How Tillage Makes Crops More Vulnerable to Drought

Key Terms and Concepts

What is Tillage: The practice of turning over and breaking up soil before planting crops. It is important because it creates a smooth seedbed, controls weeds, and mixes in crop residues. For example, a farmer may plow a wheat field in autumn and harrow it in spring to prepare for seeding. Tillage effects on soil can be measured by comparing soil bulk density or infiltration rates before and after plowing.

What is No‑Tillage: A farming method that avoids turning the soil, planting seeds directly into undisturbed ground. It is important because it preserves soil structure, retains moisture, and reduces erosion. For example, a no‑till planter will slice through last season’s corn stubble and drop new seeds into the soil without plowing. Changes in soil health under no‑till can be tracked by measuring surface residue cover and comparing moisture retention to tilled fields.

What is Soil Erosion: The process by which topsoil is worn away and carried off by wind, water, or tillage. It is important because it removes fertile soil, lowers crop productivity, and causes sedimentation in rivers. For example, after heavy rain on a bare slope, runoff may wash away rich topsoil into streams. Soil loss can be estimated with the Universal Soil Loss Equation (A = R×K×LS×C×P), where each factor represents rainfall, soil erodibility, slope, cover, and practices.

What is Tillage Erosion: The movement of soil downslope caused by the action of plows and other implements. It is important because it gradually thins soil on uphill parts and builds it up in lower areas, affecting fertility and crop yields. For example, on a gentle hillside, each pass of a plow may shift a thin layer of soil downhill. Tillage erosion rates can be modeled by Qₜᵢₗ = –kₜᵢₗ·s, where Qₜᵢₗ is soil flux, kₜᵢₗ is the transport coefficient, and s is slope.

What is Water Erosion: The detachment and transport of soil particles by raindrop impact and overland flow. It is important because it strips nutrient-rich topsoil, reduces crop yields, and can form rills or gullies. For instance, a heavy thunderstorm on a bare field can wash away several millimeters of soil. Water erosion is estimated with models like RUSLE (A = R×K×LS×C×P), where each factor stands for rainfall erosivity, soil erodibility, topography, cover, and practices.

What is Wind Erosion: The lifting and movement of soil particles by wind, often in dry or bare fields. It is important because it depletes fine soil particles and organic matter, leaving behind less fertile ground and creating dust storms. For example, during drought, loose sandy soils on a plowed field may blow away in strong winds. Wind erosion can be estimated using the Revised Wind Erosion Equation (RWEQ), which considers wind speed, soil roughness, and vegetative cover.

What is Soil Horizon: A distinct layer of soil formed by long-term processes of weathering, organic matter accumulation, and leaching. Horizons are important because they reveal soil fertility, structure, and drainage characteristics. For example, the A horizon (topsoil) is dark and rich in organic matter, while the B horizon (subsoil) is lighter with mineral accumulations. Identifying horizons helps farmers decide where roots will grow best and how deep to till.

What is Topsoil (A Horizon): The uppermost layer of soil rich in organic matter and nutrients where most plant roots grow. It is important because it supplies food and water to crops and hosts soil organisms like earthworms. For example, a 25 cm depth of dark loamy topsoil in a wheat field supports vigorous early growth. Topsoil depth is measured by digging a soil pit and noting where color and texture change, indicating the start of the subsoil.

What is Subsoil: The soil layer beneath topsoil (A horizon), often called the B horizon, containing fewer organic materials but storing water and some nutrients. It is important because deep roots can tap into its moisture during dry spells, and it provides structural support to plants. For instance, in dry summers, wheat roots may extend into the subsoil to find water when topsoil is depleted. Farmers check subsoil compaction by measuring bulk density or using a penetrometer to see if roots can penetrate.

What is Soil Translocation: The movement of soil particles from one location to another by water, wind, or tillage. It is significant because it alters soil depth and fertility patterns across a field. For example, repeated plowing upslope can shift soil downslope, making hilltops thin and valleys deeper. Soil translocation can be modeled using factors like slope gradient and tillage transport coefficients to predict long-term soil redistribution.

What is Soil Aggregate Stability: The ability of soil particles to cling together in clumps (aggregates) when exposed to water or disturbance. Stability is important because stable aggregates allow water to infiltrate, roots to grow, and reduce erosion. For example, if a handful of soil held under water remains intact for several minutes, it has high aggregate stability. Scientists measure stability using wet-sieving tests, where soil is gently agitated in water and the remaining aggregates are weighed.

