Soils Part Two

Question 1

Physical nature of typical soil

Answer 1

50% solid particles and 50% pores

Question 2

Soil texture class

Answer 2

Texture - Most fundamental inherent characteristic defined by the variously sized mineral particles.

Includes clay, clay loam, loam, sandy loam, sand, etc.

Impacts physical, biological and chemical processes and changes little over time.

Soul particles are building blocks

Question 3

Pore soaces

Answer 3

Spaces between soil particles and aggregates

Just as important as particles themselves.

Govern water and air processes.

Is where organisms live and roots grow

Impacted by relative amount of clay, silt and sand as well as aggregation. While clays are smaller than sands, theres more aggregation with large pores outside and small pore within.

While texture doesnt change with time, pore size is closely linked with managemebt

Question 4

Water, aeration and pore space

Answer 4

Gravity vs tendency of water molecules to stick to surfaces and themselves

• leads to water draining in larger pores, and holding in smaller pores
• coarse sand has many large pores, this loses water quickly. Good for aeration but can lead to drought stress and greater chemical leaching
• fine clay holds onto water, but lack of aeration can become a problem
• ideal is somewhere in between, with crumb like aggregates

Question 5

Plant water availability is a function of…

Answer 5

The soil’s water retention capacity (related to texture, aggregation and organic matter) and potential rioting volume, which is influenced by compaction.

Question 6

Infiltration capacity

Answer 6

The maximum amount of rainwater that can enter a soil in a given time, which is influenced by:
-soil type, structure and moisture content at the start of the rain.

*when fine textured soils have strong aggregates due to good management, they can also maintain high infiltration rates

Question 7

What happens to saturated and very dry soils (with low OM)

How do aggregates and surface residues mitigate these problems?

Answer 7

Saturated

• soil is soft, weaker aggregates fall apart from raindrops and water flowing past them
• No internal strength and water pressure pushes soil particles away. Erosion and mudslides become a big problem.

Very dry with low organic matter

• bonding between particles decreases because there is little pore water to hold them together
• soil becomes loose and susceptible to wind erosion

Aggregates:

• source of cohesion and prevents dispersion
• results in good tilth, implying that it forms a good seedbed after soil preparation.
• enhanced by surface residues, lack of tillage and continuous supply of OM

Surface residues and cover crops:
-Protect soil from wind and raindrops and moderate temperature and moisture extremes at soil surface

Question 8

Climatic conditions and their impacts on soils

Answer 8

Annual precipitation, rain distribution and relation to the growing season, and the intensity, duration and frequency of rain

• arid vs humid climate
• is irrigation necessary
• regular gentle showers are better than infrequent and I tense storms that can lead to runoff and erosion.

Water is most significant yield limiting factor, but too much can create anaerobic conditions.

Question 9

What happened with the dust bowl?

Answer 9

Decade of drought and unsustainable management practices led to excessive wind and water erosion, crop failures, the collapse of the agriculture industry and massive migrations out of the region.

Gave birth to the soil conservation movement

Question 10

What is erosion primarily a function of?

Answer 10

Texture (generally silts more than sands and clay)

Aggregation (strength and size of aggregates, which depends on Om)

Soil water conditions