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Lesson 2.

SOIL CHARACTERISTICS Aim: Explain the significance of soil characteristics to irrigation. UNDERSTANDING SOILS 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 through being too wet. There are basically four components to soil:

• SAND - particles between 0.02 to 2mm in diameter • SILT - between 0.02 and 0.002mm in diameter • CLAY - less than 0.002mm in diameter • ORGANIC MATTER - animal or plant material.

Too HIGH a proportion of CLAY can cause the following:

• Poor drainage • Small pore spaces (i.e. spaces between the soil particles) • This can lead to deficiency of air available to roots • Soil compaction becomes more likely • Cultivation and weeding can be more difficult • Fertilisers move slowly; they can be held tighter by the clay particles and can

become slower to work in the soil. Too HIGH a proportion of SAND can lead to the following:

• Drainage is too quick; soil becomes susceptible to drying • Soil does not hold together well; erosion can be a problem • Nutrients leach out; plants cannot fully utilise them.

Organic matter serves a number of purposes:

• Binds soil particles together, but keeps 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.

NAMING THE SOIL Soils are usually named according to textural quality. Work through the following steps to classify the soil.

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1. Place a small quantity of soil in the palm of your hand and add just enough water to make it plastic. If it doesn't stain the fingers, does not bind together and is gritty to feel, it is a sandy soil. You should be able to distinguish by the amount of grittiness, whether it is coarse sand, medium sand, fine sand or very fine sand. 2. If it does not stain the fingers, but can be rolled together into a ball that barely adheres together, it is loamy sand. 3. If it forms a more solid ball that can be rolled into a cylinder, but breaks when the cylinder is bent, and if it still feels gritty, it is a sandy loam. 4. When the cylinder is bent gently, it doesn't break and if there is no feeling of grittiness, silkiness or stickiness, it is a loam. 5. If it is similar to a loam, but there is a silky feeling, and if it cannot be polished by rubbing, it is a silty loam. 6. If the silky feeling is very strong, but otherwise it is like a silty loam, then it is silt. 7. If it is like a loam, but is sticky and can be polished, it is a clay loam. 8. If it shows the characteristics of a clay loam, but when squeezed, also has a gritty feeling, it is a sandy clay loam. 9. If, instead of being gritty, it's silky, but otherwise like a clay loam, it is a silty clay loam. 10. If the characteristic of stickiness is stronger than anything else, it is clay. 11. Organic soils have a large proportion of organic matter (more than 25%). These are usually black or brown in colour and feel silky. It is possible to get organic types of all of the above soils. DIFFERENT SOILS ARE SUITED TO DIFFERENT PURPOSES 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). IMPROVING SOILS Before deciding how to, or even whether it is necessary to improve a soil, you need to know whether a soil is good, poor or something in between. Three important qualities to consider are drainage, nutrition and structure. Drainage can be tested easily by observing the way in which water moves through soil placed in a pot and water. Soil nutrition is (to some extent) indicated by the vigour of plants growing in a soil.

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Soil structure usually changes from the surface of the soil as you move deeper down into the earth. Surface layers frequently drain better; drainage rate decreases as you get deeper. This natural change means that water moves quickly away from the surface of soil but slows down its rate of flow as you get deeper. Poor cultivation procedures in soil can destroy this characteristic of a gradation in soil structure through the soil profile. As a general rule:

• Adding sand to clay soils will improve drainage. • Adding clay or organic material to sandy soil improves the soil's ability to hold water. • Adding organic matter while improving water-holding capacity doesn't affect

drainage to the same degree as clay. • Adding sand or organic matter helps break up the soil, making cultivation easier. • Adding organic matter usually improves the nutritional status of the soil.

On a small scale adding clay to sand, or vice versa, can be reasonably easy to accomplish. On a large scale 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. Different soils have different degrees of fertility and different capacities to absorb and retain water. Therefore, 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 OF SOIL 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 aspects of the soil which must be considered include:

• Soil pH (acidity/alkalinity) • Sodicity • Salinity

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1. Soil pH Optimum pH Ranges of Some Crops

FLOWER CROP PLANTS Carnation 6.0-7.5 Orchids (most) 4.0-5.0 Chrysanthemum 6.0-7.5 Rose 5.5-6.5 Daffodil 6.0-6.5 Stock 6.0-7.5 Iris 5.0-6.5 Sweet William 6.0-7.5 Lavender 6.5-7.5 Zinnia 5.5-7.5

GRASS CROPS Turf Grass (general) 6.0-6.5 Turf - Clover 6.0-7.0 Grass - Bent 5.5-6.5 Turf - Fescue 6.0-7.5 Grass - Rye 6.0-7.0

