Understanding the Significance of Soil Management and Natural Resource Assessment in Watershed Planning

Soil, the thin layer of the earth’s crust, serves as the fundamental natural resource for restoring, maintaining, and enhancing the water resources of every watershed. Healthy soil is like a sponge-biologically active and porous, filtering nutrients and water for all plants and animals (Sachs, 1999).

Soil management places an emphasis on managing rainfall at the point of impact, thereby improving soil health, water quality, water quantity, and the health of the ecosystem. Soil management practices would minimize the impact of land uses, and the use of soil management tools would maintain healthy soils and sustain environmental quality. The overall goal is to apply techniques and practices to improve soil structure, organic-matter content, and infiltration ability of soils.

Understanding the Problems, Needs, and Opportunities

One important function of soil is its ability to act like a sponge. The porous nature of healthy soils containing organic matter will typically allow much of the rainfall to infiltrate. Rainfall infiltrates slowly to recharge groundwater supplies. The rate of water flow through the soil is much less than that of water running off the soil surface. Therefore, precipitation that filters through the soil is slower to discharge to surface waters (Pielov, 1998). Runoff from compacted soils is quicker, with higher peak flows. Less water is infiltrated, which reduces base flow conditions. This could result in such structures as bridges and culverts being underdesigned and could have profound economic and environmental impact.

In the simplest terms, healthy soils offer the opportunity to infiltrate rainfall at the point of impact, thus limiting runoff quantities and providing treatment of stormwater as it seeps into the ground. New water-quality goals, such as total maximum daily load, offer unique opportunities to apply the principles of soil management to future land development in order to contain polluted runoff and its associated pollutants. These new regulations make it imperative for water-quality issues to be addressed as soon as a raindrop hits the soil. Soil management is about protecting our waterways. It provides a key opportunity to meet the goals of the National Pollutant Discharge Elimination System Phase II, reducing polluted runoff from construction sites. It also offers practical solutions for minimizing nonpoint pollution sources from farmlands. Federal and state programs should be encouraged to install agricultural practices integrated with practices that support soil management.

The increasing demand for water resources requires decisions that cannot be made without natural resource data. Some resource data within our watersheds exist as disconnected pieces of information for answering specific questions. Because the management and use of soil affect water and other natural resources, this interaction must be taken into consideration. No existing inventories give a clear picture of the holistic soil and land-use composition that would lead us to understand how other resources are integrated and interact when soils are compacted through changing land uses. Without this information, planners cannot predict the impact or recommend best management practices for proposed land uses.

Soil and related natural resource information is useful for several levels of planning, ranging from national to local levels. Local land-use proposals must be based not only on local ecological conditions, but also on the relation between local conditions and the overall watersheds. The impact of future land uses can be reduced by first developing a soil/land-use resource inventory, then by encouraging the use of such information when reviewing future land uses.

Soil Structure

The structure of a soil influences its ability to support plant growth, receive and store water and nutrients, and resist erosion. Therefore, it is important to pay particular attention to soil structure where human activities can cause changes with either positive or detrimental impacts on the functions of soils within the watershed. Determining how soils react to these changes applied to them is a key concern (Winegardner, 1996).

The US Department of Agriculture’s (USDA) Natural Resource Conservation Service (NRCS) has been working closely with New Jersey’s Ocean County Soil Conservation District (OCSCD) in conducting soil bulk-density tests on various land uses. These field experiences on both agricultural and urban soils have led to a visual assessment of structural form, complemented by the quantitative analysis of samples. Bulk density is the key physical property of soil and will change in response to land disturbance (Sumner, 1999). It can be described as the weight of a unit volume of dry soil expressed as milligrams per cubic meter. Generally, soil having a bulk density of 1.33 mg/m³ is considered ideal, whereas bulk densities greater than 1.60 mg/m³ will inhibit plant-root penetration. Bulk density varies with the packing of soil properties. Similarly, sandy soils pack more closely, and clays cannot be packed as tightly (Sumner, 1999).

The following is a summary of the insitu bulk-density measurements tested to date. Note that the numbers in parentheses indicate how many sites were tested and that the data are averaged.

Soil conservationists use several words to explain soil properties, including the term structural stability. Structural stability is the ability of soil to retain its arrangement of solid and void space when exposed to different stresses. Stability characteristics are specific for a characteristic of each structural form and the type of stress applied (Croul, 1999). Stresses that create compaction could arise from tillage on farmlands, foot traffic on U-Pick farms, construction-equipment traffic, and lawn maintenance practices. The compaction causes a decrease in total pore space. The response of both the void and solid space to these stresses must be measured for various soils under different land management.

Soil resiliency is the ability of soil to recover its structural form through natural processes when the stresses are reduced or removed. Resiliency can normally arise from freezing/thawing, wetting/drying, and biological activity. When soils become highly compacted, however, water cannot penetrate the most restricted or compacted layer, and these natural processes to recover soil structure do not occur.

Soil texture has a major influence on the form, stability, and resiliency of soil structure (Sumner, 1999). In their simplest form, coastal plain soils are made up of single sand grains. The characteristics of their structural form are determined by the distribution of grain sizes and modification from traffic, grading, or tillage. Clay or silt-size particles may be present and can exist as coatings on the sand grains or provide some filling between the grains. This kind of structure does not shrink or swell and is not responsive to freezing. Since the organic matter is low, there is little cementation between the grains. These coastal plain soils are very vulnerable to compaction and are the least stable under a given stress. Therefore, they have little resiliency and do not recover under natural processes.

