The Physical Environment (AS); Soil (Complete) Flashcards
What is soil essential for?
Plant growth in natural ecosystems & agroecosystems
What is soil fertility?
The ability of soil to sustain plant growth
How is water content a feature of fertile soil?
-Water → essential for all organisms living in soil including plants
-Fertile soil allows good drainage so doesn’t become water logged but still retains enough water for soil biota survival
-Plant nutrients = absorbed in ionic form, dissolved in water
How are soluble materials a feature of fertile soil?
-Have macronutrients like nitrogen, phosphorus & potassium, present in ionic form ; mainly nitrates, phosphates & potassium ions
-Have micronutrients like boron, cobalt, copper, iron, manganese & magnesium
-Toxin ions like aluminium & heavy metals are absorbed → surface of mineral particles (usually clay); so can’t dissolve in water & harm soil organisms
How is air content a feature of fertile soil?
Most living organisms in soil & many processes increasing fertility = aerobic → well aerated soils likely to be more fertile
How does dead organic matter contribute to soil fertility?
-Usually have a high dead organic matter content → releases plant nutrients as decomposes
-Increases water retention & provides food for soil biota
How is pH a feature of fertile soil?
-Usually in range of pH 5. 5- 7.0 → range of tolerance for most plants, soil biota
-Acidic soils can ↑ leaching of plant nutrients & damage root cell membranes
-Alkaline conditions; phosphates become insoluble
How do soil biota contribute to the fertility of soil?
Living organisms = involved in many processes affecting soil fertility;
-Detritivores like beetle larvae, worms break up dead organic matter, release nutrients → soil. Also ↑ soil drainage & aeration by making tunnels in soil
-Decomposers like fungi break down dead organic matter; secrete digestive enzymes + rely on detritivores to physically break up dead organic matter, increase its surface area
-Nitrogen fixing bacteria convert gaseous nitrogen into ammonium ions
-Nitrifying bacteria oxidise ammonium ions → nitrite ions → nitrate ions
-Mycorrhizal fungi form symbiotic relationships w/ plant roots, aid phosphate uptake by plants
What is soil texture controlled by and what does this involve?
Proportions of different size categories of mineral particles present in soil;
- Clay; <0.002 mm diameter
-Silt; 0.002 - 0.02 mm diameter
-Sand; 0.02- 2.0 mm diameter
How do drainage rates compare in sandy and clay soils?
-Larger pare spaces of sandy soils allows rapid drainage → reduces water content, increases aeration
-Clay soils = poorly drained
How does capillary action compare in sandy and clay soils?
-Tiny pore spaces between clay particles allow underground water to rise towards surface
-No capillary rise of water in sandy soils
How does aeration compare in sandy and clay soils?
-Pore spaces in well-drained sandy soils; filled w/ air
-clay soils; more likely to be waterlogged w/ low aeration rate
How does nutrient retention compare in sandy and clay soils?
Nutrient ions absorb easily onto clay particles but not onto sand particles
How does thermal capacity differ in sandy and clay soils?
-High water content of clay soils; high thermal capacity, warm up & cool down slowly
-Sandy soils; low thermal capacity, warm up more rapidly after cold weather
How does root penetration differ in sandy and clay soils?
-Clay particles = held together by capillary water on particle surfaces; harder for plant roots to force way between soil particles
-Sandy soils = root penetration easier
How does ease of cultivation compare in sandy and clay soils?
Lack of adhesion between particles in sandy soils; easier to cultivate than clay soils
What is the composition of loam soils and why is this advantageous?
-Fairly even mix of sand, silt & clay (approx 40:40:20)
-Ideal mix of properties for cultivating most crops; good drainage,water retention, high nutrient content
What is the structure of soil and its different types?
-Soil particles form aggregates called peds
-Particles = bound together by polysaccharide gums made by decomposition, by fungal hyphae, by roots, by action of soil biota & hygroscopic clay particles
-Ped type affects soil properties & fertility. Crumb peds= small & round. Produce good drainage, aeration & easy root penetration → improve soil fertility. Platy peds= large & flat. Reduce drainage, aeration & root penetration → less fertile soils
How does depth affect soil?
-Deeper soils—> less likely to become waterlogged/dry out rapidly
-Can also aid good root anchorage
Humans affecting soil fertility; aeration by ploughing & drainage?
