Study guide for Soil Resources, AGRO 153 for exam 2
Topic 2.1 – Soil texture
1. Soil separates grouped by size:
I. Sand (coarse and the largest, 502000 micrometers)
II. Silt (medium and the second largest, 250 micrometers)
III. Clay (fine and the smallest, <2 micrometers)
2. There exist 12 textural classes as per the USDA:
II. Loamy sand
III. Sandy loam
V. Silt loam
VII. Sandy clay loam
IX. Sandy clay
X. Clay loam
XI. Silty clay
XII. Silty clay loam
3. There are 2 ways to measure Soil texture:
I. By feel
II. By sedimentation (see page 2)
III. All of these methods’ objective is to calculate how much separate is in the soil by percentage
4. How to use a USDA Soil Textural Triangle to determine a soil’s textural class? I. The triangle percentages go clockwise
II. Determine your soil’s percentage for each separate (these should be 3 percentages)
III. Using your finger or pencil, follow the diagonal line or columns, inside the triangle, corresponding to each percentage Don't forget about the age old question of What are the main sources of primitive accumulation of capital?
IV. Find out where all these three lines or columns intersect
V. That’s your soil’s textural class
5. How to calculate a percentage?
I. Find out the number of items for which you want to calculate the percentage (let’s say X)
II. Find out the total number of all items, including those that you are not concerned with (let’s say Y, so X is part of Y)
III. Divide X by Y, and multiply the result with 100
IV. Write the sign % to your final result
V. This is now a percentage
6. Size matters!
I. Coarse: Largest pore size, Low porosity, Low water holding capacity, Fast drainage, Fast infiltration
II. Fine: Smallest pore size, High porosity, High water holding capacity, Slow drainage, Slow infiltration
III. Medium: Everything is moderate, not high and not low!
7. The sedimentation method
After the soil sample has been put into the cylinder and mixed, the separated begin to settle down at the bottom of the 1000 millilitergraduated cylinder, from the most to the least dense. The timer starts when mixing stops: Don't forget about the age old question of What is the role of agencies in advertising participants?
I. After 40 seconds: Sand
II. After 2 hours: Silt
Next you have to correct the temperature of the readings. The equations for correcting these and how to find their corresponding percentage are usually given during the exam. After you have the percentage, you use the triangle to find out the textural class, as discussed on page 1 of this material
Topic 2.2 – Soil structure
8. Aggregates: Soil particles (sand, silt, clay) joined together by organic matter, fertilizer, carbonates, etc. The arrangement of these aggregates is called a structure. If you want to learn more check out Reappointment happens every how many years?
9. Aggregates may be described by their shape (type), size (class) and by their distinctness (grade)
a. Cuboidal: about equal horizontal and vertical dimensions
b. Prismatic (sometimes column): long vertical dimension and short horizontal dimension Don't forget about the age old question of What makes the jovian planets so gassy?
c. Platy: short vertical dimension and long horizontal dimension
d. Very coarse
III. Grade (how strong is the cohesion force holding the particles together): a. Weak: when disturbed, the structure breaks into a few observable aggregates
b. Moderate: when disturbed, the structure breaks into many wellformed aggregates
c. Strong: when disturbed, the structure breaks into welldefined aggregates or peds.
10. How aggregates are formed:
There are forces that are responsible for cementing the soil particles together, such as:
Note: Cementing agents in temperate regions are primarily humus and clay
11. How aggregates are destroyed:
There are some main factors responsible for destroying the soil aggregates, such as:
I. Falling raindrops
12. Importance/functions of soil aggregates:
I. Resist erosion
II. Pore size distribution (tiny pores inside an aggregate, and bigger pores between aggregates) We also discuss several other topics like Why does the southern hemisphere have less temperature variation?
III. Water and Air movement
13. Aggregate Stability: the ability of soil aggregates to resist disruption when outside forces, usually water, are applied.
14. Compaction: reducing pore space by pressing soil together, which results in an increase in the weight of the solids per unit volume (bulk density, see below for this). I. Affects root penetration
II. Affects water infiltration
III. Compaction risk increases when soil is wet
This phenomenon occurs in response to external pressure exerted on the soil, such as: a. Traffic
Topic 2.3 – Density, porosity and compaction
15. Density: how much mass of something is in a given volume (usually a unit volume) 16. Units of density: g/cm3, g/L, lbs./ft3
17. Two types of density here:
I. Particle density: the density of a soil particle alone (mass of a particle/volume of the particle). This density is constant
II. Bulk density: density of the whole soil (mass of all the solid particles in a soil/total volume of the soil). This density may change due to a change in pore size, for example. Don't forget about the age old question of What is the indus valley civilization?
