Soil Quality Indicators

Soil Quality Indicators for Whole-farm Management
in the Central Great Plains

R. Bowman, M. Sucik, M. Rosales, and J. Saunders
USDA-ARS, and USDA-NRCS, Akron, CO

The Fact: Soil quality (SQ) or soil health as it relates to the farmer/rancher/producer is the capacity of a given soil to sustain adequately plant and animal productivity without causing adverse effects on air and water quality. A more general definition would include support for human health and habitation. While air and water quality issues are more readily defined and quantified to protect human health, SQ issues are more subtle and difficult to quantify since many factors that cause soil degradation and a reduction in SQ come into play at the same time.

A quality soil is one with inherently good properties. Most medium-textural soils from native grasslands without excessive slope fall under this category during the initial years of their cultivation. If the soil is highly resilient after the sod is broken out, change is very slow. Alternatively, the soil could be very fragile and rapidly degrade under bad management. Three parameters which reflect resilience are the texture (particle size distribution), pH, and soil organic matter levels (SOM). These parameters greatly influence the physical, chemical, and biological environment for good productivity, and require some time for significant change. Soils that are too sandy will not hold much water, and soils that are too clayey will be difficult to work. Deep silt loams and loams, with adequate amounts of SOM, and pH near neutral appear desirable. As evidenced by the Dust Bowl of the 1930s, even quality soils can loose SQ because of severe erosion and decline in SOM and fertility. Thus, type of cultural and management practices will greatly determine the productivity and sustainability of a soil.

Because of the continued use of the traditional clean-till wheat-summer fallow, soils of the central Great Plains have lost over half of the original SOM content. Much of this loss occurred in the first 10 years of cultivation with more sandy soils losing SOM at a faster rate than loams and silt loams. Today the cultivated soils of the central Great Plains generally contain less than 1.5% SOM in the top 6 inches. This loss in SOM has also resulted in a loss of fertility and sometimes water storage capacity. Results from the CRP sites are showing that this loss is difficult to reverse since most of these fragile soils have lost some clay (erosion) which is the stabilizing agent for the newly processed SOM (humus).

Soil degradation is a slow process for most adversely affected parameters. We readily observe erosional gullies, and light-colored soil surface on hillsides and knolls. Water ponding and salt crusts are also easily observed. On more level soils however, negative changes in SOM, pH, texture, fertility aren=t visually observed as readily, and may require monitoring or making certain measurements. The Natural Resource Conservation Service (NRCS) Soil Quality Institute and the Agricultural Research Service (ARS) National Soil Tilth Laboratory in Ames, Iowa, and in Lincoln, Nebraska, have developed field kits and information on SQ issues that can be directly obtained through their offices, or from the NRCS Soil Quality team at Akron, CO.

Indicators are used primarily to assess changes in SQ from an earlier time to the present, or to compare an existing field against another with a different management scheme. Since climate and soil types influence changes, comparisons are best done with these factors held constant. Several SQ indicators can be qualitatively assessed in the field to conduct some of these comparisons. Some are readily assessed, and some may require the assistance of the Regional Soil Quality Team at Akron. This field evaluation does not negate the need from time to time for quantitative soil testing evaluation from a certified laboratory. Ten indicators are presented which represent various physical, chemical, and biological measurements:

3. Texture or particle size distribution 9. Salinity (electrical conductivity)

This information can be obtained directly from the county soil survey provided by the NRCS. As an example, the Weld soil, the most dominant soil series in Washington County, can be a loam or a silt loam. The family group shows that it is montmorillonitic (clay type) and of the Mollisol order. This information indicates that the top soil is of relatively good fertility. One has to remember, however, that this classification refers to the native sod characteristics which can be altered significantly by long-term cultivation. Data on water permeability and rooting depth is also provided. This information is basic for every farmer=s field, and is the first clue as to the potential for soil degradation and loss in SQ.

This information should be observed on the landscape even though it can also be found in the soil survey. Slope is important because it reflects runoff (water storage), soil depth (rooting volume), and general fertility. As an example, the Colby series is a shallow soil on knolls with poorly developed horizons (no B horizon) because of lack of water penetration and vegetation production. These soils are also low in soil organic matter, calcareous (presence of lime) near the surface with fertility problems associated with phosphorus, zinc, and iron. Unless organic matter and residue can be increased (manures and legume cover crops), productivity potential of these soils will be low because of lack of water storage. On the other hand, depositional areas or toeslopes will be very productive, and may require greater fertilization because of the extra water accumulation.

The percentages of sand, silt, and clay (texture) determines the physical and chemical properties, and ultimately, the biology of the soil. The amount and type of clay will determine fertility, water infiltration, organic matter accumulation and stabilization, erosion potential, and workability of that soil. While the soil survey gives an approximation of the texture, quick field tests based on feel for clay/silt content in a moistened soil can be performed, but may require some practice. With time organic matter and plant residues change the accommodation of the particles into what is referred to as aggregates. Thus, clay particles and other components join together to stabilize the soil against erosion, and to increase water infiltration. Generally SOM increases as clay content increases.

