What Makes Plants Thrive?

Photo credit: http://www.flickr.com/photos/quacktaculous/2558321055/

www.community-food.org

Today’s Webinar Brought to You By:

How To Ask Questions

Click on the word bubble in your webinar window to ask a question.

For technical assistance during the presentation, call 1-800-442-4614

Click on the word bubble in your webinar window to ask a question.

For technical assistance during the presentation, call 1-800-442-4614

“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?