Soil Water Relationships Dr. Aimee Taylor Soil Particles Particle Sand Silt Clay Size in mm < 2mm 0.05 mm 0.05-0.002 mm <0.002 mm Texture is determined by the relative amounts of SZC COARSE SAND Pores Movement Water, O 2, CO 2 Pore Size Distribution MICROPORE <0.05mm MACROPORE >0.05mm Size is important Sand rapid movement (Macropores) Clay slow (Micropores) Smaller pore = tightly (KPU) 1
Defined Moisture Terms Water olding Capacity Total amount of water a soil can hold at Field Capacity SATURATION FIELD CAPACITY Maximum Water olding Capacity WILTING POINT Larger Pore Sizes Smaller Pore Sizes You cannot change the textural class of your soil Can you increase water holding capacity of soil? (KPU) 2
Water Movement Infiltration Rate SANDY LOAM LOAM CLAY LOAM Check Infiltration Fast Moving Water and Slope Areas of ponding? Tramlines Compaction Surface crusting Stratification? Mark off areas and dig a pit Water hesitant to leave small pores Underlain by Clay Layer SILTY LOAM SILTY LOAM CLAY CLAY Sand Sand (KPU) 3
Preferential Flow Structural Problems Raindrop Erosion Stability of Aggregates Organic Matter Content Texture Class Surface Crusting or Protection Reduced Seedling Emergence (KPU) 4
Compaction Worst at FC Bulk Density Determination Soil is solids and pore spaces Volume is 1 cubic centimetre of soil 1cm 3 Mass is 1.33 grams Bulk Density = Mass of Soil Volume of Soil BD= 1.33g/cm 3 Compacted Soil Well Drained Soils Forms of Iron & Effect on Soil Color Photos by Jim Baker, Virginia Tech Form Chemical Formula Color Ferrous oxide FeO Grey Ferric oxide (ematite) Fe 2 O 3 Red ydrated ferric oxide (Limonite) 2Fe 2 O 3 3 2 O Yellow (KPU) 5
Depth to grey colors Drainage Class* Excessively drained Somewhat excessively drained Soil Characteristics Water is removed rapidly from soil. Effect on Cropping Will probably require supplemental irrigation. Well drained Water is removed readily, but not rapidly. No drainage required. Moderately well drained Water is removed somewhat slowly at some periods of the year. May require supplemental drainage if crops that require good drainage are grown. Well Drained Moderately Well Somewhat Poorly Poorly Drained Jim Baker, Virginia Tech Somewhat poorly drained Poorly drained Water is removed so slowly that soil is wet at shallow depths periodically during the growing season. Will probably require supplemental drainage for satisfactory use in production of most crops. Very poorly drained Free water is present at or near the surface during the growing season. *Refers to the natural drainage condition of the soil without artificial drainage. Conclusion Know your soil type texture Avoid compaction during field capacity Cover bare soil - mulching & cover crops Observe the evidence colours, water movement, ponding Increase Soil Organic Matter Increase water holding capacity Improve structure Aggregate stability Prevents loss by erosion Salinity Evaluated by EC Electrical Conductivity Salty shallow ground water? subsurface drainage Irrigate to leach salts below root zone (in early life of plants) If sodic Add Ca in form of gypsum Salt tolerant plants Keep well vegetated Wont see worms if saline O COESION -ve O -ve ADESION (KPU) 6
PORES Pore diameter 20000(µm) Nature of the pore A 20 mm crack MICROPORE <0.05mm TIGTLY ELD 2 0 MACROPORE >0.05mm SIZE IS IMPORTANT 4000 An earthworm channel (4 mm) 300 The diameter of a grass root 60-30 The smallest pore that will be air filled at field capacity 10 A fungal hypha 2 The size of a bacterial cell. The largest clay particle. The smallest pore from which a plant can readily obtain water 0.2 The smallest pore that will give up water to the suction exerted by a plant root. The pore size corresponding to the permanent wilting point 0.003 The largest pore filled with water in an "air dry" soil. The pore size is approximately ten times the diameter of a water molecule (KPU) 7