New Mexico State University

Guide A-146
Revised by Musfiq Salehin and John Idowu
College of Agricultural, Consumer and Environmental Sciences, New Mexico State University

Authors: Respectively, Extension Soil Fertility Specialist; and Extension Agronomist. Both from the Department of Extension Plant Sciences, New Mexico State University.

Introduction

Soil testing is an integral part of crop production. It guides decisions on which crop and variety to grow, how to manage water, and what nutrients the soil requires. Routine soil tests assess physical and chemical properties, while tests such as soil health can tell us about the biological status of the soil. Physical properties relevant to plant production include soil texture, compaction, soil structure, and water-holding capacity. Chemical properties include soil pH, salinity, and plant nutrients. All of these are very important for deciding which crop to grow in your soil, how much fertilizer to apply, and how to manage your soil throughout the crop growth period.

Soil testing provides a “snapshot” of conditions at the time of sampling, allowing farmers and homeowners to plan their management practices for the coming growing season. Common questions like “How much manure should I add?” or “What type of fertilizer should I use this year?” or “Do I need to lower my soil pH?” can be answered with a soil test. It is recommended to test your soil every year before the crop season to assess its chemical and nutrient status. If that is not possible, then at least every other year. While soil chemical properties and nutrient status are important to understand for your crop production, soil physical and biological properties are important for assessing and managing soil health. USDA NRCS’s dedicated soil health program can help you understand and build a soil health management plan (https://www.nrcs.usda.gov/conservation-basics/soil/soil-health).

Collecting a Sample from the Field

A soil test is only as good as the sample from which it came. One core from the corner of a field, a fairway, or one’s front yard does not represent the whole field, golf course, or yard. Samples should be taken from areas that can be treated as distinct units (management zones). The rough on a golf course differs from the green, and some areas cross soil type boundaries that vary greatly in their physical and chemical properties. It usually takes 10 to 15 subsamples taken to the same depth and combined into one sample in order to be confident in the soil test results from any given lab. Crop fields larger than 25 acres should be divided into separate sampling sections to capture differences within the field. When deciding how to divide a field, consider historical yield variation along with factors like slope, weed pressure, and soil type. The recommended number of subsamples ensures that the sample is representative of a “management zone.” This area may be treated the same for watering, fertilizer application, and yield potential.

The soil depth for sampling depends on tillage practices and the crop that is being grown. Areas that are not tilled, such as direct-seeded fields, orchards, vineyards, turfgrass, and perennial gardens, should be sampled to a depth of 6 to 8 inches. Any area that has been tilled, such as annual gardens, conventionally tilled production fields, or similar areas, should be sampled to a depth of 12 inches. It is also ideal to sample at different depths to understand the differences between them. As shown in Figure 1, a soil core sampler extending to a 12-inch depth can be divided into two depth ranges 0-6” and 6-12”) to illustrate the depth differences. Within each section, collect multiple cores and combine them in a bucket or container labeled with the sampling depth. Mix the cores thoroughly, then pull a subsample from the bucket to send to the lab (Figure 1). Before shipping, let the samples air-dry until they’re reasonably dry — spreading them on a paper towel overnight is usually sufficient. Do not oven-dry soil samples.

The second foot (12 to 24 inches) and deeper soil depths (>24 inches) are often sampled to improve salinity and nitrogen management. When sampling for soil analysis, it is important to brush aside or remove leaves and other plant residues from the surface to ensure organic or other foreign materials do not interfere with soil fertility or chemical assessment. The tool of choice is a probe that allows easy soil retrieval without leaving behind a greatly disturbed area. Some soils are too hard to push a probe into easily, and a hammer is therefore necessary. Some probes are sold with “sliding-hammer” attachments that make sampling much easier. Spades/shovel can be used if measures are taken to ensure multiple samples are taken from the same depth and limit the amount of soil per sample. For more information on proper sampling techniques, please refer to Guide A-114, Test Your Soil (https://pubs.nmsu.edu/_a/a-114.pdf) and Guide B-830 Soil Sampling in Rangelands (https://pubs.nmsu.edu/_b/B830.pdf). You can also view a video on soil sampling at https://westernsoil.nmsu.edu/soils/soil.html#started or take a free short course titled Ground Truth: Soil Sampling Essentials on the NMSU microlearning website https://nmsuondemand.nmsu.edu/browse/aces/ex/courses/ground-truth-soil-sampling-essentials.

