Soil Texture: A Master Variable in Understanding Soil Properties

1. Ch. 4 continued Soil Properties

2. Soil Texture(proportion of different-sized particles <2-mm) Master VariableBecause it affects so many other soil properties and variables Basic Soil Property Because it is not readily subject to change

3. Heavy or Light Soils These terms actually refer to texture, not weight • A heavy soil is a clayey soil • called ‘heavy’ because of the soil's ability to retain moisture and the difficulty in working the soil (and it’s heavy after rain or irrigation) • A lightsoil is a sandy soil • called ‘light’ because it is usually easy to work in the field and drains quickly • Actually, a clayey soil weighs less than a sandy soil when dry

4. Clay vs Silt vs Sand: • The surface area to volume ratio greatly increases as the particle size decreases and the shape changes from rounded to plate like • As the surface area increases, so does the ability to adsorb compounds and interact with the soil solution: more chemical reactions, more biological interactions, more surface for water to cling to

5. Original Cube= 8cm x 8cm x 8cm 8cm 2 x 2 x 2 2 x 2 x 2 Calculate: (1) total surface area of original cube (uncut) and (2) total surface area after cutting it into smaller cube of the given sizes 8cm 4 cm x 4cm x 4cm 8cm

6. Specific surface area 2 cm 2 cm 2 cm Each cube is 2 cm on each side, the same mass of material would now be present as 64 smaller cubes Surface area for one face of a small cube = 2 cm x 2 cm = 4 cm2 Surface area for each cube (6 faces, 4 cm2 per face) = 6 faces x 4 cm2/face = 24 cm2 Total surface area (64 cubes, 24 cm2/cube) = 64 cubes x 24 cm2/cube = 1536 cm2 Surface area for each face (8 cm x 8 cm) = 64 cm2 A cube has six faces Total surface area (6 faces 64 cm2 per face) = 6 faces x 64 cm2/face = 384 cm2

7. Sand and silt are largely chemically inert, while clay is chemically active (charged) • Mainly because of size, shape, and chemical characteristics of clay Cations (+) stick to clay particles because of negative charges on the clay

8. How particle size affects soil properties Specific surface area CEC Sorption capacity Stickiness Plasticity Cohesion Porosity Water holding cap. All increase as size decreases Colloidal Clay Silt Sand clay

9. How particle size affects soil properties

10. Field and Laboratory Analyses For Soil Texture (Box 4.1) • Feel method: determines the textural class in the field. • <2-mm soil sample is moistened, kneaded and 'ribboned' between the thumb and fore finger. • With practice, 'experts' can come within a few % of the separates. • For consistence, the same person should do it each time.

11. Laboratory methods are more preciseRemove cementing agents that complicate the feel method (organic matter, carbonates, Fe oxides) Soil is mixed with water and the density of the water is measured or the amount of material in suspension is determined after varying time periods.

12. Hydrometer Method Depends on the relationship between settling velocity and particle diameter (big particles fall faster) Hydrometer measures the density of liquid: more suspended soil = higher density As soil particles settle the density decreases Particles settle according to Stoke's Law. v = k d2 v = velocity of fall; k = constant (g, liquid properties, etc); d = particle diameter (assume spherical)

13. hydrometer readings are taken at various times to measure liquid density (each graduation = 1 g/L as soil colloids/liter)

14. 10,800 sec lowest density (less suspended soil) 40 sec Highest density (most suspended soil) http://www.fao.org/docrep/field/003/AC172E/AC172E06.gif

15. Example: 54% sand (40 s) 21% clay (3 h) How much silt? Silt = 100 – (21+54) = 25% Ck: 25+21+54 = 100 What texture? Sandy Clay Loam

16. Soil Texture • Texture influences most other soil properties • Soil texture does not change in nature over a short period of time • Adding organic matter may improve characteristics of soils but not change texture • Large quantities of sand, silt, or clay must be added and thoroughly mixed before the texture significantly changes • Mixing media for potting soil, golf greens, etc. is generally the only time when textural modification is economically feasible

17. Particle Density (Dp) of Soil(or specific gravity) • Mass of a known volume of dry soil solids. Doesn't vary much (for mineral soils) because the minerals that dominate (quartz, feldspar, mica) are silicate minerals • Usually taken as 2650 kg/m3 (2.65 Mg/m3 or 2.65 g/cm3) • Often written as g/cm3 (SI units use kg, Mg and m). • Mg is 1 metric ton or 1000 kg. Dp = mass of particle/volume of particle

18. Bulk Density of Soil (Db) • Mass of a volume of dry soil. • The volume includes spaces (but not water). • Db is less than particle density • Db ranges from 1.0 - 1.6 Mg/m3 • Common to assume 1.3 Mg/m3

20. Bulk density (“Db”) of soil depends on texture and soil organic matter content. Clays have lower Db values and sands have higher Db values. High organic matter content increases aggregation and porosity and thus decreases Db. • Db increases with increasing depth, compaction, and traffic and decreases with increasing clay and organic matter content. Manage soil to prevent compaction and decrease Db Never till when wet! • Maintenance of low Db is important for good soil fluid and gaseous exchange, root penetration, and plant growth.