Class 3: Soil Sampling and Testing Chris Thoreau
What is Soil Testing? What are We Testing For? Soil Sampling Methodologies Soil Testing Interpreting Soil Test Results
What is Soil Testing? Soil testing is the use of various laboratory methods to discover chemical and/or biological qualities of a soil sample as part of an overall soil management plan Soil testing can also be used to discover the presence of contaminants such as heavy metals or pollutants
Testing for chemical properties is the most common type of soil testing and includes: ph Organic Matter % Nutrients N, P, K, Ca, Mg, Fe, Cu, S, Mn, Zn, B C:N Ratio Electrical Conductivity (EC) Cation Exchange Capacity Base Saturation Particle size distribution (texture)
Soil testing involves three basic stages: 1. Sampling 2. Testing 3. Interpreting Which, ideally, lead to actions to improve our soil quality.
Soil Sampling: The process of collecting soil to be submitted for testing. The collected soil is called a sample Soil must be collected carefully to ensure a representative sample is submitted for testing
Collecting Samples 2 basic methods 1. Composite Samples: Several samples are taken from a field and then mixed together to get a representative sample Samples must be taken randomly or in a set pattern to ensure a statistically representative sample is collected as we are collecting a small amount of soil to represent a large area
Collecting Samples Random Sampling Random locations Easy way to get a representative sample Must be truly random to be accurate! Zig-zag motion can be suitable
Collecting Samples Topography Sampling Variation of random sampling method Addresses variation in field topography or field features
Collecting Samples 2. Variation Sampling: Samples are taken from a field in a fixed pattern and each sample is kept separate Good for identifying variations in a field
Collecting Samples Grid Sampling Good for uniform field(s) Does not take into consideration topography Can be used for variation sampling
Variation in Fields e.g., OM and ph
Collecting Samples Tools: Clean trowel and shovel; Oakfield probe Clean bucket Plastic bag Plus, if doing variation sampling: Individual Bags Labels Field Map
Collecting Samples in the field: After establishing sampling points 1. Remove vegetation from soil surface 2. Using shovel, dig 20-30 cm deep hole in a small area (20 cm diameter hole) 3. Mix soil in hole well 4. Using trowel, scoop up a standard, level sample and place in bucket
Where don t we collect samples from? Anomalous/non-representative locations Around trees Field edges Near manure or compost piles Around standing water
Once all samples have been taken 1. Collect about 1 litre of soil from your mixture of samples 2. Place it in a bag and label it Collection date Name of field collected from Contact info for lab
When is a good time to be sampling? Close to time when soil will be used or Spring Or, end of season to prepare for following year Test for Nitrogen: mid- or late-season when soil is warmer When soil not too wet or dry Ideally test in the conditions in which plants will be growing
Once soil sample has been collected it needs to go to a lab Where do we send our samples? Always to the same lab Request list of methods use Methods are all standardized If new lab: Confirm methods used and compare results to previous test
Once you have test results you need to know what to do with them!
ph - Measure of soil acidity Or, the amount of H+ ions in soil Must measure ph in solution and residual Scale is from 0-14 0 = acidic; 14 = alkaline/basic ph in coastal soils is usually acidic Soil Acidity is neutralized through additions of Lime (CaCO 3 ) Amount of lime required depends on soil texture and OM content
Chemistry of raising ph ph Buffering Adsorption Affinity: Al 3+ > Ca 2+ = Mg 2+ > K + (H + ) = NH + 4 > Na +
Basic Liming Recommendations Contingent on measured ph, desired ph, clay content, OM %
Table 3. Lime Application Rates to Raise Soil ph to Approximately 7.0 Existing Soil ph 5.5 to 6.0 5.0 to 5.5 3.4 to 5.0 3.5 to 4.5 Lime Application Rate (pounds per 1,000 square feet) Sandy Loamy Clayey 20 30 40 50 25 40 55 70 35 50 80 80 Lime application rates shown in this table are for dolomite, ground, and pelletized limestone and assume a soil organic matter level of approximately 2% or less. On soils with 4 to 5% organic matter, increase limestone application rates by 20%. Individual applications to turf should not exceed 50 pounds of limestone per 1,000 square feet. Avoid the use of hydrated or burned lime because it is hazardous to both humans and turf (can seriously burn skin and leaves). If hydrated lime is used, crease application rates in the above table by 50% and apply no more than 10 pounds of hydrated or burned line per 1000 square feet of turf.
Organic Matter % % is by soil weight Ideal levels = 12-20% or more If tests show amounts below this level add more organic matter!
C:N Ratio Should be 15:1 to 24:1 If higher than 24:1, be careful to not add woody or high carbon materials to soil
Electrical Conductivity (EC) Measure of salt content in soil Tests should return levels below 4 mmhos/cm Rare to have high salts in soil around here Use gypsum to remove sodium in soil Flushing water with soil important
Cation Exchange Capacity Measure of soil s ability to hold and release positively charged particles Ca 2+, Mg 2+, K +, Na +, H +, Al + Higher CEC value is better >15 meq/100g soil or 20 cmol/kg soil OM and clay both have high CEC CEC Animation
Base Saturation Fraction of cation exchange sites occupied by base cations (Mg 2+, Ca 2+, Na +, K + ) Usually expressed as a percentage Base Saturation increases with soil ph
Base Saturation Ideal proportions of Base cations 60-75% Calcium 10-15% Magnesium 3 5 % Potassium <3% Sodium <12% H +
Nutrient Levels Nitrogen Leaf and Stem growth; photosynthesis; proteins Total N % =.5 -.7%; ppm = 20ppm Phosphorus ADP and ATP synthesis; photosynthesis; N-fixation; roots 125 300 ppm Potassium Enzyme activator; metabolism; stomata regulation 150 250 ppm
Nutrient Levels Calcium Important for cell growth and structure 1500 3000 ppm Magnesium Essential element of chlorophyll; P carrier; enzyme activator 14% of Calcium levels - 200 400 ppm Copper Seed formation; chlorophyll formation; enzyme activity 5 10 ppm
Nutrient Levels Zinc Component of many proteins; toxic at high levels 50 100 ppm Iron Important for e - transport: photosynthesis; toxic at high levels 150 250 ppm Manganese Assimilation of CO 2 in photosynthesis; e - transport in PS 75 125 ppm
Nutrient Levels Boron Cell wall strength; cell division; too much is toxic 2 4 ppm Sulfate Sulfur N-fixation; chlorophyll formation; seed production 100 150 ppm
Ideals Nutrient Ratios Calcium: Magnesium = 10:1 7:1 in sandy soils Phosphorus:Potassium = 2:1 Potassium:Sulfate-sulfur = 1:1
Interpreting Soil Test Results Soil Sampling Fact Sheet Gaia College Base Saturation Soil Test Interpretations 1 Soil Test Interpretations 2 Soil Testing for Organic Gardeners