Week 9 - Nutrients Flashcards
How do plant roots absorb nutrients?
- Most nutrients absorption is passive and are taken up dissolved in water and is transported via the apoplast and symplast.
- Nutrients must be in the soil water, and that the water potential of the plant must be lower than that in the soil for water (and thus the dissolved nutrients) to move into the plant.
Describe the properties of essential nutrients?
- Plant must be unable to complete its life cycle in the absence of that particular element.
- Action of the element must be specific, and not able to be corrected or prevented by supplying any other element.
- Effect of the element on the plant must be direct. This precludes indirect effects such as alteration of soil pH, effects on concentration of other ions at the root surface, and effects on both pathogenic and non-pathogenic organisms which might limit plant growth.
Define the term macronutrient?
What are the macronutrients essential for plant growth?
Essential elements required in large amounts (>5-10 kg of element ha-1) or occur in high concentrations in normal plant tops (>0.1% of the dry matter).
These include:
C, H, O, N, P, K, S, Ca, Mg
C is obtained from CO2 in the air,
O from O2 in the air
H from water, originally from the atmosphere.
Define the term micronutrient?
Essential elements required in low concentration for normal plant growth are referred to as micronutrients (less than 1 kg of element per ha).
What are the forms of essential elements absorbed by plants?
Element Chemical formula of the form taken up by plants
Nitrogen NH4+, NO3-
Phosphorus H2PO4-, H2PO42-
Sulphur SO42-
Calcium Ca2+
Magnesium Mg2+
Potassium K+
What is required before plant uptake of nutrients can occur?
Before plant uptake can occur, the nutrient must be;
- soluble; or
- be in or capable of being turned into the plant available ionic form.
Why does elemental Sulphur not produce rapid growth response in plants?
Elemental sulphur (S) is neither soluble nor in the plant available form.
S has to be oxidised to SO42- by microorganisms before it is available so its application will not produce rapid growth response in plants.
In fact, the responses are gradual over periods of the order of months.
What happens to plant growth as nutrient supply is increased?
Zone A: The yield increases with nutrient supply, i.e. there is a nutrient deficiency. Note that this is not a straight line increase but is curved. At low nutrient supply, the increase in growth to extra nutrient is large but at higher levels of nutrient supply, extra nutrient produces less growth.
Zone B: Additional nutrient does not increase growth, i.e. the nutrient does not limit yield. The yield is limited by another nutrient, environmental or genetic factors.
Zone C: Additional nutrient decreases yield, i.e. the nutrient supply is in excess and toxicity is occurring.
Why can Molybdenum deficiency occur in acidic soils?
- There are two types of nutrient deficiency, absolute and induced. Both can occur with molyledenum.
- Some soils simply do not contain sufficient Mo to supply plants, it’s just not in the soil - an absolute deficiency.
- Molyledenum availability is high at neutral soil pH (pH 6-7). However it is highly insoluble at low pH (pH <5) so acidity can induce a Mo deficiency
Why does “blossom end” rot a Ca deficiency occur in tomatoes?
“Blossom end” rot is an example of cation antagonism (a nutrient interaction).
High concentrations of any one cation can reduce the uptake of another nutrient present in a cationic form.
Application of high rates of fertiliser can reduce uptake of Ca by tomatoes and result in “blossom end” rot.
Explain why adding one nutrient may not result in a positive yield response?
- When there are multiple deficiencies, for example, both N and P, if only N is added no response may be obtained because the P deficiency remains.
- The yield response from addition of both N and P is much greater than the response to both applied separately. This is an example of a nutrient interaction.
Why can non-essential elements cause problems in plants?
Occasionally, they cause problems when present in large concentrations in soluble forms.
Under these conditions, they result in toxicities as they interfere with normal plant metabolism, e.g. Al, Pb, and Cd.
In addition to other mechanisms, how can nutrients resist leaching via immobilisation?
Nutrients can resist leaching by being immobilised into the organic fraction, that is, incorporated into microbial tissue of microorganisms.
How do cation/anion exchange sites impact nutrient retention?
Clay minerals and organic matter, develop electro-static charge on the surface of clay sized particles.
On both of these materials, the charge is normally negative so only the cations (the positively charged ions) such as K+, Mg2+, Ca2+, NH4+ are attracted to the surface.
The link between the solid particle and cation is simply one of electrostatic attraction.
Describe the two types of charge developed on soil solids.
Permanent charge - the concentration of charge on the soil particles does not change with the conditions in the soil solution. This type of charge results from isomorphous substitution in the clay layer silicate minerals.
Variable charge - the concentration of charge on the soil particles changes with the conditions in the soil solution.
On organic matter, describe why the concentration of charge on the soil particles changes with the conditions in the soil solution.
On organic matter most of the charge is derived from the dissociation of carboxyl groups:
R - COOH ⟺ R - COO- + H+
This equilibrium equation shows that the negative charge on the organic matter will increase if the H+ concentration decreases, that is the pH is high.
Conversely the charge on the organic matter will decrease as the H+ concentration increases, that is the pH is low.
