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Colorado State University Master Gardener - Soil Water Holding Capacity and Irrigation Management Page 5 of 7 <br />These textbook figures snake a good starting point to understand irrigation management. Most automatic sprinkler <br />' systems are set to keep the lawn green in the summer. (i.e., set for the higher summer water need). Without <br />seasonal adjustments on the irrigation controller the lawn will be over-irrigated In the spring and fail by about <br />' 40%. This springtime over-irrigation Is a primary contributing factor to iron chlorosis. <br />S <br />Other factors influencin iirr! , anon ;< ana 4i: ent <br />i <br />Other factors also have a direct influence on the actual water holding capacity and irrigation demands, for <br />fgy example:. <br />! . Soil organic matter content - Since organic matter holds over ten times more water than sand, a sandy <br />i <br />soil with good organic content (around 4-5%) will hold more water than indicated in the table above. Over <br />time, clayey soils with good organic content may have an Improved soil structure, supporting a deeper <br />rooting depth. <br />Previous irrigation pattern - Plants adjust rooting depth (to the extent that soil oxygen levels allow) to <br />! where soil water is available. Frequent irrigation eliminates the need for plants to develop a deep rooting <br />' system. A shallow rooting system makes the plant less resilient to hot, dry weather. <br />• Stage of growth - The stage of growth also influences ET. Water needs increase as a plant grows In size <br />during the season, and peaks during flowering and fruit development. <br />Compared to the rooting system of a mature plant, newly planted or seeded crops don't have the root <br />system to explore a large volume of soil for water.. Recently planted and seeded crops will require frequent, <br />light irrigation. In our dry climate, even "xeric" plants need regular irrigation to establish. <br />i <br />' . Water demand of a plant - Plants vary greatly in the demand for water to 1) support growth, and 2) <br />survive dry spells (note the two are not necessarily related). <br />1 Fine-tuning for the site <br />The textbook figures are. a good starting point to understand irrigation management. When coupled <br />with careful observations a gardener can quickly fine-tune his/her irrigation schedule to the site- <br />specific irrigation demands. <br />j <br />To the observant gardener, the lawn tells you when It needs watering. The color has a subtle change from bluish- <br />green to grayish-blue and footprints are still visible an hour or more later. The observant gardener can use these <br />indicators to understand the water management for the site. <br />' Look at the historical ET for the summer. On a typical June and July day, if the lawn Is using 0.20 inches ET per <br />I day, you can estimate the water holding capacity and rooting depth by observing Irrigation needs. <br />' For example: <br />I <br />• IF.xb€l.J±rn, v?tll!_gtL... days an_ inch of vyater, and additional water won't extend the interval between <br />required irrigation, the water holding capacity (for this soil and rooting depth) Is 1 Inch. One inch w ulr d be <br />tt r..L aximum g-mo- at of w. r to aR?iy er irrigation, as ad a4r?ts1f5?vldJgacft elow the ro ir,g <br />E 3?2g.. <br />1 <br />s <br />Irrigation options include the following: <br />I <br /># o Water every 5 days applying 1 inch of water, OR <br />o Water every 4 days applying 0.8 inches of water, OR <br />' http:/,fv4nA,w.ext.colostate.ed;a/mgifiles/gardennotes/261-SoilWater.html 1/3/2048 <br />69