Soil Air

Cheatsheet Content

### Composition and Properties - **Definition:** Air found in the pore spaces of soil. - **Composition:** Similar to atmospheric air but with key differences: - Higher $\text{CO}_2$ concentration (due to microbial respiration, root respiration). - Lower $\text{O}_2$ concentration (consumed by respiration). - Higher humidity (often near 100% relative humidity). - Contains trace amounts of other gases ($\text{N}_2\text{O}$, $\text{CH}_4$, etc.). - **Variability:** Composition changes with depth, soil type, moisture content, and biological activity. ### Gas Exchange - **Diffusion:** Primary mechanism of gas movement in soil. Gases move from areas of high partial pressure to low partial pressure. - Driven by concentration gradients between soil air and atmosphere. - Slower in water-filled pores. - **Mass Flow (Convection):** Less significant, driven by pressure gradients (e.g., wind, barometric pressure changes). - **Factors Affecting Exchange:** - **Pore space connectivity:** Well-connected pores facilitate exchange. - **Soil moisture:** Water-filled pores block gas diffusion. - **Soil structure:** Aggregated soils have better gas exchange. - **Temperature:** Affects respiration rates and gas solubility. ### Importance to Plant Growth - **Root Respiration:** Roots require $\text{O}_2$ for aerobic respiration to produce energy ($\text{ATP}$). - $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$ - **$\text{CO}_2$ Toxicity:** High levels of $\text{CO}_2$ can be toxic to roots. - **Nutrient Uptake:** $\text{O}_2$ is essential for active nutrient uptake by roots. - **Water Uptake:** Impaired in anaerobic conditions. - **Microbial Activity:** Soil microbes also require $\text{O}_2$ for decomposition and nutrient cycling (e.g., nitrification). ### Soil Aeration - **Definition:** The process by which gases are exchanged between the soil and the atmosphere. - **Good Aeration:** - Adequate $\text{O}_2$ for roots and beneficial microbes. - Prevents accumulation of toxic gases ($\text{CO}_2$, $\text{H}_2\text{S}$). - Promotes healthy root development. - **Poor Aeration (Anaerobic Conditions):** - Lack of $\text{O}_2$. - Accumulation of $\text{CO}_2$ and other reduced compounds (e.g., methane, hydrogen sulfide). - Inhibits root growth and function. - Favors anaerobic microbes, leading to denitrification (loss of N) and production of phytotoxins. - Common in compacted soils or waterlogged soils. ### Managing Soil Air - **Improve Drainage:** Remove excess water to increase air-filled pore space. - **Avoid Compaction:** Minimize heavy machinery traffic, use cover crops. - **Add Organic Matter:** Improves soil structure and aggregation, creating more stable pores. - **Tillage (Controlled):** Can temporarily improve aeration but can also degrade soil structure if overdone. - **Appropriate Irrigation:** Avoid overwatering, which fills pore spaces with water. - **Deep Rooting Plants:** Can create biopores, improving aeration.