Introduction to Soil Science

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How soils supply plant nutrients An Introduction to Soil ChemistryPrepared by: Richard Stehouwer Department of Agronomy


What is soil?Soil is the unconsolidated cover on the surface of the earth. Soil is made up of mineral particles, organic particles, air, and water. Soil is capable of supporting plant growth.


Functions of agricultural soilsAnchor plant roots Supply water to plant roots Provide air for plant roots Furnish nutrients for plant growth Release water with low levels of nutrients


Soil Components The 4 parts of soil


Soil TextureThe mineral part of soil consists of sand, silt, and clay particles The amounts of each size particle determines the textural property of the soil Coarse textured, loose (more sand, less clay) Fine textured, heavy (more clay, less sand) Loamy (more even mix of sand, silt and claySand 0.1 – 0.002 in 2 – 0.05 mmSilt 0.002 – 0.0001 in 0.05 - 0.002 mmClay Less than 0.0001 in Less than 0.002 mm


Soil Structure The arrangement of sand, silt, and clay particles to form larger aggregates.Organic matter is the glue that holds the aggregates together Large pores (spaces) between aggregates are filled with air in a moist soil. Small pores are filled with water in a moist soil. Even smaller pores inside the aggregates (not shown) are also filled with water.1/10 inch


Supplying Plant NutrientsMacronutrients: (needed in large amounts) Nitrogen (N) Phosphorus (P) Potassium (K) Calcium (Ca) Magnesium (Mg) Sulfur (S) Micronutrients: (needed in small amounts) Chlorine (Cl) Cobalt (Co) Copper (Cu) Iron (Fe) Manganese (Mn) Molybdenum (Mo) Nickel (Ni) Zinc (Zn)Nutrients that plants obtain from the soil


Where do plant nutrients come from?Decaying plant litter Breakdown of soil minerals Addition by humans Commercial fertilizer Manure Lime Other


Recycling plant nutrients


Breakdown of soil mineralsCaMgKCuNiZn


Nutrient additions by humansCommercial fertilizers Nutrients are in a form that is available to plants Dissolve quickly and nutrients go into soil water Lime Dissolves slowly as it neutralizes soil acidity Releases calcium and magnesium Organic nutrient sources Manure, compost, sewage sludge Decay and nutrient release is similar to crop litter


The soil solutionSoil water is a complex solution that contains Many types of nutrients Other trace elements Complex organic molecules Nutrients in the soil solution can be readily taken up by plant roots If nutrients remained in solution they could all be quickly lost from the soil.PNiCaMgCuKZn


AdsorptionAdsorption refers to the ability of an object to attract and hold particles on its surface. Solid particles in soil have the ability to adsorb Water Nutrients and other chemicals The most important adsorbers in soil are Clays Organic matter


Surface area of clay¼ cup¼ cup of clay has more surface area than a football fieldThe large surface area of clay allows it to Adsorb a lot of water Retain nutrients Stick to other soil particles


Properties of Soil ClaysClay particles are stacked in layers like sheets of paper. Each clay sheet is slightly separated from those on either side. Each sheet has negative charges on it. Negative charges have to be balanced by positive charges called cations. 1/20,000 in


Cation Retention on Soil ClaysCopper, +2Magnesium, +2Ammonium, +1Potassium, +1Sodium, +1Calcium, +2Aluminum, +3Hydrogen, +1


Cation Retention on Organic MatterLow pH, 4 - 5 (acidic soil)Neutral pH, 7 (“sweet” soil)HydrogenNutrientsIncreasing pH increases cation exchange capacity of organic matter


Cation Exchange CapacityCation exchange capacity (CEC) is the total amount of cations that a soil can retain The higher the soil CEC the greater ability it has to store plant nutrients Soil CEC increases as The amount of clay increases The amount of organic matter increases The soil pH increases


Negatively Charged Nutrients (Anions)Some very important plant nutrients are anions. Soils are able to retain some of these nutrient anions. Retention of nutrient anions varies from one anion to anotherNitratePhosphateSulfateChloride


Phosphate retention in soil+PhosphateAluminumAluminum phosphate solid1. Formation of a new solid material2. Anion exchangePhosphate


Phosphate retention in soilIron oxide surfacePhosphate anions - Each held by two chemical bonds to the iron oxide surface3. Adsorption on oxide surfaces


Nitrate (NO3-) retention in soilsIf nitrate is not taken up by plants it is very likely to be lost from the soil Unlike phosphate, nitrate is very weakly held by soils Nitrate does not react to form new solids Nitrate is not held by oxide surfacesNO3-


Moving nutrients from soil to plantsNutrients on soil clay and organic matterNutrients in soil solution


Excessive Nutrient LoadingNutrients on soil clay and organic matterNutrients in soil solutionNutrient loss in drainage water


The black box is openSoil consists of mineral and organic matter, air and water Soils are able to adsorb nutrients and other chemicals The most important adsorbers are clay and organic matterAdsorbed nutrients are available to plants Adsorbed nutrients are not prone to loss in drainage water Soil adsorption capacity can be exceeded leading to greater nutrient loss

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Last Updated: 8th March 2018

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