Soil pH determines the availability of nutrients, and is generally a function of clay type and surfaces, and organic matter content. Certain pHs are diagnostic of certain problems. A soil pH around 7.7 indicates the presence of lime (requires a positive fizz test also). Phosphorus and micronutrient problems could exist on these soils. Above 8.4 without a fizz (application of acid on lime material releases a gas) may indicate the presence of high sodium concentration. Water infiltration could be a problem. Low pHs are associated with manganese toxicity (<5.5), and aluminum toxicity (<5.0), coupled with Ca deficiency. Plants are stunted and root growth is limited. An ideal pH is near neutral (6.5 to 7.0) even though pH limitations only become serious outside of the 5.6 to 7.6 range. Legumes generally prefer neutral to slightly alkaline soils. Many quick field tests exist for evaluation of soil pH. With our increase in cropping intensity and no-till, and more frequent N fertilization, some of our surface soils are beginning to acidify, and may require addition of limestone to avoid a decrease in production.

The SOM content, and its direction of change over time, can be diagnostic of the health of the soil. Generally, high SOM content is associated with good soil physical properties and good productivity. Quick field tests exist for relative comparisons among different cultural practices or rotations. The level of soil SOM, however, is sometimes used as a surrogate for sulfur needs and nitrate contribution, and its exact quantification once every three to five years is recommended. Organic matter changes are a direct function of crop residue remaining, tillage, and climate (water and temperature which affects decomposition). Crop residue is a direct function of cropping intensity, fertilization, tillage, and sanitation. The significance of the SOM levels is realized more fully in conjunction with the soil texture. An Ascalon sandy loam will never reach the organic matter levels of a Promise clay.

Residue and organic matter are the food earthworms need to proliferate. Their presence is rarely found in dryland conventional wheat summer fallow systems. After seven years of intensive cropping and more residue production, our plots are finally showing the presence of earthworms. Their presence can be verified directly in the late spring since they are usually within the top foot of the soil, or can be verified from their castings which they leave behind on the soil surface. These earthworms to some extent through their mixing alleviate some of the problems of fertilizer stratification created by no-till systems. Macropores are usually created from the decaying roots under no-till conditions. These residue and decaying roots also serve to promote important fungal associations which increase water, phosphorus, and micronutrient uptake. Pores also increase infiltration rate, and worms may help to break plow pans.

Depth to lime is the depth where carbonates have been leached by rainfall over many centuries. This is also the depth into which the bulk of the roots proliferate (especially cereal roots), and can demarcate the boundary between the solum (A and B horizons) and the parent material ( C horizon). In Colby soils, for instance , the depth to lime is 7 inches, the Weld, 20 inches, and the Rago which has a buried horizon, 30 inches. The productivity of these soils is usually directly related to the depth to lime.

Fertility assessment is best carried out at a soil and plant testing laboratory. However, certain quick field tests exist for soil nitrates and SOM which can be used as surrogates of soil fertility in a comparative manner. Knowing the texture and the soil pH, the depth to lime, and the topography, the previous crop and yield, can assist in the interpretation of the nitrate and SOM levels. Soil can be taken from the top foot or two feet, and the nitrate and organic matter estimated for those depths. A 1% SOM in the top 6 inches generally contributes about 30 lb of N to the crop. Knowing that about 2 lb of N is required per bushel of wheat, one can estimate the N fertilizer needs based on N in the soil profile and organic matter N contribution.

Crops differ in their tolerance to salts. While beans are sensitive, wheat is more tolerant. Salts usually accumulate from fertilizers and manures, and at times from the natural geology (shales). Low areas can show salt crusting (white deposit, saline seeps); these areas are usually devoid of plants. If the water table is not near the surface, salts can be leached, and appropriate crops can be grown. Quick field tests with electrical conductivity probes exist for salt evaluation. Where manure is rarely used and the parent material is not overlain by shales, salinity does not present a problem.

Soil aggregation is the process whereby primary soil particles (sand, silt, clay) are bound together, usually by natural forces and substances derived from root and microbial activity. Its stability is based on its (the soil aggregate) resistance to withstand wind and water forces. Thus, stable aggregates are less erosive, and create less crusting and greater water infiltration. Field tests usually employ dry sieving or aggregate disintegration in water. While some turbidity evaluation can be visually seen with the field test (soils from sod in water are less turbid than soils from cultivated sites) tests among cultivated sites are not as readily discerned, and may require laboratory evaluation. Infiltration tests basically measure the time added water takes to disappear from the soil surface, and can indicate the presence of compaction or an impermeable subsurface layer.

Since many of the parameters (texture, pH, depth to lime) of a quality soil are difficult to alter, it would appear that the direction of change in certain intermediate-term parameters such as the SOM, aggregate stability, and infiltration, could serve as field indices. It is hoped that a universal indicator for the Great Plains can be obtained from a combination of the SOM, texture, and the depth to lime.

Soil quality in the central Great Plains can be maintained or increases by: 1) increasing the cropping intensity from wheat-fallow to 4-year rotations, 2) decreasing or eliminating tillage, 3) increasing broadleaves and legumes in the rotation, 4) proper fertilization and pest control, and 5) proper sequencing for water use. We need a system whereby we keep as much cover and residue on the soil surface as possible. We cannot improve soil structure, stable aggregates, and increase water infiltration unless we build SOM; we cannot build SOM unless we produce and conserve more crop residue; and we cannot build crop residue unless we capture and conserve more water, and use that water efficiently.