What property contributes to the substantial difference in the ability of soil colloids, in comparison to the silt and sand fraction, to hold nutrients?
Clay and organic colloids have a substantial capacity to hold nutrients due to the much greater surface area. Clay has more than 1000 times the surface area of sand.
How are plant nutrients held in the soil?
Adsorbed, by electrostatic attraction to the negatively charged surfaces. This enables an exchange process to occur between the soil solution, and the colloid surface. If nutrients are too strongly adsorbed, such as when P is strongly sorbed through specific adsorption, then exchange does not readily occur.
Describe the process where plant roots may acquire nutrients from the soil solution, and how do we measure the capacity of soil to hold nutrients?
Nutrients become available in the soil solution through ion exchange processes. Cation exchange is measured using a leaching solution of a salt solution of a cation, such as ammonium chloride. Cation exchange is many times greater than anion exchange.
Name the sources that supply nutrients to the soil?
SOM, atmosphere, primary and secondary minerals.
What are exchangeable bases and which one of these is not essential for plant requirements?
Ca, Mg, K are plant nutrients, while Na is a non-essential plant nutrient.
Describe the process of N supply to plants. What form of N do plants mostly acquire from the soil solution?
Mineralization of organic matter is the major source of N to plants. However, the fixation of N by legumes is another major source, especially in some low nutrient natural ecosystems. Most plants will acquire their N as nitrate, although in some acidic soils plants have adapted to take up ammonium.
Of course some cations in the soil solution are not essential for plant growth. These can become increasingly soluble at low pH, and hence toxic to plant growth. What are these, and at what pH do these become more soluble?
Below pH the concentration in the soil solution of Al increases to a level that becomes toxic for plants. The other metal cation is Mn, which can also increase to toxic levels in acidic soils.
What property is easily measured in the laboratory and has a profound effect on many soil physical properties? What is this measurement? Hint: you will need to collect an intact core of soil.
Soil porosity can be measured/estimated by the measurement of soil bulk density.
What sort of pores will be water-filled at Field Capacity?
Meso- and micro-pores.
What pores are able to hold water most tightly against gravitational force. What is the pore size, and the matric potential of this water?
Micropores hold water that has a MP of -1500 kPa (Wilting point). This water is considered unavailable to plants. The micropores would be 100 nm.
Usually loam soil is considered to have a ‘well balanced’ distribution of pore sizes. Thus loams usually have good infiltration, and water holding capacity. However, what can be added to a loam soil to aid water retention?
Organic matter can improve the water-holding capacity.
What component of the soil may strongly affect water-stable aggregates? Especially with respect to slaking.
Organic matter
Describe the changes that occur to the distribution of pore sizes when well-structured clay is compacted? What are the changes in plant available water?
Compaction largely reduces the macroporosity, thus the infiltration/drainage will be affected. However, there is also the possibility that there some compaction of mesopores, and hence a slight reduction in the plant available water.
Hydraulic potential is the sum of which two components? This is important for predicting water movement. At what conditions is water likely to move?
Hydraulic potential is the sum of matric and gravitational potential. Water will move from a high (such as a saturated soil) to low potential (dry soil).
Soil structure can be measured in many ways, but one of the most useful is to study the soil profile. What important property does a well-structured soil (visible aggregates) have in comparison to a poorly-structured soil? What does this affect in the soil?
Well-structured soil will have a much higher porosity than soil with few large aggregates. This facilitates good aeration, water movement, biological activity and root growth.
What do clusters consist of?
Clay and organic material, including bacterial and fungal cells.
What promotes stable macroaggregates? What management technique can enhance this process?
The presence of roots and fungal hyphae can be promoted through the use of no tillage.
What are sesquioxides and in what soils are they most abundant?
These are amorphous iron oxides, which are particularly abundant in highly weathered soils.
When is goethite most likely to form?
Although these are both forms of Fe(III) oxides, the hydrated form, Goethite is predominant in moister and cooler environments.
Soils that consist of a high proportion of the clay mineral Illite are considered to have an adequate level of K. Where are these K+ ions located?
Illite contains a inter layer of free K+ ions. Weathering of the mineral releases these ions into the soil solution.
Why does Montmorillonite swell and Kaolinite not?
As montmorillonite is weakly bonded between the crystal layers, water can enter between, and enlarge the space between these layers. In comparison, Kaolinite is strongly bonded by hydrogen bonding.
Define salinity and sodicity?
Saline soils are defined when the EC> 4 dS/m. Sodic soils have ESP>6%.
What physical constraints are imposed in sodic soils? How can we alleviate these constraints?
Poor soil structure, which can lead to reduced infiltration, aeration and prone to waterlogging. Although gypsum can be useful for these soil structure, the use of organic matter can be more beneficial in the long term.
The electrical conductivity of a soil is measured in the extract of:
B)
Which of the following statement is NOT true of slaking?
B)
Which of the following is not a component potential of the total soil water potential?
B)
Which two soil physical properties are considered by the least-limiting water range?
C)
Micropores are most important for:
C)
Which of the following is true of a soil drying below field capacity?
D)
Which of the following makes water move from soil B to soil A?
D)
To generate a water retention curve of a soil, which of the following method can be used?
B)
Waterlogging of soil increases the availability of which of the following nutrients?
B)
Which of the following fertilizers is preferred for a paddy (waterlogged) soil to minimize nitrogen loss? (All the fertilizers are applied at the same rate of N)
D)
Which of the following is (are) greenhouse gasses contributed to the atmosphere by soils?
D)
0.01M CaCl2 is recommended for the measurement of soil pH because:
A)
Which of the following fertilizers acidifies soil least when applied to the soil? (All the fertilizers are applied at the same rate of N)
D)
Exchangeable cations in the soil may be determined by analysing the filtrate obtained after leaching the soil with
A)
Permanent negative charge in the soil results from
B)
To completely exchange 30 cmolc/kg of Na+ in soil, we need:
B)
For a soil with a CEC of 20 cmol(+)/kg, what is the maximum number of moles of Ca2+ that could be adsorbed by 1 kg soil?
D)
For a soil with a CEC of 200 cmol(+)/kg, what is the maximum number of moles of Ca2+ that could be adsorbed by 1 kg soil?
C)
Which of the following has the most availability of potassium (K) to plants?
D)
Which of the following has the least availability of potassium (K) to plants?
D)
Which of the following micronutrients is most likely to be deficient in acid soils?
A)
Plants that are efficient in iron (Fe) uptake in a calcareous soil are known to employ which of the following methods?
D)
Which of the following statements is correct?
D)
Which of the following crops would you recommend the farmer to grow if his paddock is contaminated with cadmium?
C)
Which of the following statements is correct?
D)
Soil compaction can adversely affect plant growth in several ways. Which of the following is least likely to be a consequence of soil compaction?
A)
From your understanding of the behaviour of ions in the soil solution, which of the following would you expect to be the most likely to contaminate groundwater?
