Making Sense of Lab Data

You’ve gotten some numbers back from the soils lab – what now? As a student, researcher, farmer, or soils enthusiast you must convert and interpret these numbers into something meaningful. First, take a look at your numbers and see if they make sense!

Below is a data set for two soil types, followed by a further description of the samples and soil series. Take some time to review the values and determine if they match the soil description. Look for relationships between the soil properties and also how values compare for the mineral soil (Crescent) vs. the organic soil (Lumbum). Some things you might think about are:

  • How do CEC and exchangeable/extractable cations relate?
    • Why might values be higher for the Lumbum soil? Do other parameters give any clues?
  • There is a significant difference in organic matter content between the two soils – can you see this reflected in other measures?
  • These soils seem very different, but the electrical conductivity (EC) is the same – how can this be?
  • How might pH affect the other values? Why are there 2 different pH measures and which one would you choose?
  • What does “available” P mean and why might you measure it?
  • How do the laboratory measurements complement the soil descriptions (e.g., texture, drainage)?
Examples of selected soil properties for two soil types commonly present in the Lower Fraser Valley, BC.
Laboratory Measurement Crescent Soil Lumbum Soil
CEC (meq/100g) 29.9 39.0
Exchangeable cations (meq/100g)
Ca 19.7 24.9
Mg 2.8 3.4
K 1.5 0.4
Na 0.1 0.2
Extractable cations, DTPA-TEA Analysis (µg/g)
Mn 7.7 40.7
Cu 1.1 10.4
Zn 1.9 18.6
Fe 238.2 401.5
Available P (mg/kg) 126.4 240.2
EC (dS/m) 2.5 2.5
Soil water content (%)a 1.4 15.8
Organic matter (%)b 11.0 60.3
pH in H2O 6.0 5.4
pH in 0.01M CaCl2 5.6 5.1
Bulk density (g/cm3)c 1.1 0.6

*Gravimetric water content. bby weight. aexcavation method for BD.

Crescent Soil
This particular sample is from the surface Ap horizon (0-15 cm) of a potato field.
Texture & description: Mainly silt loam to silty clay loam. The top 20 cm is a friable to firm, dark grayish brown, silty, cultivated layer. Soil is classified as Orthic Gleysol.
Soil moisture: Soils are moderately poorly to poorly drained; they are moderately pervious with slow surface runoff and have high soil water holding capacity.
Vegetation: Natural vegetation has been cleared and site has been cultivated. Rooting is slightly restricted below about 25 cm and severely restricted below 1 m.
Location: Crescent soils are common on Westham and Crescent islands, in the SE part of Richmond Municipality and SW of Ladner.
Land use comments: Good agricultural soils. Poor for urban and related construction.
Lumbum Soil
This particular sample is from the surface Ap horizon (0-20 cm) of a field where various vegetables are grown.
Texture & description: An organic soil. In cultivated areas, the surface (~20 cm) of Lumbum soils is partially-decomposed, black to dark reddish brown, friable organic material. Soil is classified as Typic Mesisol.
Soil moisture: Soils are very poorly drained; they are moderately pervious with slow surface runoff and very high water holding capacity.
Vegetation: A variety of vegetation is present in undisturbed areas. Rooting is mostly restricted to the upper 50 cm.
Location: Lumbum soils occur throughout the uplands and lowlands of the Langley-Vancouver area; the largest areas are on the lowlands of Richmond, Delta and Surrey Municipalities.
Land use comments:

Agricultural use is limited. Not suitable for urban and related uses.

Resources

  • Access to soil survey data, maps, and reports is available online through the B.C. Ministry of Environment.
  • Soils of the Langley-Vancouver Map Area (Report No 15 of the British Columbia Soil Survey). Volumes 1, 3, and 5 are the most relevant.

Calculating field values:

A common unit when talking about soil at the field-scale is kg per hectare. The concentration determined by lab analysis can be converted to these units if you have a soil bulk density value.

Example: Available P for Crescent soil, converting mg/kg to kg/ha

You need to know:

Bulk density for Crescent soil = 1.1 g/cm3 = 1100 kg/m3
Volume of one hectare furrow-slice that has depth of 15 cm = 1500 m3

Key things to remember when interpreting laboratory results:

  • Relate your data back to your site characteristics.
  • When doing further calculations and interpretation, remember to use the appropriate number of significant digits.

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