A Detailed Guide to the Fertilizer Requirements, High-Yield Fertilization Techniques and Timing for Walnuts in China

Walnut trees bear fruit for many years, are tall, have large canopies and deep root systems, and require good site conditions. Insufficient fertilizer supply has a considerable impact on yield and quality. So what are the fertilizer requirements of walnuts in China, and what are the high-yield fertilization techniques and timing?

Walnuts have a relatively high requirement for nitrogen and potassium nutrients, followed by calcium, magnesium and phosphorus. Every 50 kg of walnuts harvested requires the absorption of 1.4 kg of nitrogen. When nitrogen supply is insufficient, shoots grow short and thin, leaves turn yellow and drop easily, flowering is reduced, fruit drop is heavy, and alternate bearing is severe. Nutrient uptake differs somewhat among the various growth stages. Nitrogen is absorbed in greater amounts during the shoot-extending and leaf-growing stage, the flowering and fruiting stage, and the fruit pit-hardening stage; potassium absorption follows a similar pattern, but is greatest from after petal fall to the fruit pit-hardening stage; phosphorus absorption is relatively steady; calcium is absorbed in larger amounts after petal fall, making calcium deficiency likely; magnesium is also needed in larger amounts during the fruit formation stage.

For mature trees, walnut fertilization must ensure a nutrient balance for both tree growth and flowering and fruiting. Usually base fertilizer and topdressing are given equal importance, with base fertilizer accounting for about 60%, applied after the autumn harvest according to tree size. For a large tree, apply about 200 kg of organic fertilizer, 2 kg of superphosphate and 1-3 kg of potassium sulfate per tree, incorporated with deep plowing and soil improvement.

Walnut topdressing is carried out in three applications. The first is before bud break and flowering; this fertilization mainly provides nutrients for rapid new shoot growth and the flowering period. When nutrients are insufficient, new shoot growth is hindered and flowering and fruit set are poor. Therefore, apply about 1.0 kg of urea per tree, together with 0.3-0.5 kg of borax to increase the fruit-setting rate and promote fruit development. The second is after petal fall, when the fruit begins to form and swell and flower bud differentiation is also taking place; this is the period of greatest nutrient demand. When fertilizer supply is insufficient, fruit development is slow, fruit drop is severe, and flower bud differentiation is affected. Therefore, apply 0.5-1.0 kg of urea, about 1 kg of superphosphate, about 1 kg of potassium sulfate and 0.5-1.0 kg of magnesium sulfate per tree, in furrows combined with irrigation. The third is at the fruit pit-hardening stage, applying a further 0.5-1.5 kg of urea and 0.5-1.0 kg of magnesium sulfate per tree. This benefits kernel development and improves yield and quality. If tree vigor is poor after harvest, spray 0.5% urea to help the trees recover.