Methods and Recommendations for Implementing Soil and Water Conservation Engineering in Walnut Orchards

  When establishing walnut orchards on mountainous and hilly land that is dry and receives little rain, with thin soil layers, the original vegetation may be destroyed, and coupled with unreasonable cultivation, this can easily cause soil erosion. Especially during the rainy season, excessive rainfall forms surface runoff that washes away the fertile topsoil and organic matter, which not only thins the orchard soil layer, increases the stone content, and reduces soil fertility, but also exposes the walnut root systems, weakens tree vigor, reduces yield, and shortens lifespan. It can even cause mudslides or large-scale landslides, endangering the safety of the walnut orchard. Therefore, at the initial stage of orchard establishment, it is essential to improve soil conditions and enhance the drought resistance of the trees through land preparation and the construction of soil and water conservation engineering. Common types of soil and water conservation engineering include the following methods.

  1. Building terraces. Terrace surfaces are level, which facilitates cultivation, improves the site conditions for walnuts, makes full use of the abundant light and heat resources of hilly and mountainous areas, and creates a relatively ideal micro-ecological environment for walnuts. By building terraces, sloped land can be converted into platforms of varying sizes, reducing the slope gradient, shrinking the catchment area, and reducing runoff volume. This can effectively prevent soil erosion, effectively enhance the soil's water and nutrient retention capacity, raise the air temperature on the planting surface and the accumulated temperature during the growing season, and reduce the occurrence of frost damage to walnuts.

  Terraces have the characteristic of following contour lines, which facilitates soil improvement and intensive cultivation in the orchard, and also creates conditions for setting up irrigation and drainage systems and mechanized management. The terrace surfaces of mountain walnut orchards cannot be absolutely level; they should be slightly inclined to facilitate drainage of excess water. In areas with abundant rainfall and deep soil layers, inward-sloping terrace surfaces can be designed; in areas with little rainfall and shallow soil layers, outward-sloping terrace surfaces can be designed to regulate the moisture distribution on the terrace surface and facilitate soil improvement.

  2. Contour trenching. This is a simple soil and water conservation method adopted in the hilly and mountainous areas of northern China, suitable for areas with gentle slopes and relatively little rainfall. The trenching method is as follows: based on measurements along contour lines, at set intervals along the contour lines, soil is dug to form trenches, and the soil is placed on the outer edge of the trench to build a ridge, so that the cross-section of the trench and the cross-section of the outer edge continuously form a positive and negative arc shape. Walnuts are planted on the outer slope of the ridge. Within the trench, earthen dams can be built horizontally at set intervals to intercept and store rainwater and prevent soil erosion. Because the soil layer of the ridge is relatively thick and there is more moisture beside the trench, young trees grow and develop well. However, trenching increases the slope gradient both inside the trench and on the outer edge of the ridge, making the slope surface between two trenches steeper than the original slope surface and increasing the possibility of soil being washed away between the two trenches. Through contour trenching, long slopes are converted into short slopes, and direct downward flow of rainwater is converted into lateral flow. If grasses or short-stemmed crops are artificially planted on top of the ridges, between tree rows, and inside the trenches, better soil and water conservation results can be achieved. Trenching has the effects of weakening surface runoff, storing water and conserving soil, and maturing the soil. After a period of cultivation, depending on the specific circumstances, the trenches can gradually be converted into compound terraces to facilitate normal growth and fruiting of walnuts and prevent rainwater erosion.

  3. Digging fish-scale pits. For soil and water conservation engineering on hillside land, different methods can be adopted depending on the slope gradient. On slopes of 5° to 15°, trenching can be used; on slopes above 15°, terraces can be built; when the slope exceeds 25°, fish-scale pits can be dug. Fish-scale pits can be arranged in a staggered pattern. The method is to dig semicircular pits along contour lines, build outer ridges with stones or subsoil, with a ridge height of 30 to 40 cm, a pit depth of more than 1 m, and a diameter of 2 m. The pit spacing is determined according to the planting density requirements. Topsoil and subsoil are piled separately. A small ditch is opened at each of the left and right corners of the pit to facilitate the diversion and storage of runoff. When filling the pits, all the topsoil from the slope surface is backfilled, and the pit surface is inclined inward. Walnut trees are later planted on the outer side within the pits. As the walnut trees grow, the outer edge soil ridges are gradually built up year by year, so that they gradually transition into small sloping terraces.

  4. Interval-slope runoff collection and storage contour trenches. On gentle slopes of 13° to 30°, lines are drawn on the slope surface along contour lines at trench spacings of 6 to 7 m. In autumn, trenches 1 m wide and 80 cm deep are dug along these lines. When digging the trenches, the topsoil layer, the mature soil 0 to 20 cm thick, is placed on the upper side of the slope surface. The raw soil below 20 cm in the trench is dug out and turned downward to build ridges. The turned-up mature soil, together with the topsoil from the slope surface, is mixed with organic fertilizer and backfilled into the trench, and shaped into contour trenches extending along contour lines with a low inner side and a high outer side. The outer ridge is tamped firm, with a ridge height of 30 cm, to store water. Within the trench, cross dams are built at set intervals to prevent rainwater from flowing along the trench after collection, so that moisture in the trench is evenly distributed.

  Contour trenches with a ridge height of 0.3 m can ensure the safe collection of slope surface runoff and prevent soil erosion. If this method is combined with biological measures such as growing grass on field ridges, the ecological benefits of preventing soil erosion will be even better.