Guide B-830

Casey Spackman and John Idowu

College of Agricultural, Consumer and Environmental Sciences, New Mexico State University

Author:  Respectively, Assistant Professor and Extension Range Management Specialist, College of Agricultural, Consumer, and Environmental Sciences (ACES); Extension Agronomy Specialist, ACES. All from New Mexico State University.  (Print-Friendly PDF)  

Figure 1. Different tools that could be used for soil sampling. Tape on sampler represents 12 inch increments of depth. (Photo Robert Flynn).

Introduction

Rangelands make up approximately 80% of New Mexico’s land mass, with soil serving as the functional foundation. This essential natural resource provides ecological, economic, and cultural services to the state, highlighting the need for sustainable soil management. Despite its significance, soil management is often overshadowed by plant and animal management. However, without healthy soils, neither plant nor animal life could persist. Soils play a vital role in facilitating nutrient cycling, water retention and filtration, erosion prevention, and providing food and habitat for a variety flora and fauna populations.

Collecting and analyzing soil samples for appropriate properties provides land managers with the knowledge necessary for understanding the physical, chemical, and biological characteristics of below-ground resources. These properties serve as a reference to explain existing conditions and develop strategies for improvement of rangeland conditions. Moreover, they offer a framework for range management planning and informed decision-making processes.

Getting Started

Some tools are required before collecting samples in the field. The following are essential items for successful soil sampling.

1. Soil auger, probe, or a shovel and knife
2. Large bucket or container
3. Sealable plastic bags
4. Permanent marker
5. Large storage container

Different tools are available for acquiring a soil sample (Figure 1), each offering advantages depending on soil conditions. For example, a soil auger is effective for sampling soils that are difficult to penetrate. It features spiral-shaped flanges, a central shaft, and cutting tip to assist with penetration. The auger can be attached to a motor or twisted manually using T-shaped handles. Alternatively, a probe or soil sampling tube lacks spiral flanges and is generally used for less compact soils or areas with minimal obstructions. Some variations of the probe include foot platforms, which assist with applying downward pressure.

Figure 2. Open soil pit used to collect samples with the shovel and knife method.

The shovel and knife method, being the simplest and most cost-effective, consists of digging a V-shaped hole (Figure 2) and cutting thin, uniform samples from the side of the hole. This method is best suited for rocky soils but may be less effective in sandy soils, as hole edges may slough off, preventing a representative sample collection. After sample collection, open holes should be filled to prevent stepping injuries to wildlife, livestock, and other animals, especially when larger diameter samplers are used.

When collecting multiple soil samples, one or more large buckets (i.e. a 5-gallon bucket) or similar containers can be used to efficiently mix and composite collections from the same zone. It is important to properly clean the bucket or container between sampling to prevent cross-contamination. Sub-samples are then collected from bucket, stored, and transported using sealable plastic bags. Some laboratories provide soil sampling bags upon request.

If the soil is moist at the time of sampling, it is advisable to air dry it before sealing in a plastic bag. Air drying can be achieved by placing the samples in the bag, on a platform inside a closed room, and then opening the seal to allow air to dry the soil. Avoid air drying outside in direct sunlight. Improperly air-dried samples may develop mold if stored or shipped for prolonged periods due to lack of bag breathability. To mitigate mold risk, temporary breathable paper bags can be used; however, it’s important to note that glue on many paper bags is water-soluble and may disintegrate if wet soil is placed in them. For shipping purposes, the dried sample should be transferred into a sealable plastic bag to prevent spillage during transport.

Proper labeling of bags with sampling location, date, and sample number ensures accurate record-keeping and traceability throughout the sampling and shipping process. Alternatively, assigning a unique sample identification number to each bag allows for easy reference as long as a sample logbook is kept. The number can then be readily referenced to know where, when, and how samples were collected.

Field Sampling

Rangeland soils exhibit remarkable diversity, and strategic sampling will help ensure that collected samples accurately represent the landscape. Establishing a clear objective and carefully considering where, how, and when to sample will guide the development of effective soil sampling strategies.

Define an objective

Clearly defining an objective is a critical step as it lays the groundwork for the entire sampling process. Objectives determine where to sample, which soil properties to analyze, and how to interpret and implement subsequent management plans. For instance, if the overarching goal is to enhance grazing management, the focus might be assessing soil nutrient content. This allows for a better understanding of vegetation productivity, necessary for determining grazing rest periods or the formulation of a targeted soil amendment strategy. Regardless of the objective, it should always align with specific land management strategies.

Identify zones

Rangeland soils exhibit significant variability influenced by factors such as topography, vegetation, and land management practices. Categorizing areas into zones of similar characteristics, such as soil types (Figure 3), aids in targeted soil sampling. Ecological Site Descriptions (ESD) assist with this zonal partitioning by offering insights into distinct land types with specific soil and physical attributes. Accessing and understanding ESD is provided by the Natural Resources Conservation Service (NRCS) through the Web Soil Survey website (https://websoilsurvey.sc.egov.usda.gov/app/). These maps not only define zonal boundaries, but also serve as historical references for expected soil properties, land use suitability, vegetation characteristics, and various other soil attributes. However, it is essential to note that ESD and survey maps should not be the sole determinant of a zone, as management history also plays an important role. For instance, the grazing history of various pastures may differ, requiring separate sampling and partitioning of zonal areas. Other distinguishing properties used to identify sampling zones include slope of the land, soil text differences, drainage, soil amendment history, and occurrence of erosional features.

