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Updated : 21/9/2014


When roaming down the aisle of gardening products, are you slightly baffled by the three number code that appears on the labels of the fertilizer? Wonder no more!

There are three main nutrients that plants need to grow and thrive: Nitrogen, Phosphorus, and Potassium (think back to high school chemistry and remember that “K” is the symbol for potassium). All commercially packaged fertilizers have a set of numbers that correspond with these elements–these numbers indicate the ratio of that nutrient. The final number in brackets indicates the percentage concentration of the fertilizer.

The fertiliser in this example on the right contains two parts Nitrogen, 3 parts phosphate and 4 parts potassium. The concentration is 37% (37% of the content is nutrients, 63% is inert).

The amount of nutrients in 2:3:4(37) can be calculated as follows:

% Nitogen (N) = [parts nitrogen/(parts nitrogen + phosphate + potassium)] x percentage = [2/(2+3+4)] x 37 = 8,2%.

% Phosphate (P) = [parts phosphate/(parts nitrogen + phosphate + potassium)] x percentage = [3/(2+3+4)] x 37 = 12,3%.

% Potassium (K) = [4/(2+3+4)] x 37 = 16,4%.

10-10-10 contains 10 parts nitrogen, 10 parts phosphorus, and 10 parts potassium
16-16-8 contains 16 parts nitrogen, 16 parts phosphorus, and 8 parts potassium
21-0-0 contains 21 parts nitrogen and no phosphorus or potassium

When a fertilizer contains all three elements, it is called a “complete fertilizer.”

What balance of nutrients to add depends on your soil. For now let us assume that you are using  mel's mix


Nitrogen is essential for lush, green vegetation. But be careful–add too much and you’ll get lots of leaves and no fruit! Potatoes, corn, and onions need LOTS of nitrogen (about 4x’s what other vegetables require), while peas and beans need less.


Phosphorus promotes healthy root growth, flowering, and fruit. When there is an extreme deficiency, the leaves will turn purple, but moderate deficiencies result in small plants and stunted growth. This element is less available during cool temperatures, so it may be important to add it early in the spring.


Potassium promotes thick, sturdy stems in fruiting plants and higher yields from root vegetables. A deficiency leads to long, spindly stems and weak plants. This element is less available during cool temperatures, so it may be important to add it early in the spring.

Trace Elements

Besides these major nutrients, plants need several trace elements to grow well–like calcium, zinc, iron, boron, and manganese. Zinc and iron deficiencies cause the leaves to turn yellow or tan in between dark green veins.

Putting it all together

My recommendation for those using Mel’s mix is to combine 1 cup of 16-16-8 complete fertilizer with 1/4 cup of Ironite mineral supplement. Sprinkle and mix into the top 6-8 inches of soil before planting in the spring.

Be sure to purchase plant fertilizer instead of slow release lawn fertilizer. It is much less expensive to purchase bulk–either keep it for years to come, or divide among some gardening friends.

Amount Size Square Feet
1/3 cup 4′x4′ 16 sq ft
2/3 cup 4′x8′ 32 sq ft
1 cup 4′x12′ 48 sq ft

Names, formulations and characteristics of water-soluble fertilisers

1 2 3 4
Name Formulae Nutrient content (%) EC value (mS cm-1) at 250C.

Fertilisers containing calcium

Calcium Nitrate


15.5N; 19Ca;


Fertilisers containing potassium

Potassium Sulphate K2SO4 45 K; 18 S 1,54
Potassium Nitrate KNO3 13 N; 38 K 1,35

Fertilisers containing ammonium + nitrate

Ammonium Nitrate liquid



Fertilisers containing magnesium

Manganese Sulphate MnSO4.H2O 32 Mn
Magnesium Nitrate Mg(NO3)2.6H2O 11 N; 9 Mg

Fertilisers containing phosphate

Mono Potassium Phosphate KH2PO4 23 P; 28 K 0,68
Mono Ammonium Phosphate NH4H2PO4 27 P; 12 N 0,86

Fertilisers containing trace-elements

Manganese Sulphate MnSO4.H2O 32 Mn
Zinc Sulphate ZnSO4.7H2O 23 Zn
Borax Na2B4O7.10H2O 11 B
Copper Sulphate CuSO4.5H2O 25 Cu
Sodium Molibdate Na2Mo4.2H2O 40 Mo
Iron chelate (13% EDTA) Fe EDTA 13 Fe
Iron chelate (11% DTPA) Fe DTPA 11 Fe
Iron chelate (6% EDDHA) Fe EDDHA 6 Fe

Column 1: These are the common names of the fertilisers.

