Unit 10: Soil Water Properties

Unit 10: Soil Water Properties Chapter 3

Objectives • Properties of soil/water that help w/ water retention • Measurement of soil water • Amounts of water held, why is/not held • Characteristics of soil water flow • Effects of saturated, unsaturated soils • Environmental affects • Improving water-use efficiency

Introduction • Most common limit of plant growth • Irrigation has made more land productive • Many roles for water in the soil

Water Chemistry • Peculiar properties of water • Molecule so small, it should be a gas • Highest vaporization temp • Solid phase less dense than liquid • High surface tension • Greatest solute, solvent • Water held in soil due to H bonds • Bonding of water to solid particles = adhesion • Bonding of water to water = cohesion

Water Chemistry • Strong adhesion/cohesion forces cause water films in soils to be held on soil particles • More surface area of a soil > water held

Soil Water Content • Measuring Water Content • Gravimetric method – measure mass water content • Sample – weigh – dry sample – weight again • Time depends on equipment • Measures mass water content • Can also measure soil water w/ volumetric water content

Soil Water Content • Gains & Losses of Water • Measuring soil water volume can help in determining: • Amount of irrigation water needed • Amount of water evaporated • Depth that rainfall/irrigation water will wet soil

Soil Water Potential & Availability Free energy – energy to do work • Soil water has less potential to do work than water molecules in a pool of water • Can’t transport as many materials • Soil Water Potential – work the water can do as it moves from its present state to the reference state, which is the energy state of a pool of pure water at an elevation defined to be zero

Soil Water Potential & Availability • Water Potential Gradient & Water Flow • Soil water moves in response to water potential gradient • Water flows from areas of higher water potential (wetter areas) to areas of lower water potential (dryer areas) = unsaturated flow • Explains water’s ability to move upward w/ capillary action from a water table

Soil Water Potential & Availability • Water movement after rainfall or irrigation moves into & through a saturated soil by gravity • Overrides ability of water to adsorb to soil • Called saturated flow • Soil Water Classification for Water Management • Gravitational water – water that drains freely through the soil by force of gravity

Soil Water Potential & Availability • Field Capacity – measure of the greatest amount of water a soil can store under conditions of complete wetting followed by free drainage • Full saturation minus water lost to drainage • Difficult to determine average field capacity in field situations because water continues to drain & redistribute through soil following rain/irrigation

Soil Water Potential & Availability • Permanent wilting point – water held at PWP held so tight that plants not able to extract it fast enough to meet their needs • Partially explains temporary wilting (rolling) of corn – recovery at night when water transpiration slows • In conditions of true PWP – plant probably won’t recover, unless additional water added

Soil Water Potential & Availability • Plants, Wilting Point, & Available Water • Plants vary in their abilities to extract soil water • PWP - ~40-50% of field capacity • Available water capacity – amount of water that would be available to plants, if the soil were at field capacity • Difference between FC & PWP

Soil Water Potential & Availability • Capillary water & Saturation Percentage • Capillary water – held tightly in small capillary pores by H bonding • Water in minute tubes that will rise through soil matrix to needed areas • Height of capillary rise inversely related to radius of the tube • Smaller pore diameter, greater the movement

Soil Water Potential & Availability • Saturation percentage – water content of the soil when all pores are filled with water • ~ Double the amount of water at field capacity

Soils as Water Reservoirs • Water held as films on particle surfaces • Large soil pores – allow water to drain by gravity flow (sands, large aggregate soils) • Small soil pores – retain water by capillary action • >clay & humus % >water storage ability • Water held in clay soils, held very tightly • Hold large amounts of water at FC & PWP

Soils as Water Reservoirs • Medium textured soils – unique combination of have pores that hold large amounts of water, but not so tight that plants can’t get it • Largest available water capacity found in silt loams & other loamy soils • Soil organic matter, compaction, types of clay affect available water capacity

Methods of Determining Water Content or Potential • Porous Blocks • Can be used in the field to help w/ soil water measurement • Bury at various depths • Electrodes attached • Assists w/ irrigation needs • Capacitance Probes • Neutron Probes • Time Domain Reflectometry

Methods of Determining Water Content or Potential • Tensiometers • Thermocouple Psychrometers • All can perform specific soil water measurements • Predict irrigation needs

Water Flow Into & Through Soils • Saturated Flow • Water flow caused by gravity • Infiltration – water entering soil • Rapid into large, continuous pores • Reduced by anything w/ reduction in pore size • Percolation – water moving through the soil • Can carry away dissolved nutrients & salts • Leaching – removal of soluble compounds in percolating water

