Fertilising after planting: the method and its limits

At planting, Diaplant generally applies a slow-release NPK fertiliser. The rate is adjusted to the size of the plant, the soil and the product used. The aim is to make nutrients available during establishment and the first growing season, without triggering a flush of growth that the roots cannot yet sustain.

Fertilising is part of the planting operation, but it does not replace the essentials. A tree with a dry rootball, planted too deep or in saturated soil will go on struggling even if it is fed.

Nitrogen, phosphorus and potassium play a part in growth, in root function and in the plant’s regulation. In a slow-release fertiliser, these nutrients become available gradually over several months.

This pattern of release suits a newly planted tree better than a soluble dose released all at once. The plant is supported through the first phase of growth, while the roots begin to leave the rootball.

The actual duration depends on the formulation, the temperature, the moisture and the activity of the soil. That is why the choice of product and the rate are part of preparing for planting.

The fertiliser is distributed through the backfill soil or in the zone indicated for the product, avoiding concentrations directly on roots and tissue.

No layer of fertiliser is created at the bottom of the hole. Concentrated contact increases the risk of salinity and root damage.

The application is recorded with the product, the rate and the date. This record prevents another team from repeating the operation because they do not know what has already been done.

Before fertilising, the ground is examined. Texture, organic matter, pH, salinity and drainage all alter the availability of nutrients.

In a very poor soil, a fuller programme may be needed. In fertile, well-structured ground, the planting dose may be enough for the first season.

Well-rotted organic matter can improve structure and supply nutrients gradually. It should be incorporated in a balanced way through a wide volume, and not concentrated at the bottom of the hole.

Nutrients are only useful when the roots are able to function. A lack of water reduces transport and uptake; waterlogging removes oxygen and produces the same result by another route.

After planting, deep watering settles the soil and activates the relationship between water, root and fertiliser. The interval between waterings is adjusted to the drainage.

Wilted or yellow leaves do not automatically justify more fertiliser. The rootball, the moisture, the root collar and the stability are checked first.

Fertilisers are salts. An excessive rate raises the concentration in the soil and can make it harder for water to enter the roots.

The risk is greater in dry soil, in pots, where drainage is poor and when the product is concentrated in a small area.

‘More fertiliser’ does not mean faster establishment. The rate should follow the product, the soil volume and the size of the plant.

Nitrogen encourages vegetative growth. When there is too much of it, the tree can produce long shoots and soft tissue, increasing the leaf area that the roots have to supply.

In the first few months, what matters is growth in keeping with the recovery of the root system. The crown should resume activity without losing its relationship with the roots that were retained.

A tree showing more restrained growth during establishment is not necessarily failing. It may be investing in roots before extending the crown.

The initial application does not rule out a deficiency appearing. If chlorosis, small leaves or persistently weak growth show up, the pattern should be interpreted before any correction is made.

High pH, alkaline irrigation water, compaction and root damage can prevent uptake even when the nutrient is in the soil.

When the cause is not clear, soil analysis and, in some cases, leaf analysis help with the decision. The correction may involve a specific nutrient, organic matter, pH or a change in watering.

These products are not equivalent to fertilisers. Composition, evidence and conditions of use vary.

A field-grown tree already has a relationship with soil micro-organisms. An inoculant should only be used when there is a technical reason and a product suited to the context.

No biostimulant makes up for a saturated hole, a broken rootball or a lack of water.

Shrubs and perennials may respond faster to fertiliser, but they also have smaller rootballs and are more sensitive to excessive concentrations.

Mediterranean species planted in very rich soil can grow open and lose the compact structure that was intended.

In plants kept in pots, the limited volume increases the risk of salts building up. The rate and the drainage need closer control.

Lawns, vegetable gardens, orchards and other productive areas have programmes of their own. They should not dictate how the trees are fertilised simply because they share the same space.

The lawn may receive seasonal applications; a mature tree needs a plan related to the soil and to its establishment. Spreading the same product indiscriminately across every layer of planting is avoided.

What was applied is recorded and growth is observed through the first season.

The dose is not repeated as a matter of routine. A further application depends on the product, the soil, the behaviour of the plant and the maintenance plan.

In Diaplant’s practice, slow-release NPK is a normal aid to planting. Success still depends on the whole: an intact rootball, loosened soil, the root collar at the correct level, water, drainage and aftercare.

• How to assess the soil
• How to water a mature tree in its first year
• Transplant stress

1. Principles of plant nutrition, salinity and controlled-release fertilisers.
2. Methods of agronomic soil and leaf analysis.
3. Planting and fertilising procedures used by Diaplant.