Ploughing + drainage makes soils more aerobic= increases rates of N fixing, nitrification & decomposition of dead organic matter
Humans affecting soil fertility; soil nutrient levels?
-Farmers increase soil nutrient levels by adding inorganic fertilisers, organic matter, supporting natural processes increasing nutrient levels like bacterial nitrogen fixation
-Farming can also deplete soil nutrient levels due to soil erosion, biomass removal, inhibiting natural processes increasing nutrient levels/increasing leaching of dissolved nutrients
Humans affecting soil fertility; irrigation?
-Increases fertility of soil where water= limiting factor on growth
-Sufficient water allows plants to keep stomata open, continue gaseous exchange when soil would otherwise be dry
-Water dissolves nutrients, can be absorbed by plants in ionic form
Humans affecting soil fertility; soil compaction?
-Excessive use of heavy machinery/high livestock densities—> soil becomes compacted
-Aeration is reduced & waterlogging is more likely; especially when soil is wet
Humans affecting soil fertility; pH?
Controlling soil pH helps ensure nutrients= soluble but not too easily leached
How does soil erosion occur naturally and what is the rate of this?
-Soil particles are removed by wind/water
-Soil erosion rate is likely to equal/less than rate of soil formation
Types of soil erosion; what is wind erosion and why is it an issue?
-Soil= dry, low clay content—> likely to become loose w/ little cohesion between particles holding soil together
-If it’s windy, soil is unprotected; may blow away
-Can cause issues where soil is deposited as may cover crops/land in urban areas
Types of soil erosion; how is water erosion increased?
Steeper gradients
Types of soil erosion; what is rain splash erosion and why is it an issue?
-A type of water erosion
-Soil particles dislodged by splash of a raindrop. Soil particles are dispersed in all directions, those going downhill= likely to travel further
-Overtime, can cause downhill movements of large soil amounts
Types of soil erosion; what is surface runoff erosion and how is it an issue?
-Caused by surface runoff when infiltration capacity of soil has been exceeded
-Can occur when rainfall= heavy/prolonged, or soil is relatively impermeable so more water flows over ground surface
Types of soil erosion; when do slumping and landslides occur and how is this an issue?
-When soil on slopes becomes very wet, increased mass & lubrication of water—> downward movement of large soil amounts more likely
-Often occurs when deep soil on steep slopes becomes less stable post deforestation
-As roots that held soil together decompose, soil= held together less strongly & landslides following rain are more likely
How does vegetation reduce the rate of soil erosion?
-Acts as a natural windbreak, reducing wind velocity & therefore kinetic energy carrying away soil particles
-Vegetation cover & leaf litter reduce impact of raindrops on soil surface—> soil particles less likely to be dislodged
-Soil organic matter, incl colloidal material humus, helps bind soil particles together
-Plant roots hold soil together
-Plants help increase infiltration of water into soil—> reduces rate of runoff & water erosion
Humans activities increasing rate of soil erosion; vegetation removal?
Removes protection from erosion
Humans activities increasing rate of soil erosion; ploughing vulnerable soils?
Breaks up soil structure, exposing soil particles to erosion by water/wind
Humans activities increasing rate of soil erosion; overgrazing?
-If livestock density= too high, vegetation will be eaten faster than it can grow, increasing exposure of soil & risk of erosion
-Disturbance & root damage by hooves increases erosion risk
Humans activities increasing rate of soil erosion; reduced soil biota?
-Ploughing, reducing soil organic matter & use of agrochemicals can reduce # of living organisms in soil, which play an important part in reducing soil erosion;
-Detritivores & decomposers break down dead organic matter, releasing plant nutrients—> may increase vegetation cover
-Decomposition produces humus—> increases adhesion between soil particles
-Worms aerate soil, increasing drainage rate, infiltration capacity of soil
Humans activities increasing rate of soil erosion; soil compaction?
-Use of heavy machinery, high livestock densities, reduction in soil detritivores= more likely soil will be compacted
-This has smaller interstitial spaces, reduces infiltration rate so more likely rainfall will produce surface runoff & erosion
Humans activities increasing rate of soil erosion; cultivating steep slopes?
-Surface runoff water flows more rapidly down steeper slopes, has more kinetic energy to pick up and carry soil particles
-Cultivating w/ techniques disturbing soil (eg ploughing) adds to erosion risk
Effects of accelerated soil erosion; reduced productivity?
