33.. Plant Nutrition and Soil

    A. Early Views

        1. Ancient Greeks considered plants "soil-eaters" that converted soil into plant tissue.
        2. 17th Century Dutchman Van Helmont conducted an experiment.
            a. He planted a 5 pound willow tree in a pot with 200 pounds of soil.
            b. After five years of watering, the tree weighed 170 pounds; only a few ounces of soil was missing.
            c. He concluded the increase in tree weight came from water; he was unaware of substances in air.

    B. Essential Inorganic Nutrients

        1. Essential inorganic nutrients must fulfill the following criteria.
            a. They have an identifiable nutritional role.
            b. No other element can substitute and fulfill the same role.
            c. A deficiency of the element causes the plant to die.
        2. These elements are divided into macronutrients and micronutrients by concentration in plant tissue.
        3. Essential inorganic nutrients (e.g., carbon, hydrogen, oxygen) comprise 96% of plant dry weight.
            a. Carbon dioxide is the source of carbon for a plant.
            b. Water is the source of hydrogen.
            c. Oxygen can come from either atmospheric oxygen, carbon dioxide, or water.
        4. Beneficial inorganic nutrients are elements required for or improving growth of a particular plant.
            a. Horsetails require silicon as a mineral nutrient.
            b. Sugar beets show better growth in the presence of sodium.
            c. Soybeans use nickel when root nodules are present.

    C. Determination of Essential Elements

        1. When a plant is burned, most mineral elements (except for nitrogen) remain in the ash.
        2. Hydroponics is the preferred method for determining plant mineral requirements.
            a. Hydroponics is cultivation of plants in water.
            b. Nutrient requirements of plants are determined by omitting a mineral and observing the effects.
            c. If plant growth suffers, it can be concluded that the omitted mineral is a required nutrient.
            d. This works for macronutrients but impurities make micronutrient measurement difficult.

    D. Soil Formation

        1. Soil formation begins with weathering of rock by freezing, glacier flow, stream flow, and chemicals
        2. Lichens and mosses grow on barren rock and trap particles and leave decaying tissues.
        3. Decayed organic matter (humus) takes time to accumulate; its acidity leaches minerals from rocks.
        4. Depending on parent material and weathering, a centimeter of soil may develop in 15 years.

    E. The Nutritional Function of Soil

        1. Soil consists of soil particles, decaying organic matter, living organisms, air and water.
        2. Best soil includes particles of different sizes; this provides critical air spaces.
        3. Soil Particles
            a. Particles vary by size.
                1) Sand particles are larger: 0.05-2.0 mm in diameter.
                2) Sand particles are medium sized: 0.002-0.05 mm in diameter.
                3) Clay particles are smallest: below 0.002 mm in diameter.
            b. Soils lose water too readily; clay packs tight to hold water and clumps.
            c. Clay particles are negatively charged and attract positively charged ions (e.g., calcium [Ca2+]
                and potassium [K+]).
            d. In acidic soils, hydrogen ions replace positively charged nutrients and the nutrient ions float free
                and are leached; this is why acid rain kills trees.
            e. Clay cannot retain negatively charged NO3-, and nitrogen content of clay soil is low.
            f. Loam (a mixture of the three soil particles) retains water and nutrients; roots take up oxygen in the air spaces.
        4. Humus
            a. A mixture of 10-20% humus mixed with a top layer of soil particles is best for plants.
            b. Humus keeps soil loose and crumbly, decreases runoff and aerates soil.
            c. Humus is acidic and retains positively charged minerals for plants to use later.
            d. Bacteria and fungi break down organic matter in humus and return inorganic nutrients to plants.
        5. Living Organisms
            a. Small plants play a major role in formation of soil from rock and in succession.
            b. Roots of larger plants penetrate soil and weather rocks.
            c. Larger moles and badgers and smaller earthworms help turn over the soil.
            d. Soil animals, from mites to millipedes help break down leaves and other plant remains.
            e. Fungi, protozoa, algae and bacteria complete decomposition.
            f. Soil bacteria make nitrate available to plants.
            g. Some soil organisms (nematodes and insects) are crop pests that feed on roots.
        6. Soil profiles consist of horizons.
            a. A horizon is the uppermost topsoil layer that contains litter and humus.
            b. B horizon lacks organic matter but contains inorganic nutrients leached from the A horizon.
            c. C horizon is weathered and shattered bedrock.
            d. Soil profiles vary by parent material, climate and ecosystem.
            e. Grassland soils have deep A horizons from turnover of decaying grasses and lack of leaching.
            f. Forest soils have thinner A horizons but enough inorganic nutrients for tree root growth.
            g. Tropical rain forest A horizons are shallow due to rapid decomposition; the B horizon is deeper
                due to extensive leaching.
        7. Soil Erosion
            a. Erosion is caused by water or wind carrying away soil.
            b. Erosion removes 25 billion tons of topsoil annually.
            c. Deforestation and desertification contribute to erosion.
            d. U.S. farmlands lose soil faster than it is formed on one-third of cropland.