Lesson Structure:  Irrigation – Crops BHT204

There are 10 lessons:

1. Introduction
2. Soil Characteristics And Problems
3. Estimating Plant And Soil Requirements
4. Drainage – drainage systems, dams, etc.
5. Types Of Irrigation Systems
6. Hydraulics – discharge and flow rates, etc
7. Pumps And Filters
8. Selecting The Right System For The Plant
9. Trickle Irrigation
10. Design And Operation Of Systems

Please Note:  Each lesson culminates in an assignment which is submitted to the academy, marked by the academies tutors and returned with any relevant suggestions, comments, and  extra reading if required.

Learning Goals: Irrigation – Crops BHT204

• Identify and consult appropriate sources of information for the irrigation industry.
• Explain the significance of soil characteristics to irrigation.
• Determine when to irrigate in a small scale situation.
• Explain the drainage aspect of handling water.
• Explain the operation and selection of irrigation systems.
• Explain the operation and selection of trickle irrigation systems.
• Determine specifications for the design of an irrigation system.
• Explain the pumps and filters needed for handling water adequately for crops.
• Supervise the installation of an irrigation system.
• Design and operate an irrigation system for crops.

Soil Characteristics

SOIL STRUCTURE AND IRRIGATION
Soil structure has a great effect on both the soil water status and other aspects of the soil. With a desirable structure, water is held in the soil long enough for the plant to absorb what is needed but the excess drains away fast enough that the plant roots do not suffer any problems associated with being too wet.

There are basically four components to soil:

1. Sand – particle between 0.02 to 2mm diameter
2. Silt – between 0.02 and 0.002mm in diameter
3. Clay – less than 0.002mm in diameter
4. Organic matter – animal or plant material.

The nature of a soil will depend on the relative proportions of sand, silt, clay and organic matter. Soils which are very sandy are usually better for propagating seed or cuttings, but need to be kept well-watered. Sandy soils are suitable for some types of plants while heavier soils are better for other types of plants. The principle behind improving soil structure is based on correcting imbalances in the relative proportions of the four basic components in soil (e.g. to improve a sandy soil add clay or organic matter).

On a small scale adding clay to sand, or vice versa, can be reasonably easy to accomplish. On a large scale (e.g. broad-acre crops), trying to improve soils in this manner would be too expensive to carry out. Addition of organic matter to any soils, except those rare soils already high in organic matter, is generally the best way to improve a soils structure and will also help improve the moisture holding capacity of sandy soils.

Too high a proportion of clay can cause the following:
• Poor drainage
• Small pore spaces (i.e. spaces between the soil particles)
Small pore spaces can lead to:
• Deficiency of air available to roots
• Increased likelihood of soil compaction
• Difficulty with cultivation and weeding
• Slower movement of fertilisers due to them being held tightly by the clay particles and
therefore slower to work in the soil

 Too high a proportion of sand can lead to the following:
• Drainage is too quick and therefore soil becomes susceptible to drying
• Soil does not hold together well and so erosion can become a problem
• Nutrients leach out so plants cannot fully utilise them

Organic matter serves a number of purposes:
• It binds soil particles together but keeps the soil open and prevents compaction
• Restricts erosion to some degree
• Holds moisture in the soil
• As it decomposes it provides nutrients for the plants
• Slows down the rate of soil temperature changes

SOILS FOR DIFFERENT PURPOSES
Different soils have different degrees of fertility and different capacities to absorb and retain water, so an understanding of soils is of major importance to the potential irrigator. Often, little or no attention is paid to soil type when choosing an area to irrigate.

Irrigating a poor soil may be economical in the short term but can be disastrous in the long term. Soils can be characterised according to chemical as well as physical aspects.

Chemical Properties
Chemical aspects of the soil which must be considered are:

1. Soil pH (acidity/alkalinity)
2. Sodicity
3. Salinity
Soil pH
Soil pH is a measurement scale for acidity/alkalinity which goes from 0-14. Soils range between 3.5 (very acidic) to 10 (very alkaline). The “p” in “pH” stands for the negative logarithm to the base 10, and the “H” stands for H+ (the hydrogen ion). Because “pH” is the negative logarithm of the hydrogen ion concentration, a rise of 1 pH unit means a ten-fold increase in alkalinity and a fall of 1 pH unit means a ten-fold increase in acidity (in simple terms, pH is a measurement of the balance between the negatively and positively charged particles in a soil).

Soil pH can be measured in a laboratory with a pH meter on a suspension of 1 part of soil in 5 parts of water.

Field tests may be made using indicator solutions that change colour according to pH.
pH is important because it affects the availability of the various nutrients in the soil. Some nutrients become unavailable in acidic soils whilst others become unavailable in alkaline soils.
All plants prefer to grow within their own particular range of pH.

Some plants only grow well in a narrow pH range but others will tolerate a much wider range. Table 1 gives you the optimum pH ranges of some plants.

Soil pH may be lowered (made more acidic) by the addition of fertilisers such as those containing sulphur (e.g. ammonium sulphate) and may be raised (made more alkaline) by the addition of lime. The pH of irrigation water may interact with the pH of the soil to create a condition which is either good or not so good for plant growth.

Sodicity
Soils high in sodium are slow-draining and remain waterlogged after heavy rain or irrigation. When sodic soils are wetted they break down and their clay disperses. Clay dispersion can be identified by placing a small clod of the soil in a saucer of pure rain water. Within two hours, a cloud of clay particles will appear around the clod.

When Sodic soils dry out they form a very hard surface crust that makes cultivation difficult. Soil sodicity is measured in the laboratory in terms of the percentage of ions held by the soil’s clay particles, which are sodium ions. The total number of ions which can be held by the clay is referred to as the soil’s “Cation Exchange Capacity” (CEC). If sodium is more than 6% of the soils CEC it is a sodic soil, which therefore disperses when wet.

However, if saline irrigation water is used, dispersion can be prevented. A better method, though, is to prevent dispersion by the addition of gypsum Ca3 (S04) 2, lime Ca (OH)2 or other compounds containing calcium. The calcium ions will substitute for the sodium ions held by the clay particles.

Salinity
Excess salt is often found in the soils of dry areas where salt has been redistributed and concentrated by irrigation water.

Danger levels of salt concentration vary with soil texture and plant species. Soil salinity is usually expressed as the electrical conductivity of an extract from a paste of water saturated soil (EC)e which is a somewhat lower concentration than would occur in the soil solution of a wet soil…

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