| **Central Great Plains Research     Station  USDA-ARS     / NRCS /     CSU cooperating in Akron,     Colorado     1907 - 2004**     
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**Conservation Tillage Fact Sheet**

**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.**
**Soil Quality, and Quality Soil**

**A quality soil is one with inherently good properties. Mostmedium-textural soils from native grasslands without excessive slope fall under thiscategory during the initial years of their cultivation. If the soil is highly resilientafter the sod is broken out, change is very slow. Alternatively, the soil could be veryfragile and rapidly degrade under bad management. Three parameters which reflectresilience are the texture (particle size distribution), pH, and soil organic matterlevels (SOM). These parameters greatly influence thephysical, chemical, and biological environment for good productivity, and require sometime for significant change. Soils that are too sandy will not hold much water, and soilsthat are too clayey will be difficult to work. Deep silt loams and loams, with adequateamounts of SOM, and pH near neutral appear desirable. As evidenced by the Dust Bowl of the1930s, even quality soils can loose SQ because of severe erosion and decline in SOM andfertility. Thus, type of cultural and management practices will greatly determine theproductivity and sustainability of a soil.**

**Cultivated Soil versus Native Sod**

**Because of the continued use of the traditional clean-till wheat-summerfallow, 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 soilslosing SOM at a faster rate than loams and silt loams. Today the cultivated soils of thecentral Great Plains generally contain less than 1.5% SOM in the top 6 inches. This lossin 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 mostof these fragile soils have lost some clay (erosion) which is the stabilizing agent forthe newly processed SOM (humus).**

**Signals of Soil Degradation and lossin SQ**

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

**Soil Quality Indicators (SQI)**

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

**1. Soil Series and Family 7. Depth to lime (solum depth)**

**2. Topography or slope 8. Fertility (nitrates and SOM)**

**3. Texture or particle size distribution 9. Salinity (electricalconductivity)**

**4. Soil pH or reaction 10. Aggregate Stability (turbidity)**

**5. Soil organic matter content, and residue 11. Infiltration**

**6. Presence of earthworms, and macropores**

**Soil Series and Family**

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

**Topography or Slope**

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

**Texture (Particle size distribution)**

**The percentages of sand, silt, and clay (texture) determines thephysical and chemical properties, and ultimately, the biology of the soil. The amount andtype of clay will determine fertility, water infiltration, organic matter accumulation andstabilization, erosion potential, and workability of that soil. While the soil surveygives an approximation of the texture, quick field tests based on feel for clay/siltcontent in a moistened soil can be performed, but may require some practice. With timeorganic matter and plant residues change the accommodation of the particles into what isreferred to as aggregates. Thus, clay particles and other components join together tostabilize the soil against erosion, and to increase water infiltration. Generally SOMincreases as clay content increases.**

**Soil Reaction (pH)**

**Soil pH determines the availability of nutrients, and is generally afunction of clay type and surfaces, and organic matter content. Certain pHs are diagnosticof certain problems. A soil pH around 7.7 indicates the presence of lime (requires apositive fizz test also). Phosphorus and micronutrient problems could exist on thesesoils. Above 8.4 without a fizz (application of acid on lime material releases a gas) mayindicate 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 isnear neutral (6.5 to 7.0) even though pH limitations only become serious outside of the5.6 to 7.6 range. Legumes generally prefer neutral to slightly alkaline soils. Many quickfield tests exist for evaluation of soil pH. With our increase in cropping intensity andno-till, and more frequent N fertilization, some of our surface soils are beginning toacidify, and may require addition of limestone to avoid a decrease in production.**

**Soil Organic Matter**

**The SOM content, and its direction of change over time, can bediagnostic of the health of the soil. Generally, high SOM content is associated with goodsoil physical properties and good productivity. Quick field tests exist for relativecomparisons among different cultural practices or rotations. The level of soil SOM,however, is sometimes used as a surrogate for sulfur needs and nitrate contribution, andits exact quantification once every three to five years is recommended. Organic matterchanges are a direct function of crop residue remaining, tillage, and climate (water andtemperature which affects decomposition). Crop residue is a direct function of croppingintensity, fertilization, tillage, and sanitation. The significance of the SOM levels isrealized more fully in conjunction with the soil texture. An Ascalon sandy loam will neverreach the organic matter levels of a Promise clay.**

**Presence of Earthworms and Macropores**

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

**Depth to Lime**

**Depth to lime is the depth where carbonates have been leached byrainfall over many centuries. This is also the depth into which the bulk of the rootsproliferate (especially cereal roots), and can demarcate the boundary between the solum (Aand B horizons) and the parent material ( C horizon). In Colby soils, for instance , thedepth 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 tolime.**

**Fertility (Nitrates and Soil Organicmatter)**

**Fertility assessment is best carried out at a soil and plant testinglaboratory. However, certain quick field tests exist for soil nitrates and SOM which canbe used as surrogates of soil fertility in a comparative manner. Knowing the texture andthe soil pH, the depth to lime, and the topography, the previous crop and yield, canassist in the interpretation of the nitrate and SOM levels. Soil can be taken from the topfoot or two feet, and the nitrate and organic matter estimated for those depths. A 1% SOMin the top 6 inches generally contributes about 30 lb of N to the crop. Knowing that about2 lb of N is required per bushel of wheat, one can estimate the N fertilizer needs basedon N in the soil profile and organic matter N contribution.**

**Salinity (Electrical Conductivity)**

**Crops differ in their tolerance to salts. While beans are sensitive,wheat is more tolerant. Salts usually accumulate from fertilizers and manures, and attimes 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 nearthe surface, salts can be leached, and appropriate crops can be grown. Quick field testswith electrical conductivity probes exist for salt evaluation. Where manure is rarely usedand the parent material is not overlain by shales, salinity does not present a problem.**

**Aggregate Stability and Infiltration**

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

**Soil Quality Indices**

**Since many of the parameters (texture, pH, depth to lime) of a qualitysoil are difficult to alter, it would appear that the direction of change in certainintermediate-term parameters such as the SOM, aggregate stability, and infiltration, couldserve as field indices. It is hoped that a universal indicator for the Great Plains can beobtained from a combination of the SOM, texture, and the depth to lime.**

**Measures to Improve Soil Quality**

**Soil quality in the central Great Plains can be maintained or increasesby: 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 needa system whereby we keep as much cover and residue on the soil surface as possible. Wecannot improve soil structure, stable aggregates, and increase water infiltration unlesswe build SOM; we cannot build SOM unless we produce and conserve more crop residue; and wecannot build crop residue unless we capture and conserve more water, and use that waterefficiently.**
- Conservation Tillage Fact Sheet #2-98. Published by USDA-ARS, USDA-NRCS,    and Colorado Conservation Tillage Association, By: R. Bowman,    Soil Scientist, USDA-ARS, M. Sucik, Soil Scientist, M. Rosales, Agronomist and J.    Saunders, Rangeland Specialist, USDA-NRCS, Central Great Plains Research Station, 40335    Rd. GG, Akron, CO, 80720.

**[Top][ 2002 Annual Report ][ Fact Sheets ][ Feed Back ][ Field Day ][ NRCS-Akron ][ Our Research ][ Publications ][ Related Links ][ Staff Pages ]****Questions? Comments? Contact forWeb Info RequestLast edited:****Wednesday April 14, 2004** |
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