37 Soil and Plant Nutrition Flashcards
What are the basic properties of soil?
Its texture and its composition.
What are the layers of the solid called?
Horizons i.e. the A horizon, B horizon and the C horizon.
What does the ‘A horizon’ of the soil refer to?
The top soil.
This layer is made of broken down rocks, decaying organic matter and living organisms like worms. It therefore has the most nutrients for plant growth.
It is frequently weathered by wind and washed away by rain and flood.
What does the ‘B horizon’ of the soil refer to?
The layer below the top soil has less organic material than horizon A and is less weathered.
What does the ‘C horizon’ of the soil refer to?
The bottom layer of soil
It has little organic material and thus is composed mainly of broken down rocks. Note that some of these rocks migrate up to form the horizon B and horizon C.
While this region has few organic nutrients it may have expansive water tables that can be exploited by the roots of trees etc.
What does ‘hummus’ refer to?
Decaying organic material in the solid.
What is the decaying organic material in the soil called?
Hummus
What is the most fertile type of topsoil called and what is it composed of?
‘Loams’ are topsoils with roughly equal amounts of sand, silt and clay.
Why are ‘loams’ particularly fertile?
The small silt and clay particles provide the surface area for the adhesion and retention of minerals and water.
Meanwhile the larger spaces between the san particles allows good diffusion of oxygen to the roots.
Why are sandy soils not good for growth?
The large particles have a relatively small surface area and thus less water and minerals are retained.
Sandy soils often have less organic material and are also less firm so do allow the plant to anchor itself in as firmly.
Why are completely clay soils not good for growth?
Since they have poor drainage the soil becomes waterlogged and thus oxygen can not reach the roots.
Generally speaking, what properties will soil made of fine particles have?
They will be good at retaining water and nutrients as their fine particles provide a high surface area.
This will, however lead to poor drainage and thus the soil will become waterlogged. Therefore the roots will receive little oxygen.
Generally speaking, what properties will soil mad of large particles have?
There will be larger spaces between the particles so sufficient oxygen will reach the roots. This also provides good drainage so the soil won’t become waterlogged.
The large particles mean that the soil will have a lower surface are to volume ratio. This means it will be less able to hold onto nutrients and water.
Besides particle size, what factor determines how well the soil will adhere to nutrients?
Most soil particles are negatively charged and thus positive ions (cations) bind to them. This makes anions like Ca2+ and K+ easily retained by the soil and thus these minerals are not easily lost through leaching.
Anions like nitrate (NO3-), phosphate (PO4 3-) and sulphate (SO4 2-) are not attracted to the soil particles and thus are lost more easily.
How do root cells absorb minerals form the soil? (not a structural adaptation)
They perform ‘cation exchange’ to displace cations that are attracted to the negatively charged soil particles.
How does ‘cation exchange’ work?
As the root cell performs respiration it releases CO2 into the surrounding soil. This carbon dioxide reacts with water to form carbonic acid.
The carbonic acid dissociates yielding an H+ ions. These H+ ions neutralise the negative charge of the soil particle and thus cause the bound cations to be released into the water of the soil.
These displaced cations in the soil’s water then diffuse or are actively transported into the root cell.
Why is ‘cation exchange’ important?
Otherwise the cations that are attracted to the soil particles would not be free to enter the root cell.
How does soil pH affect nutrient availability?
If the soil is already acidic the H+ ions already in the soil will perform cation exchange and thus the nutrients will be freed more easily.
What soil minerals are more prone to be leeched through rain etc?
Anions as they are not attracted to the soil particles.
Besides acting as a nutrient source for plants, what is the importance of topsoil?
It prevents clay particle from packing together.
It also provides a home for bacteria and other detritivores to recycle the nutrients of the soil.
How do earthworms derive their nutrition?
From the bacteria and fungi in the soil.
What roles do worms play in improving the soil?
They consume organic material, partly break it down and then return it to the soil through egestion.
This moves organic matter deeper in to the soil. Worms also clump the soil together which provides better air pockets.