VEGETABLES Beans 5.5-7.5 Ginger 6.0-7.5 Beets 6.5-7.5 Lettuce 6.0-7.0 Broccoli 6.0-7.5 Melon 6.5-7.5 Cabbage 6.0-7.0 Onion 5.8-6.5 Carrot 5.5-6.5 Peas 5.5-6.5 Cauliflower 6.0-7.0 Potato 5.0-6.5 Celery 5.8-7.0 Pumpkin 5.5-7.0 Cucumber 5.5-6.5 Sweet Corn 5.5-7.0 Eggplant 5.5-6.5 Tomato 6.0-7.0 Garlic 5.5-8.0 Water Melon 5.0-6.5

FRUIT Apple 5.5-6.5 Nectarine 5.5-7.0 Apricot 6.0-7.0 Orange 6.0-7.0 Avocado 5.5-7.0 Passionfruit 5.0-6.0 Banana 6.0-7.5 Peach 6.0-7.5 Cherry 6.0-7.3 Pear 6.5-7.5 Grapefruit 6.0-7.5 Pineapple 5.0-6.0 Lemon 6.0-7.0 Strawberry 5.0-6.5 Lime 6.0-7.5 Tangerine 6.0-7.5

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.

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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. 2. Soil 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. If the soil is sodic a cloud of clay particles will appear around the clod within two hours. When sodic soils become very dry a very hard surface crust forms which makes cultivation difficult. Soil sodicity is measured in the laboratory in terms of the number of sodium ions held by the soil's clay particles, expressed as a percentage of the total ions held. 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 soil's CEC, it is a sodic soil that 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. 3. Soil Salinity Excess salt is often found in soils of dry areas where salt has been redistributed and concentrated by irrigation waters. Danger levels of salt concentration vary with soil texture and plant species. Soil salinity is usually expressed as the electrical conductivity (ECe) of an extract from water saturated soil paste which is a somewhat lower concentration than would occur in the soil solution of a wet soil. Below ECe2, effects on crops are negligible, between ECe2 and ECe4, sensitive crops are affected and between ECe8 and ECe16, only salt tolerant crops yield satisfactorily. It is also important to know the salinity of the irrigation water and ground water. If the ground water is salty, irrigation water percolating through the soil will raise the level of the ground water so that it is either intercepted by, or rises within the capillary fringe range of the soil surface. Evaporation is then likely to increase the concentration at the soil surface to a level toxic for plant growth.

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Tolerance of Plants to Soil Salinity: Electrical Conductivity of Saturation Extract at 10 and 50% Yield Reduction CROP uS/cm CROP uS/cm 10% 50% 10% 50% HIGH TOLERANCE Barley (grain) 10000 18000 Cotton 9600 17000 Couch Grass 8500 14700 Sugar Beet 8700 15000 Perennial Rye 6900 12200 Garden Beet 5100 9600 MEDIUM TOLERANCE Wheat (grain) 7400 13000 Safflower 6200 9900 Phalaris 6900 11100 Olive 3800 8400 Cantaloupe 3600 9100 Tomato 3500 7600 Lucerne 3400 8800 Cocksfoot 3100 9600 Cabbage 2000 7000 Potato 2500 5900 LOW TOLERANCE Grape 2500 6700 Grapefruit 2100 4900 Orange 2300 4800 Lemon 2300 4800 Apple 2300 4800 Pear 2300 4800 White Clover 2300 5700 Peach 2200 4100 Apricot 2000 3700 Avocado 1800 3370

Note 1: Crop figures are readily available, but specific figures for ornamental plants are much more difficult to obtain. Note 2: For satisfactory germination, beets require an (EC)e of not more that 3000 uS/cm - and rice, wheat and barley not more than 4000 to 5000 uS/cm. The following table includes plants which will provide a further guide to particularly salt tolerant plants (e.g. for use as windbreaks, or in gardens where salinity may be a problem): Plants Very Tolerant to High EC's Acacia longifolia var. sophorae Acacia pulchella Araucaria heterophylla Atriplex sp. Banksia sp. Callistemon citrinus Carpobrotus sp. Casuarina distyla Casuarina stricta Coprosma repens Correa alba Cortaderia sellowiana Eucalyptus camaldulensis Eucalyptus spathulata Hibbertia scandens Leptospermum lavaegatum Lippia canescens Melaleuca armillaris Melaleuca nesophila Pandanus sp. Phoenix dactylifera Rhagodia sp. Tamarix sp. Yucca sp.

