The Physical Environment (AS); Soil

Question 1

What is soil essential for?

Answer 1

Plant growth in natural ecosystems & agroecosystems

Question 2

What is soil fertility?

Answer 2

The ability of soil to sustain plant growth

Question 3

How is water content a feature of fertile soil?

Answer 3

-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

Question 4

How are soluble materials a feature of fertile soil?

Answer 4

-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

Question 5

How is air content a feature of fertile soil?

Answer 5

Most living organisms in soil & many processes increasing fertility = aerobic → well aerated soils likely to be more fertile

Question 6

How does dead organic matter contribute to soil fertility?

Answer 6

-Usually have a high dead organic matter content → releases plant nutrients as decomposes
-Increases water retention & provides food for soil biota

Question 7

How is pH a feature of fertile soil?

Answer 7

-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

Question 8

How do soil biota contribute to the fertility of soil?

Answer 8

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

Question 9

What is soil texture controlled by and what does this involve?

Answer 9

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

Question 10

How do drainage rates compare in sandy and clay soils?

Answer 10

-Larger pare spaces of sandy soils allows rapid drainage → reduces water content, increases aeration
-Clay soils = poorly drained

Question 11

How does capillary action compare in sandy and clay soils?

Answer 11

-Tiny pore spaces between clay particles allow underground water to rise towards surface
-No capillary rise of water in sandy soils

Question 12

How does aeration compare in sandy and clay soils?

Answer 12

-Pore spaces in well-drained sandy soils; filled w/ air
-clay soils; more likely to be waterlogged w/ low aeration rate

Question 13

How does nutrient retention compare in sandy and clay soils?

Answer 13

Nutrient ions absorb easily onto clay particles but not onto sand particles

Question 14

How does thermal capacity differ in sandy and clay soils?

Answer 14

-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

Question 15

How does root penetration differ in sandy and clay soils?

Answer 15

-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

Question 16

How does ease of cultivation compare in sandy and clay soils?

Answer 16

Lack of adhesion between particles in sandy soils; easier to cultivate than clay soils

Question 17

What is the composition of loam soils and why is this advantageous?

Answer 17

-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

Question 18

What is the structure of soil and its different types?

Answer 18

-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

Question 19

How does depth affect soil?

Answer 19

-Deeper soils—> less likely to become waterlogged/dry out rapidly
-Can also aid good root anchorage

Question 20

Humans affecting soil fertility; aeration by ploughing & drainage?

Answer 20

Ploughing + drainage makes soils more aerobic= increases rates of N fixing, nitrification & decomposition of dead organic matter

Question 21

Humans affecting soil fertility; soil nutrient levels?

Answer 21

-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

Question 22

Humans affecting soil fertility; irrigation?

Answer 22

-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

Question 23

Humans affecting soil fertility; soil compaction?

Answer 23

-Excessive use of heavy machinery/high livestock densities—> soil becomes compacted
-Aeration is reduced & waterlogging is more likely; especially when soil is wet

Question 24

Humans affecting soil fertility; pH?

Answer 24

Controlling soil pH helps ensure nutrients= soluble but not too easily leached

Question 25

How does soil erosion occur naturally and what is the rate of this?

Answer 25

-Soil particles are removed by wind/water
-Soil erosion rate is likely to equal/less than rate of soil formation

Question 26

Types of soil erosion; what is wind erosion and why is it an issue?

Answer 26

-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

Question 27

Types of soil erosion; how is water erosion increased?

Answer 27

Steeper gradients

Question 28

Types of soil erosion; what is rain splash erosion and why is it an issue?

Answer 28

-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

Question 29

Types of soil erosion; what is surface runoff erosion and how is it an issue?

Answer 29

-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

Question 30

Types of soil erosion; when do slumping and landslides occur and how is this an issue?

Answer 30

-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

Question 31

How does vegetation reduce the rate of soil erosion?

Answer 31

-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

Question 32

Humans activities increasing rate of soil erosion; vegetation removal?

Answer 32

Removes protection from erosion

Question 33

Humans activities increasing rate of soil erosion; ploughing vulnerable soils?

Answer 33

Breaks up soil structure, exposing soil particles to erosion by water/wind

Question 34

Humans activities increasing rate of soil erosion; overgrazing?

Answer 34

-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

Question 35

Humans activities increasing rate of soil erosion; reduced soil biota?

Answer 35

-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

Question 36

Humans activities increasing rate of soil erosion; soil compaction?

Answer 36

-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

Question 37

Humans activities increasing rate of soil erosion; cultivating steep slopes?

Answer 37

-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

Question 38

Effects of accelerated soil erosion; reduced productivity?

Answer 38

-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

Question 39

Effects of accelerated soil erosion; sedimentation in rivers & reservoirs?

Answer 39

-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

Question 40

Effects of accelerated soil erosion; Increased atmospheric particulates?

Answer 40

-Wind erosion ↑ level of atmospheric particulates
-Can make health problems like asthma worse

Question 41

Effects of accelerated soil erosion; Desertification?

Answer 41

-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

Question 42

How do long-term crops reduce soil erosion?

Answer 42

-Growth of crops not requiring frequent replanting & soil disturbance =↓ erosion rate
-Eg permanent grassland & tree crops like fruit, tea, coffee

Question 43

TBC page 165 / 169