1. The Dirt on Soil! An Introduction to Soil Chemistry
Mrs. Long
Horticulture I
Fall 2013
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For many of us soils are a black box. We put things into the box and we get things out of the box, but we don’t have a very good idea of what happens inside the box. We put seed, fertilizer, and water into the soil and out from the soil comes the crops we are growing. But what exactly happens inside that black box we call soil? Farmers and scientists and have been studying that question for hundreds of years and continue to study it today. They have learned that many complex physical, biological, and chemical processes are carried out in soil. Lets open up that black box just a little and learn something about the chemistry of soils. Knowing something about soil chemistry will help us understand how soils supply plant nutrients.

2. Soil is the unconsolidated cover on the surface of the earth.
Soil is made up of mineral particles, organic particles, air and water.
Soil is capable of supporting plant growth.
What is soil?
Before we get into soil chemistry, we need to back up a bit and think more generally about soils.
What is soil?
To a geologist soil is the decomposed surface of rocks.
To an engineer soil is the medium that must be strong enough to support a highway or a skyscraper or your house.
To scientists who study soil formation, soil is a natural body consisting of several layers and formed from weathered rocks over a period of thousands to millions of years.
Today we will consider soil from an agricultural perspective.
Soil is the unconsolidated material at the surface of the earth. (Unconsolidated simply means that it is granular material that is not cemented together like rock.)
Soil is made up of mineral and organic matter and contains both water and air.
Most importantly, soil is capable of supporting plant life. It’s the material that sustains not only the farmer’s livelihood, but that of the whole world.
It is from this perspective that we will now look more carefully at soil. First, we will consider what functions must be performed by soil used for crop production, and then we will look at how the soil accomplishes those functions.

3. Soil Components The 4 parts of soil
About ½ of the soil volume is solid particles
About ½ of the soil volume is pore space
Lets review a couple of soil science basics.
Soil is made up of 4 parts: mineral matter, organic matter, water, and air.
Mineral matter and organic matter together form the solid part of soil. Soil air and soil water occupy the spaces between the solid particles. This space is the pore space.
A good agricultural soil will be about half solid particles and half pore space.
Most of the soil solids will be mineral matter that is made up of particles of sand, silt, and clay. A small part of the solids will be organic matter. Most agricultural soils have somewhere around 2 - 5% organic matter. Organic matter is mostly made up of decomposed plant litter and roots.
Conditions for root growth will be ideal when about half the pore space is filled with water and half is filled with air. When a soil becomes compacted the mineral particles are pressed more tightly together. When this happens the soil loses pore space, and so has less capacity to store water and air.

4. Functions of agricultural soils
Anchor plant roots
Supply water to plant roots
Provide air for plant roots
Furnish nutrients for plant growth
Release water with low levels of nutrients
Soils used for crop production must perform five basic functions.
Soil must firmly anchor plant roots. It must be strong enough to hold crops and even large trees erect. Yet soil must be permeable enough to allow tiny root hairs to penetrate it.
Soil must retain rain that falls on it in order to continuously supply water to growing plants. Yet it must also allow excess water to drain. The soil must drain because it must also supply air, more specifically oxygen, to crop roots. Too much water means too little air and the crops suffocate.
Soil must supply nutrients for plant growth. To do so it must be store nutrients and then release them to the roots of growing crops.
But soil must not release those nutrients to draining water.
Soil is a truly remarkable material to be able to perform each of these tasks – tasks that sometimes seem to be in conflict with each other.
Our focus in this session will be on the last two functions of an agricultural soil.
How soil provides nutrients for plant growth, and
How nutrient laden soil can release water with low levels of nutrients.
In particular, we will consider the chemical characteristics of soil that allow it to perform these functions.