-Soil eroded → often most fertile topsoil lost
-Remaining soil may=less fertile → reduced plant growth + smaller harvests
-Erosions → leave shallower soil, root penetration may = more difficult
Effects of accelerated soil erosion; sedimentation in rivers & reservoirs?
-Soil particles carried → river may be deposited if water slows down, has less kinetic energy
-Can occur where river naturally slows down in reservoirs/on coastlines if suspended soils reach sea
-Soil that sediments in a river ↓ flow capacity, so river = more likely to overflow & cause flooding post heavy rain
Effects of accelerated soil erosion; Increased atmospheric particulates?
-Wind erosion ↑ level of atmospheric particulates
-Can make health problems like asthma worse
Effects of accelerated soil erosion; Desertification?
-Soil erosion → important part of combination of processes causing expansion of deserts
-Makes it mare difficult for vegetation to grow, contributes to further soil erosion, reduced rainfall
How do long-term crops reduce soil erosion?
-Growth of crops not requiring frequent replanting & soil disturbance =↓ erosion rate
-Eg permanent grassland & tree crops like fruit, tea, coffee
How does zero-tillage cultivation reduce soil erosion?
-Cultivation minimizing soil disturbance → reduces risk of soil erosion
-Direct drilling of seeds into ground = less disturbing than ploughing/ sowing
How does contour ploughing reduce soil erosion?
-Ploughing up & down slope → gulleys,↑ velocity of runoff + cause more rapid erosion
-Ploughing along contacts at 90° to slope ↓ erosion as water flow is stopped by ploughed furrows loses kinetic energy → can’t carry soil particles
-W/ slower flow rate → soil particles deposited in furrow. Works well unless rainfall is so heavy furrows overflow
What is tied ridging and how does it reduce soil erosion?
-Used on almost flat land
-Field divided w/ criss-cross of intersecting ridges
-Ridges retain water when it rains,↑ infiltration &↓ runoff that may have caused erosion
What is terracing and how does it reduce soil erosion?
-Where sloping land = cultivated, series of narrow fields are created w/ soil held in place by retaining walls built along contours
-Water flowing over walls flows quickly, but slows down as flows across fields
-Much of the water may infiltrate into soil
How do rows of stones reduce soil erosion?
-If gradient = gentle, rows of stones laid along contours can slow runoff &↓ erosion rate
-Any disturbed soil is deposited behind stone rows
What are windbreaks and how do they reduce soil erosion?
-Growing crop ↓ velocity of wind,↓ wind erosion but field will be exposed if plants are removed when crop is harvested
-Protection from wind is maintained if there’s hedgerows / rows of trees around field
-Hedgerow → reduces wind velocity for distance up to 30m its height
How can multicropping reduce soil erosion?
-Growing 1> crop in a field at same time can ↓ exposure to wind erosion if are harvested & re-sown at diff times b/c there’s always a crop in the field protecting the soil
-Works most easily where farming uses human labour, not machinery
-If work is done w/ machinery, wider strips of diff crops may be cultivated in ‘strip cropping’; strips more effective if arranged at 90° to prevailing wind direction
How can increasing soil organic matter reduce soil erosion?
-Any method retaining/adding organic matter will ↓ risk of erosion
-Organic matter an soil surface like mulch protects soil from wind & impact of rain drops
-As organic matter decomposes it makes humus (colloidal material) , which helps soil particles adhere to each other
How can livestock management reduce soil erosion?
-Stocking density can be controlled to ↓ soil damage, eg trampling damage
-Livestock movement can be controlled; eg ↓ access to riverbanks where trampling = more likely to cause erosion into river
-Livestock may be removed at high-risk times, eg post heavy rain
What does The Universal Soil Loss Equation (USLE) estimate and assess?
-Estimates soil loss rates & the rate of soil erosion for particular local conditions and cultivation practices
-Assesses impact of changes in farming techniques
What is the USLE formula and what does it represent?
A= R x K x L x S x C x P
A= Rate of soil erosion (annual soil loss)
R= Rainfall erosivity factor (measure of the potential ability of soil to be eroded by rain)
K= Soil erodibility factor (susceptibility of soil particles to detachment by rainfall and runoff)
L= Slope length factor (effect of slope length)
S= Slope gradient factor (steepness of slope)
C= Cropping management factor (impact of vegetation)
P= Erosion control factor (effect of type of ploughing)
