When Is the Best Time to Fertilize Walnut Trees? An Introduction to Fertilization Periods

  According to the performance of fertilizers and the different fertilization periods, they can be divided into two major categories: base fertilizer and topdressing.

  1. Base fertilizer: mainly composed of decomposed organic fertilizer, it is a foundational fertilizer that can supply walnuts with multiple nutrients over a relatively long period. Base fertilizer is generally applied in autumn, as early as possible during the period from after fruit harvest to before leaf fall. If there is no time to apply it in autumn, it can be applied in spring. Applying base fertilizer in autumn can promote flower bud differentiation and root growth, raise the tree's nutrient storage level, and benefit the growth of branches and leaves as well as flowering and fruit setting in the coming year. The earlier the autumn base fertilizer is applied, the better. Young walnut orchards can apply base fertilizer in combination with deep plowing, while mature orchards can broadcast it over the whole orchard and then shallowly turn the soil to incorporate the base fertilizer. After applying base fertilizer, irrigate thoroughly once.

  2. Topdressing: on the basis of base fertilizer, it is a quick-acting fertilizer supplemented in a timely manner according to the tree's growth and development needs. It is mainly composed of quick-acting chemical fertilizers. Topdressing can supply the nutrients needed for the tree's growth and development in the current year, which not only helps the tree grow vigorously with high yield and good quality in that year, but also lays the foundation for growth and fruiting in the coming year. It is an indispensable part of production. In areas with high temperatures and abundant rain or in sandy soils, fertilizers are easily lost, so topdressing should be applied in small amounts many times. Young trees should be topdressed fewer times, generally 2~3 times per year. As tree age increases and fruit yield rises, the number of topdressings increases; mature trees are generally topdressed 3~4 times per year. The three main topdressing periods for walnuts are as follows:

  (1) First topdressing: for early-bearing walnuts, it is carried out before the female flowers bloom; for late-bearing walnuts, it is carried out at the early stage of leaf expansion. It is mainly quick-acting nitrogen fertilizer, such as urea, ammonium sulfate, etc. Topdressing at this stage can promote flowering and fruit setting and benefit the growth and development of new shoots. For walnut trees that have entered the full fruit-bearing stage, quick-acting nitrogen fertilizer and phosphorus fertilizer must be applied before spring bud break, and the amount applied should account for more than 50% of the annual topdressing amount; otherwise, insufficient nutrients in the early stage will hinder tree growth and development and affect flowering and fruit setting.

  (2) Second topdressing: for early-bearing walnuts, it is applied after the female flowers bloom; for late-bearing walnuts, it is applied at the end of leaf expansion. It is mainly nitrogen fertilizer, combined with an appropriate amount of phosphorus and potassium fertilizer. Topdressing at this time can promote fruit development, reduce fruit drop, and benefit branch growth and lignification. The amount applied should account for 30% of the annual topdressing amount.

  (3) Third topdressing: it is mainly a topdressing for walnuts that have entered the fruiting stage, carried out after the fruit hardens in late June. It is mainly phosphorus and potassium fertilizer, with a small amount of nitrogen fertilizer applied together. The purpose of topdressing at this stage is mainly to meet the large nutrient demand for kernel development, improve nut quality, and at the same time promote flower bud differentiation, laying a good foundation for flowering and fruit setting in the coming year. This topdressing amount should account for 20% of the annual topdressing amount.

  In addition, the determination of the specific fertilization period is also affected by the nutrient and moisture conditions in the soil. For poor soil, multiple topdressings in spring are very important, while on fertile soil with high organic matter content, the number of fertilizations can be reduced. Soil moisture content affects the effectiveness of fertilizers. When the soil lacks water, fertilization is often harmful rather than beneficial. Therefore, the fertilization period should be determined according to whether water can be supplied. In water-scarce areas, fertilization should be carried out during the rainy season.