What caused the ‘American Dust Bowl?’
The destruction of prairie grass and the depletion of soil nutrients combined with a drought made the soil dusty and not held in place by plants.
This lead to giant dust storms.
What are underground water reservoirs called?
‘Aquifers’
What does ‘aquifer’ refer to?
An underground water reservoir.
What is an increase in soil salinity called?
Salinisation.
What is a particularly wasteful form of irrigation and why is it wasteful?
Simply pouring water on a field. Water is lost both by evaporation and by water runoff.
What is a less wasteful form of irrigation?
‘Drip irrigation.’ In this method perforated tubes are inserted at the soil layer where the roots are.
This minimises evaporation whilst also allowing the water to be slowly added. This reduces run off as water is added at the rate it is lost from the soil. This also prevents oxygen-defiicent water logged soils.
What are the main nutrients found in fertilisers?
N,K and P (nitrogen, potassium and phosphorus)
In what from is nitrogen added to the soil in fertilisers?
As nitrates or ammonium
In what from is potassium added to the soil in fertilisers?
‘Potash’
In what from is phosphorus added to the soil in fertilisers?
Phosphate
Besides by influence cation exchange, how does soil pH affect nutrient availability?
At pH 8, for instance, plants can absorb calcium, but iron is almost unavailable.
At pH 5 or lower, toxic aluminum ions (Al3+) become more soluble and are absorbed by roots. This impairs and the uptake of calcium.
How can plants that live in acidic soils with high Al3+ concentration survive?
They release anions that bind to Al3+ and the snake the soil safe.
What plowing technique can reduce runoff?
’No till agriculture’ in which the soil is not tilled i.e. turned over.
While tilling gets rid of weeds well it also disrupts the roots of the crops and thus makes them less able to anchor the soil in place.
How can run off be minimised in farms on hills?
With ‘contour tillage’ in which the farm is planted on flat steps down the side of the hill.
What is one way that pollute soils can be remediated?
‘Phytoremediation’ in which specific plants are grown in that soil due to their ability to absorb the particular toxins in the soil. The plants can then be removed, taking the toxins out of the ecosystem.
Why is “Thlaspi caerulescens” used in phytoremediation?
It can absorb high levels of zinc.
What is ‘phytoremediation’ a type of?
Bioremediation which includes when other organisms i.e. prokaryotes and protists are used to break down or remove pollutants from an environment.
What is an ‘essential element’ in terms of plant nutrition?
One that is required of the plant to complete its lifecycle and produce another generation.
What is a ‘hydroponic solution’?
A solution containing water and nutrients into which the plant is grown i.e. without soil.
What nutrient is a micronutrient in some plants?
C4 and CAM plants need sodium ions to regulate phosphophenolpyruvate the CO2 acceptor in these forms of photosynthesis.
What does a magnesium deficiency in plants cause?
‘Chlorosis’, a yellowing in the leaves due to an absence of magnesium which is a component of chlorophyll.
What does ‘Chlorosis’ refer to?
A yellowing in the leaves due to an absence of magnesium which is a component of chlorophyll.
What mineral deficiency is yellow leaves characteristic of?
The yellowing in the leaves (called chlorosis) is due to an absence of magnesium which is a component of chlorophyll.
Besides magnesium, what can cause ‘chlorosis’ and why?
Iron deficient as iron is a co-factor in the enzymatic reactions that form chlorophyll.
In what leaves will a nutrient deficiency be first evident?
If the nutrient is highly mobile i.e. transported by phloem the nutrient deficiency will appear first in the old leaves. This is because as the new leaves grow phloem is preferentially routed to them.
If the nutrient is not highly mobile then the old leaves will already have a stockpile in them. Therefore it is the new leaves that will show the deficiency first.
What are the macronutrients needed by plants? (in order form highest concentration to least)
Carbon. Oxygen, Hydrogen, Nitrogen, Potassium, Calcium. Magnesium, Phosphorus and Sulfur
In what form is Carbon primarily absorbed?
CO2
In what form is Oxygen primarily absorbed?
CO2