What Analyses to Request

Annual routine soil tests before crop season should include basic soil chemical properties such as pH, salinity, and plant nutrients. Key routine analyses that should be requested from any soil testing laboratory include:

• pH
• Soluble salts or electrical conductivity (EC)
• Base saturation or sodium adsorption ratio (SAR)
• Organic matter (OM) or soil organic matter (SOM)
• Inorganic nitrogen (nitrate-N)
• Bicarbonate extractable phosphorus (P) or Olsen P
• Extractable potassium (K)
• Extractable Calcium (Ca)
• Extractable Magnesium (Mg)
• Extractable Sodium (Na)
• Micronutrients such as iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), and boron (B).
• Soil texture

These tests are part of routine soil testing in many laboratories and are used in soil fertility assessment. Analyzing soil texture is not required annually, but it is important for the first time, when choosing a field or establishing a home garden, to decide on crop management practices.

There are specific ways the soil must be processed in the lab to obtain accurate results. Soil pH, soluble salts, and SAR are determined from a saturated paste extract. The saturated paste extract provides the most accurate estimate of soil salinity as plants experience it, without dilution. As New Mexico soils are high in salts, it is important to assess the soil salinity using the saturated paste extract. Most labs now determine soil organic matter content by dry combustion at a specific temperature. Inorganic nitrate-N can be determined by extracting the soil with a potassium chloride solution and testing for nitrate-N after cadmium reduction. Phosphorus is determined after extracting the soil with a solution of sodium bicarbonate (Olsen’s procedure) if the soil pH is above 7.2. A solution of ammonium acetate is used to extract potassium. Micronutrients, required by plants in small quantities, can be evaluated for plant availability by extracting with a DTPA solution. Other nutrients to consider include sulfate and boron. Please consult CR-650: Sulfur and New Mexico Agriculture (https://pubs.nmsu.edu/_circulars/CR650/index.html), for more information regarding the need for sulfate. The boron test is often offered but seldom needed in New Mexico and should only be considered if plants exhibit deficiencies or tissue testing reveals low boron levels. Boron can be toxic to plants if applied at excessive concentrations. The amount of soil lime present in the sample is also helpful when considering what plants to grow.

Saturated Paste Extract (Method S – 1.10 through S-1.60 in Miller et al. [2013])

This method is used to determine a soil’s salinity by mimicking the moisture conditions a plant’s roots actually experience. To prepare a sample, soil is air-dried, ground, and passed through a 2-mm sieve. Deionized water is then added a little at a time until the soil turns into a thick, saturated paste. You know it is ready when it glistens, flows slightly when the container is tipped, and slides cleanly off a spatula. Soil texture strongly governs the volume of water required to reach saturation: fine-textured, high-clay soils need considerably more water and a longer equilibration period than coarse, sandy soils. After mixing, the paste is capped and allowed to equilibrate—typically 4 hours for most soils, but up to 24 hours or overnight for high-clay soils—after which the saturation endpoint is reconfirmed and soil or water added as needed before extraction. The equilibrated paste is transferred to a Büchner funnel fitted with filter paper, and the solution is drawn out under vacuum (suction). The collected liquid, the saturated paste extract, approximates the soil solution composition a plant root experiences near field capacity, shortly after irrigation. Thus, this method is the reference standard for arid and semi-arid soils because it establishes a reproducible, texture-independent moisture basis for comparing salinity across soils with different textures. In New Mexico’s calcareous, salt-affected soils—where soluble salts, sodium, and gypsum content vary widely—the saturated paste extract corrects for the strong dilution effects that confound fixed soil-to-water-ratio methods (e.g., 1:1 or 1:5), making the saturation extract electrical conductivity (ECe) the standard against which crop salinity-tolerance thresholds and reclamation guidelines are calibrated. Saturated paste pH and ECe are measured directly on the extract, and the sodium adsorption ratio (SAR), which expresses the relative proportion of sodium to calcium and magnesium, is calculated from the measured soluble cation concentrations.

