🌱Intro to Soil Science Unit 7 – Soil Fertility and Plant Nutrition

Key Concepts and Definitions

• Soil fertility refers to the ability of soil to support plant growth and provide essential nutrients for optimal crop yield and quality
• Macronutrients are essential elements required by plants in large quantities includes nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)
• Micronutrients are essential elements required by plants in small quantities consists of iron (Fe), manganese (Mn), boron (B), zinc (Zn), copper (Cu), molybdenum (Mo), and chlorine (Cl)
• Cation exchange capacity (CEC) represents the soil's ability to hold and exchange positively charged ions (cations) influences nutrient retention and availability
• Soil pH is a measure of soil acidity or alkalinity on a scale from 0 to 14 affects nutrient solubility and plant uptake
  • Neutral soil pH is around 7.0
  • Acidic soils have a pH below 7.0 (e.g., pH 5.5)
  • Alkaline soils have a pH above 7.0 (e.g., pH 8.0)
• Soil organic matter (SOM) consists of decomposed plant and animal residues improves soil structure, water retention, and nutrient holding capacity
• Nutrient uptake is the process by which plants absorb essential nutrients from the soil solution through their roots

Soil Composition and Properties

• Soil is composed of mineral particles, organic matter, water, and air
• Soil texture refers to the relative proportions of sand, silt, and clay particles in a soil determines water holding capacity, drainage, and nutrient retention
  • Sandy soils have large particles and high drainage but low nutrient and water retention
  • Clay soils have small particles and high nutrient and water retention but poor drainage
  • Loamy soils have a balanced mixture of sand, silt, and clay ideal for plant growth
• Soil structure describes the arrangement of soil particles into aggregates affects water infiltration, root penetration, and gas exchange
• Bulk density is the mass of dry soil per unit volume of soil indicates soil compaction and porosity
• Soil porosity refers to the volume of pores (spaces) between soil particles influences water and air movement in the soil
• Soil water holding capacity is the amount of water a soil can retain against gravity depends on soil texture and organic matter content
• Soil color can indicate soil properties such as organic matter content, drainage, and mineral composition (e.g., dark soils often have high organic matter)

Essential Plant Nutrients

• Primary macronutrients (N, P, K) are required in the largest quantities by plants
  • Nitrogen (N) is essential for chlorophyll production, protein synthesis, and vegetative growth
  • Phosphorus (P) is crucial for root development, energy transfer, and flower and fruit formation
  • Potassium (K) is important for enzyme activation, stomatal regulation, and disease resistance
• Secondary macronutrients (Ca, Mg, S) are required in smaller quantities than primary macronutrients but still play vital roles in plant growth and development
• Micronutrients (Fe, Mn, B, Zn, Cu, Mo, Cl) are required in trace amounts but are essential for specific plant functions and enzyme activities
• Nutrient deficiencies can cause visible symptoms in plants such as chlorosis (yellowing of leaves), stunted growth, or reduced yield
• Nutrient toxicities can occur when nutrients are present in excessive amounts leading to plant damage or reduced growth
• Nutrient interactions can affect the uptake and availability of other nutrients in the soil (e.g., high potassium levels can reduce magnesium uptake)

Nutrient Cycling in Soil

• Nutrient cycling involves the transfer of nutrients between the soil, plants, and other organisms
• Mineralization is the process by which soil microorganisms convert organic forms of nutrients into inorganic forms available for plant uptake
• Immobilization is the process by which soil microorganisms convert inorganic nutrients into organic forms temporarily unavailable for plant uptake
• Nitrification is the microbial conversion of ammonium (NH4+) to nitrate (NO3-) in the soil
• Denitrification is the microbial conversion of nitrate (NO3-) to gaseous forms of nitrogen (N2, N2O) under anaerobic conditions results in nitrogen loss from the soil
• Leaching is the downward movement of soluble nutrients (e.g., nitrate) through the soil profile with water can lead to nutrient loss and groundwater contamination
• Nutrient uptake by plants removes nutrients from the soil solution which are then incorporated into plant tissues
• Nutrient return to the soil occurs through plant residue decomposition, animal manure, and fertilizer applications