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PHYSICAL PROPERTIES OF SOIL Physical aspects of soil which must be considered include:

• Soil uniformity • Available soil water capacity • Infiltration • Internal drainage

1. Soil Uniformity The more uniform the irrigation area, the easier it will be to manage. Problems of patchiness can arise, for example, in hand forming when a cut is made into subsoil that has more clay, or more sand, than the topsoil. So, it is necessary to level the land and examine the soil to the depth of the maximum cut to determine the likely effect on soil texture. 2. Available soil water capacity A high available soil water capacity is desirable for crops because the higher it is, the less likely the plant is to become stressed between irrigations. The best soils are generally loams or clay loams. The soil needs to be examined to the likely maximum rooting depth both to determine the available soil water capacity and to ensure there is no bedrock, hardpan or other root impeding layers. 3. Infiltration This refers to process of water entry into the soil. It is influenced by: a) Soil type and soil texture. Sandy soils generally have higher long term water penetration rates than clayey soils. b) The condition of the surface soil. Water will enter faster if the soil surface is friable and open or is extensively and deeply cracked. Compacted or crusted soil with few cracks has a low infiltration rate. c) The stability of the surface soil. Low water stability means that the soil crumbs do not stay together when wetted. Low water stability results in slow water penetration unless the soil is sandy. Also, it often results in the formation of a surface crust as the soil dries which will reduce infiltration at the next irrigation. d) Depth of soil above an impermeable layer. The soil may consist of light loam topsoil over a clay subsoil or bedrock. In this case, water up over the impermeable layer reduces water penetration. 4. Internal Drainage Poor internal drainage can result in temporary water logging and loss of productivity and will eventually cause permanent water logging and high salinity levels if irrigated. Conversely, soils with excessive drainage are undesirable because of the large amount of irrigation water necessary to keep them moist. Thus a slowly permeable clay layer at a depth of one metre is desirable. Observations of the effect of heavy rain on bare soil can indicate how well a soil will behave under irrigation. Does pond formation occur quickly? Does the surface soil structure remain the same when it dries out or does a crust or blocky layer develop? In order to determine the physical aspects which make a soil suitable for irrigation, one needs knowledge of soil composition, texture, structure and the moisture characteristics of soils.

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SOIL AND WATER Before designing or operating an irrigation system it is important to have an understanding of the relationship between soil, water and plants. Soil is made up of a framework of solid material comprising particles of varied sizes and shapes which fit together imperfectly to provide a complex system of pores and channels which provide space within the soil for air and water. When these pore spaces are filled with water, the soil is saturated. This may occur after irrigation or rainfall. A soil will only remain saturated if it is below the water table and cannot drain freely. The amount of water a soil can hold at saturation depends on the volume of pore space available. This is known as the saturation capacity of the soil. KINDS OF SOIL MOISTURE Soil moisture can be grouped into three types of classes:

1. Gravity water, which is the water that can only remain in the soil for a short time before it drains out under gravity.

2. Capillary water, which is the main source of water for plant growth. This occurs as thin films of water on the soil particles or as droplets in the pore space and is held in place by surface tension. When gravity has removed all the free water a balance is reached where the surface tension binds all the remaining water so that gravity is insufficient to remove it. This condition is known as field capacity.

3. Hygroscopic water, which is a thin film held very firmly to the soil particles so that plants can't remove it.

1. Gravitational water This is the water found in the larger spaces (pores) between soil particles. This water will drain away from the soil under the influence of gravity, providing there is somewhere for it to drain. • The flow of gravitational water out of a soil may be inhibited by a barrier such as a hard

pan or a high water table. • The flow of gravitational water out of a soil takes less than one day in sandy soil and

three to four days in clay soil. • Because this type of water disappears from soil quickly, it is not normally included in the

water available for use by plants. • A soil is said to be saturated when it contains the maximum amount of gravitational

water possible. Saturation capacity is therefore equal to porosity. As such, porosity can be calculated using the following formula. NB: Porosity is defined by Hartmann et al. as being "The percentage of the total bulk volume of a soil not occupied by solid particles". (Ref. Hartmann, H, Kofranke, A, Rubatzyk, V, and Flocker, W, 1988. Plant Science: Growth, Development and Utilization of Cultivated Plants. Prentice Hall) Air-filled porosity = V - M2/2.65 - (M1 - M2) x 100 volume % V Where: V = volume in cubic centimetres (= ml) M1 = weight of soil after sampling (= g) M2 = weight of soil after drying (g) (overnight at 100 to 105 degrees C, or broken up and air dried for a few days) Volume of solids = M2 (ml) 2.65

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This is for mineral soils containing only a few percentages of organic matter. (NB. 1 gram water = 1 ml of water) Air-filled porosity of a potting mix = Volume of water drained (ml) x 100 (volume %) Volume of mix (ml) Where: the volume of the mix is measured at saturated capacity For example: 120 ml of water drains from 600 ml of mix. Air-filled porosity = 120/600 x 100 = 20 volume %. That is, 20 % of the volume of the mix was air immediately after it had stopped draining. (Source: Handreck, K and Black, N, 1986. Growing Media for Ornamental Plants and Turf. New South Wales University Press) 2. Capillary water This refers to the water held in the pores between the soil particles by the force of surface tension on the individual particles.

