Fertilizer use also needs to replace those nutrients removed from the system in the harvested nuts – particularly nitrogen, potassium and calcium - as well as to help support periods of greatest needs, most notably, spring regrowth, flowering, fruit set and fruit fill.

The most required nutrients have specific roles to play in improving nut yield:

• Nitrogen is important to build the tree canopy and encourage vegetative growth leading to improved bud formation and higher yields and higher nut protein contents.
• Phosphorus is particularly important for root development, flower initiation and energy transport within the tree.
• Potassium, balanced alongside nitrogen, also boosts growth – including better water utilization. It is particularly important and needed in large quantities for nut-fill and the highest yields.
• Calcium helps build the tree and is particularly important for the development of good quality nuts with less disease.
• Boron is important for flowering and pollination, to ensure good fruit set.
• Zinc helps to maintain growth processes especially new tissue development.

Nut quality is also influenced by crop nutrition:

• Calcium and potassium help to improve good nut fill and tolerance to disease, reducing molds and storage rots.
• Nitrogen – in excess – can have a detrimental effect on nut quality. 
• Boron reduces water stage fruit split in pecans.

Nutrients such as phosphorus, magnesium and sulfur need to be applied in season at levels that maintain growth.

Early season production is mainly supported by reserves from the tree, but fertilizer nitrogen for example, applied in the spring and available from the soil, is required to meet a significant proportion of the crop’s needs from fruit set and enlargement through to harvest.

Nitrogen needs to be continuously available throughout the season and is removed in large quantities.

Between 17 and 100 lbs of nitrogen are removed for every ton of nuts harvested. This varies according to nut type; almond is the most N-demanding nut tree species.

During nut fill, nitrogen in fruit tissue is initially diluted, then builds rapidly as fruit expansion slows, ensuring good proteins and high yields. For this to happen, nitrogen must be continuously available for uptake.

Phosphorus, although needed in much smaller quantities, is critical to maintain a range of growth processes, especially strong root growth during the trees development. Potassium, like nitrogen is required at high levels. While it is utilized by the tree to support growth, potassium is also critical during nut development and fertilizer supplies can be, but not necessarily, applied slightly later than those of nitrogen.

Mean K removal in almonds and pistachios is on a par with that of nitrogen, but is lower in walnuts, pistachios and pecan. Calcium levels in the tree are up to 402 lb/ac though removals in harvested nuts are low. Calcium plays a role in supporting good fruit growth, so any calcium applied to established groves should target immediate uptake and utilization by the fruit. While levels of magnesium are normally adequate in most groves, maintenance applications are needed to maintain production and quality. Sulfur is commonly applied with potassium fertilizers in the form of potassium sulfate and so S-responses are uncommon. All data is given in elemental form, unless stated.

Micronutrients

While much lower levels of micronutrients are needed to satisfy growth and boost yield, all play a role, particularly in supporting key tissue growth.

The micronutrient taken up in greatest quantities is iron, however supplies are rarely limiting in most groves unless soil pH levels are highly alkaline.

Boron and zinc – are both required for strong tissue growth, pollination and fruiting, and need to be readily available before flowering.

Timing of boron and zinc is best prior to nitrogen application in the spring or in the autumn. Zinc is commonly used as a defoliant after harvest and its use at this time will also encourage good bud development in the following season.