Water Flow Into & Through Soils • Rate of water movement controls • % of sand, silt, clay • Which will infiltrate faster? • Which will percolate slower? • Which has highest leaching potential? • Soil structure • Organic matter – improves soil structure, increases #/size of pores • Depth of the soil to impervious layers • Amount of water in the soil – if soil is already wet/dry

Water Flow Into & Through Soils • Soil temp – warm > cold • Compaction – can reduce pore space, decrease infiltration • Permeability – the amount of saturation in the root zone (top 60”) that will affect the amount of water flowing through the soil profile • Limited by least permeable layer in the soil • Major factor in productivity of soil/suitability for development

Water Flow Into & Through Soils • Hydraulic conductivity – commonly used indicator of permeability • Permeability rates: • Impermeable - <.0015”/hr • Very slow - .0015 - .06”/hr • Slow - .06 - .2”/hr • These soils limited for campsites, playgrounds, tillage of ag fields • Moderately slow - .2 - .6”/hr • Soils < moderately slow considered insufficient for septic tank fields & irrigation

Water Flow Into & Through Soils • Moderate - .6 – 2.0”/hr • Moderately rapid – 2-6”/hr • Soils > moderately rapid also not favorable for septic tank fields, wastewater irrigation – doesn’t filter well • Rapid – 6-20”/hr • Very rapid - >20”/hr • Unsaturated flow • Water moves naturally from wetter – drier areas • Movement may not be downward

Water Uptake by Plants • Water Absorption Mechanisms of Plants • Passive absorption – caused by constant pull of water moving through plants • Plant water lost by transpiration • Drier air exerts more atmospheric pull on water, increases transpiration rates • Root extension – expansion/extension of roots into new areas in the soil ability to absorb new water as it is encountered

Water Uptake by Plants • Active absorption – plant expends energy to absorb water • Plant selects specific solubles to absorb • Helps equalize osmotic potential • Accounts for very small part of total water absorbed • Absorption through leaf stomata – plants can take in water from fog, rain, dew

Water Uptake by Plants • Depths of Water Extraction • Most plant water extracted from shallow depths • Depends on: • Saturation of the soil • Soil texture • Plants • Trees will go deep • Grasses remain shallow • Want to encourage roots to get water from deep soils – more drought tolerant

Water Uptake by Plants • When Plants Need Water Most • Visible symptoms of wilt – damage already done • Especially during critical growth periods (flowering to fertilization), rapid size increase • Plants can wilt even when soils are sufficiently wet – if climate is so hot that evapotranspiration rate > absorption rate

Consumptive Use & Water Efficiency • Evapotranspiration (ET) – water lost by evaporation from soil & transpired through plants • Occurs in dry, windy, warm conditions, soil surface moist • Can involve a large amount of water

Consumptive Use & Water Efficiency • Water Use Efficiency (WUE) • WUE – transpiration + plant growth + evap from soil + drainage loss (to produce a unit dry plant wt) • Ex. – soybeans may use ~.5”/d • Want to encourage plant available water to maximize growth by reducing evap losses, excessive drainage losses • Evap loss – keep soil canopied (soybeans) • Drainage loss – proper drainage through fields, waterways, terracing, etc.

Reducing Water Loss • Reducing Evapotranspiration • Mulches • Straw, peat, gravel, etc. • Barriers to moisture moving out of soil • Keep soil temp cooler • Long dry periods – doesn’t necessarily decrease amount of water lost (can actually increase if mulch wicks moisture from ground)

Reducing Water Loss • Fallow • Common in dryland farming • Leave land unplanted in alternating years to accumulate extra soil water • Amount of water saved is small, but enough to justify • Ex - ~4” water needed to produce wheat from seed to maturity • Each additional 1” available water increase yield 4-7 bu/ac

Reducing Water Loss • Reducing Waste & Runoff • Plant selection should carefully match soil’s water characteristics or conserve soil water • Some research into converting brushland to grasslands to help conserve soil water • Grasses root less deeply than brush • Grasses go dormant earlier in fall • Grasses intercept less precipitation, more water infiltrates soil

Reducing Water Loss • More protection from soil erosion • Found to conserve >2” more water/yr • Forests transpire much water • Also intercept rain that’s allowed to evaporate before it can reach soil • Still can’t clear-cut all forests • What consequences would there be?

Reducing Water Loss • Improved irrigation • Closely manage irrigation systems w/ better water controls • Drip irrigation – most efficient use of water, sprinkler irrigation least • Reuse of Wastewater • Municipal treatment plants, industry, irrigation tailwater • Can be high in salts/sediment • Much can be available

Reducing Water Loss • Conservation terraces • Slow water runoff • Catch basins to collect water • Soil organic matter • Positive impact on PWP • Increased organic matter %, increases ability of water to store water

Assignment

Unit 10: Soil Water Properties