18. Since a soil is made up of solid particles and pores, we can determine the porosity by subtracting the percentage soil solid from 100. The percent soil solid is given by a relation of bulk density/particle density.
19. Since the volume does not change, density changes when the mass of solid changes i.e. when you add or remove the solid particles.
I. Increasing pores number or size: Bulk density decreases
II. Increasing clay (organic matter): Bulk density decreases
Topic 2.4 – Soil color
20. Soil color is an indicator of conditions into which the soil has gone through
21. Most soil color influences are:
a. Water: darker in color
b. Organic matter: dark and black in color, like coal
c. Iron/manganese and their oxidation state: mostly reddish or brown, depending on oxidation state.
22. Using the Munsell Color chart to find out soil’s color:
I. Hue: it is the dominant spectral color. Find the page in the Munsell book that mostly matches your soil’s color
II. Value: it is the degree of lightness or darkness of the hue. Look for the color chip that mostly matches your soil’s color. On the left side of it should be a value (such as 2/, 3/, etc.)
III. Chroma: it is the intensity of the hue. After finding out your hue value, loot at the bottom for the Chroma (such as /2, /3, etc.)
IV. Now look opposite to the page for the name of that description (such as 10YR3/1=Very Dark Grey)
Topic 2.5 – Soil temperature
23. Soil specific heat: The amount of heat energy required to raise a gram of soil by 1 ℃
24. If a soil has a high specific heat, then it takes too much heat and too much time to raise its temperature. Cooling, therefore, is also slow.
25. Soils are warmed up by:
I. Altering the soil slope: to make the sun perpendicular to the soil
II. Inhibit evaporation: cover the soil with trap and condense the evaporated water
III. Making the soil darker: by a darker paint, for example
26. Soils are cooled by:
I. Modifying the slope of the soil at an angle less perpendicular to the sun II. Shading from the sun: with vegetation, structure, etc.
III. Watering the soil
Topic 2.6 – Soil water content and potential
27. Soil water is held in the pores
I. Water quantity: how much water is present
II. Water energy: how dry, moist, wet the soil is
28. There are 2 ways to quantify soil water
I. By mass:
a. Subtract the mass of the dry soil (let’s say X) from the mass of the soil before drying it (let’s say Y, and X<Y). This difference (let’s say Z) is the mass of the water that evaporated while drying the soil
b. Take the ratio of the mass of water evaporated to the mass of the dry soil (Z/X)
II. By volume:
a. Proceed as above for the method with mass, and record the mass Z. This value is numerically the same as its volume value, since the density of water is 1g/cm3
b. Calculate the total volume of the soil sample, including pores, of course (Let’s say V)
c. Take the ratio Z/V
29. Other useful calculations should be given during the exam. These are mainly conversions and other relations of the above. It is better to go through them in the book for a better comprehension of the topic.
30. Water energy: it is a measure of the potential energy of the water in the soil (how mobile it is)
31. Soil water moves from a high potential zone to a less potential zone
32. Soil water potential is measured with negative values, since the pressure is being exerted by an external agent (such as plant roots, gravity, etc.)
33. The value 5 is numerically less than 1. Do not let the negative sign confuse you! 34. There are 5 soil water conditions/constants, generally:
I. Saturation: when all the soil pores are filled with water (0 kilopascal). The excess water drains by the pull of gravity.
II. Field capacity: the soil is holding as much water as it can against the pull of gravity (33 kPa).
III. Wilting point: the soil does not contain enough water to support plant growth, and so they wilt (1500 kPa).
IV. Airdry: it is oven dry but can take up a small amount of water if it is exposed to air (3000 kPa).
V. Oven dry: all nonstructural water has been removed (1,000,000 kPa).
35. Forces that move and hold water through the soil:
I. Gravitational potential
II. Matric: due to adhesion (against roots and other structures present in the soil) and cohesion (attraction to other particles) of the soil particles.
III. Osmotic: due to the presence of salts
36. Total potential= Gravitational + Matric + Osmotic
37. Soil water moves when there is difference in potential between 2 points in the soil, from high to low potential.
38. Soil water in clay has a less potential than soil water in sand
Topic 2.7 – Soil water movement and availability
Similar to Topic 2.6 with detailed use of formulas. It is better to use the book for a better and firm understanding of the calculation work. Hope this was helpful.