SLSC 312 Midterm part 2

Soil pH to N cycle

pH formula

pH = -log [H+]

or = log 1/[H+]

High concentration of H+ ions is a _____ in soil

lower pH

How does increased clay and OM content affect pH

Higher surface acidity and buffering

What is equil’m btwn surface and solution acidity responsible for?

Ability of soil to buffer changes in pH

Sandy soils more susceptible to acidification

Clayey and high OM less

What is largely responsible for pH in soils?

Parent material

SK is neutral to alkaline (calcareous)

What does a low pH/high [H+] affect?

Mineral weathering (cations release from lattice)

Salt solubility increases (lose anions)

Root and microbial growth impeded (low CEC means more leaching)

Where is Al toxicity a problem?

In acidic soils where it’s more present in the sol’n

Also Fe and Mn

What pH do fungi dominate?

<5.5

What pH is required for nitrification and why?

Neutral

Microbes prefer neutral pH and slow nitrificatino in acidic/low pH

What is the optimal pH levels for ion uptake?

Neutral for cations

Slightly acidic for anions

How do you ctrl acidic soils?

Liming

CaCO3, CaO, chars

How do you ctrl alkaline soils?

Chem amendments, irrigation and drainage to flush out exchangeable sodium

What affects buffering capacity?

Texture and CEC

More lime needed to change a clayey soil w high CEC than a sandy soil w low CEC

What does long-term use of ammonium ferts do to the soil?

Acidifies through nitrification (creates H+ ions)

How does H2SO4 produce in the soil?

Elemental S or thiosulphate oxidizes and makes sulfuric acid

Which mineral nutr is needed in highest amts for plants?

N

Highest concs in black soil zone where high OM

What form is majority of N in the soil?

98% organic (plant unavail)

Needs to be mineralized to ammonium and nitrate

Soil N inputs and outputs

Inputs: fert, crop residue/manure, biol N fixation, atm

Outputs: crop removal, leaching, volatilization, denitrification, erosion

Open vs closed system concepts

Open: inputs - outputs = long-term fertility

Closed: inputs - outputs of avail pool = short-term fertility (growing season)

General rule for fert addition in wheat

3 lb N/bu

What is the trend in N use after applied to a wheat crop?

!st year takes up 50%, immobilizes 25%, 30% leaches/denitr

2nd year 5lb N/ac tied up in OM is released to crop

3rd-15th year 1 lb/ac tied up in OM released

Trend: amt N released to crop from SOM is greater than from immobilized fert

Symbiotic vs asymbiotic BNF

Symb: Rhizobium and legumes

Asymb: free-living bact, algae

How much N is fixed in symb fixation and what does it depend on?

75% of total N in plant

Depends on crop (perennials > annuals), Rhizobium strain, environ condns, soil condns

How do soil condns affect symb N fixation?

Greatest when avail N is at a minimum, allows Rhizobium to do its thing

If lots of avail N, plant wouldn’t send food to Rhizobium

What happens when a N-rich residue is added to a soil?

net mineralization upon addition

What happens when you add a N-poor residue to a soil?

Immobilization upon addition, eventually balances back out

What rotations show a greater and lower N loss?

Greatest in wheat-sumer fallow rotations

Then cts row cropping

Legumes least loss

What is a general way to maintain soil N?

Return the residues

Don’t burn!!

What is an atmospheric N addition?

NOx’s from industry creating acid rain

Sml amts of NOx from lightning