Column 2: In this column the chemical formulae of each fertiliser is given.

Column 3: This is the amount of nutrients in the fertiliser.

Column 4: This is the electrical conductivity of one gram of the fertiliser if dissolved in one litre of distilled water.

3 Foliar feed:

Foliar feed contains macro elements, trace elements and sometimes also growth stimulants. An endless list of products is available.

Organic alternatives

For those who prefer to fertilize only with organic materials, you have lots of options. Jerry Baker’s Backyard Problem Solver  includes tons of home made recipes to help in the garden.

There are also commercially available organic sources–blood meal (N), fish meal (N), mushroom compost (P), bone meal (P), seaweed (K), and wood ashes (K) are some examples. Just combine them in the same proportions listed above, and fertilize away!

Another obvious choice is to create and add compost to the garden. Just a quick word of warning–if you add organic materials that are not yet decomposed they can create a shortage of nutrients in the garden. But if you allow micro-organisms to work their magic, your grass clippings, food scraps, leaves, and manure can be a gold mine!

SUPER GRO 100% Liquid Organic Fertilizer

SUPER GRO is a 100% organic liquid fertilizer, which is made from poultry droppings and sea bird guano. Being made from organic matter with absolutely no chemicals added to it, it is 100% safe to use on any vegetables and of course the rest of your garden. SUPER GRO can be applied to any plant, tree, vegetable and even grass that require fertilization.

Why Organic?

Chemical fertilizers do not contain carbon, while organic fertilizers do. Organic fertilizers are every bit as strong as their chemical equivalent. While chemical fertilizers act quickly, organic fertilizers are slower to be absorbed but have the benefit of improving the soil quality.

SUPER GRO was started about fifteen years ago on the West Coast of South Africa. Its main ingredient being seabird guano gets collected from eight different sites along the West Coast under supervision of the South African Nature Conservation, to ensure that all of the birds and their nesting sites stay protected and minimal disturbance takes place. After the guano is dried and grounded it gets sent up to Johannesburg for processing and the end result is, SUPER GRO 100% ORGANIC LIQUID FERTILIZER. The factory originally only supplied farms on the West Coast of South Africa. About six years ago SUPER GRO branched out to supply nurseries and growers in the Gauteng region. To date SUPER GRO is distributed to over one hundred nurseries and growers from Pretoria, to the South and East rand as far as Vanderbijlpark and also moving into the West rand.

Tests done by the Central Analytical Laboratories in Pelindaba, reflect the following composition of macro and micro elements;

  • Nitrogen (N) 72g per litre (Macro)
  • Phosphorus (P) 45g per litre (Macro)
  • Potassium (K) 30g per litre (Macro)
  • Sulphur (S) 15g per litre (Macro)
  • Calcium (CA) 9g per litre (Macro)
  • Magnesium (MG) 7g per litre (Macro)
  • Iron (FE) 5mg per litre (Micro)
  • Iodine (IoD) 3mg per litre (Micro)
  • Marine Salt (MS) 1mg per litre (Micro)
  • Zinc (Zn) 1mg per litre (Micro)

This product is very economical for its price, as one litre concentrate dilutes with twenty litres of water. This gives you twenty one litres of mixed fertilizer to spread among your plants, trees or vegetables.

SUPER GRO comes in one, two and five litre plastic bottles, which is bottled and labelled by Indigenous and Organic Connection.

Indigenous and Organic Connection has a low carbon footprint as no electrical equipment is being used for the bottling process. The bottling of SUPER GRO is done by hand and measured correctly and the highest quality is insured.

1 Litre concentrate diluted 20:1 per 10 square meters. Repeat after 15 days Thereafter every 6 weeks Super Gro is also rich in Iron, Marine salt, and Iodine. Super Gro is made from an enriched mix of seabird guano and poultry droppings. Licence No. B3634, Act 36/1947, Group 2 Fertilizer.

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