Lesson 3 - Reading Your Soil Test

Lesson 1

A Soil Test Is a Snapshot – Not the Whole Picture

Soil tests are essential tools, but they're often misunderstood. A standard test tells you what's extractable under lab conditions – not necessarily what's available to plants in the field. Different labs use different extraction methods, which is why results from two labs on the same soil can look completely different.

The key is understanding what your specific test measures, what it doesn't measure, and how to connect test data to field observations and plant performance.

No single soil test tells the whole story. The most useful approach combines the right test for your questions with field observation and, when possible, plant tissue or sap analysis.

Lesson 2

Types of Soil Tests

Soil tests fall into three general categories, each designed to answer different questions. Click any card to learn more about when and why to use each type.

🧪

Standard Fertility Tests

Mehlich III, Bray P1, Olsen, Morgan

Measures extractable nutrients using chemical solutions. Most common and widely available.

🔬

Biological / Health Tests

Haney, PLFA, Solvita, Cornell

Measures soil biology, respiration, and plant-available nutrients in forms closer to what roots experience.

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Specialty / Targeted Tests

Saturated Paste, Total Digest, Microbiome

Answers specific questions about salinity, total reserves, or microbial community structure.

Common Methods

Mehlich III Bray P1 Olsen P Morgan / Modified Morgan Ammonium Acetate

What It Measures

Extractable levels of P, K, Ca, Mg, S, and micronutrients. Also pH, buffer pH, organic matter %, and CEC. Results indicate what a chemical extractant can pull from the soil – a proxy for plant availability.

Best For

Baseline fertility assessment, fertilizer recommendations, tracking changes over time, regulatory compliance. Most university recommendations are calibrated to these tests.

Strengths

Widely available, lower cost
Good for comparing fields
University-calibrated recommendations
Consistent methodology

Limitations

Doesn't measure biological activity
May not reflect true plant availability
Different extractants = different numbers
Misses organic nutrient pools

Common Methods

Haney Test (H3A extract) PLFA (Phospholipid Fatty Acid) Solvita CO₂ Burst Cornell Soil Health Test ACE Protein Index

What It Measures

Soil respiration (CO₂ burst), water-extractable organic C and N, plant-available nutrients using organic acid extractants that mimic root exudates, microbial biomass, and in some cases fungal:bacterial ratios.

Best For

Assessing biological activity, evaluating cover crop and compost impacts, understanding organic nutrient cycling, identifying why crops underperform despite "adequate" standard test levels.

Strengths

Measures biological activity
Accounts for organic N pools
Better predicts N mineralization
Shows soil health trends

Limitations

Less standardized across labs
Fewer calibrated recommendations
Higher cost per sample
Results can vary with sampling conditions

Common Methods

Saturated Paste Extract Total Nutrient Digest 16S/ITS Microbiome Nematode Assay Water Infiltration

What It Measures

Depends on the test: saturated paste measures actual soil solution chemistry and salinity; total digest shows total elemental reserves; microbiome tests identify bacterial and fungal species present.

Best For

Diagnosing specific problems (salinity, sodium, specific pathogens), research applications, high-value crops, or when standard tests don't explain field observations.

Strengths

Answers specific questions
High detail where needed
Can identify root causes
Useful for problem fields

Limitations

Higher cost, limited availability
Requires expertise to interpret
Not for routine monitoring
May generate more questions than answers

Lesson 3

Understanding Key Parameters

Regardless of which test you use, certain parameters appear repeatedly. Understanding what each one means – and what it doesn't – is essential for making good decisions.

Parameter Explorer

Click any parameter to learn what it measures and how to interpret it

Also: Soil Reaction

Measures hydrogen ion concentration – how acidic or alkaline the soil is. Affects nutrient solubility, microbial activity, and root function. Most crops prefer 6.0–7.0.