Healthy soils, with a good soil structure, support plant growth, cycle nutrients, receive and store water, resist soil erosion, and filter nonpoint pollutant sources. The activities of traffic and land grading and their associated soil compaction will lead to changes in the structure of the seedbed. The success of establishing plant growth in new seedbeds is dependent upon the degree of compaction and structural characteristics that control oxygen, water availability to the plants, and resistance to the penetration of roots.

Root development in plants is strongly influenced by soil structure. The root systems of many plants form a dense network in soils and lead to soil stabilization and reinforcement for streambanks and lake embankments. Soil cover provided by plants influences soil structure by reducing raindrop impact and allowing soil to absorb and retain water. However, root growth decreases as the resistance to penetration increases. Roots encounter increased resistance to penetration with increases in bulk density. OCSCD and NRCS have been using soil penetrometers to help determine root resistance. The output readings on the penetrometer measure the resistance related to root growth in a vertical direction only. These readings could be used to create a map of land use versus soil compaction problems within a subwatershed of Barnegat Bay and help provide needed natural resource data for future watershed planning. Such data could be used to map the extent of the compaction problem within a watershed and plotted on a Geographic Information System.

Soil erodibility defines the susceptibility of soils to erosion and largely depends on soil structure. Therefore, maintaining a good soil structure will help to build healthy soils, reducing the detachability of soil particles and the susceptibility of soil crusting. Soil management is an important component in preventing soil erosion, improving water management, encouraging plant growth, and improving water quality on our farms and in urban land uses.

Soil Management Components

Soil management encourages the natural function of a watershed. It supports the biological, chemical, and physical systems that are all linked through the soil so that a change in soil structure may cause changes in water and other natural resources. Applying soil management in land-use planning considers the interrelationship among proposed landscapes and provides a basis for making predictions, thus leading to better recommendations in site-plan review.

Protection of our soil resource is a key component of this approach. Such protection can prevent erosion and sedimentation, minimize flooding, and prevent water pollution. This approach requires the development of practices that will prevent soil compaction. As discussed earlier, compaction by traffic or grading is detrimental to soil structure, reducing soil porosity and the ability to grow vegetation. Decreasing the amount of clearing activities during the initial construction phase can minimize soil movement and reduce construction costs (Delaware Department of Natural Resources, 1997). While subsoiling may minimize compaction on farmlands, it is extremely costly and labor-intensive to undertake restoration of compacted soils on residential lawns. Irrigation systems, installed on many lawns, make it difficult to use aeration equipment. NRCS and OCSCD have tried to manually bore holes, and although meeting with some success, it is a laborious process. Further investigation is needed to find alternatives to restoration as well as to planning approaches so as to minimize grading, cut, and fill.

Improving soil health has social, economic, and environmental benefits. The application of the research of natural soil structure and bulk density and the development of methods for maintaining a good soil structure will lead to a reduction in runoff that transports pollutants to surface waters (Cahill Associates, 1992). That result will improve the soil’s physical characteristics and the storage of water and nutrients for plants (Zentner, 1997). This scenario is a major component of soil management.

Outcomes of Soil Management Program

• Ability to relate land use with the soil’s capabilities.
• Development of support information to guide land-use decisions.
• Soil and water management practices will be identified, developed, and evaluated for farmers and builders to better measure and minimize soil compaction.
• Soil productivity and farm profitability will increase with the development of soil management recommendations.
• Better understanding of the importance of soil management will lead to a more effective use of water resources, enhance nutrient conservation, and improve soil quality.
• Improved management practices will help restore degraded and compacted soils.
• Tools and interpretative information will be available to provide an assessment of soil health and the economic impact of improving soil quality.
• Improved design manuals and watershed models, through better engineering practices in stormwater management and soil conservation, will be available.

Conclusion

A review of scientific publications, staff experiences in the field with landscapers and contractors, and contact with professional soil scientists, planners, and engineers confirm that land development and agricultural practices have changed stormwater runoff patterns from diffused overland flows to increased and concentrated flows. Therefore, we must adopt an interdisciplinary approach toward the use of soils on farms and in urban landscapes if we are to restore the hydrologic water cycle and emulate natural drainage conditions.

What can we do to prepare ourselves for effective soil erosion control and better watershed management? What opportunities exist for future developments to reestablish a percentage of healthy soils? What benefits and costs would be associated with the pursuit of these opportunities? Besides water quality, what other benefits can be realized? These questions are fundamental when considering the integration of soil management practices into farm conservation plans and also when reviewing future development proposals. Urban developments tend to generate an increased amount of stormwater runoff and volumes along with increased quantities of pollutants contained in sediments, fertilizers, pesticides, and other toxic substances. Conventional developments strive to maximize building density, preserving open space on individual lots. Some open space is created, as required by local ordinances, for stormwater and/or recreational purposes. Stormwater management in a conventional development is concerned only with minimizing onsite and downstream flooding. The result is a constructed stormwater system to convey runoff, a detention basin and outlet structure to control stormwater release rates.

Healthy soils can provide years of essentially free stormwater management service and the conservation of water resources for future generations. Soil erosion control and administering watershed management programs require us to be concerned about infiltration and water-flow processes in soils. Infiltration is a key process because it determines how much rainfall enters the soil and how much becomes runoff. It is the key process in erosion and nonpoint pollution control because there is no erosion without runoff that transports sediment and other pollutants.

Soil management is fundamental to the well being of a watershed. To maintain productivity, to filter and regulate water flow through a watershed, and to cycle nutrients, it is critical-in the interest of human health and well being-that soil is properly managed.