Wed, 24 Feb 2016
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How to use phosphorus supply to control growth rate

When purchasing a potting medium from the majority of Australian producers, the formulation will contain adequate supplies of most essential nutrients.  These being:

  • Trace elements – iron, manganese, zinc, copper, boron, molybdenum (and nickel as needed)
  • Secondary nutrients – calcium and magnesium from the limestone and dolomite added to adjust pH, and sulphur (and more calcium) from added gypsum

In addition, the formulation will usually have starter supplies of the major nutrients nitrogen, phosphorus and potassium:

  • nitrogen in both soluble form (often urea) and slow-release forms such as urea formaldehyde and IBDU, to counter nitrogen drawdown and to supply starter amounts of soluble nitrogen.
  • phosphorus – from small amounts in the bulk ingredients and via added fertilisers
  • potassium – from pine bark, coir fibre dust and added in fertiliser salts such as potassium sulphate and potassium nitrate

More of these major nutrients will be supplied as needed via fertigation or via included controlled-release fertilisers.

In the first month or so of plant growth within the mix, the pre-planting additions will continue to supply ample amounts of all the trace and secondary nutrients, and of the major nutrient potassium, so plant growth will be controlled by the supply of nitrogen and phosphorus. If there is ample supply of both of these major nutrients present, plants can grow at the maximum rate permitted by environmental conditions such as light, temperature, humidity, day length, oxygen supply in the potting mix, carbon dioxide concentration in the air around the plants, microbial status and water supply. If all of these environmental conditions are optimised – and if ample nitrogen and phosphorus are supplied – plants will grow very rapidly, but will usually be soft, stretched and perhaps have a high susceptibility to various diseases. Under these conditions, growth rate can be reduced and controlled by one of three nutrient supply strategies (control via intermittent drought is possible, although difficult and risky).

  1. Cut the supply of both nitrogen and phosphorus
  2. Cut the supply of nitrogen
  3. Cut the supply of phosphorus

In most nurseries, it is probably one or the first two strategies that is most likely used.  Although we will be discussing the third strategy – controlling growth by carefully limiting phosphorus supply. This strategy is most useful for short term crops such as seedlings and bedding plants, however the principles apply more broadly in nurseries.

Balance between nitrogen and phosphorus

Matured leaves on healthy plants always contain more nitrogen than phosphorus. The concentration of phosphorus (P) will typically be between 5 and 15% of the nitrogen (N) concentration. Put another way, the P concentration, divided by the N concentration (the P/N ratio), will be between 0.05 and 0.15. Phosphorus-sensitive species such as proteas tend to be at the lower end of this range and more herbaceous species in the middle to upper part.

A conclusion from this is that fertilisers included in or applied to soil-less media should also have a P/N ratio roughly in the 0.05 to 0.15 range. Only where some of the supplied P is locked away from access by plant roots (as when there is soil in the medium) might the fertiliser need to have a P/N ratio above the 0.5 to 0.15 range.

Application to potted colour and vegetable seedlings in cell trays

Precise control of the P/N ratio of fertiliser applications is much easier with fertigation than where controlled or slow-release fertilisers are used. With fertigation, both the amounts of N and P supplied and the P/N ratio of the solution can be varied at will. A main aim with seedlings is to allow rapid growth, with the formation of excellent root systems, but without shoot stretching. If N and P applications are optimised, it is usually possible to achieve these results without the use of growth retardants.

A useful starting point could be to use a solution with 50 to 75 mg/L of N and a P/N ratio of 0.07. The actual amounts of N and P applied to each plant will be determined by the frequency of application and the amount of solution that enters each growing cell. A certain amount of trial and error is needed to find the optimum combination for each type of seedling. Of course if the solution is applied very frequently, the total amount of P supplied could be too high and therefore still allow stretching. The aim is to supply more than adequate N, but to keep P supply low enough that it controls growth rate. P supply will be just a little more than a level that causes deficiency symptoms.

If a fertigation solution with a P/N ratio of 0.07 still allows stretching, a next step would be to slightly reduce P supply by reducing the P/N ratio of the solution to perhaps 0.06 or even 0.05. If the seedlings start to show symptoms of P deficiency (blue-green or purple colours in older leaves, followed by yellowing), you know that you have gone too far. For some plants it might even be necessary to use a solution with a higher P/N ratio than 0.07.

Example of calculations

  • You use Secure AN-25 liquid concentrate as your source of N. This solution contains 25% w/v N (ie. 25 g N per 100 mL).
  • You use monopotassium phosphate as your source of P. This chemical contains 23.5% P. Here we ignore the potassium also supplied.
  • A 1000 L solution that contains 50 mg/L N will need an addition of 1000 x 50/1000 g of N = 50 g. This will be supplied by 200 mL of the AN-25 concentrate.

The following amounts of monopotassium phosphate are needed to produce the following P/N ratios.

P/N ratio P needed per 1000 L (g) Monopotassium phosphate needed (g P/0.235) (g)
0.05 2.5 10.6
0.06 3.0 12.8
0.07 3.5 14.9
0.08 4.0 17.0


Application to other nursery plants

The same general principles apply to other nursery plants, but with nutrients supplied by controlled-release fertilisers, the precision possible with fertigation solutions cannot be achieved. Nevertheless, an overall aim is to attempt to minimise the supply of P to just adequate for the amount of growth needed. And as flowers have much the same concentration of P as leaves, there is no need to have high P inputs to plants that are about to flower. In fact, using a fertiliser with a P/N ratio well above 0.15 has been shown to reduce flower production in a number of plants commonly grown in nurseries. When calculating the P/N ratio for controlled-release fertilisers, use the water-soluble part of the P content of the fertiliser, not the total P content. Any P within the fertiliser prill that is not soluble in water will not be released until the coating has ruptured.

More information 

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.