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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 nutrientsVital to a productive soil
Fertile soil is not necessarily a productive soilWhy?
Poor Drainage, Insects, Drought, and other factors can limit production
Existing plant-soil relationships affect soil productivityExternal factors control plant growth
Air Temperature Light Mechanical support Nutrients Water
Plants rely on soil to provide all these except light
IMPORTANCE OF SOIL FERTILITY
Water and air fill pore spaces in soilFactors 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
IMPORTANCE OF SOIL FERTILITY
UNDERSTANDING SOIL
CHEMISTRYSoil Productivity
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 infiltrationDecreases erosion lossSupplies plant nutrients
ROLE OF ORGANIC MATTER
Organic matter contains about 5% NN is not immediately availableDecomposition 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 healthySome include: phosphorus, magnesium,
calcium, sulfur, and others
ROLE OF ORGANIC MATTER
ROLE OF SOIL DEPTH Soil Depth
Depth of soil material favorable for plant root penetration
Plants need soil to be:DeepWell-drainedCorrect textureCorrect structure
Deeper soils provide for more nutrient storage
Roots can extend 3-6 feet or more when soil depth permitsRooting 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 SOIL DEPTH
ROLE OF SURFACE SLOPE Surface slope is part of what?
Topography Determines the amount of runoff and
erosion Also determines:
Irrigation methodDrainageConservation practices
The steeper the land the more management is neededMore labor and equipment costs
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 SURFACE SLOPE
ROLE OF SOIL ORGANISMS Many different organisms live in the soil
Usually in the first foot of top soilDepend on organic matter for food
Factors that affect the amount soil organisms:MoistureTemperatureAerationNutrient supplySoil pHCrop that is being grown
Some of the microscopic organisms cause many favorable soil reactionsdecay of plant and animal residuesspeed nutrient cycling.
Other reactions can be injuriousdevelopment of organisms that cause plant
and animal diseases
ROLE OF SOIL ORGANISMS
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
CHEMISTRYSoil 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 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 PH Plants will not thrive in soil that is too
acid or too alkalineThe 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 PH Several factors influence Soil pH
Organic matterParent materialRainfall
leachingForest vs. grasslandHarvesting cropsLegumesSoil depthFertilizer applicationFlooding
SOIL PH How to measure soil pH
Indicator dyespH 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 Soil pH is not the determining factor on
how much lime to addWhat 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 changeMore clay/organic matter = more pH
buffering The process is very complex
Example: Al and water
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 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 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 eyeThe smallest are called colloids
Colloids are responsible for chemical reactivity in soil
Each colloid has a negative(-) charge
CATION EXCHANGE CAPACITY The neg. charge will attract
positively(+) charged particlesWhat does that mean if they run into
another negative? Cations: Positive Charge Anions: Negative Charge
Colloids act like a magnet and holds cationsOther cations can take their placeThey are exchangeable
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 CECMore organic matter = higher CEC
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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