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Soil-Water-Plant Relationships

Jose F. De Soto

UCCE - Ventura

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Topics to be covered:

•Soils and sustainable soil management

•Water availability and conservation

•Plant – water relationship

•Irrigation methods and systems

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Soil Definitions:

• “The top layer of the earth’s surface suitable for the growth

of plant life” American Heritage Dictionary

• “ The weathered and fragmented layer of the earth’s

terrestrial surface” Introduction to Soil Physics

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Main Soil Components: Mineral Soil Particles

Sand

•Largest particle in the soil

•It feels gritty and rough

•It does not hold nutrients

•It rapidly loses water and dries out

•Plenty of aeration for good plant growth

•Easy to till

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Silt •Size is smaller than sand

•Feels smooth and powdery

•When wet it feels smooth but

not sticky

•Generally resemble sand particles

•Has some of the physico-chemical attributes of clay

Clay

•Smallest of the particles

•Particles fit tightly together

•Does not let water and air go through easily

•It’s smooth when dry and sticky when wet

•Easily molds into a ribbon when wetted

•Holds a lot of nutrients

•Difficult to till

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Soil Texture

•Qualitatively, it represents the “feel” of the soil material, whether coarse or fine and smooth.

•Quantitatively, it denotes the measured distribution or the proportions of the various size ranges of particles that occur in a given soil.

Soil Texture

•It determines water intake rates, water storage, the ease of tillage, and the amount of aeration.

•Soil texture refers only to the mineral fraction of the soil.

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Texture Triangle

•The percentage units (0-100%) of sand, silt, and clay are listed along the sides of the triangle.

http://www.public.iastate.edu/~aros

si/texture%20triangle.jpg

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Soil Structure

•It is the arrangement and organization of the particles in the soil

•There are three broad categories: single grained, massive and aggregated

Soil Structure

•Soil structure affects retention and transmission of fluids in the soil

•Affects germination, root growth, tillage and erosion

•Preferred in a loose and highly porous and permeable condition

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Sustainable Soil Management •Avoid compaction

•Aerate compacted soils

•Prevent erosion

•Use:

-organic matter,

-compost and

-mulches

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Organic Matter •Energy source for soil microbes

•Stores nutrients for plant use

•Humus = Organic portion of the soil remaining after prolonged microbial decomposition. It is the glue that binds soil particles together to prevent soil erosion

Compost

•Considered a soil amendment more than a fertilizer

•Helps restore soil biology

•Well decomposed compost is best:

- reduces tie-up of Nitrogen

- reduces potential for soil borne diseases

Amending soil with compost

•Helps form soil aggregates

•Improves soil structure and soil tilth

•Reduces surface crusting and soil erosion

•Improves air and water movement

Mulches

•Prevent raindrops from splashing on the soil surface

•↑ water penetration, ↓ erosion

•Reduce annual weeds

•Conserve soil moisture ↑ root growth

•Insulate roots from temperature extremes

•Protect plants from mechanical injury

•Improve plant establishment

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Types of Mulch

•Wood Clips

•Compost

•Ground bark

•Rice hulls

•Straw or hay

•Rocks

•Artificial – plastic, ground tires etc.,

Mulch reduces Evaporation from soil but not transpiration

The golden rule of gardening:

•“If you treat your soil well, it will treat your plants well.”

Questions?

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2. Where is the water?

Supply Sources

Los Angeles Aqueduct

Colorado River

Aqueduct State Water

Project

Local

Water Sources for Ventura County

•Ground water

•Santa Clara River (United Water)

•Lake Casitas (Casitas)

•State Water Project (Calleguas)

Water Limitations

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Water Limitations

Other water limitations

• Population growth

• Environmental issues

Key Delta Risks

Seismic Risk Bay Area Faults

0

500

1000

1500

2000

1967 1973 1979 1985 1991 1997 2003

Fishery Declines

Delta smelt

Flooding Risk Jones Tract (2004)

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Questions?

3. Plants and Water

Why plants need water?

1 - Chemical reactions in cells

Why plants need water?

