Soil Quality: Why It Varies and Why it Matters

Soil quality matters if you are gardening, growing food crops, or have pets or small children playing outside. How and when you sample your soil can impact the results of soil quality testing. Sampling the same location twice is not likely to yield identical results, but this does not mean there is something inherently wrong with sampling or lab analysis. Result variability in soil samples can occur due to environmental factors, sampling strategy, and/or sample collection differences.

In the following in-depth guide we’ll break down soil quality and variability in soil testing. You’ll learn about:

• Soil Contamination and Soil Health
• Soil Sampling Strategies
• Environmental Factors
• Sampling Choices
• Sample Collection
• Key Takeaways
• Resources for Soil Testing

Soil Contamination and Soil Health

Typically we think about soil quality as falling into two categories: 

1. Soil contamination
2. Soil health 

Where soil contamination is largely relevant to understanding the risks of exposure, soil health refers to the biology, nutrients and basic chemistry of soil used for growing flowers, other plants, and food crops. 

Ultimately, what you test your soil for — as well as when and how you test your soil — will be determined by why you are testing.

Common reasons to test for soil contamination include:

• Curiosity about contamination
• Investigating local contamination 
• Before and after contamination
• Disaster or flood event
• Your house includes a pregnant, immunocompromised, or elderly person or young child
• Old home with chipping paint
• Concern about septic tank integrity
• Purchasing a new property
• Monitoring longer-term exposure
• Persistent contamination issues

Common reasons to test soil health include:

• Curiosity about soil health
• Gardening, landscaping, and agricultural work 
• Monitoring seasonal changes to soil health
• Research into soil ecosystems

The Importance of Strategy in Soil Sampling

Soil sampling strategy simply refers to the combination of decisions you make regarding what, when, where, and how you test your soil. 

Fundamentally, the strategy you use to sample is like choosing a frame for a picture: What gets included in the frame, and what gets left out, determines what you see. 

By carefully considering environmental factors, sampling design factors (e.g. spatial and temporal variability, sample depth, and proper handling procedures), and sampling collection, you can enhance the reliability of soil assessments.

How Environmental Factors Can Contribute to Result Variability

A variety of environmental factors can influence the results of a soil test. Examples of environmental factors impacting results include:

• Local groundwater and surface water quality: Water quality can influence soil quality and vice versa.
• Local air quality: Air quality can influence soil quality through deposition of contaminants on soil surfaces.
• Previous soil use: Fertilization or contamination from industrial activity are examples of land use that can have legacy effects on soil health and contamination profiles.
• Nearby septic systems: A high density of septic systems in the area can impact contaminant levels, especially if they are improperly maintained or leaking.
  
• Nearby land use: Certain land uses can lead to seasonal or long-term variability of certain contaminants. These land use patterns may include heavy industry, dumps or landfills, hydraulic fracturing activity, mining, and agricultural practices. Some common contaminants from these sources include:
  
  
  • Nitrates
  • Coliform bacteria
  • Heavy metals—like lead, arsenic, chromium, mercury, cadmium
  • Pesticides—including glyphosate (RoundUp)
  • VOCs—including those associated with petroleum products like BTEX (benzene, toluene, ethylbenzene, xylene) and MTBE
  • Emerging contaminants—like PFAS and pharmaceuticals, which are often found in fertilizer recycled from sewage
    

• Changes to land cover or foliage: When vegetation and topsoil are removed, soil quality changes and can be more prone to contamination. Converting natural land into agricultural fields can increase nitrate contamination of soil and groundwater.
  
• Seasonal variation: Rocks and minerals in contact with the soil may contain certain radioactive elements and harmful metals, which can enter the soil in varying amounts as they dissolve.
• Weather events: Extreme weather events like hurricanes and severe flooding can disrupt soil quality as they cause “mixing” of soil with water and other contaminants and hazards.

How Sampling Choices Can Contribute to Result Variability

Your Sampling Objective Determines What Your Sample Represents

Why are you testing? Understanding the reason for taking samples helps you define your “sampling objective.” This objective will determine what you test, where you test, and when you test. These decisions impact what your results represent, and ultimately, why analytical results may vary across samples.

For example, if you’re looking to understand baseline soil health for a small garden plot before growing season, a single sample may suffice even if the results differ from the average value over time. 

(Note: a single sample can be a composite of many sub-samples to capture heterogeneity in results over your sample area). 

However, if your objective is to monitor nutrients over time and capture as much variability as possible in your soil, a single sample in a single location is inadequate. Results from samples that are taken in different ways, intending to capture different effects, are unlikely to be identical.

Sample Location Impacts Results

As noted above, many environmental factors can impact soil quality. Soils are typically heterogeneous, which means that soil properties can vary significantly across a field or plot due to factors like topography, previous land use, and management practices. 

