The term ‘trace elements’ was coined in the early days of plant nutrition science to refer to those constituents of plant ash that were present in really tiny ‘trace’ amounts. This term has stuck, but it has often been replaced by the terms ‘micronutrients’ or ‘minor nutrients’.
Gradually the ‘trace’ elements in plant ash were identified. Trials were conducted to see if they were essential to plant growth, or were just there ‘for the ride’.
Early trials revealed that the elements iron (Fe), manganese (Mn), zinc (Zn), copper (Cu) and boron (B) were essential to the growth of all plants that were studied. Then, in the 1940s, molybdenum (Mo) was added to the list. In more recent years, chlorine (Cl) and nickel (Ni) have also been included. For some plants such as sugar beet, sodium (Na) has also been found to be essential to normal growth.
As far as nurseries are concerned, the important trace elements are the first 6 listed above, plus nickel for those who grow pecans and (perhaps) walnuts. There is always plenty of sodium and chlorine (sodium chloride = common salt) present.
The trace elements needed by plants grown in containerised environments are sourced either from the bulk components of the growing medium or from additions of fertilisers or trace element mixtures.
The pine bark used in the majority of the growing media we manufacture naturally contains a small amount of each of the 6 trace elements. Therefore, is it necessary to add more when the mix is being formulated? The answer is yes, and no… It depends on the likely drift in the pH of the mix, and on the length of the growing period before sale or repotting.
Effect of pH
The availability of trace elements to plants depends in part on the pH of the mix. As pH increases, the availability of iron, manganese, zinc, copper and boron decreases. At very low pH values there is a possibility of toxic amounts of manganese, zinc, copper or boron being taken up by plants. The effect of pH on molybdenum is the opposite of this.
The main reason for stipulating that the optimum pH value of a growing medium is in the range about 5.3 to 6.3 is that in this range the availability of all trace elements is close to maximum. One of the skills needed by growing media suppliers and users is to get the pH right at the start and to keep it within this range for the duration of the production period.
Manufacturers of growing media know from analysis of their mixes by the Australian Standard method just how much of each trace element must be added for ample supply. For mixes based on Pinus radiata bark, it is essential to add extra copper and iron, but producers often add small amounts of the others as well.
Ongoing supply of trace elements
There is very little loss of the trace elements from bark-based growing media during normal irrigation. The trace elements are held fairly firmly by the humus of the bark, mostly as natural chelates. Iron, copper, zinc and molybdenum will continue to be supplied for many years. Initial manganese and boron supplies will usually last at least 12 months, but if there is heavy leaching of the growing medium with fertigation solutions, and especially if it is accompanied by acidification of the mix, there can be considerable losses, so extra of these two trace elements may be needed after 12 months.
For production runs of less than 12 months, so long as the mix has ample initial amounts of all trace elements, there is no need to add more of them. Beyond that, and in the absence of potting up with a new mix, it is desirable to use a fertiliser that supplies trace elements along with the major nutrients.
For more information, please contact one of our grower services representatives.
Please note, any advice displayed is of a general nature only, and an intending user of a product should obtain and only rely on professional advice particular to their intended purpose, climate, soil conditions and other factors.