2. Photosynthesis

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6CO2 + 6H2O +

Energy ↔

C6H12O6 + 6O2

Why plants need water?

3. Transpiration:

A plant must lose water

in order to grow

•Water lost through stomata is called TRANSPIRATION

Which is a process similar to

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Evapotranspiration Values

Crop Evapotranspiration

Et crop = Eto x Kc

Where:

Et = Evapotranspiration

Eto = Reference Evapotranspiration, or potential evapotranspiration referenced to grass, when grass Kc = 1.00

Kc = Crop Coefficient

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Crop Coefficients

Alfalfa 0.94

Avocados 0.80

Chiles 0.59

Grass 0.85 – 0.95

Lemons 0.56

Lettuce 0.63

Strawberries 0.87

What do I do with that?

•Total Eto for VC in July was 5.80 inches

•Total precipitation was 0.01 inches

•Avocado Kc is 0.80

How much do I need to irrigate in July?

(5.80 – 0.01) x 0.80 = 4.63 inches

How many gallons is an inch?

• One cubic foot contains 7.48 gallons

Then,

• 1 in/sqft =

7.48/12in = 0.62 gal

• 1 inch/acre =

0.62 gal x 43,560 sqft = 27,007 gal/acre-inch

Following with the example…

•4.63 acre-inch x 27,007 gal/acre-inch = 125,042 gallons

•125,042 gal/100 emitters/10 gal/hr = 125 hrs of irrigation needed to replace the water lost and recharge the soil profile to full capacity

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Landscape Evapotranspiration

Et = Eto x KL

Where:

Et = Evapotranspiration of Landscape

Eto = Reference Evapotranspiration

KL = Landscape Coefficient

Species Factor (0.1 – 0.9)

Density Factor (0.5 – 1.3)

Microclimate Factor (0.5 – 1.4)

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Landscape Example

• Large mature planting of Star Jazmine in full sun and little wind exposure:

• Star jasmine is classified as moderate in the WUCOLS list (0.4 to 0.60) then ks (species) = 0.5

• Is a mature plant with full canopy, average density, then kd (density) = 1.0

• Microclimate is equal to Eto (full sun, open area, no winds) and classified as average then kmc = 1.0

Then KL = 0.5 x 1.0 x 1.0 = 0.5

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Factors that determine ET

•Soil moisture

•Plant type

•Stage of plant development

•Weather factors:

- Solar radiation

- Wind Speed

- Humidity

- Temperature 55

VC Monthly Average Eto

0

1

2

3

4

5

6

Jan Mar May Jul Sep Nov

Ten yr avg

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Watering Index

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Monthly Irrigation Index without Rain

0%10%20%30%40%50%60%70%80%90%

100%

Jan.

Feb.

Marc

h

April

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Pe

rce

nt

of

Ju

ly E

T

Factors Affecting Frequency of Irrigation

• Water infiltration rate

(inch/hr):

Very slow < 0.06

Slow 0.06 – 0.2

Moderate 0.6 – 2.0

Rapid 6.0 – 20.0

Very rapid > 20.0

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Factors Affecting Frequency of Irrigation

• Soil water holding capacity:

- Saturation ( 0 kPa)

Gravitacional water

- Field capacity (-33 kPa)

Available for plant use

- Wilting point (-1500 kPa)

Capillary water

- Oven dry

Factors Affecting Frequency of Irrigation

•Plant water use

•Depth of rooting

•Irrigation method and output

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Monitor Soil Moisture

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Soil sampling tube Soil probe

Use the ‘Feel’ Test

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Dry

Medium

Wet

General principles for watering

•Irrigate established plants deeply and infrequently

•Avoid watering everyday

•Water below the current root system during each watering to encourage deep rooting

General principles for watering

•Don’t apply water at a greater rate than it can be infiltrated

•Water cycling may be necessary to avoid run-off

•Divide the total amount of water required per day into 2 - 4 cycles

•Apply water as close to initial event as possible before soil dries out

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General principles for watering

•Irrigate crop plants for optimum production

•Irrigate landscape plants for good looks and maintenance

That’s all folks !!!

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Any questions?