How to influence almond kernel size

Nitrogen is important to build the tree canopy and encourage vegetative growth, leading to improved bud formation, higher yields, and higher nut protein contents.

Key stages

Nitrogen is essential for growth and development being one of the main building blocks of proteins. It supports photosynthesis and strong productive growth leading to high almond yields.

Use of nitrogen is important to build and maintain the tree canopy. When it is in short supply, cell division and expansion in the growing tip are retarded and leaf area restricted.

Nitrogen encourages early tree development, promoting stem, shoot and leaf growth.

This carries through to the establishment phase, leading to faster girth growth and taller trees. Once established, trees with adequate nitrogen supply have higher leaf N contents and greater yields.

Nitrogen supply also influences tissue development. Trees with higher levels of fertilizer nitrogen encourage greater flower survival, leading to better fruiting.

Higher nitrogen use can improve yields in the off-year, helping to reduce the effects of alternate bearing. Regular use helps build yields year on year as the tree develops and N reserves are laid down.

Almonds are among the most efficient crops in terms of nitrogen uptake. Research in California shows that 70% or more of the applied nitrogen ends up in the almond tree.

The higher the N rate used, the higher the removal in the harvested almond. In this trial, removals peaked at 268 - 357 lb/ac annual N use and did not increase further with additional nitrogen application.

However, it is important not to apply too much nitrogen or yield and quality will deteriorate. In some instances excessive nitrogen has also been linked to increased hull rot in almonds as a result of an increased susceptibility to infection.Note that the same toxins that affect the hull, can also be found in non-fruiting spurs, causing dieback. The graph shows the percentage of spurs affected either by hull rot infection or by dieback.

Nitrogen form also influences soil pH. In drip irrigation trials on almonds, calcium nitrate helped maintain a more optimum soil pH in the upper layers of soil than ammonium sulfate. Similar deleterious effects on pH are seen from the use of urea.

The key stages for N applications are:

End of bloom to full leaf expansion. 20% of the needed N is commonly applied at this time.
Fruit growth – between full leaf expansion and shell hardening, when around 30% of total N should be used.
Kernel filling when a further 30% of the estimated total should be applied, dependent on revised yield estimates according to the season.
Early post-harvest – the remaining 20% of N is used to set up the tree for good yields in the following year

In this almond trial, labeled N fertilizer was applied at different dates during 1980 (March, June, August or December), and N recovery in the following year’s fruit investigated. Applications of nitrogen made in August result in a higher N recovery in fruit harvested the following year.

Fruit N uptake from earlier N applications in the previous year were lower and December applications resulted in reduced N utilization in the following year.

Foliar nitrogen applied in the summer is more likely to end up in the kernel than soil applied nitrogen, particularly when this is applied too late in the season for uptake. The uptake of foliar applied urea is particularly fast. In this trial, about 80% was taken up within 48 hours after application. 14 days after application, 70% of the applied urea had been translocated from the leaves to other plant organs.

In almond, about 80% of applied nitrogen is taken up into the fruit within 130 days after full bloom.