Soil pH (Method S–1.10 in Miller et al. [2013])

Among common household items, vinegar, cola, and coffee are acidic, whereas antacids, detergents, and bleach are alkaline. New Mexico soils are mostly alkaline (>7.0), which can limit nutrient availability, especially phosphorus and micronutrients, even if these nutrients are abundant in the soil. Most other nutrients are likewise affected by pH; only nitrogen, potassium, and sulfur are largely unaffected. Soil pH in New Mexico normally ranges from 6.5 to 8.4. Mountain soils, which receive more rain or snow, generally fall below 7.0, and some garden soils amended with large amounts of organic matter may also be lower, though not always. For New Mexico soils, pH should be determined on the saturated paste (pHs), which best reflects field conditions.

Knowing the soil’s lime content is also helpful when considering gypsum as a reclamation tool. Gypsum is a calcium mineral (CaSO₄) and is often used to reclaim sodic soils. However, if the soil already has a high lime content (CaCO₃), alternative sources should be explored, such as elemental sulfur or sulfur-based fertilizers.

Calcium Carbonate (Carbonate Qualitative Test, Method S–13.05; Gravimetric Determination, Method S–13.10 in Miller et al. [2013])

Across much of New Mexico, soil pH is governed by the presence of lime (calcium carbonate). Lime buffers the soil against pH change and can tie up phosphorus and iron, making them unavailable to plants. The carbonate content can be measured at a soil-testing laboratory—qualitatively by an effervescence test (S–13.05) or quantitatively by gravimetric determination (S–13.10). A quick field check uses household vinegar: pour a little on the soil, and effervescence (fizzing) indicates lime is present.

A soil containing 5% lime in the top 6 inches holds roughly 2,296 pounds of lime per 1,000 square feet, and soils with more than 3% lime usually have a pH above 7.5. All this lime must be neutralized before soil pH can be lowered. Elemental sulfur is a common amendment used to acidify soil but requires time to dissolve the lime; no pH change occurs until the lime has been consumed—often making acidification impractical. It is more economical to grow plants tolerant of alkaline, high-lime soils. See the USDA PLANTS Database at https://plants.usda.gov for information on plant tolerance to alkaline soils.

Soil Organic Matter (SOM) (Total Organic Carbon, Dry Combustion, Method S–9.30 in Miller et al. [2013])

Soil organic matter reflects far more than visible compost and roots; it is the stabilized fraction of carbon-rich material that governs many soil functions. In the 4th-edition Western Region methods, organic matter is best determined by high-temperature dry combustion (S–9.30), in which a soil sample is combusted, and the evolved carbon dioxide is measured to quantify total carbon, with total nitrogen determined simultaneously. Because dry combustion recovers total carbon, the inorganic carbonate (lime) fraction must be removed by acid pretreatment—or measured separately and subtracted—before the result represents true organic carbon. This step is essential for New Mexico’s calcareous soils, where untreated samples would otherwise substantially overstate the organic matter content. Organic carbon is then converted to organic matter using a conventional factor (commonly ×1.724). Dry combustion has largely replaced the older Walkley-Black wet-oxidation procedure, offering better precision, no hazardous dichromate waste, and simultaneous measurement of carbon and nitrogen. As a general guide, roughly 30 pounds of plant-available nitrogen can be credited to the soil each year for every 1% organic matter in the top foot. Knowledge of organic matter content helps explain salinity behavior, water-holding capacity, and nutrient reserves. Organic matter benefits soil and crops by:

• Strengthening soil aggregates, which improves tilth and structure.
• Improving aeration and water infiltration.
• Increasing water-holding capacity (0.08 to 0.19 inches per 1% SOM).
• Providing a large number of nutrient-exchange sites.
• Buffering against rapid changes in soil pH.
• Forming stable organic compounds that can increase micronutrient availability.
• Supplying plant nutrients (about 0.7 pounds N per 1,000 square feet per year per 1% SOM per foot of soil).
• Providing a food source and habitat for soil microorganisms, earthworms, and other beneficial organisms.

Nutrient Analyses

Three primary nutrients should be evaluated routinely: inorganic nitrogen, phosphorus, and potassium. Five micronutrients that are routinely requested are iron, zinc, copper, manganese, and boron. The nutrients are interpreted according to the likelihood that plants would respond to additional fertilizer. A soil that is ranked low means there is not enough of that nutrient for the plant to grow correctly, and that the plant would benefit from an application of that nutrient. If the nutrient is ranked highly, there is little chance the plant would respond to additional amounts of that nutrient (i.e., it would be a waste of money to add more).

