THE WATER-SOIL-PLANT RELATIONSHIPRELATIONSHIP

“Maximizing Avocado Production” Maximizing Avocado Production Series

Presented by—Gary Bender and Mary Bianchi

Water• Essential for all plant lifep• Important in mineral transport• Principle medium for chemical processesPrinciple medium for chemical processes

that support plant metabolism• Acts as a solvent for dissolved plant sugars p g

and minerals transported throughout the plant• Under pressure provides physical support

h lto the plant• Through transpiration, provides cooling to

maintain favorable temperatures necessarymaintain favorable temperatures necessary for plant functions.

LightAssimilation

Photosynthesis Water Status

Temperature

Growth ReproductionMaintenance Protection

Fruit Productivity

Shoot & FlowersRoots

RespirationNutrient Movements

RepairDefense

Productivity

Production Model:Which things can’t be changedg gWhich things can be changed

Slide courtesy of M. Arpaia and R. Heath, UCR

Water Deficit ConsequencesConsequences

From U of Florida IFAS Extension

lower leaf surface of ‘Hass’ avocado leafControls gas exchange in & out of leaves

t

[CO2]in

[CO2 ]out stomataopen = easy movementclosed = no movement

[CO2]in[H2O ]in

[H O ]

Bulk Air Boundary Air Internal Air

leaf[H2O ]out epidermis

Slide courtesy of M. Arpaia and R. Heath, UCR

Transpiration• Water vapor is released from the stomata. • This creates a gradient in the water conducting

vessels. Water is “pulled’ up through the plant to replace the– Water is pulled up through the plant to replace the water lost from the leaves.

• Guard cells regulate the size of the stomatalfopening, and rate of water

vapor loss;• Transpiration helps to coolTranspiration helps to cool

the leaves.

Avocado• Evolved in cool/moist and warm humid subtropicsp

– Able to grow in semi-arid CaliforniaSoil moisture must be adequate

– Efficient control of water loss from leaves• Waxy leaves and fruit• Dense hairs on young leaves

– Seasonal soil moisture requirements for different climatesdifferent climates

– Dry soil moisture conditions can lead to fruit loss at critical stages g

Evapotranspiration• In addition to water loss through theIn addition to water loss through the

stomata, water is also lost directly from the soil through evaporation.

• A smaller amount of water can also be lost as evaporation from plant surfaces other than the stomataother than the stomata.

• The combined water lost through the stomata, soil and plant surfaces is called , pevapotranspiration

Ratio of evaporation and transpiration changes—transpiration changes

through the life of the orchard

At t R diAtmometer ReadingsSLO County Spring 2009

Thi t idThings to consider—Applied Water

• 80-90% of applied water can be transpired.

• 10% of applied water is• 10% of applied water is lost through evaporation from the soil.

• Irrigation efficiency >10% may be lost by leaching below the root zonebelow the root zone.

• Additional water is necessary to leach the salts out of the root zone.

Soil• Four componentsp

– Minerals, organic matter, water (with dissolved minerals) and air.

• The percentage of these components vary greatly depending on the soil texture and str ct restructure.

• An active root system requires a delicate balance between these four componentsbalance between these four components, but the balance between the liquid and air phase is most critical.

Solids in Soils• The solid portion of the soil consists of:• The solid portion of the soil consists of:

– Sand– SiltSilt– Clay– Organic matter

• Soil texture is classified according to the relative quantities of sand, silt and clay.

S il T t T i lSoil Texture Triangle

USDA-NRCS

Organic Matter• Plant and animal residues, living soil

organisms, and materials they synthesize. • In an avocado grove, largely decomposing

l d b h f th d tleaves and branches from the avocado tree itself.

• Organic matter in Western USA soils tends• Organic matter in Western USA soils tends to be low (1-2% organic matter).

• Most groves can benefit by adding additional g y gorganic matter to the soil

• Adding organic matter improves the water and nutrient holding capacity of the soil.

Soil Organic Matter• Helps strengthen soil aggregates, improving p g gg g , p g

soil structure• Improves aeration and water infiltration• Increases water-holding capacity• Improves cation-exchange capacityp g p y• Provides buffering in soil when acid or

alkaline forming materials are added• Provides a source of some plant nutrients• Provides a food source for soil

microorganisms

Organic matter is now availablefrom local composting operations

Cation Exchange Capacity

• Clay and organic particles have negative p gcharges around the edges which attract positively-chargedpositively-charged nutrients, such as calcium, potassium

d iand magnesium.

UC ANR Publication 3382

Sand• Sand particles are larger components of a soilSand particles are larger components of a soil

Particle size ranges from 0.05-2.0 mm.

– Sandy soils can still be highly erosive (single y g y ( ggrained)

• Sandy soils have lower cation-exchange capacity (CEC) of any soil texture– lower nutrient holding capacity

• Total porosity and pore arrangement determine water holding capacityholding capacity

• Sandy soils drain rapidly

• Good aeration• Oxygen available yg

to roots. • Clays have

more pores

Silt• Particle size between sand and clay and y

generally between 0.002-0.05 mm in size• Primary mineral, like sand

– Quartz, feldspar, hornblende

– Slight negative charge, like clayg g g y

• Silt is the component most often deposited by flooding– Sands falls out, clays stay suspended

Clay• Clay is the smallest soil component andClay is the smallest soil component and

defined as being smaller than 0.002 mm.• Greater water holding capacity because g p y

of smaller pore spaces.• Because of it’s greater water holding g g

capacity, clay can also be much slower to drain than other soil types.

• Saturated clay soils– can have reduced oxygen content which is

h f l f iharmful to root function.

