Best tillage practices for eucalyptus growth and productivity: A review on the Brazilian experience

ABSTRACT Commercial eucalyptus forestry has significantly increased in the last decades to supply the growing demand for forest goods (pulp, paper, sawmill, by-product industries, and biomass for bioenergy and biofuels). Among factors most influencing forest productivity, the soil physical environment is very important and can be harmful or beneficial to trees. In the effort to increase environmental and economic sustainability of commercial plantations, in this literature review, we summarize relationships between soil type and properties, forest rotation, and forest growth and productivity. Mechanized soil tillage must consider soil type, particularly land slope, soil drainage and depth, along with forest rotation (first or higher-order). Soil surveys, including soil physical properties (e.g., compaction, granulometry including gravel) and morphological attributes (e.g., horizons, solum depth, subsurface drainage impedance), are thus essential for foresters to make knowledgeable decisions on soil tillage, provided tillage is the single most costly practice in eucalyptus forestry. Subsoiling is the most common soil tillage for eucalyptus, but it is best for deep, drained soils. Existing scientific publications show, for clayey cohesive or compacted soils, tillage depth might be of 0.70 m (deep subsoiling) to ameliorate compacted soil of low microporosity and restriction to root growth, but 0.50 m (shallow) subsoiling plus ridding produce similar results. The latter, nonetheless, has a higher operational cost. Downslope subsoiling on sloppy lands increases the risk for soil erosion, which may be reduced by interrupting ripping, mechanized hole- or pit-drilling with low operational cost. Chiseling performed for fertilizer application (minimum 0.25 m deep) might be the only tillage required for non-compacted sandy soils. Mechanical or manual pitting could also be an option for second or higher rotations, but results show crop early-growth is restricted, possibly because of root confinement. For second or higher rotations, stumps require cutting to allow cross-slope tractor traffic and tillage. Subsoiling plus ridging or bedding is required in low-drainage or shallow soils to increase the soil volume to be explored by roots. The ridges can be built by grade bedding or terracing plows. In areas with waterlogging, drainage and ridging without subsoiling are necessary. Research opportunities include further studies for slopy lands and low-drainage or compacted soils, tillage affecting soil erosion and eucalyptus productivity, equipment for special tillage, and mapping compaction risk and special tillage needs.

摘要 近几十年来，商业桉树林业规模显著增长，以满足日益增长的林产品需求，包括纸浆、纸张、锯木、副产品工业用材，以及用于生物能源和生物燃料的生物质。在影响森林生产力的诸多因素中，土壤物理环境至关重要，其对树木既可能产生不利影响，也可能带来益处。为提升商业人工林的环境与经济可持续性，本文献综述总结了土壤类型与性质、森林轮伐期，以及森林生长和生产力之间的关联。机械化土壤耕作（Mechanized soil tillage）必须综合考量土壤类型，尤其是土地坡度、土壤排水性与土层深度，同时结合森林轮伐期（首轮或多轮伐期）制定方案。因此，土壤调查——包括土壤物理性质（如压实度、含砾粒的粒度组成）与形态特征（如土壤发生层、土体厚度、地下排水阻抗）——是林务员做出合理土壤耕作决策的必要依据，毕竟耕作是桉树林业中成本最高的单一作业环节。深松耕作（Subsoiling）是桉树种植中最常见的土壤耕作方式，但它最适用于深厚且排水良好的土壤。现有科研成果表明，针对黏结性或压实的黏质土壤，耕作深度需达到0.70米（深层深松），以改善低微孔率且限制根系生长的压实土壤；而0.50米（浅层）深松配合起垄也可达到相似效果，不过后者的作业成本更高。坡地顺坡深松会加剧土壤侵蚀风险，可通过中断裂土作业、采用低成本的机械化穴坑钻孔来降低该风险。为施肥而进行的凿形耕作（Chiseling，作业深度至少0.25米），或许是非压实砂质土壤所需的唯一耕作方式。机械或人工挖坑也可作为第二轮或多轮伐期的备选方案，但研究显示作物早期生长会受到限制，这可能是因为根系活动空间受限。对于第二轮或多轮伐期，需伐除树桩以允许拖拉机沿横坡通行并开展耕作作业。在排水不良或浅层土壤中，需采用深松结合起垄或作床的方式，以扩大根系可探索的土壤体积。垄台可通过平地作床或梯田犁进行构筑。在涝渍地区，无需深松的排水与起垄作业是必要的。未来的研究方向包括：针对坡地、排水不良或压实土壤的进一步研究，耕作对土壤侵蚀与桉树生产力的影响，专用耕作设备的研发，以及压实风险与特殊耕作需求的制图。