Researching suitable N-P-K ratios for indoor plants recently, I was reminded that product labelling is not always consistent across the globe. Nitrogen, phosphorus and potassium are three essential elements required by plants to support their growth and it is essential that interior plantscapers understand just how much they are applying to their plants. Deficiency of these elements usually shows up in older leaves, a useful diagnostic tip especially for those interior plantscapers just starting out. 

An N-P-K ratio essentially refers to the percentage of nitrogen (N), phosphorus (P) and potassium (K) found in a fertilizer product. American interior plantscaper (retired) and IPA Honorary Member, Kathy Fediw says that most professionals in her country recommend a fertilizer with a 24-8-16 N-P-K ratio for indoor plants (or one with a 3-1-2)¹. She also says that a 20-20-20 fertilizer, or a 15-30-15 fertilizer are considered good all-purpose fertilizers and can also be used on indoor plants¹.

However, Dr Samuel Stacey, a Technical Manager with ICL Specialty Fertilizer, said

“That those ratios appear to be based on oxide analysis, rather than elemental analysis which is used in Australia. Fertilizer labelling regulations differ here to those used in the United States and the European Union, which means that the N-P-K numbers quoted on labels are different. The American ratios are therefore different to what an Australian grower would see.

The Association of American Plant Food Control Officials’ AAPFCO Product Label Guide states that the grade statement on a fertilizer product in the United States must refer to the percentage of Total Nitrogen (N), Available Phosphate (P₂O₅) and Soluble Potash (K₂O) in the same terms, order, and percentages as in the guaranteed analysis².

However, here in Australia our fertilizer product labels must include the concentration for all elements in the elemental form (e.g. P, K) and not the oxide form as is the case in some countries (e.g. P₂O, K₂O). For solid fertilizers, percentage or grams per kilogram on a weight for weight basis as supplied. (e.g. 4.1% w/w 41g/kg w/w). For liquid fertilizers, on a weight for volume basis, as percentage weight for volume, with the method to be stated. (e.g. 4.1% w/v). The concentration may in addition be stated as grams per litre or milligrams per litre in the same manner (e.g. 4.1% w/v and (optionally) 41g/L)³. “

What is a suitable N-P-K ratio for indoor plants and why?

Dr Stacey says that plants normally require higher levels of nitrogen and potassium than phosphorus. His colleague Robert Megier, ICL’s Turf & Ornamental Regional Manager says that an N-P-K ratio of 4-1-3 or 6-1-4 would work well for most indoor plants.

So nitrogen, phosphorus, and potassium are obviously important in maintaining plant health? But what is it they do exactly? And why do symptoms of their deficiency appear in older leaves first?

Speaking with Michael Harding, EcoGro Director and IPA member said:

 “Nitrogen is essential in the formation of protein, and protein makes up much of the tissues of most living things. Phosphorus stimulates root growth, helps plants set buds and flowers, and produce seeds. It also helps plants use other nutrients more efficiently and helps turn energy from the sun into usable energy for your plants.

Potassium helps plants grow strong stems and keep growing fast. Plants lacking in potassium do not have enough energy to grow properly grow, their roots are not well formed, and they have weak stems, the edges of older plant leaves appear burned, and potassium deficient plants cannot regulate and use water efficiently, Michael adds.

In addition to nitrogen, phosphorus and potassium, plants require additional elements, including oxygen (0), carbon (C) and hydrogen (H). Elements taken up from the soil or growing media are generally categorised as either macro or micro-nutrients. Macronutrients include nitrogen (N), phosphorus (P), potassium (K), as well as calcium (Ca), magnesium (Mg) and sulphur (S). Micronutrients (or trace elements) include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), silicon (Si) and zinc (Zn). These elements are required by plants in smaller amounts than macronutrients.

Movement of these macro and micronutrients through a plant occurs through its conductive tissues, i.e. its xylem and phloem. Whilst most movement from the roots occurs in the xylem, lateral movement and redistribution of nutrients in plants is largely via the phloem.

