Introduction to soil water relationships. Particle density ( r s ) Definitions Mass of soil particle divided by volume of soil particle Specific gravity, SG = ratio of mass of soil particle to mass of equal volume of water of water at 4°C
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Particle density (rs) • Definitions • Mass of soil particle divided by volume of soil particle • Specific gravity, SG = ratio of mass of soil particle to mass of equal volume of water of water at 4°C • Particle density in cgs or tonnes m-3 numerically equal to SG • mean particle density depends on: • ratio of OM to mineral matter • constitution of soil minerals • constitution of OM
Determination • SG bottle • boiled water to remove dissolved air • de-aerate for several hours with vacuum pump to remove air trapped between particles • problem of floating OM Typical values • organic matter = 1.3 g cm-3 • quartz = 2.66 g cm-3 • average for clay = 2.65 g cm-3 • orthoclase = 2.5 to 2.6 g cm-3 • mica = 2.8 to 3.2 g cm-3 • limonite = 3.4 to 4.0 cm-3 • Fe (OH)3 = 3.75 cm-3 • normally taken as 2.65 cm-3
Bulk density (rb) and related parameters Bulk density rb = mass of solids total volume Value is effected by particle density, degree of compaction, organic matter content
Typical values: • 0.9 for organic soil (peaty) to 1.8 for compacted sand • Sand generally has a higher density than clay - why? • What do we mean by heavy & light soils? • Determination: • soil coring devices • problems of compaction • oven drying at 105°C • gamma ray transmission
Gamma ray transmission • measures density – • 2 probes - transmitter & detector
Wet v dry bulk density Ms + Mw Vt
Coefficient of linear extensibility (COLE) Bulk density changes in swelling - shrinking soils. COLE is a measure of this Compares dry with saturated soil after it comes to equilibrium. Cracks complicate the problem of determining BD of swelling soils. Even allowing for cracks the overall density may be higher on shrinking as the surface becomes lower.
Total pore space (T) = volume of (air + water) vol. of (air + soil + water)
Volume of (air + water) = total volume (air + soil + water) - volume of soil where Vt and Vs are the volumes of the total sample and the soil particles respectively Vs = Ms /rs and Vt = Ms /rb where Ms is the mass of oven dry soil and rs and rb are the particle density and bulk density respectively. So:
and so: and : rb = (1-T)/rs Used in agricultural (soils) research especially for compaction studies. Typical values 0.3 to 0.6. Often expressed as a %.
Packing density measure of compaction of particular texture class • Void ratio (e) • Used mainly in engineering applications • e = volume of (air + water) • volume of soil • e = T/(1-T) [void ratio] • Typically 0.3 to 2.0 • Air filled porosity • = volume of air • volume of total
Moisture content and related parameters (a) Volumetric basis: volume of water volume of total qv = Vw/Vt (b) Gravimetric basis: mass of water mass of soil qm = Mw/Ms
As Vw = Mw/rw and Vt = Ms/rb then and so qv = qmrb/rw = qmrb/1 (not dimensionally correct) in metric measurements - density of water is 1 Often expressed as depth/depth for example mm/m
Degree of saturation (s) degree of saturation = volume of water volume of (water + air) s =qV/T Liquid ratio Liquid ratio = volume of water volume of solid
An example to try A hole 30 cm X 30 cm x 30 cm is dug in a field. The wet soil weighs 50.55 kg. The soil is taken back to the laboratory and oven dried. The final weight is 38.34 kg. (a) What is the bulk density (b) What was the moisture content in the field (i) by volume (ii) by weight (c) If the mean particle density is 2.64, what is the total pore space
Graphical representation .... Q. Why is the moisture content less at depth?)
