soil macronutrients (N, P, K) Flashcards
what are 4 inputs of P for soils
- weathering of rocks (phosphate)
- litter and necromass SOM
- fertilisers
- atmospheric deposition (dust)
what form of P do plants need
inorganic phosphate
Describe how P is mineralised
through the decomposition of SOM microbes mineralise into phosphate (PO4 -3_
decribe the process of P immobilisation
microbes compete with plants for phosphate in soils.
are there organic forms of P in soil
yes, organic P is dissolved in water and can be taken up by microbes and not plants
what is adsorption if P and when does it occur
a form of P fixation it is an abotic process, it occurs in low pH (acidic soils) where Fe/Al surfaces can adsorb P.
what are the 2 main losses of P
- through leaching and runoff with high rainfall (low dissolved P)
- through erosion loss of particles (high P in solid fraction)
do old or young soils have more P? eg think of AUS
P is mainly in primary minerals therefore P is higher in young soils and in weathered old soils ie australia, P is low because of lack of primary minerals.
what phosphate ions occur in the soil and how do thier concentrations differ with pH
HPO4 2-
H2PO4-
PO3 4- (only in high pH)
why is P have low concentrations in solution?
mobility is low because it reacts with soil particles .
what are 6 plant strategies to increase P uptake?
- symbiotic: root hairs and mycorrhizal fungi (AMF) hyphae
- increase root surface area with root hairs
- organic acids released in acidic soils to chelate with Fe or Al
- organic acids to dissolve Ca in alkaline soils
- phosphatase enzymes to increase P decomposition
- enhance growth of microbes that uptake soluable P
what is precipiation and how does it occur (P)
a form of P fixation
an abotic process in high pH soils (akaline) where Ca minerals precipitate with P
Australian plant adaptation to low P
form cluster roots (dense structures) that solubilise P
what are the 2 forms of P fixation
- low pH (fe, al and mn) adsoptio this decreaes availaibility
- high pH (ca, mg) precipitation to form ca/mg phosphates this decreases solubility
best pH for P availability
6-7 (neutral)
what does the P fertilisers debate disuss?
most fertilisers derived form rock (mainly in morocco)
dissagreement on amount of stock left. p mining increasing exponentiuall and need other alternatives maybe waste?
what form of K is in soils
there is only one form K+ cation. there is no gas phase and not involved in OM can be exchanged with other cations on clay or OM complex
3 inputs of K into soils
- mainly through weathering of primary minerals feldspar and mica and secondary clays
- rainfall washing K from leaves into soil
- fertilisers
3 loss of K
- harvest
- erosion
- runoff
biggest is harvest, low for erosion and runoff
what is the purpose of K in soils
K is a macronurtient for plants essential to activating enzymes for photosythesis, metabolism. it does not get incorporated into plant structures
how can K increase tolerance
drought tolerance: ion regulates osmotic water potential
pest tolerane study
sign of K deficiency
high mobility plants locate K to young leaves when stressed therefore symptoms in oldest leaves (K needed for plant functions)
what is luxury consumption?
when plants uptake more K than needed and does not add to their plant growth however can help with future stresses
why are K fertilisers necessary
plants uptake lots of K and K is removed once harvested. K fertilisers are easy to make and ulimited.
inputs (weathering and deposition) are slow processes therefore K fertilsier is needed to replace.
where is the biggest stock of N
atmosphere in form of n2 gas ~80% of air is N.
how does N enter the soil
N is not added through weathering but through biological fixation and deposition.
- fixation: microbes fix N2
- deposition through rainwater
describe ammonification
N in SOM is decomposed by microbes and mineralised into NH4 + (ammonia)
describe nitrification, what conditions are needed, and what can it cause
occurs under aerboic conditions
as O is needed for process.
ammonium mineralised into nitrite(NO2- ) (quickly) into nitrate (NO3 -). therefore more nitrate in soils and nitrite can be toxic in high Concentrations
causes acificication (H+) needed buffering
what is the available nitrogen pool
plants can only uptake mineralised forms eg ammonium and nitrate
how is N immobilized (2 ways = abiotic and biotic
nitrogen taken up microbes anfd compete with plants unavailable to pltants when locked by SOM
abotic process of immbolising ammonium fixed within caviities of clay structure between 2 layers
4 ways N is lost
- leaching, nitrate anion is mobile and can leach
- volatilisation, ammonia gas in soils with high pH
- anammox and nitrification, (process where ammonia oxidised by nitrite instead of O to form N2 gas. ammonium and nitrite transformed into gas
- denitrification nitrate turned into gas (O removed) with under anaerobic conditions. higher than 75% moisture all N2 produced no N is fixed
what microbe relationships fix N
symbiosis with plants eg legumes and non legumes (angiosperms), some are assciative symbotic and some non symbotic
how does N get fixed process
strong triple bond between N2 gas requires a lot of energy, microbes get this with symbiosis carbon from plant. most is through symbiotic relationship
how does high N availability effect N fixation
N fixation decreases with available N in soils.
human sources of N in atmoshere
through human acivity, agriculture (ammonia fertiliser and NH3 from livestock gas.
fossil fuels also release NO and NO2
how can N losses be controlled
- slow release fertiliser
- urease inhibitors to slow hydroloiss and prevent ammonia volatilisation in high PH soils
- Nitrifiacation inhibitors, inhibit acivity of some microbes to slow process of nitrification and prevent nitrate leaching by keeping as ammonium
what is eutrophication
high level of N into surface water cuases alagal blooms which deplete O when die.
what is volatilisation
forming ammonia from ammonium in basic soils (high pH)
