Fertilizer Rate and Irrigation Duration Affect Leachate Volume, Electrical Conductivity, and Growth of Gardenia jasminoides

An increasing number of laws and regulations regarding runoff and water use are necessitating container nursery growers to irrigate more efficiently.  However, over-irrigation and intentional leaching are still common in the industry.  Leaching of fertilizers often leads to the need for additional fertilizer applications, which are costly for the grower and the environment.  By reducing fertilizer application rates and irrigating more efficiently we believe that salable plants can be produced with little or no irrigation-induced leaching.  In this study, we related fertilizer application rate and irrigation duration to leachate volume, leachate electrical conductivity, and plant growth.  A soil moisture sensor-controlled irrigation system was used to irrigate Gardenia jasminoides 'Madga I' (sold as Heaven Scent).  Controlled release fertilizer was applied at 100%, 50%, and 25% of the label rate and irrigation durations were 2, 3, 4, or 5 minutes.  All plants within an experimental block were irrigated when the volumetric water content of the control plants (2 minute irrigation duration, 100% fertilizer treatment) reached 35%. At that time, plants in all treatments were irrigated. This provided excessive irrigation to plants irrigated for 3, 4, or 5 minutes. Leachate was collected biweekly and included leachate caused by rainfall. Leachate volume was greatest for plants receiving the 5 minute irrigation for all fertilizer treatments.  The cumulative leachate volume was 15, 12.5, 10.5, and 9 L/plant for the 5-, 4-, 3-, and 2-minute irrigation treatments, respectively.  Electrical conductivity (EC) of the leachate was highest with the 100% fertilizer rate and decreased with reduced fertilizer rate.  Fertilizer rate and the interaction of fertilizer rate with irrigation duration had a significant effect on shoot dry weight.  Average shoot dry weight was 18.7, 25.3, and 27.3 g per plant for the 25%, 50%, and 100% fertilizer treatments, respectively.  Using 3-minute irrigation cycles, shoot dry mass of plants grown with 50% fertilizer was only 0.2 g lower than that of plants grown with 100% fertilizer, while with 4-minute irrigation cycles, this difference was only 1.1 g. This shows the potential for reduced fertilizer use with moderate irrigation applications.  In this study, we have shown that reduced fertilizer application rates can be used along with moderate irrigation durations to reduce leaching of nutrients, without negatively impacting plant growth.