What is Soil Moisture: The amount of water held in the pores between soil particles, often expressed as volumetric water content (percentage). It is critical because plants need water for photosynthesis and nutrient uptake; without enough moisture, crops suffer stress. For example, sensors at 10 cm depth may report 25% moisture, indicating sufficient water for cereals. Soil water balance is tracked by ΔS = P – ET – R – D, where ΔS is change in storage, P precipitation, ET evapotranspiration, R runoff, and D drainage.

What is Infiltration: The process by which water on the soil surface moves into and through the soil profile. It is important because rapid infiltration reduces runoff and erosion, recharges deep soil layers, and supplies water to plant roots. For example, a no‑till field with residues may have an initial infiltration rate of 2 cm/h, while a compacted tilled field may only absorb 0.5 cm/h. The Green‑Ampt infiltration equation (f = Kₛ·((ψf + Δθ·L)/L)) estimates how fast water can enter the soil.

What is Runoff: Water that flows over the land surface because it cannot infiltrate into the soil. It is important because runoff can cause soil erosion, carry nutrients and pollutants into waterways, and reduce water available to plants. For instance, if heavy rain falls on a saturated or compacted field, most water becomes runoff instead of soaking in. Runoff volume can be calculated by the SCS Curve Number method: Q = (P – 0.2S)² / (P + 0.8S), with P precipitation and S potential retention.

What is Root Penetration (Root Depth): The depth and extent to which plant roots grow into the soil. It is important because deeper roots can access water and nutrients when the surface dries out, improving drought resilience. For example, wheat roots reaching 80 cm can tap subsoil moisture during dry spells, while in compacted soils roots may stop at 30 cm. Farmers measure root depth by digging a profile and examining the deepest roots; root depth can be related to yield using simple regression equations.

What is Crop Yield: The amount of grain or biomass harvested per unit area (e.g., tons per hectare). It is important because it measures agricultural productivity and determines farm income and food supply. For instance, a cereal field yielding 5 t/ha in a dry year under no‑till versus 4 t/ha under tillage shows yield benefits of moisture conservation. Yield response to water stress can be modeled as Y = Yₘₐₓ·(1 – k·WD), where Yₘₐₓ is potential yield, k a stress coefficient, and WD water deficit fraction.

What is Biomass: The total mass of living plant material above ground in a given area, often measured as dry weight. It is important because biomass indicates plant growth vigor, carbon sequestration, and potential yield. For example, clipping and drying a 1 m² plot of barley might yield 1 kg of dry biomass, reflecting healthy growth. Models like AquaCrop simulate biomass as Bₜ = Bₜ₋₁ + WP·Tr, where WP is water productivity and Tr is actual transpiration.

What is Drought: A prolonged period of below‑normal precipitation leading to water scarcity for plants and humans. It is important because drought stress during critical growth stages (e.g., heading in cereals) causes flower abortion and reduces yields. For example, if rainfall from April through July is 25% below average, crops may experience severe water stress. Drought indices like the Standardized Precipitation Index (SPI) or Palmer Drought Severity Index (PDSI) help farmers monitor and respond to dry conditions.

What is Bulk Density: The mass of dry soil per unit volume (including pore space), expressed in g/cm³. It is important because high bulk density indicates compaction, reduces pore space for air and water, and limits root growth. For example, a healthy loam might have bulk density of 1.2 g/cm³, while a compacted field may reach 1.6 g/cm³. Measured by drying a soil core of known volume and dividing mass by volume (BD = dry mass ÷ core volume).

What is Hydraulic Conductivity: A measure of how easily water moves through soil pores, usually in cm/h or m/day. It is important because it influences infiltration, drainage, and availability of water to roots. For instance, sandy soils have high conductivity (e.g., 5 cm/h) and drain quickly, while clayey soils may have low conductivity (e.g., 0.1 cm/h) and hold water tightly. Field measurements use infiltrometers or are estimated by pedo-transfer functions based on texture, such as the Saxton–Rawls equations.

What is Soil Organic Matter: Decomposed plant, animal, and microbial residues in the soil, often expressed as a percentage of soil mass. It is important because it improves soil structure, water-holding capacity, nutrient release, and supports soil life. For example, a topsoil with 5% organic matter feels crumbly and holds moisture, while 1% organic matter soil is hard and prone to erosion. Soil organic carbon (a major component) relates to water-holding capacity: AWC ≈ 0.25 × (Organic Carbon, %).

What is Colluviation: The accumulation of soil and sediment at the bottom of slopes due to erosion upslope. It is significant because it creates deeper, more fertile soils in low-lying areas but thins soil on hilltops. For example, over decades, soil may move from a hillside into a valley, making valley soils 50 cm deeper than hilltop soils. Rates can be inferred by comparing soil depth across a slope or using radionuclide tracers (e.g., ^137Cs) to track sediment movement.

