Fundamentals of Soil Science

Irrigation

Teacher Objectives:

1) Explain soil and plant relationships in moisture management

2) Describe the benefits and use of irrigation

3) List and evaluate sources of water

4) Explain methods of water application

5) Describe how to make efficient use of water

6) Explain the meaning and use of Chemigation

Reasons to Learn

Why is it important to be able to understand soil moisture management?

Why is it important to identify soil water?

Why is it important to understand the significance of the root zone?

Why is it necessary to identify soil moisture balance and know when to apply water?

Why is it important to be able to understand the uses of irrigation?

Reasons to Learn

Why is it important to know what irrigation scheduling is and its purpose?

Why is it important to understand what quality irrigation water is and why it is necessary?

Why is it important to know what the sources of irrigation water are?

Why is it important to realize the different application methods used in irrigation?

Why is it important to understand what it takes to make efficient use of water?

Why is it important to know what chemigation is and what it is used for?

Soil Moisture

Management:

Water in the soil is absorbed by the roots of plants

Plants may be 90% water Plants lose water to transpiration

The loss of water through the stomata of the leavesThis serves to regulate the temperature of the plant

(liken to human perspiration)

Soil Moisture

Management:

*Moisture is provided by:

Precipitation- the natural way water is deposited on earth rain, snow, etc.

Irrigation- the artificial way water is deposited on earth

Soil Water

Soil moisture- the moisture level found in the soilMoisture is held in three ways:

*-Gravitational-water that moves downward through the soil*-Capillary- water that is held in the pore spaces between

the tiny soil particles*-Hygroscopic- water that is held so tightly by soil particles

that it is unavailable to plants

Soil Water

Available soil moisture- moisture in the soil that is available for plant use

When moisture levels are high plants can easily extract water

As the moisture is used the soil tension increases

Soil Water

Soil moisture tension- the force by which soil particles hold onto moisture

As moisture decreases tension increases due to the lack of moisture

Soil texture influences water availabilitySandy soils hold less water and result in quicker wilting points

than loam or clay soilsSoils high in clay content hold water well but too much water

is also a problem

Root Zone

Root zone- soil level at which roots are found, this level variesSome plants have deep root systems allowing them access to

larger areas for water and nutrientsOther plants have shallow systems allowing access to a limited

area for water and nutrients

Root Zone

Soil moisture management is based on the moisture in the root zone The highest volume of moisture is extracted from the upper part (top 1/4)

of the root zone which provides 40% of the plant available water The lower ¼ of the root zone provides only 10% of plant available water

Soil Moisture Balance

water taken up by plants and lost by evaporation must be restored to the soil by rainfall or irrigation

maintaining good soil moisture balance requires knowledge of the water needs of plants and the current soil moisture level

Soil Moisture Balance

plants require different moisture loads at different times during growth and throughout maturity

Often times the current soil moisture level is in-sufficient for growing plants and will require supplemental irrigation to improve soil moisture level

When to apply Water

Wilting point- point at which plants begin to wilt do to the low level of available moisture in the soil

Occurs quicker In sandy soilsIrrigation should be done before the wilting point Once wilting occurs often time irreparable damage has occurred

When to apply Water

How to determine when to apply water without stressing plants:Ribbon test- small amounts of soil are rolled between the thumb

and finger the feel and appearance of the soil are used to asses the moisture content.

Moisture Sensors- electronic instrument stuck into the soil that sends signals to a computer about the moisture content of the soil

Sap Flow Sensors- measures the movement of water inside the stem of a plant.

When to apply Water

Tensiometer- device that measures the “pull” of soil particles using a vacuum gauge.

Moisture meter- assesses moisture based on the flow of low-level electric current between two electrodes.

Info Sources such as land grant soil laboratories or private soil laboratories

Use of irrigation

Irrigation is an important part of soil moisture management and plant growth and provides other benefits: Provides adequate water that would be otherwise unavailable to improve or

allow plant growth Promotes plant growth by applying fertilizer and growth regulators into the

water

Use of irrigation

Disposes of waste water Protects plants from extreme cold temperatures Reduces dust

Deficiencies in water influence plant growth and damages productivity. It also increases the susceptibility of the plant to disease and pest infestation.

