How soils supply plant nutrients How soils supply plant nutrients An Introduction to Soil Chemistry Prepared by: Richard Stehouwer Department of Agronomy
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How soils supply plant nutrientsAn Introduction to Soil ChemistryPrepared by:Richard StehouwerDepartment 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 rootsSupply water to plant rootsProvide air for plant rootsFurnish nutrients for plant growthRelease water with low levels of nutrients
Soil ComponentsThe 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 soilCoarse textured, loose (more sand, less clay)Fine textured, heavy (more clay, less sand)Loamy (more even mix of sand, silt and claySand0.1 0.002 in2 0.05 mmSilt0.002 0.0001 in0.05 - 0.002 mmClayLess than 0.0001 inLess than 0.002 mm
Soil StructureThe 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 humansCommercial fertilizerManureLimeOther
Recycling plant nutrients
Breakdown of soil mineralsCaMgKCuNiZn
Nutrient additions by humansCommercial fertilizersNutrients are in a form that is available to plantsDissolve quickly and nutrients go into soil waterLime Dissolves slowly as it neutralizes soil acidityReleases calcium and magnesiumOrganic nutrient sources Manure, compost, sewage sludgeDecay and nutrient release is similar to crop litter
The soil solutionSoil water is a complex solution that contains Many types of nutrientsOther trace elementsComplex organic moleculesNutrients 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 WaterNutrients and other chemicals
The most important adsorbers in soil areClays Organic matter
Surface area of clay cup cup of clay has more surface area than a football fieldThe large surface area of clay allows it toAdsorb a lot of waterRetain nutrientsStick 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.
Cation Retention onSoil ClaysCopper, +2Magnesium, +2Ammonium, +1Potassium, +1Sodium, +1Calcium, +2Aluminum, +3Hydrogen, +1
Cation Retention onOrganic MatterLow pH, 4 - 5(acidic soil)Neutral pH, 7(sweet soil)HydrogenNutrientsIncreasing pHincreases 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 asThe amount of clay increasesThe amount of organic matter increasesThe 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 phosphatesolid1. Formation of a new solid material2. Anion exchangePhosphate
Phosphate retention in soilIron oxide surfacePhosphate anions -Each held by two chemical bonds to theiron 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 soilUnlike phosphate, nitrate is very weakly held by soilsNitrate does not react to form new solidsNitrate 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 waterSoils are able to adsorb nutrients and other chemicalsThe most important adsorbers are clay and organic matterAdsorbed nutrients are available to plantsAdsorbed nutrients are not prone to loss in drainage waterSoil adsorption capacity can be exceeded leading to greater nutrient loss
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For many of us soils are a black box. We put things into the box and we get things out of the box, but we dont have a very good idea of what happens inside the box. We put seed, fertilizer, and water into the soil and out from the soil comes the crops we are growing. But what exactly happens inside that black box we call soil? Farmers and scientists and have been studying that question for hundreds of years and continue to study it today. They have learned that many complex physical, biological, and chemical processes are carried out in soil. Lets open up that black box just a little and learn something about the chemistry of soils. Knowing something about soil chemistry will help us understand how soils supply plant nutrients.Before we get into soil chemistry, we need to back up a bit and think more generally about soils.What is soil?To a geologist soil is the decomposed surface of rocks. To an engineer soil is the medium that must be strong enough to support a highway or a skyscraper or your house.To scientists who study soil formation, soil is a natural body consisting of several layers and formed from weathered rocks over a period of thousands to millions of years. Today we will consider soil from an agricultural perspective.Soil is the unconsolidated material at the surface of the earth. (Unconsolidated simply means that it is granular material that is not cemented together like rock.) Soil is made up of mineral and organic matter and contains both water and air. Most importantly, soil is capable of supporting plant life. Its the material that sustains not only the farmers livelihood, but that of the whole world. It is from this perspective that we will now look more carefully at soil. First, we will consider what functions must be performed by soil used for crop production, and then we will look at how the soil accomplishes those functions.Soils used for crop production must perform five basic functions.Soil must firmly anchor plant roots. It must be strong enough to hold crops and even large trees erect. Yet soil must be permeable enough to allow tiny root hairs to penetrate it.Soil must retain rain that falls on it in order to continuously supply water to growing plants. Yet it must also allow excess water to drain. The soil must drain because it must also supply air, more specifically oxygen, to crop roots. Too much water means too little air and the crops suffocate.Soil must supply nutrients for plant growth. To do so it must be store nutrients and then release them to the roots of growing crops. But soil must not release those nutrients to draining water. Soil is a truly remarkable material to be able to perform each of these tasks tasks that sometimes seem to be in conflict with each other.Our focus in this session will be on the last two functions of an agricultural soil. How soil provides nutrients for plant growth, andHow nutrient laden soil can release water with low levels of nutrients. In particular, we will consider the chemical characteristics of soil that allow it to perform these functions.
Lets review a couple of soil science basics. Soil is made up of 4 parts: mineral matter, organic matter, water, and air.Mineral matter and organic matter together form the solid part of soil. Soil air and soil water occupy the spaces between the solid particles. This space is the pore space. A good agricultural soil will be about half solid particles and half pore space. Most of the soil solids will be mineral matter that is made up of particles of sand, silt, and clay. A small part of the solids will be organic matter. Most agricultural soils have somewhere around 2 - 5% organic matter. Organic matter is mostly made up of decomposed plant litter and roots. Conditions for root growth will be ideal when about half the pore space is filled with water and half is filled with air. When a soil becomes compacted the mineral particles are pressed more tightly together. When this happens the soil loses pore space, and so has less capacity to store water and air.Soil texture refers to the amount of various size mineral particles t