Understanding soil chemistry
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Understanding Soil Chemistry. Importance of Soil Fertility. 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?

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Understanding Soil Chemistry

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Understanding soil chemistry

Understanding Soil Chemistry


Importance of soil fertility

Importance of Soil Fertility

  • 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


Importance of soil fertility1

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


Importance of soil fertility2

Importance of Soil Fertility

  • Water and air fill pore spaces in soil

    • Factors that affect water relationships also affect soil air

  • 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


Understanding soil chemistry1

Understanding Soil Chemistry

Soil Productivity


Role of organic matter

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 loss

    • Supplies plant nutrients


Role of organic matter1

Role of organic Matter

  • Organic matter contains about 5% N

    • 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


Role of soil depth

Role of Soil Depth

  • Soil Depth

    • 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


Role of soil depth1

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


Role of surface slope

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


Role of surface slope1

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


Role of soil organisms

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


Role of soil organisms1

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


Role of nutrient balance

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


Understanding soil chemistry2

Understanding Soil Chemistry

Soil pH


Soil ph

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-


Soil ph1

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


Soil ph2

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


Soil ph3

Soil PH

  • Several factors influence Soil pH

    • Organic matter

    • Parent material

    • Rainfall

      • leaching

    • Forest vs. grassland

    • Harvesting crops

    • Legumes

    • Soil depth

    • Fertilizer application

    • Flooding


Soil ph4

Soil ph

  • How to measure soil pH

    • 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


Lime requirement

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


Lime requirement1

Lime requirement

  • Lime requirement

    • 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?


Lime requirement2

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


Cation exchange capacity

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


Cation exchange capacity1

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


Cation exchange capacity2

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


Cation exchange capacity3

Cation Exchange Capacity

  • Leaching is reduced by higher CEC

  • Knowing our CEC will help determine how much fertilizer to apply and how often


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