Figure 3. A rangeland area (462 ac) with four different soil types (orange lines separate the soil type boundaries). (Diagram John Idowu)

Determine sample frequency

There is no established standard for the frequency or quantity of soil samples needed within a designated rangeland zone due to inherent zonal variability and microsites. As a general guideline, collecting as many samples as feasible and composite them into a unified, mixed sample for subsequent analysis is recommended. Some soil laboratories suggest compositing a minimum of 10-15 samples per management zone. The more composited samples from a management zone, the more reliable the results will be. Compositing serves to mitigate soil variability by creating an average representation of soil properties in a given area. A subset of this composite sample can then be submitted for analysis.

Figure 4. Soil sampling techniques within a zone: a) zig-zag pattern, b) grid pattern, c) transect, and d) random pattern (diagram by Casey Spackman).

To prevent sampler bias, maintaining a consistent distance between samples is crucial. Employing a systematic approach, such as following a zigzag path, implementing a grid system where samples are collected within defined squares, or collecting at designated intervals along a transect (Figure 4), ensures a representative sampling process that effectively captures soil variability. It is advisable to avoid sampling in animal congregation areas, such as supplementation sites, as this can bias results from analysis. For instance, these sites may have higher than normal mineral or nitrogen content due to supplementation stations or increased feces occurrence. Additionally, scraping or removing above-ground vegetation or litter prior to sampling is advised, as its presence can bias organic matter results.

Depth

The appropriate depth for soil sampling depends on the specific objectives and nature of the soil profile under consideration. Some sampling endeavors may target the surface soil (0- 6 inches), while others may seek a deeper exploration (exceeding 12 inches). Terrain characteristics also play a role. For instance, slopes or hilltops may have a shallow impermeable layer, limiting penetration to superficial depths. Bottoms or swells might allow for deeper penetration and a deeper soil assessment.

A common soil sampling depth is 6 inches (Figure 5), as it is most affected by various land management practices. This layer provides valuable information on nutrient availability, organic matter content, and soil texture. Extending the sampling depth by an additional 6 inches offers further insights into nutrient distribution and plant rooting patterns, recognizing that nutrient distribution is not uniform throughout the soil profile and can exhibit patterns of accumulation or leaching.

Regardless of the sampling depth, composited samples must be collected consistently and at similar depths. Although samples may originate from different depths, they should be composited and analyzed separately (e.g., 0-6 inches as one sample and 6-12 inches as another). This practice enables an understanding of distinct soil functions at various depths, contributing to a more comprehensive assessment of the soil profile.

Some soil testing laboratories provide information on specific depths for sample collection, especially for soil amendment recommendations. It is recommended that sampling depth is approved by the soil laboratory prior to sampling to ensure quality assessments and recommendations.

Timing

The timing of soil sampling is influenced by different factors, including the objective, environmental conditions, or management practices. Sampling during the growing season provides insights into soil conditions affecting plant health, while considering seasonal variability inherent in rangelands captures fluctuations in soil moisture, temperature, and plant growth patterns. Precipitation events can influence nutrient leaching, erosion, and soil structure, while temperature changes affect soil moisture content and plant growth, directly impacting soil conditions. Sampling that coincides with specific environmental or management events, such as grazing or rainfall, allows for an assessment of changes over time. Whether aiming for regular monitoring or targeting specific plant phenological influences (e.g., plant growth stage), the timing of soil sampling plays a critical role in understanding the dynamics of rangeland soils. Consistency in collection timing, such as sampling around the same time each year, enhances the reliability and comparability of soil data over time.

Analysis and Tests

Understanding the chemical, physical, and biological properties of soils through analyses and tests is essential for effective rangeland management. These properties offer valuable insights that empower landowners, ranchers, and land managers to make informed decisions aimed at optimizing rangeland conditions and productivity. The following soil assessments can be conducted in-the-field or through a certified laboratory to inform rangeland management practices. For interpretation of analysis and tests refer to NMSU Circular 676.

Figure 5. Soil sampling tube collection at a depth of six inches. (photo Casey Spackman)

A. Chemical measurements

Soil pH:  Measures the acidity or alkalinity of soils, which influences microbial activity, nutrient availability, and growth of plants. Microbes break down organic matter and transform or release nutrients that are then available for plant uptake. However, they can only persist in a given pH range, emphasizing the importance of proper pH levels in the soil.

Electrical Conductivity (EC):  This is a measure of soil salinity. This is done by measuring the ability of the soil to conduct an electrical current, indicating salt and nutrient concentrations. In arid and semiarid regions like New Mexico, EC values are used to assess the salt concentrations within the soil. High EC is indicative of a high salt concentration in the soil which can negatively influence plant growth.