5. Soil Structure. The arrangement of sand, silt, and clay particles
Soil Structure The arrangement of sand, silt, and clay particles to form larger aggregates. How they fit together!
Organic matter is the glue that holds the aggregates together
Large pores (spaces) between aggregates are filled with air in a moist soil.
Small pores are filled with water in a moist soil. Even smaller pores inside the aggregates (not shown) are also filled with water.
Individual particles of clay, silt, and sand stick together into larger particles called aggregates. Aggregates can take on many shapes and sizes, but in a good topsoil they tend to be small crumb-like particles. Small aggregates, like those shown in this diagram tend to clump together into still larger aggregates. Soil structure refers to the arrangement of individual particles of sand, silt and clay into small aggregates, and the arrangement of small aggregates into larger aggregates.
Clay is important in soil structure because it is sticky and makes individual particles clump together. Aggregates, and the individual particles in them, are often coated with soil organic matter. The soil organic matter acts like a glue that strengthens the aggregates, and helps to hold them together.
The spaces between aggregates are called pores. These spaces are also an important part of the soil. They are not empty, but are filled either with water or with air. In a soaking wet soil (just after a rainfall or snowmelt), all the pores will be filled with water. As the soil drains due to the force of gravity, water in the larger pores moves downward to tile lines or groundwater. When the water drains out it is replaced by air. These drained, air-filled pores are shown in yellow in this diagram. Water in smaller pores is held more tightly by the soil and does not drain (blue areas in this diagram). Water will be lost from the smaller pores due to plant uptake of water and evaporation. In addition to the pores shown in this diagram, there are also very small, even microscopic pores within each of the aggregates. These are pores that occur between the individual particles of silt and clay that make up the soil aggregates. These very small pores will be filled with water in all but the driest soil.
1/10 inch

6. Soil Structure
Sand
Loose
Silty
Granular
Clayey
Platy or
Blocky

7. Soil Texture
The mineral part of soil consists of sand, silt, and clay particles
Sand – Largest particles – low moisture holding capacity
Silt – Medium particles – good moisture holding
Clay – Smallest particles – Hold a lot of water
Loam – equal parts sand, silt, clay – ideal texture!
1/100 in
Sand
0.1 – in
2 – 0.05 mm
Silt
0.002 – in mm
Clay
Less than in
Less than mm
Soil texture refers to the amount of various size mineral particles that are present in the soil.
Soil mineral particles are separated into sand, silt, and clay on the basis of the particle diameter. This diagram shows the size of sand, silt and clay relative to each other.
Sand is the largest and gives soil a gritty feel. Particles larger than 1/10 inch would be considered gravel. The sand particle in this diagram represents a fine sand particle – about 1/100 inch in diameter.
Silt is intermediate in size between sand and clay. Soil with a lot of silt has a floury feel. Sand and silt provide a skeleton for the soil. The main function of sand and silt in soil is to give strength. Sand and silt contribute very little to the capacity of soil to retain water and nutrients.
Clay is the smallest of the mineral particles, and makes soil sticky when wet. Clay particles are microscopic, so individual clay particles cannot be seen by the naked eye. The black dots in this diagram are representative of the largest clay particles. Relative to the size of the sand and silt in this diagram, the dots are larger than most clay particles.
The relative amounts of sand, silt, and clay give the soil its textural property. A loose, coarse textured soil has a lot of sand and less silt and clay. A fine textured soil is heavy and has a lot of clay and less sand. A loamy soil has a more even mix of all three.
When you scoop up a handful of good topsoil you see crumbs or granules of soil that are much larger than the individual particles shown in this diagram. That brings us to the topic of soil structure.

8. Where does soil come from?
Parent Material
Mechanical weathering – breaks down rock into smaller pieces without changing soil chemistry
Chemical weathering – breaks down rocks into smaller pieces by chemical reaction
Biological weathering – results from activities of living organisms

10. Mechanical Weathering

11. Chemical Weathering

12. Biological Weathering

13. Homework for 04/ Bring a small soil sample from your yard at home.
Make sure its clean.
Bring in a full quart-sized ziploc bag full of soil
YOUR NAME has to be on it!!

14. Soil Sampling
5.02 Discuss the soil profile and soil sampling for surface and subsurface layers.
10/31/12

15. Before we start today: Soil Particle Size Lab (Part I)
Take your soil sample (1 per group)
Place a cup full of soil into a jar
Fill the jar to the top with water and tighten the lid.
Shake for 3 minutes.
Set to side (labeled) until tomorrow!

16. Soil Layers Soil layers are called horizons.
Layers parallel to earth’s surface
Defined usually by color, texture and physical features.

