(Simple presentation)

Some twenty years ago, gardeners did not have many choices. There were only a handful of fertilizer products on the market to choose from, and from those very few were liquid fertilizers. Today, there are hundreds of competing products, each promising the heaven on earth. We have normal, slow release fertilizer in powder or pellets, for the garden or for pots, liquid fertilisers, etc. Are we better off today?

Despite all major developments and improvements in testing, production and presentation, not all products available today carry information related to the content of the product and/or how to use it.

Recently, I purchased 1l of expensive liquid fertilizer produced by a well-known international company. The 1l container shows no info about the content and there are no "how to use it" instructions. I use 2.5ml per litre; however, I have no idea if I used too much or too little. The only message on the bottle is: “Increased root mass gives superior growth and yield.” After a year, I noticed no difference.

Fertilizers are organic or inorganic, natural or synthetic products, which when added to the soil, can supply one or more of the nutrients required by a plant in order to grow, remain healthy and flower.

There are two types of fertilizers: 1. Organic Fertilizers (contains enriched organic matter). 2. Inorganic Fertilizers contain compounds, synthesized using a process that produces ammonia as the end product. Ammonia is then used as a feedstock for other nitrogen fertilizers, such as anhydrous ammonium nitrate and urea.

Fertilizers are divided into two major groups:

1. Basic fertilizers: These products supply in pre-arranged proportions the macro-nutrients: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulphur (S). The most commonly used macro-nutrients are: Nitrogen (N), Phosphorus (P) and Potassium (K) and their concentration is presented as N-P-K; or N-P-K-S when sulphur (S) is added. Other macro-nutrients are: carbon (C), hydrogen (H), oxygen (O) and CO2.

2. Complex fertilizers. Complex fertilizers supply in addition to the above mentioned macronutrients, other nutrients that are needed in smaller amounts, known as micronutrients. Although soils and water contain some micronutrients, there is a need to make as many micronutrients as possible available. Micronutrients are added on basic fertilizers either as a part of the recipe or at the buyer’s request based on soil requirements.

The micronutrients are: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). Deficiency or oversupply in one or more of the above can cause visible blemishes, such as yellowing of the leaves, etc.

What does N-P-K mean?

I am told that it means different things to different people. Some claim that it is calculated differently in North America to Europe. I am not quite sure about that.

According to Wikipedia: “NPK” rating (or N-P-K) is used to label fertilizers based on their relative content of the chemical elements nitrogen (N), phosphorus (P) and potassium (K) that are commonly used in fertilizers. The N value is the percentage of elemental nitrogen by weight in the fertilizer. The values for P and K represent the amount of oxide in the form of P2O5 and K2O that would be present in the fertilizer if all the elemental phosphorus and potassium were oxidized into these forms”.

The value for Nitrogen (N) is simple: It represents the wt.% of elemental nitrogen present.

The values for Phosphorus (P) and Potassium (K), are complicated and confusing for number of reasons.

A chemical analysis presents Phosphorus as P2O5, and Potassium as K2O. In N-P-K, phosphorus is presented as the wt.% of an equivalent quantity of P2O5 and Potassium as an equivalent quantity (wt.%) of K2O. Plants are only interested in the “elements” Phosphorus and Potassium.

To determine the P (wt.%) content, the P2O5 (wt.%) given must be converted to P (wt.%). It is done by multiplying the amount of P2O5 (wt.%) x 0.44 Why 0.44? The atomic weight of oxygen is 16 and that of phosphorus is 31. P2O5 has 2P and 5O; 2P = 62 and 5O = 80; the molecular weight of P2O5 = (62 + 80) = 142; 62 (2P): 142 (P2O5) = 0.44 (figures rounded up).

To determine K (wt.%) content, the K2O (wt.%) given is multiplied 0.83.

When a fertilizer contains 100 (wt.%) P2O5, it contains is 44 (wt.%) elemental Phosphorus. That is the amount of Phosphorus available.

Taking into account that only soluble nutrients are useful and can be absorbed by plants, the next question is: How many wt.% of the amount of Phosphorus available is soluble? Most fertilizer products present both figures; the amount available and the amount soluble.

Insoluble nutrients can accumulate in the soil and can contain minute amounts of metals. The presence of these metals in larger amounts in the soil can be harmful to the plants and to the growers’ health alike, that’s where charcoal is useful.

More and more manufacturers present the nutrients (chemical analysis results) in grams/Litter (g/L). I like this way.

To avoid misunderstandings, I would like to see two statements: 1. The analysis represents individual elements and not oxides (K = K and not K = K2O); and 2. 100% of the wt.% given of every element is water soluble.