Horticulture - Unit 4
Environmental Requirements for
Good Plant Growth
The Plant Environment
• In order to grow properly, plants require a
certain environment.
• This environment is divided into two parts:
• The underground in which roots grow and
• The aboveground in which the plant visibly
The Underground Environment
• Rhizoshere: The 24 inches of soil just
below the earth’s surface.
• Soil is made up of sand, silt, clay, organic
matter, and pore spaces which hold air and
Solid portion = 50%
Air & Liquid =
Water = 25%
Mineral Matter = 45%
Air = 25%
Organic Matter = 5%
• Soils are classified according to the
percentage of sand, silt, and clay they
• Soil particles vary greatly in size. A sand
particle is much larger than silt. Clay
particles are by far the smallest.
• Clay particles hold water and food elements
much more effectively than larger particles.
• A certain amount of clay in all soil is
important for this reason.
Soil continued
• Soils also vary greatly in general
• Some soils are formed from rock breaking
down, others are formed as certain materials
are deposited by water.
• A normal soil profile consists of three
• 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 material.
• The solid matter is mainly minerals and a small portion of
organic matter.
• The other 50% is pore space.
• The pores are small holes between solid matter and are
filled with water and air in varying amounts.
• Ideal water/air ratio is half and half
• The amount of water and air depends on the soil structure
and type of soil.
• Sandy soils have large pores and water is lost quicker
(these are called well-drained soils)
• If soils contain too much clay, they may not drain well
enough to allow enough oxygen in pore space.
Types of Water in Soil
• Gravitational: Water that is unable to hold against
the force of gravity. It is and becomes part of
ground water. It is of little use to the plant
because it drains away taking soluble plant food
elements with it.
• Capillary water: Is held against the force of
gravity. It is held in the small pores of the soil.
There are three types:
• Free moving: Moves in all directions
• Available or field capacity: Water left after capillary
movement stops - Roots move toward it.
• Unavailable: Held tightly and can only be moved as
• Sandy Soil: Sandy or light soils include
soils in which clay or silt make up less than
20% of the material by weight. These soils
drain well, but have little capacity to hold
moisture and plant food.
• Clayey Soil: Must contain at least 30%
clay. It is a heavy soil which has poor
drainage and aeration capacities. Clayey
soils tend to hold too much moisture.
• Loamy soils: The most desirable for
general use. It is a mixture of equal parts of
sand, silt and clay.
Soil Improvement
• Soils may be improved by adding increased
drainage, irrigation methods and organic
matter and plant food.
• Add organic matter
• Use of tile drains to remove water from the
• Raising plant beds
• Place ditches between planting beds
Moisture Retention
Adding organic matter
Fertilize to add plant food
Disease control
• Use resistant varieties of crops
• Use chemicals sparingly
• Pasteurize soil used in container
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 the leaves.
• Yellow or pale green indicates a nitrogen
• Purple color on the underside, shows a
phosphorus deficiency.
• By the time these symptoms appear,
damage has already been done.
• A soil test would have determined the plants
Advantages and disadvantages of
planting media mixes.
Mix is uniform
Mixes are sterile
Soil less mixes lighter in
weight therefore easier to
• Good moisture retention
and drainage are possible
through the proper
• Disadvantages:
• Since they are light, the
containers may be blown
• Mineral content is low Minor plant food elements
may be missing
• Plants may hesitate to
extend roots when
transplanted to soils.
Content of Mixes
• Perlite: a gray-white material of volcanic origin. Used to improve
• Sphagnum moss: the dehydrated remains of acid bog plants, used in
shredded form. Used for covering seed because it has good moisture
• Peat moss: Partially decomposed vegetation that has been preserved
underwater. High moisture holding capacity.
• Vermiculite: Very light, expanded material with a neutral pH. Has a
very high moisture-holding capacity.
• Limestone: Ground natural limestone.
• Tree bark: usually the bark of pine or oak trees broken into small
• Slow releasing fertilizers: Contain plant food which is gradually made
available to plants.
Plant Food and Fertilizers
• Water is the most important plant food. It
makes up 90% of the weight of plants.
• Water is the most limiting factor of plant
• All food elements are dissolved in water
and move into the plant in a soluble form.
• Only approx. 1% of the water absorbed is
used by the plant.
• 99% is lost through the process of
• Transpiration of water is high when soils are wet
and the stomata open wide to allow more water to
• It is estimated that there are 250,000 stomata in 1
square inch of the underside of an apple leaf.
• For each 10 degree increase in temperature C, the
loss of water is doubled.
• A single corn plant can use 2 quarts of water per
• Stomata open in response to light.
• Transpiration is of little use. The cooling is
Plant Food Elements
• Major Elements
• Required in large
– nitrogen
– phosphorus
– potassium
• Minor Elements
• Required in smaller
• Encourages above ground vegetation
growth and gives a dark green color to
• Produces soft, tender growth
• Seems to regulate the use of other major
• Too much nitrogen may lower plants
resistance to disease, weaken the stem,
lower the quality of fruit, and delay maturity
or hardness of tissue.
Encourages plant cell division
Flowers and seeds will not form without it
Hastens maturity
Encourages root growth and strong roots
Makes potassium more available
Increases plants resistance to disease
Improves the quality of grain, root and fruit
– Increases soluble salt
which can dry out roots
by pulling water from
the roots
– Purple coloring on
underside of leaves
– Reduced flower, seed
and fruit production
– Susceptible to cold
– Susceptible to disease
– Poor quality fruit and
Rarely available in sufficient amounts
Encourages resistance to disease
Encourages strong roots
Essential for starch formation
Necessary for chlorophyll development
Essential for tuber development
Encourages efficient use of carbon dioxide
• Acts as plant food
• Affects soil acidity
• Furnishes calcium which is important in
formation of plant cell walls.
Soil Acidity (pH)
Most plants grow best in pH from 5.6 to 7
Soil at 7 is neither acid or alkaline (basic)
Values lower than 7 indicate acid soils
Above 7 indicate alkaline soils
To lower acidity, use materials such as
sulfur, iron sulfate or aluminum sulfate
• To increase acidity, apply lime
The Environment Above the
Plant diseases
Gases or air particles
• The temperature of the air has one of the
strongest effects on plant growth
• Some plants such as lettuce, cabbage and
kale grow best in cool temperatures
• Corn, beans and tomatoes prefer hot
• Generally, plant growth increases up to a
temperature of about 90 degrees
• Light must be present before a plant can
manufacture food.
• Some plants prefer full sunlight, others
prefer shade
• Light also affects plants other ways. Thr
response to different periods of day and
night is called 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.
• Dehlias develop fibrous root systems during
long days but develop thick storage organs
when days shorten.
• The moisture level in the air
• Most plants are not affected greatly by
minor changes
• When humidity is very high (80-100%),
problems such as the spread 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.
Gases and Air Particles
• Carbon dioxide is vital for plant growth
• Greenhouse operators find that adding
carbon dioxide to the air increased growth
to plants more than enough to pay for it
• Some air pollutants cause damage to the
plant (Sulfur dioxide from coal furnaces and
carbon monoxide from cars)
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.
• Examine label on plant food. List all plant
food elements and record the percentage of
each. Identify major and minor elements.
• Using a soil test kits, test a sample for pH
level, phosphorus and potassium.
• Complete self-evaluation.

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