Soil and

Plant Nutrition

Early scientists had interesting ideas about plant growth. Aristotle thought the “stuff” for plant growth came from

the soil, and Jean Baptiste van Helmont tested the hypothesis that plants absorbed soil for growth, but only a small amount of soil disappeared (started with 200 lbs

and after five years, had 169 lbs. 3 oz. of soil and an almost 200 lb tree), so he concluded that the water he

provided caused the growth.

Later, Stephen Hales suggested that plants are nourished by air. So just what materials are used by plants to support growth? Van Helmont was partially correct, water is used (in fact, @ 80-90% of an herbaceous plant is water) and some minerals are also absorbed from soil making up about 4% of the dry mass of the plant (this would account for some of the “lost” soil), and Hales was also partially correct in that CO2 is incorporated into glucose making up about 96% of the dry mass of the plant (making structures like cell walls or stored for future energy use like starch).

MineralsAll organisms require minerals for growth.

•1. Mineral nutrients are essential chemical elements that plants absorb from the soil as inorganic ions.

•2. Although more than 50 elements have been found in plant tissues, many of these are most likely not essential to plant growth.

•3. Essential nutrients are those that are required for a plant to grow, complete its life cycle, and reproduce. Three criteria for essential nutrients are: (1) the nutrient is required for growth or reproduction, (2) no other element can substitute for it, and (3) it is required for a specific structure or metabolic function not because it aids in the uptake of a different essential nutrient. This can be determined by using hydroponics (growing plants in water and adding nutrients in a controlled manner)

Nutrients need by plants can be categorized as either macronutrients that are needed in relatively large amounts (nine of the essential elements are macronutrients), and micronutrients that are needed in relatively small amounts (eight of the essential elements are micronutrients).

Studies have indicated that most plants need 17 essential nutrients.

Plant Essential Nutrients by Source - Essential nutrients are required by all plants to complete their life cycle Macronutrients - air and water Carbon (C), Hydrogen (H), Oxygen (O)

Macronutrients - soil Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca),Magnesium (Mg), Sulfur (S)

Micronutrients - soil Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Nickel (Ni),Boron (B), Molybdenum (Mo), Chlorine (Cl)

Plant beneficial nutrientsBeneficial nutrients enhance the growth of some plants.

Cobalt (Co), Silicon (Si), Vanadium (V), Sodium (Na)Mnemonic Devices

Mineral deficiencies most commonly result in stunted growth and discolored leaves. But the symptoms do vary depending on the element, its role in plant growth, and its mobility within the plant.

Plant Nutrient Type Visual symptoms

Nitrogen

Deficiency

Light green to yellow appearance of leaves, especially older leaves; stunted growth; poor fruit development.

Excess

Dark green foliage which may be susceptible to lodging, drought, disease and insect invasion. Fruit and seed crops may fail to yield.

Phosphorus

Deficiency

Leaves may develop purple coloration; stunted plant growth and delay in plant development.

ExcessExcess phosphorus may cause micronutrient deficiencies, especially iron or zinc.

Potassium

Deficiency

Older leaves turn yellow initially around margins and die; irregular fruit development.

ExcessExcess potassium may cause deficiencies in magnesium and possibly calcium.

Calcium

Deficiency

Reduced growth or death of growing tips; blossom-end rot of tomato; poor fruit development and appearance.

ExcessExcess calcium may cause deficiency in either magnesium or potassium

Magnesium

Deficiency

Initial yellowing of older leaves between leaf veins spreading to younger leaves; poor fruit development and production.

Excess

High concentration tolerated in plant; however, imbalance with calcium and potassium may reduce growth.

Sulfur

Deficiency

Initial yellowing of young leaves spreading to whole plant; similar symptoms to nitrogen deficiency but occurs on new growth.

ExcessExcess of sulfur may cause premature dropping of leaves.

IronDeficiency

Initial distinct yellow or white areas between veins of young leaves leading to spots of dead leaf tissue.

ExcessPossible bronzing of leaves with tiny brown spots.

Manganese

DeficiencyInterveinal yellowing or mottling of young leaves.

ExcessOlder leaves have brown spots surrounded by a chlorotic circle or zone.

ZincDeficiency

Interveinal yellowing on young leaves; reduced leaf size.

ExcessExcess zinc may cause iron deficiency in some plants.

Boron

DeficiencyDeath of growing points and deformation of leaves with areas of discoloration.