Soil pH and Nutrient Availability

• Soil pH affects the solubility and availability of plant nutrients in the soil solution
• Most plant nutrients are optimally available in the pH range of 6.0 to 7.0
• Acidic soils (pH < 7.0) can have reduced availability of macronutrients (N, P, K, Ca, Mg, S) and increased solubility of micronutrients (Fe, Mn, Zn, Cu)
  • Liming is the application of calcium and magnesium compounds (e.g., limestone) to raise soil pH and reduce acidity
• Alkaline soils (pH > 7.0) can have reduced availability of micronutrients (Fe, Mn, Zn, Cu) due to their precipitation as insoluble compounds
  • Sulfur application can be used to lower soil pH in alkaline soils
• Soil pH can also affect the activity and diversity of soil microorganisms which play crucial roles in nutrient cycling and soil health
• Regular soil pH testing is important for monitoring and managing soil acidity or alkalinity to ensure optimal nutrient availability for crops

Fertilizers and Soil Amendments

• Fertilizers are materials applied to the soil to supply plant nutrients and improve soil fertility
• Inorganic fertilizers are manufactured from synthetic or mined sources and provide readily available nutrients (e.g., urea, ammonium nitrate, superphosphate)
• Organic fertilizers are derived from plant or animal sources and release nutrients slowly as they decompose (e.g., compost, manure, bone meal)
• Soil amendments are materials added to the soil to improve its physical, chemical, or biological properties (e.g., lime, gypsum, peat moss)
• Fertilizer application rates should be based on soil test results, crop nutrient requirements, and yield goals to avoid over- or under-fertilization
• Timing and placement of fertilizers are important for optimizing nutrient uptake and minimizing losses (e.g., split applications, banding, fertigation)
• Slow-release or controlled-release fertilizers can provide a more gradual and consistent nutrient supply to plants reducing leaching and runoff losses

Soil Testing and Analysis

• Soil testing involves collecting soil samples and analyzing them in a laboratory to determine nutrient levels, pH, and other soil properties
• Soil sampling should be representative of the field or area being tested taking into account soil variability and management history
• Soil test results provide information on nutrient deficiencies or excesses, pH, organic matter content, and cation exchange capacity (CEC)
• Interpretation of soil test results requires knowledge of crop nutrient requirements, soil type, and local conditions
• Soil testing should be conducted regularly (e.g., every 1-3 years) to monitor changes in soil fertility and adjust management practices accordingly
• Plant tissue analysis can complement soil testing by providing information on the nutrient status of the growing crop
• Nutrient budgeting is the process of balancing nutrient inputs (e.g., fertilizers, manure) with nutrient outputs (e.g., crop removal, leaching) to optimize nutrient use efficiency and minimize environmental impacts

Sustainable Soil Management Practices

• Cover crops are planted between main crop cycles to protect soil, suppress weeds, and improve soil health (e.g., legumes, grasses, brassicas)
• Crop rotation involves growing different crops in a planned sequence to break pest and disease cycles, improve soil structure, and enhance nutrient cycling
• Conservation tillage practices (e.g., no-till, strip-till) minimize soil disturbance, reduce erosion, and maintain crop residues on the soil surface
• Integrated nutrient management combines the use of inorganic and organic fertilizers, along with other soil management practices, to optimize nutrient use efficiency and soil health
• Precision agriculture technologies (e.g., GPS, variable rate application) enable site-specific management of nutrients, water, and other inputs based on soil and crop variability within a field
• Soil erosion control measures (e.g., terracing, contour farming, buffer strips) help to reduce soil loss, maintain soil fertility, and protect water quality
• Agroforestry practices integrate trees and shrubs with crops or livestock to improve soil health, biodiversity, and economic returns (e.g., alley cropping, silvopasture)
• Soil health assessment tools (e.g., soil health cards, soil quality indicators) can be used to monitor and evaluate the impact of management practices on soil physical, chemical, and biological properties