Ag Science

Soil and Plant Growth

The Plant Environment

• In order to grow properly, plants require a certain environment.

• Divided into two parts:

• The underground - roots grow and live.

• The aboveground – visible plant

The Underground Environment• Rhizoshere: The 24 inches of soil just

below earth’s surface.

• Soil is made up of sand, silt, clay, organic matter, and pore spaces

Solid portion = 50%

Mineral Matter = 45%

Organic Matter = 5%

Air & Liquid = 50%

Water = 25%

Air = 25%

Soil

• Soils are classified according to the percentage of sand, silt, and clay they contain.

• Soil particles vary in size. Sand > Silt > Clay

• Clay particles hold water and food elements much more effectively

Soil continued• Soils composition

• Some soils are formed from rock breaking down, others are formed as certain materials are deposited by water/wind

• Soil profile consists of three layers:

• Topsoil-the depth normally plowed or tilled

• Subsoil-a well defined layer under topsoil

• Soil bedrock or lower subsoil

The Ideal Soil• The ideal soil is about 50% solid & 50% pore

space • The solid matter is mainly minerals and a small

portion of organic matter.• The pores are small holes between solid matter

and are filled with water and air• Ideal water/air ratio is half and half• The amount of water and air depends on the soil

structure and type• Sandy soils have large pores (well-drained soils)• If soils contain too much clay, they do not drain

well

Types of Water in Soil• Gravitational: Unable to be held against the

force of gravity. Becomes part of ground water

• Capillary water: Held in pore space against the force of gravity

• There are 3 types:• Free moving: Moves in all directions• Available (field capacity): Water left after

capillary movement stops - Roots move toward it.

• Unavailable: Held tightly and can only be moved as vapor

• Sandy Soil: Sandy or light soils include soils where clay or silt make up less than 20% by weight. Drain well but hold little moisture or nutrients

• Clayey Soil: Must contain at least 30% clay. Poor drainage and aeration

• Loamy soils: Most desirable for general use. Mixture of equal parts of sand, silt and clay.

Soil Triangle

Soil Improvement

• Large Scale

• Soils may be improved by adding increased drainage, irrigation methods and plant food

• Small Scale

• Organic matter and plant food

Drainage

• Drain tile

• Add organic matter

• Raising plant beds

• Place ditches between planting beds

Moisture Retention

• Irrigate

• Leaving land fallow

• Adding organic matter

• Mulch

• Fertilize to add plant food

Disease control

• Use resistant varieties of crops

• Use chemicals sparingly

• Pasteurize soil – Soil or sand should be heated thoroughly

for 30 minutes at 180 degrees F

Nutritional Deficiencies• Nutritional or plant food deficiencies often

show up on leaves

• Nitrogen = yellow/pale

• Phosphorus = purple on underside

• By the time these symptoms appear, damage already done

• Soil Testing

Plant Food Elements

• Major Elements• Required in large

amounts– nitrogen

– phosphorus

– potassium

• Minor Elements• Required in smaller

amounts– calcium

– magnesium

– sulfur

– iron

– manganese

– boron

– copper

– zinc

Plant Food and Fertilizers• Water = most important plant food. Makes

up 90% of the weight of plants.

• Most limiting factor of plant growth.

• All food elements are dissolved in water and move in a soluble form.

• Only 1% of the water absorbed is used by the plant.

• 99% is lost through the process of transpiration.

Transpiration• Transpiration of water is high when soils

are wet (stoma fully opens)• An estimated 250,000 stomata in 1 square

inch of the underside of an apple leaf.• For each 18 degree increase F, the loss of

water is doubled.• A single corn plant can use 2 quarts of

water per day.• Stomata open in response to light.• Transpiration is of little use. The cooling is

minimal.

Nitrogen• Encourages above ground vegetation

growth and gives a dark green color to leaves.

• Produces soft, tender growth

• Regulates the use of other major elements.

• Too much nitrogen may lower disease resistance, weaken the stem, lower the quality of fruit, and delay maturity

Phosphorus

• Flowers and seeds will not form without it

• Quickens maturity

• Encourages root growth

• Makes potassium more available

Phosphorus

• TOO MUCH:– Increases soluble salt

which can dry out roots by pulling water from the roots

• TOO LITTLE:– Purple coloring on

underside of leaves

– Reduced flower, seed and fruit production

– Susceptible to cold injury

– Susceptible to disease

– Poor quality fruit and seeds

Potassium• Rarely available in sufficient amounts

• Encourages resistance to disease

• Encourages strong roots

• Essential for starch formation

• Necessary for chlorophyll development

• Encourages efficient use of carbon dioxide

Lime

• Acts as plant food

• Affects soil acidity

• Furnishes calcium which is important in formation of plant cell walls.

Soil Acidity (pH)

• To lower acidity, use materials such as sulfur, iron sulfate or aluminum sulfate

• To increase acidity, apply lime

The Environment Above the Ground

• Temperature

• Light

• Humidity

• Plant diseases

• Insects

Temperature

• The temperature has one of the strongest effects on plant growth

• Lettuce, cabbage and kale grow best in cool temperatures

• Corn, beans and tomatoes prefer hot weather

• Plant growth increases up to about 90 degrees

Light

• Light must be present for plant to manufacture food.

• Some plants prefer full sunlight, others shade

Photoperiodism• Definition: The response of plants to

different periods of light and darkness in terms of flowering and reproductive cycles.

• Short Day: Flower only when days are short and nights are long. (chrysanthemum and Christmas Cactus)

• Long Day: Flower when days are long and nights are short. (lettuce and radishes)

• Indifferent: Plants that do not depend on periods of light to flower.

Other Reactions to Light

• Plants grow toward their source of light because the plant stem produces more growth hormones on the shady side.

Humidity

• The moisture level in the air

• When humidity is very high (80-100%), problems of fungal disease may occur.

Plant Diseases and Insects• Any time a plants is

suffering for disease or insect damage, production will suffer.

• Leaf damage reduces ability to produce food

• Stem damage may girdle (circle) or clog up a stem and kill the entire plant.

Student Activities• Bring in one jar 1/2 full of soil. Add water

to 2 inches from top. Replace lid and shake well. Next class, examine layers. Classify soil according to texture triangle.