Soil Quality and Crop Production

Soil and Land Quality for Crop Production

This chapter focuses on providing a good soil quality environment for crops, emphasizing the crucial activities occurring in the belowground root system.

Gaseous Exchange in the Effective Rooting Zone

Root Respiration

• Process: Roots respire, consuming sugars produced in leaves through photosynthesis (a combination of $CO<em>2$ and water) and oxygen from the soil. This process converts sugar into energy forms, including ATP, $CO</em>2$, and water.
  • $Sugar + O<em>2 \rightarrow ATP + CO</em>2 + H_2O$
• Requirements:
  • Oxygen must diffuse down to the root system to sustain root respiration.
  • Carbon dioxide ($CO_2$), a product of root respiration, must be able to escape the soil and move into the air.
• Recycling: Some $CO_2$ produced by root respiration diffuses to the aboveground plant parts, where it can be absorbed by shoots and recycled into photosynthesis, becoming part of new sugars.

Biological Nitrogen Fixation (BNF) for Legume Crops

• Key Gas: Nitrogen gas ($N_2$) is essential for symbiotic biological nitrogen fixation in legume crops, where it diffuses into root nodules.
• Process: Biological nitrogen fixation converts diatomic nitrogen gas ($N<em>2$) – a form the plant cannot use – into the ammonium ion ($NH</em>4^+$) – a usable form – in the presence of a reductant (hydrogen source).
• Impact of Soil Moisture:
  • If soil is too wet (waterlogged), small pores are filled with water instead of air.
  • $N_2$ gas diffuses very poorly through water.
  • Consequently, nitrogen fixation may not occur in wet soils, leading to inadequate nitrogen fertilization for the legume crop.

Ideal Soil Physical Properties

• Balance of Solid and Airspace: Good physical properties require an optimal balance between solid soil particles (sand, silt, clay, organic matter) and airspace.
• Airspace Composition: In ideal soil, the void space (airspace) should be approximately $50\%$ water and $50\%$ air.
• Pore Balance: A good balance between large pores (macropores) and small pores (micropores) is desired, aiming for about $50\%$ macropores and $50\%$ micropores.
  • This balance facilitates good gaseous exchange and efficient water movement.

Soil Water Dynamics and Availability

• Soil Particle Charges: Soil particles (e.g., clay, organic colloids/humus) have negative charges, balanced by positive ions like potassium ($K^+$), ammonium ($NH_4^+$), calcium ($Ca^{2+}$), and magnesium ($Mg^{2+}$).
• Water Zones Around Soil Particles:
  • Plant Unavailable Water: A tightly held film of water surrounding soil particles, held at tensions of at least $15$ bars. Roots cannot access this water.
  • Plant Available Water: Water held between $15$ bars and approximately $0.33$ to $0.1$ bar tension. The exact range ($0.33$ or $0.1$ bar) depends on soil texture (sand, silt, clay proportions). This water is available for crop uptake.
  • Gravitational Water: Water held less tightly than $0.33$ to $0.1$ bar tension. This water occupies larger macropores and drains relatively quickly (within 2-3 days of rainfall cessation) below the effective rooting zone, making it generally unavailable to crops.
• Key Soil Moisture Concepts:
  • Field Capacity (FC): The water content in soil after gravitational water has drained (about 2-3 days post-rainfall). Water at field capacity is held at $0.33$ to $0.1$ bar tension or greater.
  • Permanent Wilting Point (PWP): The soil moisture content at which plants permanently wilt because water is held too tightly (approximately $15$ bars tension) for roots to absorb.
  • Available Water Calculation: Plant available water is the difference between field capacity and the permanent wilting point: $Available~Water = FC - PWP$.
• Water Uptake by Roots:
  • For roots to absorb water, the water films surrounding both the soil particle and the root must be overlapping.
  • Roots are dynamic and continually grow, exploiting new soil volumes to access available water as the soil dries and water films thin.
  • Irrigation becomes necessary when the plant available water zone becomes too thin to sustain the crop until natural rainfall.
• Capillary Flow: Good soil quality with a balance of large and small pores allows for efficient capillary flow of water, enabling movement from lower soil regions upward and laterally (e.g., from row middles where water is reserved to the crop rows).

