Chapter D9. Irrigation scheduling

1 D9. Irrigation SOILpakChapter D9. IrrigationschedulingPURPOSE OF THIS CHAPTERTo explain how to plan and schedule your Irrigation programCHAPTER CONTENTS factors affecting Irrigation intervals influence of soil water using the water balance method for schedulingASSOCIATED CHAPTERS A3 Features of soil D1 Soil examination and structural rating D2 Soil texture tests D7 Cultivation and soil structure THE IMPORTANCE OF DETERMINING YOUR WATER NEEDSAll plants need water to grow and produce good yields. When plantsare water stressed they close their stomata (the small holes in the leafsurface) and cannot photosynthesise effectively.

2 Best growth can beachieved only if plants have a suitable balance of water and air in theirroot zones. Some stages in the growth of a crop are particularlysensitive to moisture shortages sufficient to hinder crop growth can occur withoutproducing obvious wilting of foliage, while waterlogging can causelarge yield reductions too. The grower must therefore rely on someother method of determining the water needs of the crop to avoidproduction or quality losses. This requires an understanding of themovement and storage of water in the root zone of the crop and the rateof water use by the AFFECTING THE Irrigation INTERVALThe interval between irrigations and the amount of water to apply ateach Irrigation depend on how much water is held in the root zone andhow fast it is used by the crop.

3 This is determined by soil texture soil structure/water penetration depth of effective root zone of the soil the crop grown the stage of development of the soils are composed of solid particles of various sizes, organicmatter, and pore spaces that hold air and water. The size of these pores,and the amount of water they hold, depends on the texture and structureof the Irrigation SOILpakSoil textureSoil texture refers to the feel of the soil. There are three types ofparticles that make up the soil; these are classified as sand, silt or claydepending on their size. The proportion of these particles in the soildetermines the feel or texture of the soil and the size of the microporesbetween the particles, as well as the amount of water that can be storedin them (Table D9 1).

4 Soils are classified into texture classes such as clay loams, heavyclays, loams and sandy loams, as determined by the proportion of sand,silt and clay in the D9 1. Available moisture according to texture (mm of plant-available water per metre). Note that this is not a complete list of soiltexture textureRangeAverageSandup to 6549 Loam155 to 172164 Clay loam155 to 172164 Clay137 to 147137water and air can movefreely through the soilinternal drainage is goodwaterFigure D9 1. A well structured soilroot growth isnot restricted See Chapter D2 for moreinformation on soil STRUCTURESoil structure refers to the natural aggregation of soil particles thatare stable when wetted.

5 The size and shape of these aggregates affectthe way they stack together and the size of the pore spaces betweenthem. A well structured soil (Figure D9 1) contains many pores thatD9. Irrigation SOIL pakroot growthis restrictedto cracksinfiltration of water is slow andwater storage is poorwaterloggingis likelyFigure D9 2. A poorly structured soilwill hold water and air and aid infiltration of water into the poorly structured soil (Figure D9 2) has fewer large pores; it willhave a reduced water-holding capacity and poor water infiltration, andwill probably restrict root is clear from Figure D9 1 that a well structured soil will giveplant roots an environment favourable to healthy growth and theextraction of water and structures of many soils have been damaged by years ofcultivation and compaction by traffic; others have been damaged bysodicity.

6 This has led to the development of soil crusting and hardpans,which reduce water penetration and retention and restrict root results in a reduction in plant natural features of a soil may affect its structureindependently of the soil management. For example, a sodic soil islikely to be poorly structured and will restrict air and water movementthrough the WATER CONSIDERATIONSB efore you consider your soil moisture status you need tounderstand a few terms. These include field capacity, wilting point,saturation, available soil water and refill after a soil has been irrigated it is at saturation (FigureD9 3).

7 Almost all of the soil pore spaces are filled with water and verylittle air remains. If the drainage is adequate, water will drain awayfrom the larger pore spaces and allow some air to enter the soil. Thistakes about 48 hours, depending on the soil type. If the internaldrainage of the soil is restricted the soil becomes waterlogged. With noair in the soil, the roots begin to die from lack of capacityOnce the soil has drained by gravity for about 48 hours, water isheld in the pore spaces by surface tension around the soil particle, andlittle further drainage will take place. This condition is called fieldcapacity, and at this time the soil is holding as much water as it canD9.

8 Irrigation SOILpakFigure D9 4. Soil at field capacity (good balance of waterand air)thick layer of water held to the surface of soil particleswater totally fills soil poresFigure D9 3. Saturation (soil contains no air)soil particles(Figure D9 4). In soil that is well structured there is also enough air inthe soil to supply plant roots with the oxygen needed for them to liveand pointAs plants use water from the soil the roots are working against thesurface tension that holds the water in the soil. That is, the roots aresucking water from the pore spaces within the soil. Naturally, plants useD9. Irrigation SOIL pakthe most easily extracted water first, and as the soil dries out they mustwork harder to get water.

9 Water is extracted from the soil until a point isreached when the plant s root system can no longer obtain water fromthe soil. This is called the wilting point for that soil (Figure D9 5). Thesoil is not totally dry, but the remaining water is held so tightly that theroots cannot extract it. As a soil approaches wilting point the plantgrowth slows. Plants do not grow in a soil at wilting point and will dieif moisture is not replenished. Irrigated agricultural crops should neverbe allowed to approach the wilting D9 5. Soil at wilting point (no water available to plants)Available soil moistureThe amount of water held in the soil between the field capacity andthe permanent wilting point is known as the available soil amount of water in a soil can be expressed as a percentage, or inmillimetres per metre of soil.

10 For example, a soil with 15% water has150 mm of water per metre of available water-holding capacities vary within each textureclass due to variations in soil structure and soil organic matter. A soilwith better structure and more organic matter will have more pores ofthe right size to hold plant-available water than poorly structured matter retains water and binds soil particles to improve pointIdeally, aim to keep the water content of the soil close to the fieldcapacity for as much of the season as possible, without saturating thesoil for periods of greater than 15 hours. A useful strategy to avoid cropstress is to define how much water your soil holds between fieldcapacity and wilting point and then aim to replenish the water stored inD9.