Excess

Leaf tips become yellow followed by necrosis. Leaves get a scorched appearance and later fall off.

Soil differs in quality depending of the relative proportions of sand (large particles), silt (fine particles), and clay (finest particles). Plants do best in loam soils that have about the same proportions of sand, silt, and clay. Knowledge of these basic physical properties of soil (texture and composition) helps people understand why it is important to conserve soil, and understand the limitations of different soil types and help promote sustainable agriculture. Agronomy is the study of food crops and other plants used for humans and includes the study of soil.

Soil Conservation

Soil Formation is a very slow process that involves: Physical events like fragmentation (by weathering e.g, by temperature changes and abrasion) of the parent material consisting of rocks or other geological deposits and Biological events: Organisms (e.g., lichens) also play a role in weathering, Humus is the resulting decaying organic matter that is formed and constitutes an important ingredient of the soil. Humus is important in that it consists of essential nutrients as well as changes the pH of the soil, and other organisms also play a role in soil formation.

Erosion

the wearing away and transportation of soil by water or by wind. Erosion can be natural or accelerated by human activities. This occurs on a large scale. It is estimated that between 1992 and 1997, erosion resulted in the loss of 1.9 million tons of topsoil from U.S. farmland.

Desertificationthe conversion of cropland, pasture, and rangeland into desert or land too arid to be farmed. Causes include: drought, global warming, overgrazing, overcropping semi-arid lands and deforestation. This is occurring in many regions: the U.S. (an example would be the Dust Bowl), Africa, Australia, Brazil, Iran, Afghanistan, China and India. This is a very serious problem

Farmland Conversion

The loss of fertile farmland to development (farmland conversion) is a major problem in many regions. In the U.S. for example, 1.3 million acres of farmland are developed each year (homes, businesses etc.)

Irrigation

The amount of irrigated cropland is substantial and produces @ 33% of our food. However, since the 1980s land irrigation has decreased. There are many problems with irrigation including salinization and overuse of water resources.

Waterlogging

Approximately 10% of the world’s irrigated cropland suffers from waterlogging that suffocates plants and makes soil difficult to cultivate.

Salinization

(a result of mineral buildup in irrigated soil after evaporation) also reduces food production. About ¼ of the world’s lands are affected.

Possible solutions

Soil erosion and desertification can be reduced or stopped by various methods: minimum tillage (decreasing land disturbance), contour farming (planting crops perpendicular to the slope), strip cropping (planting multiple crop species in alternating strips), terracing (using small earthen embankments that run across the slope of the land), gully reclamation (using rapidly growing plants), and shelterbelts (rows of trees planted around fields).

Farmland Conversion can be reduced by zoning restrictions, Growth Management Plans etc.

Water should be used more efficiently to prevent salinization and loss of this valuable resource. Special drainage systems could also help reduce salinization.

Appropriate use of fertilizers is needed to replace nutrients in soil. Nitrogen, Phosphorus, and Potassium are common components of synthetic fertilizers that have greatly increased crop yields. However, these elements cause other environmental problems (e.g., eutrophication of lakes and ponds). The use of organic fertilizers from animal and plant waste is advocated for sustainable development. In addition, crop rotation especially involving the periodic planting of legumes that replenish Nitrogen, can reduce the use of synthetic fertilizers.

Instead of converting non-farm land, the productivity of existing agricultural land could be increased. This could be accomplished by the cultivation of hybrids, selective breeding for high production and pest resistance, and genetic engineering (although controversial). The Green Revolution has increased the productivity of a great deal of land, BUT it is becoming apparent that these increased yields may be temporary and that the methods used in the recent past actually cause the deterioration of land by soil degradation, and erosion

Protecting wild stocks that give rise to cultivated species requires habitat protectionThe utilization of native species (e.g., the winged bean of the tropics) could help meet the increasing food demands.

Aquaculture is a relatively new way to produce a high protein food source, fish and preserve native fish stocks.

Various pests destroy at least 30% of crops. Pest management is thus necessary, but alternatives to the application of broad spectrum, potentially harmful should be utilized for sustainability.

Also eating lower on the food chain would increase total crop yields due to the low productivity associated with meat production.The implementation of new government policies is necessary for sustainability to be realized.

Phytoremediation can be used to reclaim some areas that have become unfit for agriculture or wildlife due to the activities of humans that result in contamination with heavy metals or organic pollutants. This is a new nondestructive method for reclaimation. It involves using certain plants that extract these pollutants from the soil.