What is Soil Truncation: The thinning or removal of surface soil layers (especially topsoil) by erosion, tillage, or construction. It is important because losing topsoil reduces nutrient availability, water storage, and crop productivity. For instance, a long‑tilled slope may lose 10 cm of topsoil over decades, leaving subsoil with lower fertility exposed. Soil truncation is assessed by measuring soil depth at different points or by examining where original horizons have been cut off in a pit.

What is Tillage Transport Coefficient (kₜᵢₗ): A number quantifying how much soil is moved by tillage per unit slope and time (e.g., kg·m⁻¹·yr⁻¹). It is important for predicting tillage erosion and long-term soil redistribution on slopes. For example, a chisel plow operating at a given speed may have kₜᵢₗ = 500 kg·m⁻¹·yr⁻¹, indicating substantial downslope movement. In models, tillage flux is Qₜᵢₗ = –kₜᵢₗ·s, where s is slope gradient (rise/run).

What is Slope Gradient: The steepness or incline of land, expressed as a percentage (rise over run × 100) or an angle in degrees. It is important because steeper slopes increase runoff speed and erosion risk, both by water and tillage. For example, a 10% slope rises 10 m for every 100 m horizontally, making it prone to faster water flow and soil movement under plowing. Slope gradient is measured with a clinometer or GPS-based digital elevation model and used in erosion equations like LS in RUSLE.

What is Soil Compaction: The increase in soil bulk density and decrease in pore space due to heavy machinery, livestock, or repeated tillage passes. It is important because compacted soils have poor drainage, limited root growth, and reduced aeration, impairing crop health. For instance, tractors working on wet fields can form a dense layer that water cannot penetrate, causing runoff and root restriction. Compaction is detected by measuring bulk density or using a penetrometer; management includes controlled traffic and deep‑rooted cover crops to break up hard layers.

What is Plough Pan: A dense, compacted layer of soil that forms just below the regular tillage depth, often between 15 and 30 cm deep. It is significant because it restricts water infiltration and root penetration, leading to reduced yields and waterlogging above the pan. For example, repeated plowing at the same depth compacts soil just beneath that depth, causing surface water to pool after rain. Detection is by digging a pit or using a penetrometer; alleviation involves deep‑ripping when soils are dry or adopting no‑till to avoid forming the pan.

What is Ap Horizon: The upper soil layer that has been disturbed by plowing (denoted by “p” for plowed) and is rich in organic matter and nutrients. It is important because crops establish roots in this horizon, drawing water and nutrients for early growth. For instance, in a conventionally tilled wheat field, the Ap horizon may extend 20 cm deep and appear dark and crumbly. Loss of Ap horizon material through erosion can be tracked by measuring its depth annually and noting when it mixes with subsoil.

What is Plough Horizon: Another name for the Ap horizon, referring to soil mixed by regular plowing to a uniform depth. It is important because its properties—texture, structure, fertility—govern early crop growth and seed placement. For example, a plough horizon in a barley field may be 25 cm deep of fine, loose soil, ideal for seed germination. Over time, tillage erosion can thin this horizon on slopes, so monitoring its depth helps manage erosion and maintain productivity.

What is Crop Productivity Model (AquaCrop): A computer model developed by the FAO to simulate crop growth and yield based on water availability and soil conditions. It is important because it predicts how different soils, climates, and practices like tillage affect yield, helping farmers plan irrigation and management. For example, AquaCrop can simulate how a 30 cm loss of topsoil reduces wheat yield in a dry year. The core equation is B = WP·Tr, where B is biomass, WP is water productivity (kg m⁻³), and Tr is actual crop transpiration.

What is Tillage‑Induced Soil Redistribution Model (SPEROS‑C): A spatial model that simulates how tillage moves soil across a landscape over time based on elevation data and tillage practices. It is significant because it predicts long‑term changes in soil depth and fertility due to repeated cultivation on slopes. For instance, SPEROS‑C can map how a hillside loses 10 cm of topsoil over 50 years of plowing, depositing it downslope. Its main equation is Eₜᵢₗ = kₜᵢₗ·(∂²h/∂x²), where Eₜᵢₗ is tillage erosion rate, kₜᵢₗ the transport coefficient, and h elevation.

What is Soil Parent Material: The geological material (e.g., glacial till, loess, bedrock) from which soil horizons develop through weathering and organic processes. It is important because parent material influences soil texture, mineralogy, drainage, and fertility. For example, soils formed on loess tend to be deep and loamy, while those on clayey glacial till may be dense and poorly drained. Farmers use soil surveys to identify parent material, guiding decisions on amendment needs and crop suitability.