Sampling haphazardly in a field or methodically in uniform patterns (like sampling along a line) may introduce bias in sample results.

Sample Timing and Seasonal Variability Impacts Results

When you test can also impact results. Comparing results from different time periods will likely result in differences that need to be contextualized based on when they were sampled. 

Reducing result differences  introduced by temporal changes in soil quality requires an understanding of seasonal effects (which can impact moisture content and mobilization of contaminants). 

Soil Sample Collection Approach

There are multiple ways to collect a single sample of soil. What soil goes into the bag you send to the lab will ultimately impact your ability to compare those sample results to the results of another sample. Examples of how to collect a sample include:

• Single-location Sample: A soil sample collected from one specific location, representing only that precise location. Samples may not be representative of the overall soil quality if there is substantial variation in the soil.
  
• Composite Sample: A mixture of multiple subsamples (often 15-20 subsamples collected per sample) from different locations within an area. This is used to represent average conditions. Composite samples may under-represent a highly contaminated hot spot if averaged with subsamples that have low contamination profiles.
  
• Incremental Sampling Methodology (Advanced): A structured approach using many small samples collected across a defined area and combined to produce a representative sample with reduced variability. Expertise in soil sampling methodology is required.

For most informational testing, a single sample or composite sample at an appropriate sample depth is recommended. 

Sample Depth Impacts Result Variability

Sample depth refers to how deep the sample is taken, which can affect contaminant concentrations and soil characteristics. The best sample depth for a particular situation will depend on what information you are seeking from your sample. 

SimpleLab Guide to Soil Testing

How Sample Collection Can Contribute to Result Variability

Reading sample instructions carefully and planning ahead will ensure that you sample properly and helps minimize the chance of result variability across samples due to improper collection. 

A few key areas to consider for sample collection include:

Areas to Avoid

Avoid collecting samples in areas that are clearly not representative of your soil. Examples include:

• Areas that are wet
• Areas that previously had livestock
• Areas with erosion
• Areas where wood or other items were previously burned

An exception to this is whether you are explicitly testing these “areas to avoid” because you plan to use the soil for a new purpose.

Contamination

Analysis for some contaminants can be particularly vulnerable to contamination—bacteria and PFAS are both good examples. It can be easy to contaminate a sample container with bacteria if you happen to have the target species on your hands (a common occurrence). 

PFAS are common culprits for cross-contamination as well due to the fact that they are present (or suspected to be present) in so many everyday items, like certain rain jackets, fast food wrappers, the ink from some permanent markers (like Sharpies), etc. Writing on a sample bottle with a permanent marker suspected to contain PFAS or wearing clothing treated with PFAS while sampling can impact your results.

Take precautions to minimize cross-contamination according to sampling instructions.

Headspace

Depending on the type of analysis used to measure volatile organic compounds (VOCs), sample collection may require that you leave no space in the sample container above the soil itself. Space above the soil sample is called “headspace” and this happens when the container is not filled entirely to the top. This is a problem for VOC detections because they can leave the soil and remain in the air in the sampling container. 

Additional Resources

If you need assistance that is more specialized to your area, your local cooperative extension office is likely a great resource. They will be familiar with soils in your area and the common issues people have, and often offer soil testing services. 

If you’re in doubt, it’s always a good idea to take more samples if possible—the more data the better when you’re trying to understand variability in results across samples. All in all, soil chemistry is complicated and small changes in soil quality are to be expected.

Key Takeaways:

• Soil quality isn’t fixed — results can change based on when, where, and how you sample.
• Your testing goal drives your sampling strategy — choose locations, depth, and timing to match your objective.
• Environmental factors matter — weather, land use, and local conditions can all influence results.
• Good technique reduces variability — follow instructions, avoid contamination, and consider composite or incremental sampling.

Resources for Soil Testing

1. EPA’s "Soil Screening Guidance: Technical Background Document"
   This document explains variability in contaminant concentrations and how sampling strategy impacts reliability.
2. ITRC Soil Background and Risk Assessment Guidance
   Explains how environmental factors and historical land use affect contaminant profiles.
3. Purdue Extension – Soil Sampling for Home Lawns and Gardens
   Discusses practical soil sampling techniques for homeowners, with attention to sampling variability.
4. NRCS Soil Health – Indicators and Assessment
   Offers insight into biological and chemical factors that define soil health, emphasizing testing considerations.
5. University of Minnesota Extension – Soil Testing Guide
   A comprehensive guide on soil sampling methods, including composite sampling for residential use.
6. University of California Agriculture and Natural Resources – Soil Management Resources
   Includes strategies to assess soil fertility and contamination across seasons and land use types, with recommendations for different sampling goals.