Liebig's Law of the Minimum, often simply called Liebig's law, states that growth is dictated not by total resources but by the scarcest resource which becomes the limiting factor (Justus von Liebig, 1873). This is often depicted as Liebig's barrel where the individual staves in a barrel represent nutrients or other limiting factors.
A large amount of nutrients are taken up to build the frame and the canopy of the tree and retained as reserves.
For example, as much as 200kg N/ha is found in almond tree roots and a similar quantity in the rest of the tree’s vegetative components.
Some of this nitrogen is remobilised after winter to promote early leaf production, tree growth, flower bud and fruit formation, and fill.
Alongside nitrogen, significant amounts of other nutrients are also needed. Calcium is particularly important in supporting the tree’s structure with almost 400kg/ha required in a hazelnut orchard.
Every season around 15-20% of nutrients held in tree reserves are recycled through the soil as a result of pruning and leaf fall and this needs to be taken into account when calculating fertiliser requirements.
Fertiliser 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.
Nutrients such as phosphorus, magnesium and sulphur need to be applied in season at levels that maintain growth.
Early season production is mainly supported by reserves from the tree, but fertiliser 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 8.5 and 50kg nitrogen is removed for every tonne 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 maintaining a range of growth processes, especially strong root growth during the tree's 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 fertiliser 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 450kg/ha (Figure 18) 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.
Sulphur is commonly applied with potassium fertilisers in the form of potassium sulphate and so S-responses are uncommon. All data is given in the elemental form unless stated.
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 the 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.