What is Soil Carbon Sequestration: The process of capturing atmospheric CO₂ and storing it as organic carbon in soil through practices like no‑till, cover cropping, and adding organic amendments. It is vital because it mitigates climate change and improves soil fertility and structure. For instance, converting to no‑till may increase soil carbon by 0.3 t ha⁻¹ yr⁻¹, enhancing water retention and productivity. Changes in soil carbon are measured by repeated sampling and analysis, with ΔC = C₍final₎ – C₍initial₎ showing net sequestration.

What is Soil Water Storage Capacity: The total amount of water a soil profile can hold between field capacity (when drainage stops) and permanent wilting point (when plants cannot extract water). It is critical because it determines how long crops can survive without rain or irrigation. For example, a 1.5 m deep loam with available water capacity of 0.2 m³ m⁻³ can store 300 mm of plant‑available water. Available Water Capacity (AWC) is calculated as AWC = θ_fc – θ_wp, where θ_fc is volumetric water content at field capacity and θ_wp at wilting point.

What is Soil Nutrient Availability: The concentration of essential elements (N, P, K, micronutrients) in forms plants can absorb from the soil. It matters because plant growth depends on nutrients; deficiencies limit yield, and surpluses can cause toxicity or pollution. For example, phosphorus is most available at pH 6.0–7.5; outside this range, it may bind to minerals and become inaccessible. Farmers assess nutrient availability with soil tests reporting concentrations (e.g., mg P kg⁻¹), guiding fertilizer rates; plant uptake can be estimated as Uptake = Concentration × Biomass.

What is Soil Conservation: A set of practices aimed at preventing soil erosion, maintaining fertility, and preserving soil health. It is important because it sustains agricultural productivity, protects water quality, and supports ecosystems. For instance, contour farming plows along slope lines to slow water flow and reduce erosion. Conservation success is measured by comparing soil loss (t ha⁻¹ yr⁻¹) before and after practices against tolerable soil loss thresholds.

What is Conservation Agriculture: An approach combining minimal soil disturbance (no‑till), permanent soil cover (residues or cover crops), and diversified crop rotations to enhance soil health and productivity. It is significant because it reduces erosion, builds organic matter, conserves water, and often improves yields over time. For example, planting cover crops like legumes after maize harvesting and seeding soybeans directly through the residue maintains soil cover year-round. Indicators of success include increased soil organic carbon, reduced runoff, and more stable yields under stress.

What is Soil Fertility: The ability of soil to supply essential nutrients in adequate amounts for plant growth. It is important because fertility directly affects crop health, yield, and quality. For example, a loamy soil with pH 6.5, 4% organic matter, and balanced N–P–K levels supports high cereal yields, while a sandy, acidic soil may need lime and fertilizer amendments. Fertility is assessed by lab tests (e.g., available P in mg kg⁻¹), CEC (cmol kg⁻¹), and organic matter percentage, guiding nutrient management.

What is Soil Compaction Threshold: The maximum bulk density or penetration resistance beyond which roots cannot grow freely or water cannot infiltrate effectively. It is critical because exceeding this threshold restricts root development, reduces yields, and increases runoff. For example, maize roots may stop growing when bulk density exceeds 1.6 g/cm³, causing stunted plants. Compaction thresholds vary by texture—sandy soils tolerate higher bulk densities (~1.8 g/cm³) than clay soils (~1.4 g/cm³)—and are determined by field or lab tests.

What is Cover Cropping: The practice of planting specific crops (e.g., legumes, grasses) during fallow periods to protect and improve soil. It is important because cover crops prevent erosion, add organic matter, fix nitrogen (legumes), suppress weeds, and improve soil structure. For instance, planting clover and rye after wheat harvest shields soil in winter and provides green manure in spring. Benefits are measured by biomass production, nitrogen fixation (e.g., 80–120 kg N ha⁻¹), and improvements in soil organic carbon over seasons.

What is Soil Water Deficit: The difference between water held at field capacity and the current soil moisture, indicating how much water is lacking for optimal crop growth. It matters because it quantifies plant water stress and guides irrigation decisions. For example, if field capacity is 25% volumetric water content and current moisture is 10%, the soil water deficit is 15% units. Models track daily SWD as SWDₜ = max[0, SWDₜ₋₁ + ETₚ – P], where ETₚ is potential evapotranspiration and P is effective precipitation.

What is Soil Texture: The proportion of sand, silt, and clay particles in a soil, defining classes like loam, sandy loam, or clay. It is important because texture influences water retention, drainage, nutrient-holding capacity, and root growth. For example, a loam soil with roughly equal sand, silt, and clay balances water storage and aeration, ideal for most crops, while a pure clay holds water but drains slowly. Texture is determined by laboratory particle-size analysis or the “feel method,” and guides irrigation and tillage practices.