Inorganic Nitrogen (N) (Nitrate-N, Method S–3.10; Ammonium-N, Method S–3.50 in Miller et al. [2013])

Inorganic-N is the sum of nitrate-N and ammonium-N. However, most inorganic N in the soil is present as nitrate-N. Therefore, it is often measured only as nitrate-N, and N fertilizer rates are adjusted based on soil nitrate-N. A common extraction method uses potassium chloride (KCl) to recover inorganic N from soil. The fertilizer nitrogen needed is the difference between the nitrogen already in the soil and the amount required to support a given crop; fast-growing plants generally require more nitrogen than slow-growing ones. Nitrogen applications may initially be reduced or eliminated when the soil tests high in inorganic-N. Fertilizer should be applied when temperature and moisture conditions favor active growth, not during periods of stress or inactivity. Heavy rain or excessive irrigation leaches nitrate-N from the soil, so samples taken before a leaching event overestimate soil inorganic-N and underestimate the nitrate-N the crop will subsequently need.

Phosphorus (P) (Method S–4.10 in Miller et al. [2013])

Because western soils are typically alkaline, plant-available phosphorus should be determined with a sodium bicarbonate extractant (Olsen procedure), which correlates well with plant-available phosphorus in high-pH soils. New Mexico soils low in organic matter are usually also low in available phosphorus. Soil chemistry in New Mexico favors the formation of apatite, a calcium-phosphate mineral that plants cannot use, so plant-available phosphorus is typically low in alkaline soils and generally must be supplemented. Phosphorus fertilizer can be applied at roughly one-fifth the nitrogen rate in routine applications.

Potassium (K) (Method S–5.10 in Miller et al. [2013])

The accepted method for plant-available potassium is extraction with water or ammonium acetate solution. The water-extractable potassium recovers only a fraction of the extractable potassium in most soils; the value of any method ultimately depends on how well reported values relate to plant response. As with other nutrients, a low potassium rating generally indicates a good chance of crop response to added fertilizer.

Micronutrients (Method S–6.10 in Miller et al. [2013])

For Western soils, iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) are commonly assessed using the diethylenetriaminepentaacetic acid (DTPA) extraction method (Method S–6.10). This chelating extractant provides an index of potentially plant-available micronutrients, particularly for mildly acidic to calcareous soils. Results are typically reported as mg kg-1 soil or ppm (parts per million) and should be interpreted using crop-specific calibration data, soil pH, texture, organic matter, and carbonate content when deficiency is suspected.

An alternative multi-element method is DTPA–sorbitol extraction (Method S–6.12), which estimates potentially available Zn, Mn, Fe, Cu, and B in a single extraction and is commonly analyzed by an instrument called Inductively Coupled Plasma (ICP) equipped with Optical Emission Spectroscopy (OES). This instrument measures the elemental concentration in a soil extract. In addition, sorbitol allows boron (B) to be included with the other micronutrients. This method is useful where laboratories want a combined micronutrient extraction, but values should not be directly interchanged with standard DTPA or hot-water B results unless the laboratory has established method-specific interpretations.

Boron can also be measured separately using the hot-water extraction method (Method S–7.10). This method estimates relative soil B bioavailability using 0.02 M CaCl2 with hot-water extraction, followed by azomethine-H colorimetry or ICP-OES analysis. Hot-water B is used primarily to evaluate potential crop B deficiency, while saturation-paste soluble B is more appropriate for evaluating excessive or potentially toxic soluble B conditions. Because the range between B sufficiency and toxicity is narrow, soil-test results should be interpreted with caution and, when possible, confirmed by plant-tissue testing and crop-specific recommendations.

Sulfate (SO₄-S) (Method S–11.10 in Miller et al. [2013])

For western soils, sulfate-S is measured by extracting the soil with a calcium phosphate solution. This test provides an estimate of the sulfur available for plant uptake, but it should be treated as a general guide rather than an exact measure, as crops can also receive sulfur from organic matter decomposition, irrigation water, fertilizer, and other sources. For most crop nutrient recommendations, this extractable sulfate-S test is the most useful method. For more on sulfur in New Mexico, see CR-650: Sulfur and New Mexico Agriculture (https://pubs.nmsu.edu/_circulars/CR650/index.html).

