Soils Workshop 2008
Soil Fertility Session
13th August 2008
John Standley
Soil Carbon
• Labile carbon – oxidised with dilute
potassium permanganate; the fraction
changing most rapidly
• Organic carbon – oxidised by potassium
dichromate in sulphuric acid; about 70% of
total carbon
• Total carbon – converted to carbon dioxide
at high temperature; may include
carbonate
Organic carbon in soils
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Comparisons - surface and subsoil
Different soils
Uncleared land or by trees
Under pasture or lawn
In cultivated paddocks or gardens
Bare soil
See S-O3, S-O4
Soil pH and liming
• Some soils may be too acid for some
plants
• Adjust pH by adding lime. Does gypsum
have the same effect?
• Compare the three soils without and with
the addition of lime
• Experiment with different rates
• See S-O6
Fertilisers and pH
• Compare ammonium sulphate, ammonium
nitrate, potassium nitrate and urea
• Check the effect of urea on the three soils
• Experiment with your soils
• See N-O6
Urea changes
• Expect urea to be converted ammonium to
make soils more alkaline
• Expect ammonium to be converted to
nitrate
• Check what has happened with the three
soils
• Beware of units – measuring N (14) or
NO3 (62) ppm?
Fertiliser experiments
• Grow various plants with various fertilisers
and record what happens
• Compare rates and plot growth response
curve
• Maybe in conjunction with maths teacher
run a replicated trial, fertiliser rates
replicated three or four times to separate
treatment and environmental effects
Nutrient removal
• Consider removal by various crops
• Consider fertiliser analyses and how much
needs to be applied
• Consider manures
• See N-O5
Manure composition
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An example
Cattle manure from feedlots
Nitrogen 2%
Phosphorus 0.8%
Potassium 2.3%
Compare with inorganic fertilisers and
calculate how much will be required
Deficiency symptoms
• What does the plant tell us?
Mycorrhizae
• Have a look
• Experiment with linseed in pots
• Soil “as is”, sterilised , with and without
inoculum added
• Also with and without P fertiliser
• Could consider zinc too
• See O-O6
Nodulation
• Examine the roots of grasses and legumes
• Grow soybeans with and without rhizobial
inoculum
• Examine nodules
• With and without N fertiliser
• See O-O7