17. O horizon The O horizon contain Organic matter Uppermost dark Colored
Most fertile
Made of leaves, twigs, waste, decaying matter

18. A horizon Topsoil Porous mix of humus Just below the O
Home to earthworms and microorganisms
Brown in color

19. B horizon Subsoil Clayey with high mineral content Brownish red
Receives a lot of leached material

20. C horizon
Parent material
Clumps of unweathered rocks

21. R horizon
Bedrock
Cannot be dug by hand
Very few roots can penetrate

25. Purpose Determines levels of fertility in soil
Use this information to make accurate fertilizer recommendations for growing plants
A soil test shows deficiencies, excesses and imbalances can be avoided.
This step leads to higher plant yields and quality by following recommended application rates.

26. The Soil Testing Laboratory
Kits are available at local garden centers
Submit samples to NC Dept of Agriculture and Consumer Service Agronomic Division
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The Soil Testing Laboratory is located on the campus of the University of Georgia at 2400 College Station Road in Athens. It is equipped with the most modern instruments available for rapid and accurate soil analysis. Analysis results and fertilizer recommendations are returned to your county extension agent for dissemination and adjustments, if necessary.
The laboratory offers a number of tests to meet specific soil and cropping circumstances.

27. Lime & Fertilizer Recommendations
Information from a soil test will help you select the proper liming and fertilization program to obtain optimal growth of lawn, garden and ornamental plants.

28. Step 1 Create a visual grid of the area to be planted.
5-10 subsamples from the site
Sample before the growing season

29. Step 2 Use a stainless steel soil- sampling probe
Take the surface sample to tillage depth
4 inches for lawns
6-8 inches for crops

30. Step 3
Mix soil samples together.

31. Step 4
Remove any:
Grass
Rocks
Any other material
SOIL ONLY!!

32. Step 5 Place soil samples in a box Get one from County Extension
Also need Soil Sample Information Sheet

33. Step 6 Send soil tests to Raleigh Free for residents
NCDA&CS Agronomic Services Division Mailing Address: 1040 Mail Service Center, Raleigh NC Physical Address: 4300 Reedy Creek Road, Raleigh NC Phone: (919) ; FAX: (919)

34. Classwork
Perform the:
Soil Testing Webquest (Activity )

35. Today’s Lineup Making “Dirt” Bedrock at bottom - R Parent material - C
Subsoil - B
Topsoil - A
Organic matter – O

36. Types of Horticulture Soils

37. Peat Moss Organic Partially decomposed material from Swamps
Holds moisture

38. Perlite Inorganic Natural volcanic material
Helps with soil aeration and water holding capacity

39. Organic Organic Matter Compost Dead plant or animal tissue
Contains Carbon!
Decayed organic matter
Used for soil conditioning
Fertilizer

40. Bark Organic Larger matter Promotes drainage
Mainly for trees and shrubs

41. Sphagnum Moss Organic Dehydrated bog plants Holds moisture Very acidic
Hanging baskets

42. Horticulture Soils Advantages Disadvantages Sterile pH is neutral
Disease and weed free
Good for starting plants
Easy to mix
Improves plant uniformity
Expensive
Light weight

43. Hydroponics
The process of growing plants without soil.

44. Advantage of Hydroponics
No soil & problems associated with soil.
Easy to control nutrient content of plants.

45. Disadvantages of Hydroponics
Plant support must be provided with strings, wires, or stakes.
Water quality must be high.
Diseases spread through water.
More moisture or humidity in air to cause favorable environment for disease organisms.
More expensive.

46. Hydroponic Lettuce

47. Background on Today’s Lab

48. pH Soil pH is a measure of the acidity or alkalinity of soils
pH ranges from 0-14, 7 being neutral
Ideal pH is

49. pH
Lime
Calcium/Sulfur
Raises the soil pH
Lowers the soil pH

50. Soil Texture Triangle Give names to soils combos Loam Clay Sand Silt
Sandy Clay
Silty Clay
Clay Loam
Sandy Clay Loam
Silty Clay Loam
Loam
Sandy Loam
Silty Loam
Sand
Loamy Sand
Silt

52. Physical Properties of Soil
Permability
Rate at which water moves through the soil

53. Physical Properties of Soil
Water Holding Capacity
Ability of soil to hold water for plant use

54. Physical Properties of Soil
Porosity
Amount of air space between particles