1. Understanding Soil Chemistry

2. Objectives Describe the meaning and importance of soil fertility;
Explain the role of organic matter, soil depth, surface slope, soil organisms, and nutrient balance in soil productivity;
Explain the role of soil reaction (pH) and liming in soil chemistry; and
Explain the role of soil colloids, ions, and the cation exchange capacity (CEC) in soil chemistry.

3. Understanding Soil Chemistry
Soil Fertility

4. Importance of Soil Fertility
Ability of a soil to provide nutrients for plant growth
Involves storage and availability of nutrients
Vital to a productive soil
Fertile soil is not necessarily a productive soil
Why?
Poor Drainage, Insects, Drought, and other factors can limit production

5. Importance of Soil Fertility
Existing plant-soil relationships affect soil productivity
External factors control plant growth
Air
Temperature
Light
Mechanical support
Nutrients
Water
Plants rely on soil to provide all these except light

6. Importance of Soil Fertility
Water and air fill pore spaces in soil
Factors that affect water in the soil also affect air in the soil
Nutrient availability is influenced by soil and water balance and temperature
Root growth is influenced by soil temperature as well as soil, water, and air

7. Understanding Soil Chemistry
Soil Productivity

8. Role of organic Matter What is organic matter?
Plant, animal, and microbial residues
Varying stages of decay
Adequate levels of O.M. benefit soil by:
Improving physical condition and tilth
Increases water infiltration
Decreases erosion
Supplies plant nutrients

9. Role of organic Matter
Organic matter contains about 5% N(nitrogen)
N is not immediately available
Decomposition must occur first
Fertilizer (inorganic) N is needed for non- legume plants
Organic matter also provides soil with a variety of other mineral elements that plants need to be healthy
Some include: phosphorus, magnesium, calcium, sulfur, and others

10. Role of Soil Depth Soil Depth Plants need soil to be:
Depth of soil material favorable for plant root penetration
Plants need soil to be:
Deep
Well-drained
Correct texture
Correct structure
Deeper soils provide for more nutrient storage

11. Role of Soil Depth
Roots can extend 3-6 feet or more when soil depth permits
Rooting depth is affected by
Physical barriers
Chemical barriers
High water tables
Hardpans, Shale beds, Gravely Layers and accumulations of salts are very hard to correct

12. Role of surface Slope Surface slope is part of what?
Topography
Determines the amount of runoff and erosion
Also determines:
Irrigation method
Drainage
Conservation practices
The steeper the land the more management is needed
More labor and equipment costs

13. Role of surface Slope
At certain slopes, soil is no longer suitable for row crops.
What are some row crops?
Potential productivity is largely determined by ease of erosion along with percent of slope

14. Role of Soil Organisms Many different organisms live in the soil
Usually in the first foot of top soil
Depend on organic matter for food
Factors that affect the amount soil organisms:
Moisture
Temperature
Aeration
Nutrient supply
Soil pH
Crop that is being grown

15. Role of Soil Organisms
Some of the microscopic organisms cause many favorable soil reactions
decay of plant and animal residues
speed nutrient cycling.
Other reactions can be injurious
development of organisms that cause plant and animal diseases

16. Role of Nutrient Balance
Nutrient balance is a vital concept in soil fertility and crop production
Nitrogen is the first limiting nutrient in non- legumes
Without adequate amounts of the other nutrients, N cannot do its best
As Nitrogen fertilization raises yields, the crop demands more of the other nutrients

17. Understanding Soil Chemistry
Soil pH

18. Soil pH The measure of acidity or alkalinity of the soil
pH is determined by the concentration of hydrogen ions(H+) and hydroxyl ions(OH-) in the soil solution
Pure water has an equal number of H+ and OH-

19. Soil ph Another term for alkaline is basic Acid = high in H+
Basic = high in OH-
A soil with a pH of 6.0 has 10 times as much active H+ than neutral

20. Soil ph
Plants will not thrive in soil that is too acid or too alkaline
The soil pH directly affects the availability of mineral nutrients
Example
Low pH: phosphates may be bound chemically in the soil so plants are unable to access them
Also means that nitrate production in the soil is reduced

21. Soil PH Several factors influence Soil pH Organic matter
Parent material
Rainfall
leaching
Forest vs. grassland
Harvesting crops
Legumes
Soil depth
Fertilizer application
Flooding

22. Soil ph How to measure soil pH Which do you think is more accurate?
Indicator dyes
pH meter
Which do you think is more accurate?
The meter
If our soil’s pH was too acidic what would we add to amend it?
Lime

23. Lime requirement
Soil pH is not the determining factor on how much lime to add
What else do we need to consider?
Texture of soil
Amount of organic matter
The amount of clay and organic matter in a soil will determine the ability of a soil to resist pH change
More clay/organic matter = more pH buffering
The process is very complex
Example: Al and water

24. Lime requirement Lime requirement Lime reduces soil acidity
Amount of Ag limestone needed to establish the desired pH range for the cropping system being used
Lime reduces soil acidity
Look at Al and water example
Uses Ca ion
Soil will become more and more acidic if a liming program is not followed
We do not apply lime to soils here in Northern Utah very often… Why?

25. Lime Requirement In the case of high pH
Lowering pH is not usually economical
These are generally high in salts
Find plants that tolerate high salt levels

26. Understanding Soil Chemistry
Cation Exchange Capacity

27. Cation Exchange Capacity
As soils form
Some particles are broken down into extremely small particles
Chemical weathering breaks them down until they cannot be seen with the naked eye
The smallest are called colloids
Colloids are responsible for chemical reactivity in soil
Each colloid has a negative(-) charge

28. Cation Exchange Capacity
The neg. charge will attract positively(+) charged particles
What does that mean if they run into another negative?
Cations: Positive Charge
Anions: Negative Charge
Colloids act like a magnet and holds cations
Other cations can take their place
They are exchangeable

29. Cation Exchange Capacity
Can a soil hold more cations than it has colloids? no
We need to know our soil’s CEC
Higher CEC = more cations can be held
Depends on clay and organic matter
Lots of clay = higher CEC
More organic matter = higher CEC

30. Cation Exchange Capacity
Leaching is reduced by higher CEC
Knowing our CEC will help determine how much fertilizer to apply and how often