Optimal Range

6.0 — 7.0 for most crops

Low pH Effects

Al/Mn toxicity, reduced P availability, less bacterial activity

High pH Effects

Micronutrient lockup (Fe, Mn, Zn), reduced P availability

What It Doesn't Tell You

How much lime is needed – that's what buffer pH is for

Buffer pH

Also: Lime Index, SMP Buffer

Indicates the soil's resistance to pH change – how much lime is needed to raise pH. Soils with more clay and organic matter buffer more strongly.

What It Tells You

Lime requirement to reach target pH

Higher Buffer pH

Less lime needed (lower buffering capacity)

Lower Buffer pH

More lime needed (higher buffering capacity)

Common Methods

SMP, Sikora, Adams-Evans – don't mix between labs

Organic Matter

Also: OM%, SOM, LOI

Percentage of soil that is decomposed plant and animal material. Affects water holding, nutrient retention, biological activity, and soil structure. Changes slowly over years.

Typical Range

2–6% for most agricultural soils

Each 1% OM Holds

~20,000 gal water/acre, ~1,000 lb N reserve

Methods Vary

LOI vs Walkley-Black give different numbers

What It Doesn't Tell You

Quality of OM, how active it is, or how fast it's cycling

CEC

Also: Cation Exchange Capacity

The soil's ability to hold positively charged nutrients (Ca, Mg, K, Na, H). Higher CEC = more nutrient storage but also more lime/fertilizer needed to move the needle.

Low CEC (<10)

Sandy soils – nutrients leach easily; apply little and often

Medium CEC (10–20)

Loams – balanced retention and availability

High CEC (>20)

Clay/high OM – strong retention, higher rates needed

Units

meq/100g or cmol/kg (same thing)

Base Saturation

Also: % Ca, Mg, K, Na, H saturation

Shows what percentage of CEC is occupied by each cation. Some consultants target specific ratios; research suggests ranges matter more than exact ratios.

Calcium

60–80% typical; affects structure and pH

Magnesium

10–20% typical; excess can tighten soil

Potassium

2–7% typical; luxury uptake above ~5%

Caution

Ideal ratios are debated – don't over-engineer

Phosphorus

Also: P, Bray P, Olsen P, M3-P, H3A-P

Critical for root development, energy transfer, and reproduction. Numbers vary dramatically by extraction method – never compare Bray to Olsen directly.

Mehlich III

30–50 ppm often adequate for row crops

Bray P1

Best for acid soils (pH <7.2)

Olsen P

Best for alkaline soils (pH >7.2)

What Tests Miss

Organic P pools, mycorrhizal delivery, stratification

Potassium

Also: K, Exchangeable K, M3-K

Important for water regulation, disease resistance, and grain fill. Tests measure exchangeable K, but much more K exists in mineral forms that release slowly.

Low

<120 ppm – likely response to K

Adequate

150–250 ppm for most crops

Watch For

High K can suppress Mg and Ca uptake

K:Mg Ratio

Some target 0.3–0.5; balance matters for animal feed

Calcium

Also: Ca, Exchangeable Ca

Major structural cation affecting cell walls, root growth, and soil structure. Usually abundant but can be limiting in very acid or sodic soils.

Typical Range

1,000–4,000 ppm in most soils

Base Saturation

60–80% is common; higher in calcareous soils

Ca:Mg Ratio

Historically targeted 7:1; research suggests wider range OK

True Deficiency

Rare in field crops; more common in fruits/vegetables

Magnesium

Also: Mg, Exchangeable Mg

Central atom in chlorophyll, essential for photosynthesis. Also affects soil structure – excess Mg can make soils tight and sticky.

Typical Range

100–500 ppm; higher in clay soils

Base Saturation

10–20% is typical target

High Mg Soils

Can have structure problems; gypsum may help

Deficiency Signs

Interveinal chlorosis on older leaves

Sulfur

Also: S, SO₄-S, Sulfate

Essential for protein synthesis. Increasingly deficient as atmospheric deposition has decreased. Soil tests often underestimate plant-available S.