Irrigation and Climate

Climate and weather affect moisture management in uncontrolled environments. Irrigation can have an effect on both controlled and uncontrolled environments Irrigation in a greenhouse is imperative for plant survival Irrigation is necessary in areas that receive only seasonal precipitation in

order for plants to grow and be productive year round

Irrigation Scheduling

Irrigation scheduling- water scheduled to be delivered to plants at specific times when the plant requires Water supply increased before peak need Efficient use of scarce water supply May be mandated by local authorities in very dry regions

Quality Irrigation Water

Quality is extremely important Chemical composition is a factorWater with the lowest amount of harmful chemicals should be

selected

Soluble Salt Problems

Most chemicals in water are soluble salts known as ions. Using water high in salt content leaves the salt residual in the soil. Too much salt is a huge problem. The common chemicals that form salts are:

Calcium- plants need some but not a lot

Magnesium- does not damage plants and they use some of it

Sodium- the single most damaging salt in the soil, a part table salt, can be combatted somewhat by adding gypsum and flooding to leach some of the sodium out of the soil

Chloride- the other part of table salt, damaging and even toxic to many plants

Sulphate- needed by plants but can be a problem if too much is available

Soluble Salt Problems

Salt affects the availability of water to reach plant roots. Too much salt will actually pull water away from roots.

Other Water Contents

May contain non-salt minerals, gases, heavy metals, and other pollutants

Most of these are not a problem but can become problems if the amounts rise to quickly before they can be leached out

Carbonates can become a problem if they are combined with sodium, they become toxic to plants

The accumulation of heavy metals is detrimental to organisms that consume plants grown on these soils.

Pesticide residues, detergents, and run-off is also a potential hazard

Sources of Irrigation

Surface Water Surface water- the accumulated water from rainfall,

melting snow, springs, etc. Large reservoirs are often organized into districts based off of

the districts use history. Acre foot- amount of water needed to cover one acre of

land with 1-ft of water Water rights are important and sometimes worth more than the

land around them

Ground Water

Ground water- water held in spaces between sand, rocks, and other underground formations Involves drilling into an aquifer and pumping water out Less likely to be polluted but may contain salts and other chemicals that can

build up in the soil

Application Methods

The method chosen method should be selected on an economic basis

*Flood Irrigation

*Sprinkler Irrigation

*Trickle Irrigation

*Sub-surface drip Irrigation

*Micro-spray Irrigation

Flood Irrigation

Flood irrigation- the covering of the surface of a field with water and allowing it to soak into the soil.

Border irrigation- small earthen ridges that are used to direct water where it is needed.

Furrow irrigation- a combination of flood and border irrigation where water is diverted down rows or furrows. Flood irrigation is more economical to use on large scale operations like cropping.

Sprinkler Irrigation

Sprinkler irrigation- applying water through the air in droplet form by the use of sprinklers Works well on land that is not level Operates on higher pressure often made by pumps

Sprinkler Irrigation

Types: Portable Side roll wheel move systems Travelling guns Center pivot systems Linear move systems

Low Volume Irrigation

Application of lower water volumes over an extended time uses emitters for precision watering above or below ground

Trickle Irrigation

Trickle irrigation- the application of water to the root zones of plants AKA drip irrigation makes efficient use of limited water More expensive because of the equipment and installation costs Normally used with high value plants

Emitter- a small orifice that releases irrigation water

Sub-surface drip Irrigation

Similar to regular drip irrigation except the emitters are below the surface

Water is delivered directly to the root zone

Micro-spray Irrigation

Sprays low amounts of water Emitters produce a mist Used also to protect crops from freezing

Making efficient use of Water

Irrigation water is valuable and should not be wasted Using water efficiently increases production and

decreases costs and losses Sprinkler irrigation should be done in the cool of the day

with little to no wind

You should monitor moisture at the root zone, once the water reaches this point enough water has been applied

Making efficient use of Water

Systems should be kept in good working condition and be checked regularly faults in the system will cost time and money

Water should be applied uniformly Apply only the necessary amount of water this will add

productivity, slow runoff and slow the accumulation of undesirable compounds

Chemigation

Chemigation- the application of agricultural chemicals mixed with irrigation water Chemical is injected into irrigation water system Can be used with multiple irrigation systems Chemical must be water soluble or stay in suspension Must utilize a backflow prevention device to keep chemicals out of water

source

Summary

*Irrigation is the artificial application of water for plant growth. It adds moisture to the soil making it available to plants. Adequate moisture is particularly important in the root zone.

Summary

*Water needs of plants need to be met to ensure productivity and viability. Irrigation use varies with the climate. In some cases irrigation is the sole source of soil moisture, while in others irrigation is a supplement to natural precipitation.

Summary

*Irrigation scheduling provides plants with needed water at different times depending on plant need and soil moisture. Water high in salt content or other damaging chemicals should be avoided.

Summary

*Irrigation water is usually drawn from two main sources: ground and surface water. Complex systems such as dams, canals, and ditches have been built to carry irrigation water.

Summary

*Water is applied in three major ways: flood, sprinkler and trickle. Each of these systems varies based on plant needs, terrain, and the size of the area being irrigated. These systems can also be used to apply other chemicals, such as fertilizer and insecticide, by chemigation.