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SOILS AND SOIL PHYSICAL PROPERTIES Outline

A. What is soil? B. Where does it come from? C. What is it made of, and how do different soils behave? D. Soil structure E. Soil temperature F. Soil microorganisms

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!A. What is soil?

a) Definitions Different concepts = Different definitions • *PLANT GROWTH • Engineering • Pedological (soil is a distinct entity) !b) What are the functions of soil? i. Support growth of higher plants ii. Primary factor controlling fate of water in hydrologic system iii. Nature’s recycling system iv. Habitat for organisms v. Engineering medium

B. Soil Formation !

Soil-forming factors !a) Parent material: E.g., rock, alluvium b) Climate: Temperature, moisture, seasonal distribution c) Living organisms: Plants, animals, microorganisms d) Topography: Sloped or flat e) Time: How long the soil has been forming? ! Our Western soils: (arid = less

developed, sandier (sometimes), more Ca, K, P etc.) Makes organic farming a little easier.

C. What is in soil? !a) 40–50% mineral Gravel, cobbles, stones, boulders, sand, silt, clay !!b) 0–10% biological Flora and fauna (living things) and formerly living things. This is the organic fraction. The more living things + humus (well-rotted formerly living things), the

better. !!

c) ~50% pore space Air and Water (fills the spaces)

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!!1. The main types of mineral particles

a) Mineral particles are divided into 3 categories: sand, silt and clay. *All are negatively charged.

b) Sand is (by far) the biggest: 0.05 mm to 2.0 mm c) Silt is in the middle: 0.002 to 0.05 mm d) Clay is (by far) the smallest: < or = .002 mm

Soil texture• The relative percentages (by volume) of sand, silt

and clay is the soil’s texture • Most plants grow best in loam (40% sand, 40%

silt, 20% clay). Why? • Pore spaces in loam are balanced: water holding

and drainage, root penetration, and the right size homes and tunnels for the soil flora and fauna

• Loam is best for agriculture: Loams can be tilled without compacting or drying out too much

• Soil texture triangle (text p. 4). Cabrillo soil texture (from soil test)

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Mineral surfaces have important chemical and physical properties

*Surface area increases greatly as particle size decreases. So, particles with the greatest surface area for a given volume are the most imprortant. Clay has the greatest surface area:volume of any mineral particle (by far)

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Something about the humungous surface area and

platelike structure of clay...• lots more places for water to ‘stick’; plus, water

sneaks into the very small spaces between the plate-like particles

• lots more negative charges to attract and hold positive nutrient cations

• But, the water that sneaks in between plates means clay drains poorly

• The platelike structure allows easy compaction, that is difficult to reverse.

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Compare to sand:

• Very few spots for water to stick • Very few negative charges for nutrients

to ‘stick’ !

• So sand holds water and nutrients poorly

• It does drain well, so at least it’s hard to drown your plants in sandy soils!

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Compare to Humus

• Humus, because of its filamentous, matrix structure, has even more surface area to volume than clay

• So, it has more spots for water and nutrients to ‘stick’

• Not platelike, so water doesn’t get trapped. So, humus also drains well and doesn’t compact either!

• The magic of organic matter revealed!!12

D. Soil Structure a) ‘Arrangement of soil particles into aggregates’ (groups of particles). See text, p 5. !ii. Go for granular! (like a ‘half a grain of rice’ iii. Aggregation results from 1)Physical forces (pushed together) and 2)Binding agents: (the matrix and ‘glues’ of organic matter) !

1)Physically formed aggregates = weak. 2) Binding agents = stable !

Granular soil ! Ideal for plant growth: just the right porosity for air and water and roots. !Granular, loam soil = best structure + best texture. High

humus, granular, loam soil = the ultimate!

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Physical forces!ii. Clay (type and amount) (positive) !iii. Calcium (positive) and sodium (negative) !iv. Climate, including wetting and drying, freezing and

thawing (positive) !v. Tillage (positive or negative) depending on the soil

moisture, and implement

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Summary• Humus is really great! • Use it to maintain and improve soil health • Eg.: Use humus to improve poor textured soils: !Sandy + low organic matter = well drained, low moisture-holding !Sandy + high organic matter = well drained, moderate moisture-holding !Clay + low organic matter = poorly drained, high moisture holding !Clay + high organic matter = moderately well drained, high moisture holding

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!1. Biological activity: Soil temperature affects plant growth and microbial activity, and therefore organic matter decomposition (humus formation), and nitrogen availability (mineralization) are dependent on it. !a) Lower temperatures, moisture available. Organic matter accumulates, but doesn’t break down very quickly, so both humus formation and nitrogen availability are restricted. ! Conventional farmers apply mineralized nitrogen, so low temperature not important. Organic growers don’t have economical, soil applied mineralized nitrogen fertilizers and may need to apply foliar fertilizers, or use soil warming techniques (eg. plastic mulch). !!!

E. Soil Temperature !b) Higher temperatures, moisture available. Organic matter accumulation and decomposition is balanced. Humus forms, nitrogen availability is maximized. !c) High temperatures, but moisture restricted (Deserts): plant growth is insufficient to supply organic matter. Any organic matter that forms gets oxidized quickly when moisture is available. !d) Low temperatures, but moisture restricted (Tundra): plant growth is slow. Any organic matter that forms accumulates. !!!!! !18

!F. Soil microorganisms

!Who are they? Bacteria, fungi, actinomycetes, (algae, protozoa) !!!!!!!!!

Bacteria !!!What do they do? Decomposition of organic materials, release of nutrients, fixation of nitrogen from the atmosphere (free-living bacteria vs rhizobia). !