Sensor Integrated Automatic Irrigation System to Reduce Runoff and Nutrient Loss without Affecting Plant Growth

Container grown woody ornamentals were irrigated with four different treatments based on daily water use (DWU) to study the impact on plant growth, leachate electrical conductivity and pH, and runoff water volume and nutrient content. A completely randomized design was used with 4 overhead irrigation treatments: 1) irrigation scheduled to replace 100% DWU per application (100DWU); 2) irrigation alternating every other application with 100% replacement of DWU and 75% DWU the following application (100-75); 3) irrigation scheduled on a three-application cycle replacing 100% DWU followed by two applications replacing 75% DWU (100-75-75); 4) irrigation scheduled on a four-application cycle replacing 100% DWU followed by three applications replacing 75% DWU (100-75-75-75). The substrate volumetric moisture content (SVMC) was determined by soil moisture sensors (Model 10 HS, Decagon Devices, Inc., Pullman, WA 99163) placed in a subset of containers. Plant DWU was calculated as the difference between SVMC 1 hour after irrigation and SVMC immediately before irrigation the following day. The irrigation amounts were scheduled by a programmed data logger (CR3000, Campbell Scientific Inc., Logan, Utah 84321) based on the highest DWU calculation from the sensors in each zone. Irrigation applications were separated by at least 24 hrs. Hibiscus syriacus ‘Bricotts’, Euonymus alatus ‘Select’, Weigela  florida ‘Alexandra’, Spiraea japonica ‘Yan’ were grown in 10.2-L (#3) containers in 2011. The average daily irrigation amount applied for 100DWU, 100-75, 100-75-75, and 100-75-75-75 were 513, 424, 473, and 423 mL, respectively. Viburnum dentatum ‘Ralph Senior’ potentilla fruticosa ’Happy Face’ and Thuja occidentalis ‘Sunkist’ were grown in 10.2 L containers from June to October in 2012. The average daily irrigation amount applied for 100DWU, 100-75, 100-75-75 and 100-75-75-75 were 900, 980, 970, and 910ml, respectively with little differences among treatments. There were no differences in final plant growth, final electrical conductivity and pH, and runoff NO₃ and PO₄ concentrations among treatments for both years. The seasonal average runoff NO₃ and PO₄ loads among treatments in 2011 was not different; in 2012, the seasonal average runoff NO₃ load of 100–75 for 2012 was the highest versus 100-75-75-75 which was the least compared to other treatments.  PO₄ load was not different among treatments. Seasonal average runoff volume was similar for both years. These results indicate that irrigation water could be applied at plant DWU or a slight deficit without affecting plant growth.