Botany · Ch 6 — Principles of Ecology
Edaphic Factors
Edaphic Factors
Edaphic factors are the abiotic factors tied specifically to soil — its physical and chemical composition in a given area — and the dedicated study of soil is called pedology. Soil itself is the weathered superficial layer of the Earth's crust in which plants can root and grow; it is a complex composite of mineral particles, soil water, soil air and soil organisms all mixed together. Soil originates from parent rock and forms gradually through weathering, at a rate that depends on local climate and ecology; biological weathering specifically happens when bacteria, fungi, lichens and plants help break down rock by producing acids and other chemical compounds. Once formed, soil is described by its texture — the relative mix of clay, silt, sand particles it contains — with loamy soil (roughly 70% sand and 30% clay/silt, or both) considered ideal for cultivation because it drains well while still retaining enough moisture, and its porosity allows both good aeration and easy root penetration. Based on where they grow relative to soil conditions, plants are further classed into groups such as halophytes (saline soils), psammophytes (sandy soils), lithophytes (rocky surfaces), chasmophytes (rock crevices), cryptophytes (below the soil surface), cryophytes (ice surfaces), oxylophytes (acidic soils) and calciphytes (calcium-rich alkaline soils). Several specific edaphic factors control which plants a given soil can support: soil moisture and soil water (soil water, held mainly as capillary water between soil particles, is the single most important water source for most land plants); soil reaction (whether the soil is acidic, alkaline or neutral — crop plants generally do best between pH 5.5 and 6.8); soil nutrients (the soil's fertility, i.e. its ability to supply minerals and organic nutrients as ions); soil temperature (which governs the geographical distribution of plants — low soil temperature reduces water and solute uptake by roots); soil atmosphere (oxygen and carbon dioxide held in the pore spaces between particles); and soil organisms (the bacteria, fungi, algae, protozoans, nematodes, insects and earthworms that live in and shape the soil). Soil is not u …
| # | Soil type | Particle size | Relative proportion |
|---|---|---|---|
| 1 | Clayey soil | Less than 0.002 mm | 50% clay and 50% silt (cold / heavy soil) |
| 2 | Silt soil | 0.002 to 0.02 mm | 90% silt and 10% sand |
| 3 | Loamy soil | 0.002 to 2 mm | 70% sand and 30% clay/silt or both (Garden soil) |
What this figure shows. A vertical cross-section of soil stacked into five labelled horizons from top to bottom: the O-horizon (organic humus layer, split into O1 freshly-fallen litter and O2 decomposed matter, usually absent in agricultural/desert soils), the A-horizon (leached topsoil rich in humus and minerals, split into A1 dark organic-rich and A2 pale mineral-rich sub-layers), the B-horizon (subsoil accumulation zone rich in iron/aluminium/silica clay, poor in humus), the C-horizon (partially weathered parent-rock fragments with lit …