Soil and Water Kelly Young

UA Cooperative Extension

Objectives: • Discuss soil components, texture,

chemistry • Consider relationships between soil,

water, plants and air • Learn about the properties of water • Compare and contrast fertilizers • Suggest soil amendments

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Ideal Soil Composition

Desert soils have significantly less than 5% organic matter. Why?

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• Maricopa County soils: –Mineral –Alkaline –Arid –Probably some caliche –May be rocky and shallow,

particularly in foothills –May be saline –May be heavy

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Soil Texture

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beachball

frisbee

dime

Soil texture affects water movement

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Determine your soil texture

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Soil Texture Triangle

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Soil Compaction

• Compaction reduces pore space: – Restricts H2O and O2 – Poor root

development

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Capillary Action • Cohesion – “like sticking

to like” – Water molecules stick

together

• Adhesion – “sticking to unlike” – Water molecules stick to

certain surfaces

• Capillary action – drawing of water in a narrow tube

Soil-Plant-Air Continuum

Stomata Factors affecting opening and closing: • Light, especially blue light • Water • Temperature • CO2

Water moves down its concentration gradient • Concentration of water is – Highest in soil – High in plant – Low in air

When transpiration > water uptake by roots, wilting occurs.

Water loss • Evaporation – change of water phase

from liquid to gas • Transpiration – evaporation of water

from leaves • Evapotranspiration (Et) – combined loss

of water directly from soil evaporation and transpiration

Factors that affect Et

• Temperature • Relative humidity • Wind speed • Light intensity • Type of plant

Terry’s 1-2-3 Rule Watering depth 1. 1 ft - Flowers, vegetables and other small

annuals 2. 2 ft – Shrubs 3. 3 ft – Trees

Root volume • V = ½( 4/3 π r3) Π ≈ 3.14 r = ½ diameter

If a tree has a 12’ diameter:

r = 6’ The root volume

would occupy approximately

452 cubic feet.

Root volume V = ½( 4/3 π r3)

Π ≈ 3.14 r = ½ diameter

Assume 5 years later the tree has a 20’ canopy: r = 10’ Now, the root volume would occupy

approximately 2093 cubic feet!

Soil Chemistry

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atom

• Cations: – NH4

+, K+, Fe++, Ca++

• Anions: – NO3

-, SO42-

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Cation Exchange Capacity

Clay particle

Organic matter and clay carry a negative charge

Plant Nutrients

Macronutrients • C • H • O • N • P

• K • Ca • Mg • S

Micronutrients • Fe • Zn • Mn • B • Mo • Cu 31

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Nitrogen deficiency

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Phosphorus deficiency

pH is a measure of acidity/alkalinity 34

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Zn deficiency Fe deficiency

• Add nutrients to the soil • Organic and inorganic forms • Usually salts • Can burn plants • Must be watered in

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Fertilizers

Fertilizer analysis N-P2O5-K2O (nitrogen-phosphate-potash)

Complete fertilizer Incomplete fertilizer Slow release fertilizers

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“Organic” versus “Chemical” fertilizers Organic

Manure, compost, fish emulsion, etc. Sometimes recycles waste May be resource intensive Can be bulky, heavy Micronutrients Pathogens, weeds Salt

Chemical Ammonium nitrate, urea, superphosphate, etc. Can be energy intensive to make Lighter weight Salt Must be purchased Salt

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Salinity • Na, Ca, K

and other salts accumulate in soils

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How does soil become saline?

• Shallow watering • Fertilizers • Irrigation water quality • Application of other salty

substances to soil.

Soil Amendments

• Used to modify soil chemistry –Gypsum (Calcium sulfate) –

temporarily removes Na from soil –Soil sulfur – may eventually

reduce pH after many yearly applications

–Organic matter 43

What should be added to the native soil when planting trees and shrubs in the landscape?

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1. Organic matter 2. Fertilizer 3. Organic matter

and fertilizer 4. Nothing, only

native soil should be backfilled into the planting hole.

Possible causes of “unthriftiness”

• High soil salinity

• Root diseases • Root parasitic

nematodes

Questions? Contact me at kyoung@arizona.edu.

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