Water Holding Capacity of SoilCapacity of Soil

Inches of Water Held per Foot of SoilSand 0.5-0.7Sand 0.5 0.7Fine Sand 0.7-0.9Loamy Sand 0.7-1.1Loamy Fine Sand 0 8 1 2Loamy Fine Sand 0.8-1.2Sand Loam 0.8-1.4Loam 1.0-1.8Silt Loam 1.3-1.8Clay Loam 1.3-2.1Silty Clay 1.4-2.5y yClay 1.4-2.4

Soil Texture Plant-available t f t

Gallons of t biwater per foot

of soil depthwater per cubic

foot of soil

Sand 0 5 1 0 0 33 0 66Sand 0.5 – 1.0 0.33 – 0.66

Sandy loam 1.0 – 1.5 0.66 – 1.00

Clay loam 1.5 – 2.0 1.00 – 1.33

Cl 1 5 2 5 1 00 1 66Clay 1.5 – 2.5 1.00-1.66

Maximum Water Application RateApplication Rate

Maximum Rate of Irrigation -Inches per Hour, Bare SoilSand 0.75Fine Sand 0.6Loamy Sand 0.5yLoamy Fine Sand 0.45Sand Loam 0.4Loam 0.35oa 0 35Silt Loam 0.3Clay Loam 0.25Silty Clay 0 2Silty Clay 0.2Clay 0.15

Soil• Has a profound impact on plant productionHas a profound impact on plant production.• Maximizing yields are largely dependent on

optimizing cultural practices specific to the p g p ptype of soil present.

• In most avocado groves, there are differences in soils in different areas of the grove.

• In one two-acre UC test plot in Valley Center there were 5 different soil typesthere were 5 different soil types

The Soil-Water RelationshipS t t d S il ll th fill d• Saturated Soil—all the pore spaces are filled with water.

• Plant limiting roots need oxygen to respire• Plant limiting—roots need oxygen to respire.• Drainage of water due to gravity—dependent

on soil texture and structure and their effecton soil texture and structure and their effect on pore space.

• Field capacity, the water is retained in the soil pores through both cohesion and adhesion.

Plant Available Water

UC ANR Publication 3382

Water Availability• After gravity has drained the larger pores, g y g p ,

the remaining water is held within the soil by a force known as adhesion between the soil particle and water and cohesion ofsoil particle and water, and cohesion of water particles to each other – Field Capacity

• More than 90% of plant available waterMore than 90% of plant available water is held at higher soil moisture levels

• As the water is extracted from the soil, the thickness of the film decreases which requires more energy to remove.Pl t d t ti t il t t ti• Plant adaptations to soil water extraction.

Water Availability• The permanent wilting point of a soil isThe permanent wilting point of a soil is

defined as the point where a plant is incapable of extracting any more water from the soilfrom the soil.

• Hygroscopic water is very closely bound to the soil particle and not available toto the soil particle and not available to the plant.

Available Water

TensiometerT b ith l• Tube with a porous clay cup, attached to a vacuum gauge.

• The whole system is filled• The whole system is filled with water and sealed tightly.

• Installed in the soil in theInstalled in the soil in the wetted rootzone, effective from 3 cb to 50 cb.

• They break suction in dry soil.

How a Tensiometer Works

Gypsum Blocks• Blocks are buried in the

soil with wires leading to the surface. A il i t h• As soil moisture changes the moisture in the blocks changes.

• When soil is wet the electrical resistance i lis low.

• As the soil becomes dry the resistance indry, the resistance in the block increases.

Comparison of Two MethodsTensiometer Gypsum Block• Effective for soil

moisture from 0-50 cb, but will break suction

• Read by a meter that is small and portable, can have manybut will break suction

and fail if the soil gets drier than 80 cb.

can have many stations.

• Inaccurate in highly• Re-fill as needed and

pump to remove air bubbles

Inaccurate in highly saline soils and very sandy soils.

bubbles• Should be pulled out of

the ground once a year

• Blocks need to be replaced every two to three yearsthe ground once a year

for maintenance.to three years.

Other Methods for Soil Moisture MeasurementMoisture Measurement

• Neutron Probes. – Radioactive source in a tube that is lowered

i t th d N t itt d l dinto the ground. Neutrons emitted slow down as they collide with hydrogen atoms in the soil. The slower neutrons are counted by the ydetector unit.

Accurate but Expensive

• Thermal Dissipation Sensor– Thermal Dissipation Sensor measures soil

water by measuring dissipation of heat from ate by easu g d ss pat o o eat oa porous ceramic block or disc.

High cost and careful calibration of each sensor.

Other Methods for Soil Moisture Measurement Moisture Measurement

• Time Domain ReflectometryG t hi h d i l th t

-continued

– Generates a high-speed microwave pulse that travels down a transmission line buried in the soil.

– The velocity of the pulse in the soil is determined– The velocity of the pulse in the soil is determined primarily by water content.

– The returning pulse is detected, and via a g p ,microprocessor, travel time of the wave is used to calculate water content of the soil.

• A function of hydrogen in solution with an Soil pH

y gacid soil having more hydrogen ions.

• Measured on a scale from 1 through 14, ith 7 b i t l 1 b i hi hl idiwith 7 being neutral, 1 being highly acidic,

and 14 being highly alkaline. - A pH of five is ten times more acidicA pH of five is ten times more acidic

than a pH of six • Affects nutrient availability, microbial

activity and the solubility of toxic ions.• Nutrient availability is greatest for avocado

at a pH of 6 0 to 6 5at a pH of 6.0 to 6.5.