The Physical Environment (AS); Soil (Complete)

Plant growth in natural ecosystems & agroecosystems

The ability of soil to sustain plant growth

-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

-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

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

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

-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

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

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

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

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

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

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

-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

-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

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

-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

-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

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

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

-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

-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

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

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