Specific Benefits of Maintaining Good Soil Quality

Good soil quality encompasses improvements in the physical, chemical, and microbiological properties of the soil. As Herman Warsaw, the world corn champion, stated, "I try to do right by my soil, knowing that if I do the very best job I can of maintaining good soil quality, good yields will follow."

• Increased Water Infiltration: A higher proportion of rainfall and irrigation water moves into the soil profile to recharge pores. This prevents lateral runoff or movement downhill, ensuring water is available to crops.
• Increased Soil Aeration: Improved gas exchange, allowing oxygen to diffuse into roots for aerobic respiration, nitrogen into nodules for BNF (in legumes), and $CO_2$ to diffuse out of the soil (from root and microbial respiration).
• Increased Nutrient Retention: Essential plant nutrients ($N, P, K, Ca, Mg, S, Cu, Zn, Fe, Mn, B, Cl, Mo$) are retained within the effective rooting zone. This prevents costly nutrient leaching into groundwater, which can lead to contamination and eutrophication of waterways.
• Increased Soil Microbial Activity: Well-aerated soil fosters healthier aerobic microorganisms. These microbes are crucial for breaking down crop residues (preventing pests and diseases) and building up the humus fraction, enhancing overall soil fertility.
• Increased Natural Control of Pests: Good soil quality contributes to natural pest control, reducing the reliance on potentially environmentally damaging pesticides. This is key for stable, sustainable agroecosystem yields.
• Increased Warming of Soil in Spring: Well-drained soil with a good balance of pores warms up faster in spring. This allows for earlier planting, leading to faster seedling growth and better competition against weeds.
  • Corn Example: Earlier corn planting enables the crop to develop a full leaf canopy sooner, maximizing light absorption before the longest day (June 21st). This leads to better harvesting of solar energy and higher grain yields.
• Reduced Water and Wind Erosion: A good granular soil structure, fostered by balanced soil pores, leads to healthier crops and less vulnerability to erosion by water and wind.
• Decreased Fuel Consumption for Tillage: Loose, friable soil with good granular structure offers less resistance to tillage equipment. This reduces the 'draft' requirement, lowering fuel consumption and energy costs for farming operations.
• More Efficient Use of Crop Growth Factors: Overall, good soil quality ensures a more efficient utilization of all critical crop growth factors: water, light, essential nutrients, and space.

Optimizing Crop Planting and Yield

Corn Yield as a Function of Planting Date

• Yield Curve: Plotting corn yield (vertical axis) against planting date (horizontal axis) typically shows an optimal planting window, with yields decreasing on either side.
  • Too Early Planting: Often results in a poor stand due to cold, wet soil conditions, potential pests, or poor seed germination, inhibiting early crop growth.
  • Too Late Planting: Leads to poor harvesting of the sun's energy because the crop does not develop its full leaf canopy early enough to intercept maximum sunlight. This can result in a significant and rapid yield reduction.
• Geographic Variation: The optimum planting date shifts geographically; for example, it's earlier in Eastern North Carolina and later in Western North Carolina (e.g., April 7th for early, May 7th for late in Raleigh).

Leaf Area Index (LAI)

• Definition: Leaf Area Index ($LAI$) is a unitless ratio of the total leaf area of a crop to the land area it covers. For instance, an $LAI$ of $4$ means four units of leaf area per one unit of land area.
• Yield vs. LAI: Crop yield (e.g., corn, soybeans) typically increases with $LAI$ up to an optimum point, then declines.
  • Optimum LAI: For corn and soybeans, the optimum $LAI$ is around $4$. At this point, approximately $95\%$ of incident light is intercepted by the canopy.
  • Below Optimum LAI: Too few plants or insufficient leaf surface area mean less light energy is absorbed, reducing photosynthetic output and yield.
  • Above Optimum LAI: Excessively high plant density ($LAI$ greater than $5$) can lead to lodging, where plants have thin, spindly stems that fall over easily, reducing harvestable yield.
• Connection to Soil Quality: Achieving the optimal $LAI$ for $95\%$ light interception by the critical transition from vegetative to reproductive growth (e.g., tasseling and silking in corn) requires good soil physical condition. A well-drained soil warms quickly in spring, allowing for timely planting. This ensures that the crop develops its canopy effectively, maximizing light capture and yield potential. Therefore, maintaining good soil physical, chemical, and microbiological properties is fundamental to achieving the correct $LAI$ at the right time.