Laboratories

Soils are complex biological, chemical, and physical systems. Diligence in interpreting soil test results can save hundreds of dollars every year. The only way to know what a soil is truly like is to start with a soil sample. Several commercial labs are listed in Table 1, along with test packages or names that would be useful for interpreting New Mexico soils. Be aware that procedures should be similar to those suggested in this document. Individual labs vary in the services they offer, prices, and the time they require for analyses. The list of laboratories in Table 1 is not all-inclusive, and the list of services may change over time. To select a lab, consider convenience, services offered, and quality. Consider using laboratories that participate in the North American Proficiency Testing Program (Table 1). This program assists soil, plant, and water testing laboratories in their performance through inter-laboratory sample exchanges and a statistical evaluation of the analytical data.

Quality Assurance

Soils are heterogeneous, and replicates of the same sample will never yield exactly the same analytical result from one laboratory to another. The North American Proficiency Testing Program guidelines have been developed for the agricultural laboratory industry by representatives from groups familiar with and involved in standardizing methods and developing nutrient recommendations for soil and plant analysis methods within the U.S. and Canada. It is operated as an activity of the Soil Science Society of America and overseen by an oversight committee comprising representatives of the aforementioned groups. These include regional soil and plant analysis workgroups, scientific organizations, state/provincial departments of agriculture, and private and public plant analysis labs.

Finally, keep a record of your lab results. If you need help interpreting the results, please consult with your New Mexico State University Cooperative Extension Service county office.

For Further Assistance

Clients are encouraged to contact their county Extension agent with specific questions, or Dr. Musfiq Salehin, who has assisted with soil test interpretations in New Mexico, at salehin@nmsu.edu or 575-748-1228.

References

1. Flynn, R.P., & Ulery, A.L. (2011). An introduction to soil salinity and sodium issues in New Mexico [Circular 656]. Las Cruces, NM: New Mexico State University Cooperative Extension Service. https://pubs.nmsu.edu/_circulars/CR656.pdf 
2. Flynn, R.P., Ulery, A.L., & Lindemann, W.C. (2010). Sulfur and New Mexico agriculture [Circular 650]. Las Cruces, NM: New Mexico State University Cooperative Extension Service. https://pubs.nmsu.edu/_circulars/CR-650.pdf 
3. Glover, C.R., & Baker, R.D. (2000). Test your soil [Guide A-114]. New Mexico State University Cooperative Extension Service. https://pubs.nmsu.edu/_a/a-114.pdf 
4. Miller, R. O., Gavlak, R., & Horneck, D. (2013). Soil, Plant, and Water Reference Methods for the Western Region (WREP-125).
5. Spackman, C., & Idowu, J. (2024). Soil sampling on rangelands [Guide B-830]. New Mexico State University Cooperative Extension Service. https://pubs.nmsu.edu/_b/B830.pdf 
6. Thien, S.J. (1979). A flow diagram for teaching texture by feel analysis. Journal of Agronomic Education, 8, 54–55
7. USDA–NRCS. (n.d.). Soil Health. https://www.nrcs.usda.gov/conservation-basics/soil/soil-health 

Original Author: Robert Flynn, Extension Agronomist, Department of Extension Plant Sciences, New Mexico State University.

Musfiq Salehin is an Extension Soil Fertility Specialist at of Extension Plant Sciences and Assistant Professor at the Agricultural Science Center at Artesia, NM in NMSU. He earned his master’s degree in Plant and Environmental Sciences from New Mexico State University and his Ph.D. in Agronomy from Texas A&M University. His research program integrates soil fertility, nutrient management, and climate-smart agricultural practices to enhance the productivity and sustainability of semi-arid and subtropical cropping systems.

John Idowu is an Extension Agronomist in the Department of Extension Plant Sciences at NMSU. He earned his master’s in agronomy from the University of Göttingen in Germany and his Ph.D. in land management from Cranfield University in the UK. His research and Extension activities are focused on sustainable crop production and soil management in New Mexico.

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September 2026. Las Cruces, NM.