Week 4 - soil water

calc water content, availability, nutrients

How is water in soil used?

• lack of soil water = limiting global soil productivity

• irrigation: increases soil water

  • salinization problems, expensive

• dissolves nutrients into soil for plants and throughout the plant

2 methods of measuring soil water

1. traditional = gravimetric (in the lab)

2. modern = time domain reflectometry (in the field)

What is the mass gravimetric water content?

Ө_m = (wet soil – dry soil) / dry soil 

• soil gravimetric water content = mass of water/mass of oven dried soil

• no container weight

  • post baking → do bulk density + soil density calcs

What is the volumetric water content

Ө_v = (water weight/dry soil weight) x (bulk density of soil/water density) 

commonly used numbers

• density of water = 1.0g/cm³ 

• bulk density = 1.3g/cm³ 

• standard for oven dry soil = 20 to 25g wet soil dried at 105C for 48 hours/24h for sandy soil 

• weight = g 

What is water potential?

• water potential determines how tightly water is bound in the soil 

  • the availability of water for bio processes 

• diff in energy level of water from one condition to another 

• potential = neg value 

  • the more tightly water is held = the more work needs to be done to move the soil from the soil to a pool of water at the zero state 

  • removal of water from the soil by plant uptake or evaporation

What is total water potential Ψw?

Ψ_w= Ψm + Ψs + Ψp + Ψg 

• Ψm matric potential – water is in contact with solid particles 

  • clay, sand, water absorbed to soil surface 

• Ψs solute (osmotic) potential – effect of dissolved substances on the ability of water to work 

  • usually really small unless salt/solutes are present (can ignore?) 

• Ψp pressure potential – pressure from gases 

  • can igore unless wetland/peatland 

• Ψg gravitational potential – effect of vert position on water 

  • can ignore? 

• basically Ψw ≈ Ψm

What is water potential gradient? What is saturated vs unsaturated flow?

• WPG = water potential gradient 

  • water moving from a wet area to a dry area 

  • diff in total water potential/distance 

• unsaturated flow = wet to dry 

• saturated flow = is saturated but moves under force of gravity 

How to measure soil water flow?

• water further away from soil particle = higher water potential 

• water adj to soil particle = low potential (water held strongly to soil part) 

Water potential classification (3)

1. gravitational water

• drains from saturated soil by gravity

• potential depends on soil type

  • loamy + clay = -33 kpa, sandy = <-10 kpa

2. field capacity

• optimal water content (extra water already drained)

• usually reach at 48h, sandy = 24h

• depends on soil texture

3. permanent wilting point

• water unavailable to plants

• potential is at -1500kpa

water flow in + through soil (3)

1. infiltration

• water mvmt into upper soil layer

• from rail or irrigation water on soil surface

2. percolation

• movement of water through already wetted soil 

3. leaching

• percolating water moves through soil carrying dissolved nutrients and salts

Factors controlling water movement in soil

1. soil texture

2. structure

3. SOC

4. depth to bedrock

5. amt of water in soil

6. soil temp

7. soil compaction

fix soil = inc SOC

what is WUE

soil water use efficiency

• how efficiently water is used by plants for transpiration, plant growth, evaporation, drainage loss

How to reduce soil water loss? evaporative water loss?

change: evapotranspiration, runoff, percolation

use: mulching (organic or inorganic)

soil temperature (organic mulch = lowers soil temp) (inorganic mulch = increase soil temp)

What does organic mulch do?

• tree leaves + branches on soil surface

• adds OM, soil nutrients

• replaced annually

• crops seeded into mulch

What does inorganic mulch do?

• volcanic rock 

• thickness of rock mulch affects crop production

  • too big = blocks crops from growing, too small = cant maintain anything

what is land fallowing

• the practice of leaving land unseeded for a long time so that the soil regains its fertility

• bare soil = allows water to accumulate 

• remove weed from taking water from soil

Which nutrients are used in photosynthesis?

C, H, O

makes of 90% of plant dry weight

what is mass flow?

The movement of dissolved plant nutrients in water flowing toward the root (area of low nutrient concentration)

mechanisms of nutrient uptake

1. nutrients to roots by mass flow

2. diffusion: mvmt of dissolved plant nutrients in water

• plant root = area of low nutrient concentration

3. root interception: when root surface comes into contact with soil colloids

root hairs

roots vs leaves in nutrient absorbtion

roots: not well understood how nutrients get absorbed into roots

speculation of diff pathways for diff nutrients

leaves: absorb a small quantity of nutrients compared to roots via plant stomata

C + other nutrients (dry deposition, leaching)

what is nitrogen in soil?

• most deficient nutrient in soil in temperate zone 

• the limiting nutrient 

• supplemented via external supplies 

  • fertilizer, manure, compost 

what is nitrogen fixation?

• primary source of N from atmosphere 

• microbial action takes N from atmosphere and transforms to a usage form for plants 

  • symbiotic: formation of root nodules by certain plants that house bacteria fixing N 

  • non-symbiotic: bacteria have no specific association with plants and exist independently in soil 

what is nitrogen mineralization?

• plants cannot take N in organic forms → must be converted to inorganic N  

• mineralization process 

• dep on C:N ratio of organic material (affects rate of activity) 

  • high ratio = slow decomposition rate, low ratio = fast 

  • 20:1 benchmark for mineralization vs immobilization

nitrification + denitrification

nitrification: oxidation of ammonium to nitrate by microbes 

• when soil moisture <60% 

• in 2 days avg, slower in less optimal env conditions

dentrification: nitrate to nitrogen by microbes 

• middle step produces N₂O if dentrification is incomplete (GHG) 

How is nitrogen leached?

• ammonium/nitrate are soluble in water 

  • leached from soil 

  • not held on to cation exchange site 

  • leads to eutrophication 

• plants cant take everything, remainder of N and NO3 leaches 

what is inhibition nitrification?

decrease the rate of nitrification of ammonium (NH4+) → reducing N2O emissions

how is nitrogen lost via N2O gas?

• losses of N via N2O can occur either via nitrification (soil water <60%) or denitrification (soil water >60%) 

• 273x global warming potential of CO2, a lot from agriculture 

how is phosphorous used?

• pollutant = eutrophication 

• plant cell division, root growth, general plant growth 

• plants differ in ability to absorb P 

phosphorus and plant uptake

• low solubility = difficult uptake 

• fertilizers combine w/ cations forming low solubility substances 

• low C/P ratio allows for mineralization 

• not clear if plants can take up organic P 

how to ensure sufficient p for plant uptake

• maintain soil ph between 6-7 

• high OM input from fresh plant residues 

• addition of P fertilizers 

potassium in soil

• highly soluble 

• enough, rate of K release from decomp = rate of plant require K

• can be taken up in excess amts without affecting yield 

• excess may affect Mg absorption 

• losses of K = leaching, K fixation 

potassium and plant growth

plant cell division

carbohydrate formation

enzyme actions

translocation of sugars

regulation of plant water (osmosis)

sulphur in soil

• essential  

• legumes, cabbage, onions need high S

• deficient in S = light green  

sources of sulphur

SOM → microbes break down organic S into soluble forms

• A horizon

• sulphur is low in soils of arid and semi arid regions

soil minerals = inorganic form of S

• but easily solubilized

• accumulate as salts on surface in arid regions

atmosphere

• industrial emissions, acid rain