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.