Nutrients:  Assesses levels of essential nutrients such as nitrogen (N), phosphorus (P), potassium (K), and other micronutrients critical for plant growth. Understanding where excess or deficient nutrients exist can guide management practices, such as applying soil amendments, implementing revegetation efforts, or altering grazing management strategies.

Organic Matter (OM): Determines the percentage of OM in the soil, which affects soil structure, water retention, and nutrient cycling. The OM binds soil particles into aggregates, altering the structure. Subsequently, porosity, water-holding capacity, microbial activity, and nutrient cycling all increase, benefiting plant growth and soil health.

Cation Exchange Capacity (CEC):  Measures the soil’s ability to hold and exchange essential nutrients, such as potassium, calcium, magnesium, providing insights into soil fertility and nutrient retention. Soils with higher CEC can hold more nutrients, more readily supplying nutrients to plants for growth.

Total Carbon and Nitrogen:  Measures the total carbon and nitrogen content in the soil, providing insights into nutrient cycling and OM deposition. Carbon serves as an energy source for soil microorganisms, influencing microbial activity and OM decomposition, while nitrogen is essential for plant growth and productivity.

B. Physical Measurements

Soil Texture:  Identifies the proportion of sand, silt, and clay in the soil, influencing water retention and drainage, nutrient-holding capacity, and helps determine which plant species are likely to thrive on a particular site. Soil types often have different textures, subsequently determining pore size and water movement. Additionally, some plant species have varying soil texture preferences, helping to explain the prevalence of one species over another.

Water Infiltration Rate:  Assesses how quickly water moves vertically into the soil. The rate is dependent upon soil texture, structure, and soil moisture content. Healthy soils can retain large amounts of water in soil pores, reducing surface accumulation, limiting erosion risk, and providing water for plant growth.

Bulk Density:  Determines the compactness of the soil, aiding in understanding root growth and water movement. Bulk density helps identify areas prone to compaction, which can lead to reduced water infiltration and plant growth. Management practices can be implemented to minimize compaction in sensitive areas.

Soil Moisture Content:  Monitors the amount of water present in the soil at a given point in time aiding in the understanding of nutrient turnover, microbial activity, and plant growth. Soil processes heavily depend on water, thus inferences can be made about the current soil condition and implement adaptive management practices to benefit soil health.

Water Holding Capacity:  Assesses the ability of the soil to retain water against the force of gravity. Although similar to soil moisture content, it is distinct in that it measures the maximum amount of water that a soil can retain at any point in time. It is influenced by soil texture, structure, and OM content. Water holding capacity is important for nutrient availability, plant growth, and many other soil processes.

C. Biological Measurements

Microbial Biomass and Activity:  Assesses the abundance and activity of soil microorganisms, crucial for nutrient cycling and overall soil function. Microbes transform inaccessible nutrients into plant-available forms, suppress plant pathogens, decompose organic material, and degrade pollutants, all to facilitate a healthy soil environment and support plant growth.

Soil Respiration Rate:  This is a measure of how biologically active the soil is and also tracks the process of organic matter breakdown. The measurement captures how much carbon dioxide is being emitted from the soil as soil microbes undertake respiration. This can be measured in the field or the laboratory, depending on the method and equipment used.

Active carbon:  This measures the fraction of the soil organic matter that serves as a readily available food source for soil microbes. Active carbon is an indicator of soil microbial activity in the soil and the higher the active carbon, the more microbial activity in the soil. This carbon fraction is also called labile carbon and there are different methods used in the laboratory to assess it. Two popular methods are particulate organic matter and permanganate oxidizable carbon.

Protocols for Physical and Biological Measurements

Soil physical and biological measurements may require different sampling methods depending on specific measurement being taken. For instance, bulk density is typically sampled using previously described equipment producing soil cores, whereas infiltration measurements are conducted in the field using ring infiltrometers. Some laboratories are capable of analyzing soil texture and water capacity from the same samples for soil chemical analysis; however, soil biological analysis often entails more expensive procedures, necessitating special handling during sample collection. For additional guidance with physical or biological soil measurements, contact your local county extension office.

Conclusion

Understanding and evaluating soil properties is critical for effective rangeland management, enabling informed decision-making focused on enhancing the productivity, sustainability, and resilience of rangeland ecosystems. Equipped with the appropriate tools, sampling strategies, and soil tests, land managers can tailor their practices to suit the unique characteristics of the soil. Overall, prioritizing soil assessments and understanding their significance in rangeland management can ensure the long-term health and vitality of rangeland ecosystems. 

Casey Spackman is an Assistant Professor and Extension Range Management Specialist at New Mexico State University. He earned his Ph.D. at Utah State University. His Extension efforts aim to assist producers, land managers, and agency personnel in monitoring and developing management objectives that maintain or improve natural resource health and sustainability.  

To find more resources for your business, home, or family, visit the College of Agricultural, Consumer and Environmental Sciences on the World Wide Web at pubs.nmsu.edu

Contents of publications may be freely reproduced for educational purposes. All other rights reserved. For permission to use publications for other purposes, contact pubs@nmsu.edu.

New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.

September 2024 Las Cruces, NM