Title : Cover crops in almond orchards - Supporting their services and mitigating competition by adjusting fertilization
Abstract:
Introduction: Conventional chemical control of weeds in almond orchards grown in the Mediterranean region can deteriorate soil quality over time. The practice of cover crop (CC) intercropping in orchards is viewed as a cost-effective and alternative method for promoting sustainability by improving soil fertility, effective weed suppression, and supporting the beneficial functions of soil organisms. Despite the perceived benefits, some literature reports yield reductions in orchards utilizing cover crops. Our initial findings indicate that incorporating CC in and out of the tree lines of almond orchards may result in a substantial (37% in 2021 and 45% in 2022) reduction in yields.
Objectives: This work aimed to enhance the positive services of CC intercropping in almond tree lines by increasing the annual fertilizer load and compensating for their potential disservices.
Methodology: This study was carried out at the Model Farms of the Neve Yaar research station in the Yizre'el Valley, Israel. A self-fertilized almond "Matan" cv was planted in January 2019. We used a two-factor factorial design, including twenty individual plots (eight trees each), with five replicates per treatment. Ryegrass CC was planted on the tree line, while the non-cover crop (NCC) line was treated with herbicide. The second factor we investigated was fertilization levels. We used two distinct levels of fertilization: the common best practice rate (F1, 100%) and 20% more than the common best practice rate (F2, 120%). Our experiment consisted of four distinct treatment groups, each with a unique combination of CC presence and fertilization level (CC + F1, CC + F2, NCC + F1, and NCC + F2). The soil at the site is classified as clay grumusol (Vertisols), pH 7.5-8.3, EC 0.39-0.99 ds/m, soil organic carbon 0.58-1.08%, total nitrogen 200-500 mg/kg, K 245-519 mg/kg, and P-PO4 15.6-56.3 mg/kg. We used ammonium sulfate nitrate (12%) and Sarit liquid fertilizer with NPK and micronutrients. The soil was sampled prior to the initiation of the fertigation season (March) and at its end (November). Monthly stem water potential measurements were recorded from April to September. April and July leaf samples were analyzed for total nitrogen content (TN). Yield was analyzed from 20 individual trees (5 per treatment) and from each plot.
Results: The results revealed that cover crops increased nitrate, mineral nitrogen, total organic carbon, total nitrogen, and organic matter levels compared to plots without cover crops at 20cm soil depth. Fertilization rates did not significantly impact nitrogen, organic carbon, and organic matter levels at a depth of 20 cm in the cover crop plot. Notably, the interaction of cover crops and fertilization rates significantly increased soil nutrient levels in CCF2 compared to no cover crop plots, suggesting potential nutrient provision to cover crops, thus reducing competition. While NCCF1 had low nitrate, mineral nitrogen, total organic carbon, total nitrogen, and organic matter levels, no significant variations were found in pH, EC, C/N, or K at any soil depth. Almond leaf nutrient analysis indicated no statistical difference in leaf N content between April and July, though content decreased in July compared to the April sample in all treatment. Leaf water potential measurements showed no significant differences among treatments from April to August, but in September, the stem water potential was lower in the CCF2 treatment. While cover crops had no negative impact on almond leaf status, they aided in retaining soil water and preventing evaporation, resulting in a lower soil water potential than NCCF1 and NCCF2. We observed significant yield variations among different groups (p<0.0068). NCCF2 had a higher yield, while CCF1 had a lower kernel yield. Interestingly, there was no statistical difference between CCF2 and NCCF1. Despite applying new fertigation, the yield remained lower in CCF2 compared to NCCF2, although CCF2 had a relatively higher yield than CCF1. The adjustments made to fertigation during the 2023 growing season may not have impacted that season's yield. Almond trees rely on stored nutrients from the previous year (2022) to support their growth and fruit production. Therefore, the yield in 2023 may have been influenced by the nutrients stored from the previous year.
Conclusion: Based on observations, there was no competition for water, but there appears to be nutrient competition in CCF1 between the cover crop and almond trees, potentially leading to a decrease in yield. However, it's crucial to note that these conclusions are based on one season, and it cannot be conclusively stated that adjusting fertigation levels has no impact on almond yield, as improvement was observed in CCF2. Furthermore, we conclude that any impact from the new fertigation in the 2023 growing season may manifest in vegetative growth rather than yield, with the effects potentially visible in the next growing season.
