Soils: Formation, Classification and Properties
Domain focus
Soils tests formation, classification and the physical, chemical and biological properties that control drainage, nutrient supply and productivity.
Core rules
- Soil forms from parent material under climate, organisms, relief and time; these factors explain texture, depth and drainage.
- Texture is the sand, silt and clay proportions; structure is how particles aggregate. Texture affects water holding, aeration and workability.
- Chemical fertility depends on pH, cation exchange capacity (CEC), base saturation and nutrient availability. Lime raises pH and can reduce aluminium toxicity.
- Biological activity decomposes organic matter, cycles nutrients and builds structure; earthworms, bacteria, fungi and mycorrhizae are important.
- Irish soil groups include brown earths, podzols, gleys, peats and alluvial soils; drainage and organic matter are key classifiers.
Worked example
A soil has CEC 20 cmol/kg and base saturation 60%. Exchangeable bases = 0.60 × 20 = 12 cmol/kg. To reach 80% base saturation, bases must be 0.80 × 20 = 16 cmol/kg, a deficit of 4 cmol/kg. The lime recommendation would then be calculated from the soil's lime requirement test, not from this deficit alone, because lime neutralises acidity and supplies calcium, and over-liming can reduce availability of some micronutrients.
Traps
Do not confuse texture with structure. pH is logarithmic: pH 5 is ten times more acidic than pH 6. Do not assume lime always helps; it can worsen micronutrient deficiency. Check units for CEC and do not ignore organic matter or drainage.