Lecture 29- Soils Flashcards
What are the three key macro nutrients for plant growth
Nitrogen, potassium, phosphorous
Why are micronutrients important?
To help plants carry out key enzyme functions
Why are carbon, oxygen and hydrogen not considered major soil macro nutrients?
They are predominantly gathered from atmospheric and water sources by plants, and not the soil
What is a cation and what is an anion?
A cation is a negatively charge ion, a cation is a positively charged one
Saplings the yield response to micronutrient contamination?
Stage 1. Deficiency. This is where increasing concentrations cause an increase in plant weight/ growth
Stage 2. Adequate. The concentrations are in sufficient quantities that any increases do not have an impact on plant growth
Stage 3. Toxicity. Concentrations get too high and cause toxic conditions, causing a decrease in plant weight
Explain the yield response to micro contaminants
As the plants don’t rely on them for growth, increases in the concentrations of micro contaminants have no impacts until reaching toxicity levels
Why are cancerous soils often micronutrient deficient?
The high ph and cancerous content cause micronutrient deficiencies. These micronutrient deficiencies result from less h+ ions biding to colloid surfaces, meaning micronutrients can
Where are alkaline soils and associated micronutrient deficiencies most common?
Middle East, North Africa, south west USA, Australia
What is PH?
A measure of the H+ ion concentration in solution. More hydrogen atoms means a more acidic solution
Why is ph considered a master variable in chemical reactions?
Many chemical and biological reactions are dependant on it
What is the ph range for mineral dominated soils?
4.0 to 10.0
What is the common ph range for humid regions?
5-7
How do soils act as a buffer zone?
They act as a buffer between the atmosphere and groundwater, retaining nutrients and supply them in a steady flow
Within soil, what absorbs ions, molecules and gas?
Clay minerals and organic matter
What causes soil to have its sorptive properties?
Large surface area of clay and hummus, electrical charges
What properties cause greater sorptive properties?
- Divalent ions are more preferable than mono charged ones
2. Large cations are preferred over smaller ones of the same charge, due to their greater surface area
What is cation exchange capacity?
A sum of the total cations that a soil can absorb
What is cation exchange capacity dependant on?
- Negative charge density
- Number of exchange sites
- Soil ph
How does cec impact a soils buffering capacity?
Soils with a greater cec have a greater buffering capacity
What are the three types of acidity in soils?
- Active acidity
- Reserve acidity
- Exchangeable acidity
What is active acidity in soils?
Quantity of H+ ions in soil water solution
What is the reserve activity in soils?
The amount of H+ ions held on soil surfaces via adsorption. This has a greater capacity than soil water.
What is the exchangeable acidity of soils?
The proportions of H+ and aluminium ions held within the soils diffuse layer. These can easily move into solution
How does the addition of acid to soils impact ions on cation exchange sites? Refer to active and reserve acidity
The addition of little acid will cause displacement of the cations bound to the soil surface (by H+ ions). This means there is no change in the soil water PH (which is the measured part) as the H+ ions are held on colloid surfaces and not in solution. When the reserve acidity is full, H+ ions are then encore rates into active acidity, altering PH.
Name the three ways of neutralising soil acidity , and one mitigation technique
- Liming. This is commonly the addition of limestone and Dolomites to soils to replace H+ ions with ca and mg on colloid surfaces.
- Addition of gypsum to reduce al toxicity
- Addition of om, as this increase the cec of soil (more surfaces) and increases base saturation which decreases acidity.
- Simply plant plants that are better adapted to acidic conditions
Explain the difference between adsorption, desorption, and absorption
Adsorption is where molecules adhere to the surface of a phase.
Desorption is when molecules are released from the surface of a phase.
Absorption is when the molecules are drawn into the bulk of the phase.
What is complication and how does it impact movement in soils?
A complex is a structure consisting of a central atom bound to surrounding molecules (e.g a ligand). Complex substances have decreased mobility, reducing the bioavailability of nutrients within the soils
How do reduced soils impact soil chemistry?
They alter the form of metallic minerals present, and their subsequent reactivity
Regarding all the nutrients within soils, which store can plants access?
Plants can ONLY access macro and micro nutrients available in soil water. This is due to the buffering capacity of the soil
What is bioavailability of nutrients a function of? (9 factors)
- Total concentration of the nutrient in soils
- microbial factors (influencing whether nutrients are broken out of bonds)
- Plant factors
- Redox potential of soil
- salinity
- soil ph
- Organic matter content (influxes cec)
- clay content
- Oxide content
Which factors control the cec of soils?
- soil ph
- Organic matter content
- clay content
- Oxide content
Why are agricultural soils typically more acidic than natural ones?
When plants die, they “replace” cations that counter acidity (eg. Ca2+) however, when plants are taken away for harvest they do not return these ions to the soil. This means there is nothing remaining to counter acidity, increasing acidity of soils
What are colloids and why are they important in soils?
The most chemically active part of the soil, due to their large surface area. These large surface areas increase the cec, attracting soil nutrients and ions. This governs physical and chemical properties in the soil
Why are anions often less present in soil than cations?
Cations are able to bond to particle surfaces due to differences in charge, whereas anions can’t. This makes them substantially more vulnerable to leaching processes
What does a high cec value indicste > 25?
This indicates a high clay or organic matter content that can old a lot of cations
What does a low cec value (<5)indicate?
A Sandy soil it’s limited organic matter that cannot hold many cations
