Autumn Fertilization for Citrus to Boost Yield

Following the Start of Autumn, citrus fruits enter a critical stage of development. To ensure an adequate flower load for the coming year and mitigate alternate bearing, in addition to thinning heavily fruiting trees, it is essential to apply autumn fertilization in a scientifically sound and well‑timed manner.

The Importance of Applying Autumn Fertilizer to Citrus Trees

Starting from the autumn shoot‑growth stage, citrus fruits undergo a second phase of enlargement that demands substantial nutrients, and the emergence of new autumn shoots likewise requires ample nutrition. During this period, the root system must allocate absorbed nutrients both to support new shoot development and to fuel fruit expansion. If root vitality is insufficient—particularly as temperatures drop in autumn and dry conditions set in—the roots can easily decline, impairing their ability to uptake nutrients. This leads to yellowing of the autumn shoots, commonly referred to as “autumn yellowing.”

“Autumn yellowing” is a sign of poor root development in citrus trees, a condition commonly observed in orchards that have long relied on chemical fertilizers and where soil quality has deteriorated. Trees exhibiting yellowing of their autumn shoots tend to show weakened vigor later in the season and produce a higher proportion of unripe fruit; if left unmanaged, they become more susceptible to Huanglongbing (HLB) infection. Therefore, safeguarding autumn shoots is a critical step in tree health management and in extending the productive lifespan of citrus orchards.

Quick-release nitrogen, phosphorus, and potassium are fundamental.

Nitrogen: Citrus trees begin to reduce their uptake of nitrogen after July. A moderate supply of nitrogen is required during the autumn shoot‑emergence stage, but excessive nitrogen should be avoided during the later stages of shoot growth and fruit enlargement, as it can negatively affect color development and fruit maturation.

Phosphorus: Phosphorus primarily promotes flower bud differentiation, apical bud development, and root formation, and also aids in fruit coloring. It should be supplemented especially during the autumn shoot‑growth period and in early spring.

Potassium: Potassium serves as both a target and a transport vehicle for nutrient mobilization. Citrus trees exhibit increased potassium uptake after June, with peak absorption occurring in September; therefore, potassium fertilizer application can be appropriately increased during this period. However, it should be noted that excessive potassium can inhibit the uptake of calcium, magnesium, and other nutrients, and may also lead to rough, thickened fruit peel.

Micronutrients and trace elements are equally essential.

Calcium supplementation is also essential during the autumn shoot‑growth phase. As autumn ushers in the citrus tree’s final peak of root growth for the year, calcium application helps stimulate the development of fine roots; a robust root system is the key to strong, vigorous autumn shoots.

During the autumn shoot‑growth period, magnesium and micronutrients should also be supplemented. If new shoots develop small leaves, yellowing leaves, or mottled foliage, and the autumn shoots fail to turn green, this is not due to insufficient fertilizer but rather to nutritional imbalance—specifically, a deficiency of magnesium and essential micronutrients.

The months of July to September mark the period of rapid fruit enlargement in citrus, as well as the phase when summer shoots mature and autumn shoots emerge. During this time, both air and soil temperatures are high, root absorption capacity is strong, and uptake of nitrogen, phosphorus, and potassium is substantial, resulting in high fertilizer efficiency. Nutrients absorbed by the roots are translocated throughout the tree more rapidly, leading to a marked increase in nutrient consumption.

At this stage, applying autumn shoot fertilizer in conjunction with irrigation to combat drought has a significant effect on both fruit enlargement and the growth of autumn shoots. June and July represent the critical period when citrus trees have their highest demand for nutrients, while June through September is the key phase for building tree vigor; during this time, fruit expansion, flower bud differentiation, and the emergence of autumn shoots all require adequate nutrition.