SOIL LAB BACKGROUND INFORMATION
Soil Texture and Particle Size Information
Soil texture affects:
 Affects soil’s ability to hold water…ability to hold water affects plant’s growth and development
 what you can grow in your soil
 the soil quality
 porosity
 permeability
Soil texture relates to porosity:
Sand particles are generally round, while silt and clay particles are usually thinner and flatter.
 Soil with larger, round particles (like sand), more space is available for the water and air that
plants need.
 If the air space between the particles is larger (sand grains), it provides good aeration.
 Clay – holds water –water does not pass through
 Sand – does not hold water – water passes through easily
Differences among Sand, Silt, and Clay textures
Clay
 heavy, difficult to work.
 dry out slowly
 leave a hard crust that does not allow the rain to penetrate.
 can hold water and nutrients between layers
 have more surface area than sand or silt- more area is available for plant nutrients to stick to.
Sand
 Loose and single grained.
 Dries out quickly
 Does not hold a lot of nutrients
Soil Texture and Management
 Coarse Textured Soils (sands, loamy sands)
o Hold low amounts of water and nutrients - may have to irrigate and fertilize frequently
o Aren’t eroded easily - allows water to infiltrate; less runoff
o Very permeable - good for waste disposal only if a deep soil
 Fine textured soils (clays, sandy clays, silty clays)
o Hold large amounts of nutrients and water - may hold things too tightly
o Easily eroded - slow infiltration; high runoff means much erosion.
o Slow permeability – holds onto a lot of water, plants can become oversaturated
o May shrink/swell
 Medium Textured Soils (loams, clay loams, sandy loams, silt loams)
o Properties fall in between the other two.
Soil Particle Size
 Affects permeability
 Larger particle size (sand) = higher permeability
 Smaller particle size (clay) = lower permeability
Soil Porosity and Permeability Information
Soil Permeability
Permeability - ability of soil to transmit water or air, this is affected by texture,
porosity and structure:
 if permeability is high : water moves quickly, drains fast
 if permeability is low : water moves slowly, drains slow, soil can become
waterlogged and plant roots can die due lack of oxygen
 High permeability (sand) = water passes quickly
 Low permeability (clay) = water passes slowly
Factors affecting permeability
1. Pore size present in the soil - Water moves faster through larger pores than smaller
pores. - the bigger the pore the more material it will move. High porosity= faster
movement of materials
2. Soil texture -Pores size decreases with smaller particle size. As the size of the separates
decrease so does the pore size, but remember that the surface area will increase
quickly.
Soil texture Permeability
Sand
very high
Sandy loam
high
silt, loam
high
silt loam
med-low
Clay
low
Increasing Permeability
 Create large continuous pores throughout the soil using worms and insects
 Increasing organic matter forms large pores when plant residues are added or left
on the soil surface.
Soil Porosity
Soil texture relates to porosity and the ability to hold water and nutrients
Soil texture
sand
silt
clay
Pore size
large pores
medium pores
fine pores
Amount of water held in soil
Low water holding
High water holding, plants can use most of water
High water holding, water not available to all plants
 High porosity= large pores=soil cannot hold a lot of water and nutrients
 Low porosity= fine pores= soil can hold onto water and nutrients
Chemical properties of soils (NPK and pH)
Nitrogen
Nitrogen importance
 Main element that affects plant productivity.
 Directly responsible for vegetative growth and green color in plants.
 Plant growth
 Building blocks of proteins and enzymes
 Stimulates root growth and development
 Helps the uptake of other nutrients
Nitrogen loss from the soil system
 Leaching- loss of nitrogen due to excess water
 Crop removal
 Soil erosion and runoff
Nitrogen and soil texture
Coarse-textured soils (sandy soils) have a lower water holding capacity and, therefore, a
greater potential to lose nitrate from leaching when compared with fine-textured
soils.
Sandy soils are also easily eroded and subjected to nitrogen loss through runoff.
Nitrogen levels
 Low nitrogen= poor growth and pigment change, yellowing of leaves
 High nitrogen= pollution of water, poisoning of grazing animals, excessive plant
overgrowth
 Can be replaced by fertilizers and planting of legumes
Phosphorus
Phosphorus importance
 Make up DNA
 Important for plant reproduction and photosynthesis
 Essential for strong roots, flower and fruit development.
 Important for biological energy transfer processes that are vital for life and growth.
 Results in higher crop production, improved crop quality, greater plant
strength, increased root growth, and earlier crop maturity.
Phosphorus levels
 Low phosphorus=plants susceptible to disease
 High phosphorus= inhibit absorption of zinc, manganese and iron, leaves turn
yellow, plants weaken
Potassium
Potassium Importance
 Provides environment for plant metabolism and producing sugars
 Reduces frost damage
 Increase plant’s ability to survive cold temperatures, and survive droughts.
 Boosts plant’s resistance to disease and insect damage.
Potassium levels
 Low potassium-low metabolism, leaves turn brown
 High potassium- inhibit absorption of other nutrients
Increasing potassium levels
 Weathering of feldspar and micas
 Adding organic material
 Increasing soil moisture
 Fertilizer
Changing N–P–K
 Fertilizer = replaces lost nutrients
 Packages contain 3 numbers = % of N-P-K
Ex. 5-10-10 = 5% N, 10% P, 10% K
 The key is to match the correct fertilizer to the specific deficiencies of the soil
Ex. Soil with N-P-K results of L-L-H would require 15-15-5 fertilizer
Ex. Soil with N-P-K results of H-M-L would require 5-10-15 fertilizer
pH
pH Importance
 How acidic or basic the soil is. Soils 0-6= acidic, 7=neutral, 8-14= basic
 Most plants cannot grow in soils too acidic or basic (pH of 5.5-7.5 range is good)
 pH too high or low prevents plants from using nutrients like N, P, or K.
Factors affecting pH
 Parent material - type of rocks from which the soil developed.
 Rainfall - soils under high rainfall conditions are more acid than soils formed under
dry conditions.
 Soil organic matter - soil organisms are continuously decomposing organic matter.
The net effect of their activity is that hydrogen ions are released and the soil
becomes more acidic.
Native vegetation - the type of the native vegetation under which the soil was
formed affects the pH of the soil. Soils formed under forest vegetation tend to be
more acidic.
Increasing the Soil pH- make more basic, raise pH number
 Adding lime
 Adding wood ashes
Decreasing the Soil pH make more acidic, lower pH number
 aluminum sulfate
 sulfur