Measurement of soil moisture • Laboratory • definitive • weigh, oven dry at 105°C for 24 hours, reweigh • if volume of hole from which sample was taken is known, bulk density can be calculated and hence volumentric moisture content Field methods Include: • neutron scattering • gamma ray transmission • time domain reflectometry • all need calibration against laboratory method
H scatters and slows neutrons very effectively - elastic collisions with atomic nuclei • called “thermalisation” of fast neutrons - come to same thermal (vibrational) energy as atoms at ambient temperature • hydrogen, has nucleus of about same size & mass as neutron and so has much greater thermalising effect on fast neutrons than any other element • method detects mostly H atoms not water per se • single probe containing radioactive source of high-energy neutrons such as radium-beryllium or americium-beryllium or caesium-137 • thermal neutron density easily measured • thermal neutron density may be calibrated against water concentration on volume basis of other sources of H are constant
Time domain reflectometry • measures dielectric constant - ability of soil to transmit electromagnetic (radar) waves - • mostly but not entirely dependent on water
Simple parameters to characterise H2O & O2 availability • Soil water potential • matric potential • gravitational potential • pressure potential
Note on units Soil water potential is the energy density - usually per unit volume Since dimensions of energy is ML2T-2 (force x distance) dimensions of soil water potential has dimensions of ML-1T-2 Pressure is force per unit area so has units of MLT-2/L2 = ML-1T-2 Soil water potential thus has same units as pressure. It can this be expressed as bars, cm H2O, cm Hg, atmospheres SI unit of Pressure, and so energy density, is the Pascal 1 kPa = 10 mb, 1 bar = 100 kPa
Capillarity and adsorbed water combine to produce matric potential
Permanent wilting point • Usually taken as 15 (1500 kPa) bars, but may be more, • e.g. 20 bars (2000 kPa). • Water held between 1500 and 2000 kPa negligible • in virtually all soils. • PWP strongly correlated with clay. • In reality, a dynamic property which depends on: • potential evapotranspiration, • unsaturated hydraulic conductivity of the soil, • type of plant.
Field capacity • the upper limit of available water; • traditionally defined as the moisture content of a soil 48 hours after saturation and subsequently being allowed to drain; • a high proportion of irrigation water added above field capacity is “wasted”; • FC has also been considered to be: • 0.33 bars [33 kPa] in USA or • 0.1 bars [10 kPa] in the UK • FC also sometimes considered as the mean soil moisture • content in winter (cold climates) when the potential • evapotranspiration is small (and so drainage is main factor • governing equilibrium moisture content.
The tension equivalent to FC will be at least equal to the air entry potential - see below. FC, PWP and AWC are strongly dependent on texture, OM and BD
Air capacity Defined as the air content (%) at field capacity. Used in poaching studies. Low air capacity usually means poor aeration. Available water capacity Difference between FC and PWP (%) often x soil depth to give mm
Exercise The moisture content of a soil at field capacity was found to be 27.3% by weight. At wilting point, the moisture content was 19.7%. After oven drying of a volumetric sample, it was found that the bulk density was 1.42 g cm-3. What is the available water capactiy as a percentage of the volume? A crop has a rooting depth of 1.5 m. How much water is potentially available to the crop in mm equivalent. If irrigation is to take place when the AWC is depleted by 40%, how much water would need to be added?
Effect of bulk density on air capacity, wilting point & field capacity
Wet year Any suggestions? Dry year
Dynamic nature of FC, PWP, AWC • It is important to realise that FC, PWP and AWC are • commonly conceived as static soil properties but that • in reality, the are used as proxies for characteristics of dynamic system. • They do not take into account: • field conditions such as underlying horizons; • rainfall and or irrigation frequency and amount; • hydraulic conductivity of the soil; • run-off characteristics; • roots extension; • water infiltration and redistribution; • drainage from soil profile; • some water may drain at the same time as • evapotranspiration takes place; • ground cover changes;
crop height changes • climate, especially evapotranspiration rate effect the • values • Beware of too simplistic a view. • Even so, FC, PWP and AWC are very useful concepts.
Measurement of soil potential Tensiometers After Richards, 1965
Electrical resistance methods • Gypsum blocks • Granular Matrix Sensors • e.g. WATERMARK sensor from Irrometer Co, USA
Relationship of soil water potential to soil vapour pressure If vapour between soil particles is in equilibrium with held water, the vapour pressure is influenced by the “pull” of the soil water ... where : Ytis the sum of matric and osmotic potential is the density of the water at the prevailing temperature, R is the Universal Gas Constant M is the molecular weight of water T is the Temperature (°K) e is the vapour pressure in the soil pores e0 is the saturated vapour pressure of free water at the particular temperature
The phenomenon is used as the basis of: • (a) the determination of the potential of a soil in the • laboratory (often in order to determine the moisture • release characteristics) by allowing a filter paper of • known pore size / moisture release characteristics to • come into equilibrium with the moist air over the soil • which is also in equilibrium with the soil water • potential. • (b) to determine the soil water potential in the field by • determining the humidity of the soil air using a • thermocouple psychrometer
Moisture release characteristics • Determination • pressure plate apparatus • sand; sand/kaolin bath apparatus • filter paper - allow to come into equilibrium and weigh paper • solution - mixture so that vapour pressure is known and this can be equated to soil potential, allow soil to come into equilibrium with solution • use of pF scale
Filter paper method top filter paper not in contact - measures sum of matric and osmotic potential of soil bottom filter paper is in pore contact so measures matric potential