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 live. • The aboveground in which the plant visibly exits. 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 water. Solid portion = 50% Air & Liquid = 50% Water = 25% Mineral Matter = 45% Air = 25% Organic Matter = 5% Soil • Soils are classified according to the percentage of sand, silt, and clay they contain. • 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 composition. • 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 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 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 vapor • 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. Drainage • Add organic matter • Use of tile drains to remove water from the soil • Raising plant beds • Place ditches between planting beds Moisture Retention • • • • Adding organic matter Mulch Irrigate Fertilize to add plant food Disease control • Use resistant varieties of crops • Use chemicals sparingly • Pasteurize soil used in container gardening 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 deficiency. • 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 needs Advantages and disadvantages of planting media mixes. • • • • Advantages: Mix is uniform Mixes are sterile Soil less mixes lighter in weight therefore easier to handle • Good moisture retention and drainage are possible through the proper combination • Disadvantages: • Since they are light, the containers may be blown over • 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 aeration. • Sphagnum moss: the dehydrated remains of acid bog plants, used in shredded form. Used for covering seed because it has good moisture retention. • 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 pieces. • 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 growth. • 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. Transpiration • Transpiration of water is high when soils are wet and the stomata open wide to allow more water to escape. • 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 day. • Stomata open in response to light. • Transpiration is of little use. The cooling is minimal. 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 Nitrogen • Encourages above ground vegetation growth and gives a dark green color to leaves. • Produces soft, tender growth • Seems to regulate the use of other major elements. • Too much nitrogen may lower plants resistance to disease, weaken the stem, lower the quality of fruit, and delay maturity or hardness of tissue. Phosphorus • • • • • • • 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 crops 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 Essential for tuber 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) • • • • • 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 Ground • • • • • • Temperature Light Humidity Plant diseases Insects Gases or air particles Temperature • 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 weather • Generally, plant growth increases up to a temperature of about 90 degrees Light • 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. 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. Humidity • 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.