• The soil is holding the maximum level of capillary water when all the gravitational water has drained away.

• Capillary water is the main type of water used by plants. 3. Hygroscopic water This refers to an extremely thin layer of water held by forces of adhesion between soil particles and water molecules.

• It is not usually available to plants even when the soil is air dry. There are some plants such as xerophytic desert plants that are capable of extracting such water.

• Hygroscopic water moves under the influence of gravity as a vapour. • The hygroscopic percentage constant is defined as the moisture content of the soil

when it is about air dry. It can only be removed by drying the soil in an oven, if this constant is required for calculations.

TRANSPIRATION AND WILTING POINT Plants use water through a process known as transpiration whereby the plant acts as a pump, drawing water against the forces holding it in the soil, into the plant roots and transporting it through the stems to the leaves where it is lost to the atmosphere via evaporation, an important factor which is influenced by climatic features such as temperature, humidity and wind. If temperature and evaporation are high then a plant will require more water from the soil than when they are low. Free water is readily utilised by plants, however increasing suction is required to remove the water held by surface tension. When plants reach a stage where they can no longer draw enough water to provide for their needs then the plant may begin to droop.

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This is known as the wilting point. If water becomes available at this stage they will recover, however if the plant continues without water they will reach a point at which they will not recover. This is known as permanent wilting point. The difference between permanent wilting point and the soil moisture content, or field capacity, is known as the available water and is thus the water available for plant needs. The amount of water held in a soil and the amount that is tightly bound will vary from soil to soil. The following table indicates typical soil moisture quantities for some different soils expressed as a percentage by weight of dry soil.

SOIL TYPE SATURATION FIELD CAPACITY

PERMANENT WILTING POINT

AVAILABLE WATER

Fine Sand Sandy Loam Silty Loam Clay Loam Clay

15-20% 20-40% 30-50% 40-60% 40-70%

3-6% 6-14% 12-18% 15-24% 25-45%

1-3% 3-8% 6-10% 7-10% 12-20%

2-3% 3-6% 6-8% 8-14% 13-20%

A FEEL-TEST FOR ESTIMATING SOIL MOISTURE LEVEL It is possible to gain a reasonable understanding of a soil's water capacity by taking a small sample and rolling it between your fingers. Table: Feel Test Characteristics of Soils

How Moist? What It Feels Like % Moisture of Field Capacity

Dry

Powdery and dry 0%

Low Crumbles and doesn't adhere into a ball, even loosely

Below 25%

Reasonable Crumbles but will adhere in a ball

25 to 50%

Good Adheres into a ball with a little pressure

50 to 75%

Excellent Forms a pliable ball which can be rolled into a cylinder

75 to 100%

Too wet When squeezed, water drips from the soil

Over 100% (Over field capacity)

FERTIGATION Fertigation is the process of fertilising and irrigating at the same time. This is done with the aid of fertiliser injectors. When preparing the concentration for the fertiliser required, the usual process is to make up a stock solution which is then injected into the irrigation water through a ‘proportioner’. It is important to ensure that no precipitant in the stock reaches the injector. A common injector dilution ratio is 1:200.

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Water supply authorities require a physical break-check valve or a tank between the injector and the water supply. To supply the fertiliser/irrigation to the plants the usual methods include either hand held hoses, or application through drippers or emitters. It is extremely important to remember that drippers can block hence filters are important to ensure no precipitant is carried through the water to block the emitters. Overhead irrigation is regarded as the preferred and cheapest way to apply irrigated water and liquid feed to large numbers of plants in small pots over a large area. When applying liquid feed, irrigate until you know that the pots have received the required amount of feed. The following manufacturer data sheets are examples of different types of fertigation equipment that is available:

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SET READING Refer to, and read any reference material you have access to that relates to the aim of this lesson. This may include any of the following:

• Books in your own possession, or which you find in a library • Periodicals you have access to (i.e. magazines, journals or newspapers) • Websites

Spend no more than 2 hours doing this.

SELF ASSESSMENT Perform the self assessment test titled ‘Self Assessment Test 2.1.’ If you answer incorrectly, review the notes and try the test again.

SET TASK 1. Phone your local government Agriculture Department and find out what physical and chemical soil tests they conduct to determine the suitability of a soil for irrigation (with respect to irrigated crops if you cannot get details for gardens). 2. Carry out a simple soil moisture-holding test that can be done in the field (i.e. use the feel test in your lesson notes). Test at least three different samples of soil. Record your results. 3. Contact a company that deals with fertigation equipment. Ask to be sent brochures or technical information on their products. How do they work? What are their benefits?

ASSIGNMENT Download and do the assignment called ‘Lesson 2 Assignment’.