Overhead sprinkler irrigation strategies to reduce water and nitrogen loss from container-grown plants
The overall objective of this study was to determine the influence of intermittent water application via overhead irrigation on water and N leaching, substrate water distribution, and plant growth in a pine bark substrate. Intermittent irrigation was defined as the application of a plants' daily water allotment in multiple applications with prescribed intervals between applications. Specifically, experiments were conducted to determine: 1) the influence of water application rate and pre-irrigation substrate moisture content on water application efficiency, 2) the influence of the number of intermittent applications and interval duration between applications on application efficiency, 3) substrate water distribution after intermittent irrigation, 4) efficiency of intermittent irrigation in unamended and sand-amended pine bark, and 5) influence of intermittent irrigation on plant growth. In all experiments, Marigold (Tagetes erecta L. 'Apollo') was grown in unamended or sand-amended pine bark in 3.8-liter containers. After saturation, bark was allowed to dry via evapotranspiration to targeted moisture deficits after which irrigation treatments were applied via a simulated overhead irrigation system. In experiments evaluating intermittent irrigation, water was applied intermittently (multiple applications with intervals between applications) or continuously (single application). After 1 h drainage, collected leachate was measured and analyzed (in two experiments) for electrical conductivity, N0₃-N and NH₄-N and water application efficiency (percentage of applied water that is retained in the substrate) determined. There was an inverse linear relationship between water application rate and efficiency. Leachate N (total) increased linearly as application rate increased. Efficiency increased curvilinearly and leachate N (total and concentration) increased linearly as pre-irrigation substrate moisture content decreased. Efficiency of intermittent irrigation was greater than continuous irrigation over a range of pre-irrigation substrate moisture contents, and in both unamended and sand-amended pine bark. Intermittent efficiency increased linearly as interval duration between applications increased. Efficiency of five 60 ml applications was greater than three 100 ml applications. Gravimetric moisture content of the bottom third of the substrate was greater with intermittent than continuous irrigation. Intermittent efficiency was greater in pine bark than in sand-amended pine bark when the water deficit from container capacity was the same for both substrates. Repeated intermittent irrigations resulted in less water and N leaching compared to continuous irrigation. Intermittent irrigation did not result in substrate nutrient accumulation and had no adverse effect on plant growth. Fresh and dry root weights and shoot and root N concentrations were greater with intermittent irrigation than continuous irrigation.