Soil Texture Soil “Texture” or the name of the soil is determined by using the soil texture triangle which is a unique three sided graph. There are two methods to determine soil texture; the field method where we actually feel the moistened soil to try and estimate the amounts of sand silt and clay in the sample. And the laboratory method that we will be doing in class. In the lab method we will separate each of the components into layers and determine the % of each. The soil texture is important to us in agriculture because it affects everything from the water and nutrient holding capacity to how hard the soil is to”Till” or work. This is a “Profile” or cross section of soil showing different layers of soil and on the bottom the parent material that the soil was made from. The different layers are often different textures.

What is Soil Texture ? * Soil texture is a use classification of soils.. *It is based on its relative content of sand, silt and clay or the % of each of these particles in a given amount of soil. *At one extreme we have pure sand, and at the other, pure clay. Most soils are mixtures that fall between these extremes *This is how we determine the common name that most of us apply to a particular type of soil, ie. “Sandy”, “Clay” , “Silty” or “loam”.

So What? Soil Texture determines the uses and the limitations of the soil.

The Type of agricultural crops that are best suited for a type or “Texture” of soil Ask the students what the crops are and then to speculate on how the soils might be different for each of the crops shown. They could jot this down on their notes page. Call on a couple of students to get their Ideas on each picture. Help them refine answers. Root or tuberous crops such as potatoes do better in light “Sandy” soils. One reason is they are also easier to clean when processing and fewer imperfections in the tubers. Heavy or clay soils are better for crops that require a lot of water like rice. This is because clay soils hold water better and infiltration rate is slower we get standing water. Trees and vines need a deep well rounded soil or “Loam” that has the good drainage characteristics and air movement of the sand but also has the water holding capacity of the clay so that it doesn’t have to be irrigated as often as a straight sandy soil.

The type of trees and shrubs that will grow in the soil. Have the students speculate as to why the vegetation is different in the foreground and back ground in the slide. Students could pair share and jot down their answers in their notes. Call on several of the groups to get their responses. The texture of the soil will affect the type of vegetation because of water holding capacity and how fast water runs off of the soil. Here we see brush growing on a heavier textured (clay) soil and pine and fir on the lighter textured soil.

How fast rainfall will run off of the soil. Heavy soils ( clay) = more total water holding capacity but Percolation rate and “Infiltration rate are slower. This results in more runoff. Light soils ( Sand) = less water holding capacity but higher Infiltration” rates and higher “Percolation” rates. This results in less runoff. I use a sponge and a piece of cardboard at the sink to illustrate good and bad infiltration rate. Hold the sponge under a trickle of water and show that it absorbs the water rapidly like a light or sandy soil would. Then hold the cardboard under the water at the same rate. The water will run off rapidly illustrating how a heavy or clay soil will not allow water to infiltrate rapidly and it will not percolate or go thru the cardboard very fast either.

The type of irrigation that can be used on the soil. Different irrigation systems allow the water to infiltrate the soil surface at different rates. Have the students speculate which type of system might work best on different textures of soil.

How hard it will be to work or till the soil. The texture of the soil will affect how much friction there is on the surface of the tillage tool. Have students speculate on which soil texture heavy or light will have more or less friction. Light has less. Heavy has more based on the number of soil particles that come into contact with the surface of the tillage tool. More friction = more horsepower needed to pull the same tool or implement thru a heavy soil than a light textured soil.

The nutrient holding capacity of the soil. Clay particles have a negative charge. Nutrient compounds normally have a positive charge. Clay soils or “heavy”soils are better at holding nutrients. I have usually already gone over the label on fertilizer bags and quiz the students about the numbers on the bag. More clay = more nutrient holding capacity in a given soil. The opposite is true for light soils. Light soils might need several applications of less nutrients per application because the soil can’t hold them and they will get flushed past the root zone by rain or irrigation. Sandy soils or “light” soils don’t hold nutrients as well.

Soil Texture Triangle Sand: Silt: Clay: Texture 25% 65% 10% Silt Loam Tell the students that we have determined the amount of each of the soil particles in the soil to be those listed on the slide. We are going to see how to plot them on the triangle to determine the soils name or “Texture”. This will be where the lines intersect. I usually use a ruler or yard stick to mirror the line as it comes in to emphasize that the line is coming in parallel to each of the three bases of the triangle. Sand: Silt: Clay: Texture 25% 65% 10% Silt Loam

Soil Texture Triangle Sand: Silt: Clay: Texture 35% 45% 20% Loam After this slide I would pass out the work sheet and have the students due the first problem. I would put the answer key up on the screen and pull down the block covering the 1st answer. And continue from there. Sand: Silt: Clay: Texture 35% 45% 20% Loam