Adequate

>10–12 ppm SO₄-S for most crops

Why Tests Miss S

Most S is organic and releases during the season

High Risk Conditions

Sandy soils, low OM, high rainfall, no manure history

N:S Ratio

Target 10–15:1 in tissue for optimal protein

Soil Respiration

Also: CO₂ Burst, Solvita, Microbial Activity

Measures CO₂ released by soil microbes – an indicator of biological activity and potential nutrient cycling. Only available on biological/health-focused tests.

What It Indicates

Microbial biomass and activity level

Low (<40 ppm CO₂-C)

Sluggish biology, slow cycling

Ideal (80–150+)

Active biology, good cycling potential

On Haney Test

Drives Soil Health Calculation score (1–50 scale)

WEON / WEOC

Also: Water Extractable Organic N/C

Measures the organic nitrogen and carbon that dissolves in water – the fraction most available to microbes and closest to what roots "see" in the soil solution.

WEON

Organic N pool that will mineralize; reduces fert need

WEOC

Labile carbon feeding microbes; indicates cycling potential

WEOC:WEON Ratio

High ratio = N immobilization; low ratio = N mineralization

On Haney Test

Used to calculate organic N credit

Lesson 4

Why Numbers Look Different Across Labs

The same soil sent to two different labs can produce wildly different numbers – not because one is wrong, but because they're using different extraction methods. Each method "asks" the soil a different question.

Parameter	Mehlich III	Haney (H3A)	Why They Differ
Phosphorus	30–50 ppm adequate	Often 5–20 ppm	H3A uses weaker organic acids; shows only readily-available P
Potassium	150–250 ppm adequate	Often lower	H3A doesn't release K from clay interlayers
Nitrogen	Not typically measured	Inorganic + organic N	Haney includes WEON – organic N pool
Biology	Not measured	CO₂ burst, WEOC	Standard tests ignore biological activity

Important: Don't panic if your Haney P is lower than your Mehlich P. They're measuring different things. Haney shows what's immediately available; Mehlich shows what can be extracted with stronger acids. Both are useful.

Lesson 5

When Soil Tests "Lie"

Soil tests don't lie – but they don't tell the whole truth either. Here are common situations where test results don't match field performance:

High Test Levels, Poor Crop Response

Nutrients show adequate but crops are hungry. Possible causes: low biological activity isn't cycling nutrients, pH is locking out availability, compaction is limiting root access, or there's an antagonism (high K blocking Mg, for example).

Low Test Levels, Healthy Crops

Tests show deficiency but crops look fine. Possible causes: active biology is cycling nutrients faster than tests capture, mycorrhizal associations are delivering P, organic pools are mineralizing during the season, or the test method doesn't suit your soil type.

Inconsistent Results Year to Year

Same field, same lab, different numbers. Possible causes: sampling depth or timing changed, soil moisture at sampling was different, different area of field sampled, or real changes from inputs.

Always pair soil test data with field observation and, when possible, plant sap or tissue analysis. The goal is triangulation – no single data source tells the complete story.

Lesson 6

Choosing the Right Test

Different questions require different tests. Use this guide to determine which test type best fits your situation.

Which Soil Test Do I Need?

Answer questions to get a recommendation

What's your primary goal?

Lesson 7

A Practical Interpretation Process

When a soil test arrives, work through it systematically instead of jumping to specific numbers:

Soil Test Review Checklist

Start with pH. Is it in range for your crop? If not, other numbers may be misleading.

Check organic matter. Is it building, stable, or declining? This drives long-term trends.

Look at CEC. High CEC means more buffering; adjust rate expectations accordingly.

Review macros (P, K, S). Note any obvious deficiencies or excesses.

Check base saturation balance. Look for major imbalances, not perfect ratios.

Scan micronutrients. Flag anything clearly low (Zn, Mn, B are common issues).

If Haney: review biology indicators. CO₂ burst, WEOC:WEON, Soil Health score.

Connect to field observations. Do the numbers match what you're seeing?

Knowledge Check

Test Your Understanding

5 questions to reinforce key concepts

Up Next

Lesson 4: Understanding Plant Sap Analysis

Learn how to read real-time feedback from the plant itself and connect sap data to in-season management decisions.

Continue to Lesson 4 →