the dirt on soil! an introduction to soil chemistry

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The Dirt on Soil! An Introduction to Soil Chemistry. Mrs. Long Horticulture I Fall 2013. 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. - PowerPoint PPT Presentation


The Dirt on Soil! An Introduction to Soil Chemistry

Mrs. LongHorticulture IFall 2013The Dirt on Soil!An Introduction to Soil Chemistry1Before giving this program, add your name and county to the Prepared by list in the space below Department of Agronomy There are no animations or slide builds included in this program. Feel free to add your own. Delivery of many slides could be improved with some animation or slide build.

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.

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.2Before 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.Soil ComponentsThe 4 parts of soil

About of the soil volume is solid particlesAbout of the soil volume is pore space3Lets 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.

Anchor plant rootsSupply water to plant rootsProvide air for plant rootsFurnish nutrients for plant growthRelease water with low levels of nutrients Functions of agricultural soils4Soils 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.

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. Soil StructureThe arrangement of sand, silt, and clay particles to form larger aggregates. How they fit together!1/10 inch5Individual particles of clay, silt, and sand stick together into larger particles called aggregates. Aggregates can take on many shapes and sizes, but in a good topsoil they tend to be small crumb-like particles. Small aggregates, like those shown in this diagram tend to clump together into still larger aggregates. Soil structure refers to the arrangement of individual particles of sand, silt and clay into small aggregates, and the arrangement of small aggregates into larger aggregates. Clay is important in soil structure because it is sticky and makes individual particles clump together. Aggregates, and the individual particles in them, are often coated with soil organic matter. The soil organic matter acts like a glue that strengthens the aggregates, and helps to hold them together.The spaces between aggregates are called pores. These spaces are also an important part of the soil. They are not empty, but are filled either with water or with air. In a soaking wet soil (just after a rainfall or snowmelt), all the pores will be filled with water. As the soil drains due to the force of gravity, water in the larger pores moves downward to tile lines or groundwater. When the water drains out it is replaced by air. These drained, air-filled pores are shown in yellow in this diagram. Water in smaller pores is held more tightly by the soil and does not drain (blue areas in this diagram). Water will be lost from the smaller pores due to plant uptake of water and evaporation. In addition to the pores shown in this diagram, there are also very small, even microscopic pores within each of the aggregates. These are pores that occur between the individual particles of silt and clay that make up the soil aggregates. These very small pores will be filled with water in all but the driest soil. SandLooseSiltyGranular ClayeyPlaty or Blocky

Soil Structure

6The mineral part of soil consists of sand, silt, and clay particles

Sand Largest particles low moisture holding capacitySilt Medium particles good moisture holdingClay Smallest particles Hold a lot of waterLoam equal parts sand, silt, clay ideal texture!Soil TextureSand0.1 0.002 in2 0.05 mmSilt0.002 0.0001 in0.05 - 0.002 mmClayLess than 0.0001 inLess than 0.002 mm1/100 in7Soil texture refers to the amount of various size mineral particles that are present in the soil.Soil mineral particles are separated into sand, silt, and clay on the basis of the particle diameter. This diagram shows the size of sand, silt and clay relative to each other. Sand is the largest and gives soil a gritty feel. Particles larger than 1/10 inch would be considered gravel. The sand particle in this diagram represents a fine sand particle about 1/100 inch in diameter.Silt is intermediate in size between sand and clay. Soil with a lot of silt has a floury feel. Sand and silt provide a skeleton for the soil. The main function of sand and silt in soil is to give strength. Sand and silt contribute very little to the capacity of soil to retain water and nutrients.Clay is the smallest of the mineral particles, and makes soil sticky when wet. Clay particles are microscopic, so individual clay particles cannot be seen by the naked eye. The black dots in this diagram are representative of the largest clay particles. Relative to the size of the sand and silt in this diagram, the dots are larger than most clay particles.The relative amounts of sand, silt, and clay give the soil its textural property. A loose, coarse textured soil has a lot of sand and less silt and clay. A fine textured soil is heavy and has a lot of clay and less sand. A loamy soil has a more even mix of all three.When you scoop up a handful of good topsoil you see crumbs or granules of soil that are much larger than the individual particles shown in this diagram. That brings us to the topic of soil structure. Parent MaterialMechanical weathering breaks down rock into smaller pieces without changing soil chemistryChemical weathering breaks down rocks into smaller pieces by chemical reactionBiological weathering results from activities of living organisms

Where does soil come from?8

9Mechanical Weathering

10Chemical Weathering

11Biological Weathering

12Bring a small soil sample from your yard at home.Make sure its clean.Bring in a full quart-sized ziploc bag full of soilYOUR NAME has to be on it!!

Homework for 04/135.02 Discuss the soil profile and soil sampling for surface and subsurface layers.

10/31/12Soil Sampling14Soil Particle Size La


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