what makes plants thrive?
DESCRIPTION
Erin Silva from the UW-Madison agronomy department discusses what you need to do to help your plants flourish.TRANSCRIPT
What Makes Plants Thrive?
Photo credit: http://www.flickr.com/photos/quacktaculous/2558321055/
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“What Makes Plants Thrive?” Why start the discussion with soil?
Organic Agriculture (USDA National Organic Program)
Recognition that healthy soils are the foundation of sound, sustainable agriculture system primary focus of organic farming is to use
practices that build healthy soils
National Organic ProgramSection 205.203(a)
Select and implement tillage and cultivation practices that maintain or improve the physical, chemical, and biological condition of soil and minimize soil erosion
Soils: More Than “Just Dirt” Aim of organic and sustainable farming: to
improve the soil Encourage soil organisms Supply nutrients in sufficient quantity without
oversupplying Maintain tilth and minimize compaction Build organic matter
Outline for today What is soil? Soil textures Soil organic matter Soil fertility management in relation to above
factors
What is a soil? Characteristics of an ideal soil Components of a soil Appearance of a soil Soil texture/ soil type
Characteristics of an ideal soil What are characteristics of ideal soils?
Fertile Deep Well drained/ aerated High in organic matter Friable
soil is easily worked
Components of Soil
Mineral
Organic
Water
Air
Soil Air Oxygen supports soil life
Roots Microbes
Air can be displaced by water
Management Question: The management of what important factor to
related to plant growth must be considered in relationship to soil air?
Irrigation! Saturated soils – over-irrigation, poor drainage Water fills in all the air spaces – no room left for air Affects root growth/health, soil biological activity,
available nitrogen
Mineral and organic components
A substance with a complex structure consisting of: Broken down rock particles (mineral component) Organic matter
Ideal soil is about 5-6% Function of organic matter
Source of food for soil microorganisms Source of nutrients for plants Holds minerals against loss from soil due to leaching Holds water where plants can access it
Soil Profile Topsoil
It is the darkest layer of the soil because it contains organic matter and humus.
Contains majority of feeding roots of plants
Subsoil Low in nutrients Contains few
microorganisms Structure of subsoil is
important for drainage
Example of a soil profile
• Note the dark organic horizon at the surface and changes in color and structure down the profile
Soil texture
Determined by mineral component Coarseness or fineness of soil
Soil texture Clay
Soil particles are less than 0.002 mm in size
Does not drain easily and is difficult to work
Root growth is poor due to small spaces between soil particles
Soil texture Silt
Particle size is between 0.002 to 0.05 mm
Feels silky to the touch
Soil texture Sand
Sand particles range in size from 0.2 mm for the very finest sand to 2 mm for the coarsest
Feels gritty if rubbed between your fingers
Warms up and dries early in spring
Low in nutrients
Soil Types Most soils are a mixture of different soil textures
Often a soil type will be dominated by a particular texture Can group soil types by how well they drain and major
texture class they contain Heavy soils
Contain a high proportion of clay Light soils
Contain a high proportion of sand Important to know the soil type because it will determine the
management practices you need to use
Why is soil texture important? The texture will determine how much air is in
soil and how well water flows through the soil Determines how easily the soil is worked
How might you manage a clay soil different than a sandy soil?
Irrigation – sandy soils will hold less water Types of crops – some crops do better in a
lighter soil (root crops, squashes) Fertility – may need to do in-season
applications in sandy soil Cultivation/cover cropping –
Beware of erosion!
Pause for Questions....
Soil Organic Matter Many soil properties impact soil quality, but
organic matter deserves special attention Affects several critical soil functions, can be manipulated
by land management practices, and is important in most agricultural settings across the country
Because organic matter enhances water and nutrient holding capacity and improves soil structure, managing for soil carbon can enhance productivity and environmental quality, and can reduce the severity and costs of natural phenomena, such as drought, flood, and disease
Soil Organic Matter Living
Roots Earthworms and insects Microorganisms
Dead Fresh plant residues
Releases nutrients as it decomposes – allows for slow, steady release Recently deceased soil organisms Active organic matter
Very Dead Well decomposed organic materials Humus
Slow to decompose – may last 100’s of years Hay, cornstalks Not well understood
Characterization of Organic Matter:
Humus
Organic Matter
Generally 0-6% Reservoir for nutrients, improves soil
aeration, drainage, water holding capacity Very complex compounds
Difficult to characterize chemically Biologically controlled
Very dynamic- differs depending on moisture, temperature
Humus Very well decomposed
organic matter refers to any organic matter that has reached a point of
stability, where it will break down no further and might, if conditions do not change, remain as it is for centuries, if not millennia
Mineralization of organic matter to soil humus releases some amounts of nitrogen, phosphorous and sulfur from organic forms, and can dramatically influence the availability of micronutrients
Relative amounts of OM throughout US
Pause for Questions...
16 essential elements: C, H, and O: not consider minerals Macronutrients:
Primary macronutrients: N, P, and K Needed in relatively large amounts
Secondary macronutrients: Ca, Mg, S
Micronutrients: Cl, Fe, B, Mn, Zn, Cu, Mo Required in small amounts
Supplied through: Compost, cover cropping, fertilizers (both organic and
synthetic)
If any one is missing or low, plant productivity is limited
How do plants take up nutrients? By roots Must be plant available
Soil solution and nutrient uptake
Ions Cation
Calcium, magnesium
Anion Certain forms of nitrogen
Nutrients in Ionic phase Magnets: opposites attract
Molecules: opposites also attract
Soil acts as a magnet Attracts and retains positively charged ions in
soil solution Prevents them from moving downward with
water
Clay and Cation Exchange Capacity
Brady & Weil, 2004. Elements of the Nature and Properties of Soils
Cation Exchange Capacity (CEC) It is the ability of clay, organic matter, and humus to
attract positive ions Clay, organic matter, and humus all have negative
charges on their surfaces Most nutrients (P, K, Ca, etc) are cations or have positive
charges “Opposites attract”
The higher the cation exchange capacity, the more closely the nutrients are held and the less likely they are to be lost to leaching
Soils with high CEC Hold more nutrients Avoid rapid changes in levels of nutrients in
soil solution
High vs. Low CEC
Soil organic matter and CEC Structure of soil organic matter is extremely
complex, with that can interact with cations (plant required nutrients)
Often contributes from 30 to 90% of the total charge present in the soil surface, even when present in relatively low amounts.
pH Dependent Charge on OM
What else has positive charge and affected by CEC? Water
OM vs. Clay Because of it complex and open structure, the
ability of humus to pull water from the surrounding saturated atmosphere of the soils approaches 80 to 90% of its weight, as compared to 15 or 20% for soil clays.
Humus does not readily fix exchangeable cations, as do some clays, but maintains these ions in an easily exchangeable form
Management High CEC soils (high clay content, moderate to
high organic matter content) high water holding capacity – be careful not to over-
irrigate low leaching potential for cationic nutrients - less
frequent need for lime and fertilizers (except N) their physical properties may make it difficult for a
farmer to cultivate or maintain good aeration
Management Low CEC soils (high sand, low organic
matter) need for more frequent irrigation may need to apply fertilizer throughout season to
meet plant demand may be more prone to erosion can be easier to work
Questions?