Impact of organic matter amendments on soil and tree water status in a California orchard

Abstract

Permanent crops like almond (Prunus dulcis) require significant water inputs for economic yields and long-term productivity. This demand creates a challenge in drought-prone regions like California. Use of organic matter amendments (OMA) can improve water use efficiency by conserving soil moisture and reducing tree water stress. The majority of almond orchards in California are no-till with irrigation targeted on a narrow tree berm where OMA is applied as surface mulch. We examined the effects of composted dairy manure (CM) and the timing of its application on soil moisture, soil water retention and tree water status in a young orchard planted in 2014. Treatments including Fall-applied CM (October 2015 and 2016), Spring-applied CM (April 2016 and 2017) and an unamended control were monitored during the 2016 and 2017 growing seasons. Fall-applied CM was more readily incorporated in soil organic matter of the 0–60 cm rooting zone as evidenced by significantly greater soil organic carbon (SOC) for Fall versus Spring-applied CM in 2016 (p < 0.05). Fall-applied CM significantly increased soil volumetric water content (VWC) by 22% from 0 to 150 cm depth during the driest period of year one and tended to make midday stem water potential (SWP) less negative relative to the control. Fall-applied CM tended to increase VWC in 2017, but treatment differences were no longer significant. Differences in VWC and SWP between Fall-applied CM and the control were most apparent at low VWC (<10%). Two years of Fall-applied CM increased soil water retention between 0 to ˜100 kPa at 0–10 cm depth by 13% compared to the control. These results demonstrate Fall-applied CM was more effective at enhancing soil moisture retention and reducing tree water stress compared to Spring-applied CM. We also conclude OMA use may buffer against periods of limited water supply for young trees.