So called phloem-mobile elements are present in soluble forms in plants. These elements include magnesium (Mg), nitrogen (N), phosphorus (P) and potassium (K). Their deficiency symptoms usually first appear in recently matured and older leaves.

Phloem-immobile elements are present in plants in insoluble forms. These elements include calcium (Ca), iron (Fe) and boron (B). Their deficiency symptoms appear in youngest leaves first.

In between, we have those elements with intermediate phloem mobility: copper (Cu), manganese (Mn), molybdenum (Mo), sulphur (S) and zinc (Zn).

So if you are trying to determine why a plant’s health is poorly, look not only at what colour its leaves are compared to its healthy counterpart, but also the location of those leaves on the sickly plant. For more information on deficiency symptoms associated with the absence of macronutrients refer to Table 1.

Table 1. Essential Elements: Functions and Deficiency Symptoms associated with plant macronutrients

Element (macronutrient)	Functions	Deficiency symptoms
Nitrogen (N)	Component of amino acids, proteins, nucleotides, nucleic acids, chlorophylls, and coenzymes	General chlorosis, especially in older leaves; in severe cases, leaves become completely yellow and then become tan as they die; some plants exhibit purple coloration due to accumulation of anthocyanins
Phosphorus (P)	Component of energy-carrying phosphate compounds (ATP and ADP), nucleic acids several coenzymes, phospholipids	Plants dark green, often accumulating anthocyanins and becoming red or purple; in later stages of growth, stems stunted; older leaves become dark brown and die
Potassium (K)	Involved in osmosis and ionic balance and in opening and closing of stomata; activator of many enzymes	Mottled or chlorotic leaves with small spots of necrotic tissue at tips and margins; weak, narrow stems; mostly older leaves affected
Sulphur (S)	Component of some amino acids and proteins and of coenzyme A	Young leaves with light green veins and interveinal areas
Magnesium (Mg)	Component of the chlorophyll molecule; activator of many enzymes	Mottled or chlorotic leaves; may redden; sometimes with necrotic spots; leaf tips and margins turned upward; mostly older leaves affected; stems slender
Calcium (Ca)	Component of cell walls; enzyme cofactor; involved in cellular membrane permeability; component of calmodulin, a regulator of membrane and enzyme activities	Shoot and root tips die; young leaves at first hooked, then die back at tips and margins, developing cut-out appearance at these sites

Reference: Table adapted from  Raven, PH, Evert, RF & Eichhorn, SE 2005, Biology of plants, W.H. Freeman and Co, New York, (Table 29-2. Essential Elements: Functions and Deficiency Symptoms)

Further reading

To find out more about the pros and cons of some fertilizers commonly used indoors including controlled release, water-soluble, foliar and organic applications, read IPA’s article, ‘What makes a fertilizer suitable for application in interior plantscapes?’, in the August 2021 issue of www.hortjournal.com.au

Your recommendations?

Share your stories and/or your favourite tips for using fertilizers indoors on IPA’s Facebook page https://www.facebook.com/interiorplantscapeassociation

Postscript

Is it spelt ‘fertilizer’ or ‘fertiliser’? I am an ‘s’ person myself. However, you will find that the Australian fertilizer industry uses the ‘z’. So that is what I stuck with here. And do not get me started on ‘sulphur’ or ‘sulfur’!

Written by:

Gabrielle Stannus / Inwardout Studio  (IPA Committee Member)

References

1. Fediw, K 2015, The Manual of Interior Plantscaping: A guide to design, installation, and maintenance, Timber Press, Portland, Oregon (page 187)

2. Association of American Plant Food Control Officials (AAPFCO) 2019, AAPFCO Product Label Guide, viewed 14 July 2021, http://www.aapfco.org/pdf/product_label_guide.pdf (page 3)

3. Fertilizer Australia 2018, Code of Practice for Fertilizer Description & Labelling, May 2018, viewed 14 July 2021, https://fertilizer.org.au/Portals/0/Documents/COPs/National%20Code%20of%20Practice%20for%20Fertilizer%20Description%20and%20Labelling.pdf?ver=2017-06-23-114549-300