What is Conservation Buffer: A planted strip of vegetation (grasses, trees, shrubs) between farmland and sensitive areas (streams, slopes) to trap sediment, filter pollutants, and slow runoff. It is important because buffers protect water quality, reduce erosion, and support wildlife habitat. For instance, a 10 m grass strip along a creek can trap 50–75% of sediment from adjacent fields. Effectiveness is measured by reductions in sediment load (t ha⁻¹ yr⁻¹) and improvements in water clarity downstream.

What is Soil pH: A measure of soil acidity or alkalinity on a scale from 0 to 14, with 7 being neutral; below 7 is acidic, above 7 is alkaline. It is important because pH affects nutrient availability and microbial activity; extreme pH levels can lock up nutrients or cause toxicity. For example, phosphorus is most available at pH 6.0–7.5; outside this range, it binds to other minerals. Soil pH is tested using a pH meter or indicator dye in a soil-water mix, and adjusted by adding lime to raise pH or sulfur to lower it.

What is Agroecological Resilience (Soil Health): The capacity of soil to function as a living ecosystem that supports crop growth, filters water, and cycles nutrients sustainably. It is important because healthy soils buffer against drought, reduce erosion, and maintain productivity over time. For example, a no‑till field with 4% organic matter, balanced nutrients, and active microbial life shows high resilience to stress. Soil health is assessed by indices combining indicators like organic carbon, bulk density, aggregate stability, pH, and microbial biomass, guiding management practices.

What is Watershed Management: The coordinated planning and management of land and water resources within a drainage basin to protect water quality, control flooding, and sustain ecosystem services. It is important because activities on land—like tillage, deforestation, or urbanization—directly affect runoff, sediment, and nutrient loads in downstream waters. For example, implementing no‑till and buffer strips across farms in a watershed can reduce sediment load by 40–50%. Tools like GIS and models (e.g., SWAT) simulate how land use changes impact hydrology and guide policy for best management practices.

What is Soil Quality: A measure of how well a soil performs functions like producing crops, filtering water, and supporting biodiversity. It encompasses physical, chemical, and biological properties, such as structure, nutrient levels, pH, organic matter, and microbial activity. For instance, a high-quality soil has good aeration, balanced nutrients, pH near 6.5, and active earthworms, supporting stable yields. Soil quality is evaluated by composite indices that combine key indicators, helping farmers compare fields and track changes over time.

What is Soil Service Functions: The roles soils play in ecosystems and society, including provisioning (food production), regulating (water filtration, carbon storage), supporting (nutrient cycling, habitat), and cultural (heritage, aesthetics). It is important because soils underpin agriculture, water quality, and biodiversity. For example, a healthy prairie soil filters pollutants, stores carbon, supports plant growth, and preserves archaeological artifacts. Trade‑offs between functions are assessed by indicators like crop yield, infiltration rate, carbon sequestration, and species diversity, guiding land management that balances multiple goals.

What is Soil Physical Degradation: The decline in soil structure and function due to processes like compaction, crusting, and loss of organic matter. It is important because degraded soils have poor infiltration, restricted root growth, and are prone to erosion, reducing crop productivity. For instance, repeated heavy machinery passes on wet fields can compact soils, leading to waterlogging and shallow roots. Physical degradation is measured by increases in bulk density, reduced porosity, lower infiltration rates, and higher penetration resistance; remediation includes deep ripping and adding organic amendments.

What is Soil Biogeochemical Cycling: The movement and transformation of elements (carbon, nitrogen, phosphorus, etc.) through soil, living organisms, the atmosphere, and water bodies. It is vital because it maintains soil fertility, supports plant growth, and regulates greenhouse gas emissions. For example, when cover crop residues decompose, microbes mineralize nitrogen into ammonium and nitrate, making it available for the next crop. Models describe nutrient pools and fluxes with equations like dC/dt = I – R – L, where C is soil carbon pool, I input, R respiration loss, and L leaching, helping guide sustainable nutrient management.

What is Sustainable Soil Management: The use of soil resources to maintain productivity and ecological functions for current and future generations through practices that conserve soil, water, and biodiversity. It integrates techniques like crop rotation, cover cropping, residue retention, reduced tillage, and balanced fertilization. For example, a sustainable farmer may rotate maize with legumes, use no‑till planting, and apply composted manure to build soil organic matter. Success is gauged by long-term yield stability, soil organic matter trends, nutrient balances, and biodiversity measures, ensuring food security and environmental health.