What is the best time to apply potassium fertilizer to citrus trees?

Potassium is one of the elements essential to plants. Potassium fertilizer helps enlarge the fruit, so when growing citrus you need to seize the right timing for application. When is the best time for potassium fertilizer on citrus?

1. Best time for potassium fertilizer on citrus

Potassium fertilizer on citrus is generally applied during the fruit-swelling and fruit-filling period in June-July and after the fruit harvest is finished in early November. Potassium fertilizer must be applied together with nitrogen and phosphorus fertilizers to give full play to its fertilizing effect.

2. Citrus fertilizing methods

Citrus fertilizing methods generally include soil application and root-outside topdressing, also called foliar feeding.

1. Soil application must be based on the distribution characteristics of the roots, placing the fertilizer within the root distribution layer so the roots can absorb it easily, which is why it is also called “rhizosphere fertilization”. The fibrous roots (absorbing roots) of citrus are mostly concentrated a little beyond the outer edge of the canopy, and the root system has a tendency to grow toward fertilizer, so its growth direction often shifts to the fertilized spot. For young trees, fertilization should be used to induce horizontal roots and absorbing roots; generally, organic fertilizer should be applied slightly deeper and slightly farther than the concentrated root layer to induce the roots to grow deeper and wider and form a strong root mass. For mountain orchards, base fertilizer should be applied in deep trenches 30-40 cm deep, while topdressing is generally applied in shallow trenches. The trenching position for fertilization should be outside the vertical projection of the canopy (commonly known as the drip line at the outer edge of the canopy), and fertilizer must never be applied directly under the tree trunk.

Fertilizing methods: (1) Ring trench fertilization. Along the canopy drip line, dig a ring-shaped fertilizing trench 15-20 cm deep and 30 cm wide, then cover with soil after application. Mostly used for young trees.

(2) Radial trench fertilization. Dig trenches within the canopy starting 1-1.5 m from the trunk, and according to the size of the canopy, dig 4-6 trenches radiating outward; the depth and width of the trenches are the same as for ring trenches, but note that they should be shallow on the inside and deep on the outside to avoid injuring large roots. The position should be changed every year.

(3) Hole fertilization. Within the canopy area, dig several holes, then cover with soil after application, alternating the positions each year.

(4) Strip trench fertilization. This is a commonly used method: on the outer edges of both sides of the canopy, dig strip trenches 20-30 cm deep and 30 cm wide, with the trench length depending on the size of the canopy. Rotate the position each time.

2. Root-outside topdressing is simple and easy to carry out, uses a small amount of fertilizer, takes effect quickly, can promptly meet the urgent needs of the fruit trees, and can avoid the fixation of certain elements in the soil that makes them ineffective. However, root-outside topdressing cannot replace soil application.

After fertilizer is sprayed onto the leaves, it is generally absorbed within 15 minutes to 2 hours; young leaves absorb faster than old leaves, and the underside of leaves absorbs faster than the upper surface. Therefore, spray more on the underside of the leaves. In addition, young shoots, branches, trunks and fruit can all absorb it.

Root-outside topdressing plays an important role in strengthening shoots, promoting flower bud differentiation, and protecting flowers and fruit.

Fertilizers and concentrations used for root-outside topdressing: urea 0.3-0.5%, monopotassium phosphate 0.2-0.4%, magnesium sulfate 0.5-1%, zinc sulfate and manganese sulfate 0.1-0.2% (plus 0.25-0.1% slaked lime), borax 0.1-0.2%, ammonium molybdate 0.05%.

Citrus fertilization needs to be combined with the growth condition, so pay more attention in daily management.