficiency symptoms look similar, and they also may 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 further up the stem may be 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 P for the plants. For pictures of nutrient deficiencies on field crops, visit https://www.extension.iastate.edu/Publications/IPM42.pdf.

Using the simple tools and observations suggested above, you can evaluate your soil’s health. Soil health scorecards or soil quality books provide a place to record field notes and assessment information to allow you to compare changes that occur over the years. You also can make up your own assessment sheets.

Table 22.2: Examples of Nutrient Deficiency Symptoms
NutrientDeficiency 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).

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