Effects of mulching on soil properties and environmental stress in Satsuma mandarins (Citrus unshiu)

The northern California foothills have produced high quality Satsuma mandarins (Citrus unshiu) for over 120 years, despite the stress of cold, wet winters and hot, dry summers. Climate change compounds tree stress with variable rainfall, temperature fluctuations, and frequent winds. Limited soil fertility, organic matter, and irrigation water have also become challenging. In recent years, foothill citrus growers have used organic mulch to mitigate these issues. In 2017, mulching trials were established in four commercial mandarin orchards to determine whether mulch can mitigate water use and tree stress, while improving productivity, soil and tree health. Mulch and control treatments were laid out in a randomized complete block design of three sets of trees using the middle tree as the data tree. Mulch was composed of 50% wood chips and 50% composted horse manure, applied in spring under the canopy. We measured the effects of mulch on soil moisture, temperature, organic matter, and microbial activity; as well as visible tree health by rating, fruit drop, yield and quality. Mulch treatments maintained significantly higher soil moisture; reducing irrigation needs, tree stress, and fruit drop. Mulch significantly increased winter soil temperatures, providing some frost protection when temperatures declined rapidly. Water, temperature, and other environmental stresses increase fruit drop, impacting yield and return. Despite wide temperature fluctuations and very low humidity, mulch reduced fruit drop significantly. Collectively, the results suggest the effects of climate change on mandarin production can be mitigated with mulching. This paper presents the impact of mulch on fruit drop, soil, water, and temperature parameters, and presents guidelines for optimal soil conditions and timing of mulch applications.