W2, Soil testing

Question 1

Why is soil testing for micronutrients not necessarily the best option?

Answer 1

• uptake is highly dependent on crop species/cultivar
• amounts taken up are small, and there are analytical issues with determining such trace levels in soil
• the cost of correcting deficiencies is small, and so high precision isn’t required

Question 2

Fill in the blank…

A soil test is only as good as the method of ________ and soil _________ prior to chemical analysis.

Answer 2

A soil test is only as good as the method of sampling and soil preparation prior to chemical analysis.

Question 3

What factors determine the depth that you should sample soil to?

Answer 3

• Crop type
  
  • deep-rooted crops require deeper sampling
• Soil type
  
  • distinct horizons in the soil profile may require separate sampling
    
    • e.g. duplex soils will have a huge difference in clay content, and therefore nutrient dynamics, between layers.
• Analyte (i.e. what you’re testing for)
  
  • e.g. nitrate (N) or sulfate (S) may require deeper sampling because they’re both negatively charged and are (therefore) leached from the topsoil quickly.

Question 4

Explain what the ‘maintenance’ application of a fertiliser refers to.

Answer 4

Supplying only what is going to be/has been removed at harvest.

Question 5

What are the differences between EC_e and EC_1:5 and why would you use one over the other?

Answer 5

• EC_1:5 is easy and doesn’t require any special equipment guesswork (what constitutes “glistening”).
• An EC_e value is going to be higher than EC1:5 because the solution is more concentrated, but this is closer to what’s actually experienced by the plants
• Conversion from EC_1:5 to EC_e requires a conversion factor based on soil texture (e.g. sand = 12.5, loam = 8.0, light clay = 7.0, heavy clay = 6.0)

Question 6

Explain the process for testing EC_e.

Answer 6

1. Dry soil at 40°C
2. Grind and/or sieve to < 2 mm
3. Add soil to a mixing bowl
4. Add water and mix the soil with a flat spatula until the soil just “glistens”
5. Remove water using suction over a filter
6. Measure EC using a calibrated electrode

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Question 7

What’s the single most useful measure of soil nutrient and toxicity status?

Answer 7

pH.

Question 8

How is the critical value on soil tests determined?

Answer 8

Experimentation.

The critical value is the [nutrient] that achieves 90% of the potential (maximum) yield.

Question 9

What are the basic principles of soil testing (e.g. where you should and shouldn’t sample from)?

Answer 9

• Sampling should be representative of the area being examined, meaning:
  
  • the sampling area should be relatively homogeneous
  • areas to be avoided:
    
    • where you’re likely to intercept fertiliser (especially important for immobile nutrients like P)
    • near any type of road (too much disturbance)
    • under trees in pastures (rest places for animals = ↑ urine/faeces concentration)
    • near fences (same reason as for near roads)
    • near steep slopes (nutrient concentrations in these areas aren’t representative because a lot of nutrients can be brought from another area in the catchment)
    • near any infrastructure, construction, pipelines, sheds, etc.
    • flooded/waterlogged areas
    • areas used for depositing limestone, gypsum, fertiliser, or any type of agrochemicals
    • near termite mounds (common in Oxisols) (termites being soil from deep in the subsoil, so samples in these sites won’t be representative)
• Zig-zag sampling patterns are more likely to pick up spatial variability ( than transect patterns
• If samples are going to be mixed (to reduce analysis costs), the greater the spatial variability, the more sampling that needs to be done
• Samples should be analysed ASAP (extractable nutrient concentrations change markedly over time)

Question 10

Which will give a higher value, pH_w or pH_Ca? How much do they differ?

Answer 10

pH_w is about 0.5-0.8 units higher than pH_Ca

(Ca replaces some of the H+ on the CEC, causing the H+ to partitiion to the solution, causing the pH to decrease.)

Question 11

Explain the process for testing EC_1:5.

Answer 11

1. Dry soil at 40°C
2. Grind and/or sieve to < 2 mm
3. Add soil to test tube
4. Add water (1:5 soil:water)
5. Shake for at least 1 hour
6. Let solids settle (or preferably filter)
7. Measure EC of supernatant using a calibrated electrode

Question 12

What are the problems associated with testing for [soil N]?

Answer 12

1. N is mostly in organic form and requires mineralisation before plant uptake
2. The rate of mineralisation is dependent on climatic conditions
3. The amount of N retention is dependent on climatic conditions (e.g. high rainfall = N leaching)

Question 13

What methods can be used to test EC?

Answer 13

• EC_1:5
• EC_e
• EM38 (uses electromagnetic field, can drag behind vehicle)

Question 14

Explain the process for testing pH.

Answer 14

1. Dry soil at 40°C
2. Grind and/or sieve to < 2 mm
3. Add soil to test tube
4. Add water or 0.01 M CaCl₂ solution (usually 1:5 soil:water)
5. Shake for at least 1 hour
6. Let solids settle (or filter)
7. Measure pH of supernatant using a calibrated electrode

Question 15

What are some principles for soil testing around row crops?

Answer 15

• Soil amelioration, especially to depth, can only really be done before the crop is established
• Sampling has to mimic what the roots are going to encounter/explore, so for perennial row crops like grapevines, it’s essential that soil is sampled to an adequate depth
• Need to consider how far away from the row the crop is likely to access nutrients (so you know how far out you need to sample to).
  
  • Irrigation type is an important determinant for this