lecture 10 Flashcards
how are soils grouped into textural classes?
based on % of sand, silt, and clay
* At each point it is 100% of one type/soil kind
Loam - middle area
how is soil structured? look at diagram
- Aggregation of mineral particles with Organic Matter into larger units
- Glue that holds aggregates together = OM and clay
- Pores= spaces between aggregates
Always has air bc particles don’t fit perfectly together
describe porosity
Pores contain and affect water and air movement
* Size of pores and total pore space - influenced by soil texture
coarse (sandy) soil pores?
- Coarse texture (sandy) soils have:
○ Larger pores but less total pore space
Affects water infiltration and percolation, well aerated
Fine textured (clay) soils pores?
- Fine textured (clay) soils have
○ Smaller pores but more total more space
Affect h2o infiltration and percolation, tend to waterlog
what is permeability? how does it differ for clay vs sandy loam
- Permeability - how fast air and water moves through soil
- Clay loam - takes longer and half the depth of sandy loam
Matters for the availability of water for organisms - if the drainage or retention of moisture is useful or not
- Clay loam - takes longer and half the depth of sandy loam
describe soil water - saturated, field capacity, unsaturated
- Saturated soil - pores are full of water, water drains under the influence of gravity (saturated flow)
- Once all gravitational water has left the soil, the soil is at field capacity (no more free drainage)
Unsaturated floe will continue (e.g through the roots) until wilting point - water is held too tightly by soil particles to be removed by plant
- Once all gravitational water has left the soil, the soil is at field capacity (no more free drainage)
water holding capacity factors? (3)
- Field capacity - enhanced by clay - large pore volume and surface area
- Measure by drying soil and weighing
- PWP - also increases wih more clay
- Water available for plant use = difference between water at field capacity and PWP
- Part of why loam soils are good for growing
- Organic content of soils Functions
○ Important as a nutrient source
○ Holds water (much like clay) ;impacts on field capacity and wilting point
○ Contributes to aggregation, cation exchange capacity (CEC)
○ Energy for soil microorganisms -decomp
Mycorrhizae - fungi that form symbiosis with plants increasing plant water and nutrient absorption
how are the chemical properties of soil neceessary - what does composotion vary with?
- Soil solution= soup of dissolved chemicals available for plants and soil organisms; contrains ions
- Composition varies with time + depth
○ Cations (+) = AL3, NH4,K,MG2,CA2
○ Anions (-) = CL-, NO3, SO4 2 . HCO3
- Composition varies with time + depth
inputs and depletions of chemicals in soil?
- Inputs: from atmosphere, transformations
Depletions - through plant uptake, transformations to solids, losses from ecosystems
what is CEC? what does it indicate? when is it high
*
* Cation exchange capacity
Ability of the soil to hold onto nutrients (cations) for plant use and prevent leaching ebyond roots
* Good indicatior of soil fertility, quality, productivity
Clay and humus (OM) - begatively charged (higher CEC)
what is CEC influenced by?
○ Soil texture CEC (high number = more cations held)
§
○ Strength of absorption - the strength of the bond of the cations
○
○ The relative concentration of cations in the soil solution
CEC levels of different soil types
Sands (light-colored) 3-5
Sands (dark-colored) 10-20
Loams & silt loams 10-25
Clay and clay loams 20-50
Organic soils 50-100
strength of cation bonds from strong to weak
Al+3 >
H+
Ca2+ >
Mg2+ >
K+=NH4+
Na
what is AEC? ex of limiting anions?
- Hold onto or retain anions
- Some very important plant nutrients are aninons (most limiting from most negative to least)
Phosphate HP042-
Sulfate S042
Chloride Cl-
Nitrate N03-
- Some very important plant nutrients are aninons (most limiting from most negative to least)
when are anions lost most? how do they relate to PH
- Most negatively charged - loss of anions
Some soils (e.g. with high AL and Fe oxides) have charges that vary with ph (low ph = high AEC/more positive charge) ; can retain some of these nutrient anions
why is nitrate limiting
- Nitrate is very weakly held by soils (anion)
○ Very soluble, does not react to form new solids (prone to leaching)
If nitrate is not taken up by plants it is very likely to be lost from the soil
what is ph
(H+ activity)
what does PH affect
Affects:
-Nutrient availability
-Organisms (often narrow
range of tolerance)
what is PH affected by?
Affected by:
-Organic acids released by
plants & soil microbes
-Plant litter -> conifer &
moss litter are more acidic
-Parent material
