Lecture 3: Chemical and bio properties of soil

Question 1

Key macronutrients for soils:

Answer 1

• nitrogen (growth)
• phosphorus
• potassium
• sulphur
• calcium
• iron
• copper

Question 2

What are Exchangeable cations:

Answer 2

• positively charged ions that are loosely attached to the negative surface sites of clay particles or organic matter by electrostatic forces
• main cations = Ca2+, Mg2+, Na+, NH4+, K+

Question 3

what is Cation Exchange capacity and why is it needed

Answer 3

• total capacity of soil to hold exchangeable cations
• influences soil ability to hold essential nutrients
• indicator of water retention - allows water adheration

Question 4

causes of soil acidifcation

Answer 4

• insufficient use of nitrogen -> when NH4 used, breaks into NO3- and H+, H+ can build up if NO3- leaches
• removal of plant material -> plant material usually slightly alkaline, removing will leave excess H+ in soil

Question 5

what are the effects of soil acidification

Answer 5

• aluminium becomes soluble, soluble Al retards root growth and limits access to water and nutrients
• decreased availability of major nutrients
• lower microbial activity -> especially less nitrogen fixing rhizobia (killed)

Question 6

How to fix acidification

Answer 6

• ameliorating/liming (CaCO3) -> most economical
• add species to soil that will increase pH

Question 7

causes of increased soil salinity

Answer 7

• clearing
• using shallow rooted annuals -> use less rainwater which then adds to groundwater, leads to slat being brought up to surface and left when water evaporates

Question 8

impacts of soil salinity

Answer 8

• salt destroys protective plant cover
• plant death
• waterlogging
• increased water usage for farmers

Question 9

how to fix soil salinity

Answer 9

• increase water use by planting perennials
• manage soil water content (drain)
• enhance remnant vegetation
• re-plant trees on recharge zones
• plant salt tolerant species

Question 10

Phosphorus cycle process

Answer 10

• decomposed by organisms, released to soils or added to reservoirs
• runoff and erosion, leaching and plant uptake = out of soil
• manures, plant residues, fertilizers, atmospheric deposition = into soil

Question 11

what is the Phosphorus buffering Index

Answer 11

PBI (phos buffering Index) = capacity of soil to fix P.
- Higher = need more P supplied in fertilizers to maintain P level
- how much P soil absrobs

Question 12

Why can legumes fix atmospheric Nitrogen

Answer 12

• symbiotic relationship with rhizobium bacteria -> live in nodules
• they fix it into an inorganic state so plants can absorb it
• N -> NH4+ -> NO3-
• nitrate is highly mobile in roots but leaches quickly due to this, ammonium less so due to pos charge so leaching less -> thus need legumes

Question 13

why is nitrogen so important?

Answer 13

• needed in highest amounts
• major component of chlorophyll
• major component of amino acids -> needed to build proteins for growth

Question 14

why are bilogical components so important to soils

Answer 14

humus formation, OM breakdown, N fixing and cycling, soil structure maintenance
- macrofauna break down OM, microfauna aid nutrient transfer, mesofauna spread disease or prevent it
- fungi agregate the soil, decomposers
- worms and insects aerate the soil