All bags or packets of fertilisers all have one thing in common, they all have a number ratio printed clearly and in plain sight on the packaging. For most people these numbers mean very little, and concentration on the buying decision can often be driven by other aspects of the packaging or trust in a particular brand.
These numbers on bags of fertilisers represent a very important ratio of all the three major nutrients required by all plants, including lawns. These major nutrients are (in order) Nitrogen - Phosphorous - Potassium, and the ratio on the bag of fertiliser is telling us the ratio in which these major nutrients are present in each bag of fertiliser we are considering purchasing.
The ordering of the nutrients is always the same across all brands and all fertiliser types. So a ratio of 10 : 4 : 4 will represent 10 parts Nitrogen - 4 parts Phosphorous - 4 parts Potassium.
Each major nutrient has a specific job to do, and each one can help our lawns or plants in specific ways. So if we can diagnose a problem of nutrients lacking in our soils, we can easily rectify it by purchasing an appropriate fertiliser for our needs.
Right now you may be thinking that 10 + 4 + 4 only equals 18 parts in total, and it may also occur that another 82% of the bag of fertiliser is left unaccounted for - so the question is… what's in the other 82%.
The rest of the fertiliser may include things such as minor nutrients and trace elements in tiny amounts, which still leaves a large portion of the fertiliser left unaccounted for.
The rest of the fertiliser is made up of various fillers which bulk up the fertiliser to dilute it's intensity. Without these fillers being present - the fertiliser would be too strong, would have to be applied in minuscule amounts - and would pose a grave risk of damaging or killing a lawn if they were applied even slightly over recommended application rates. So fillers are there for our own benefit as security against us damaging our own lawns.
Lawns and all plants don't just require these 3 major elements, they also require many other nutrients and minerals such as Iron or Manganese. These nutrients are often present in many of the better lawn fertiliser but can be ignored and left out by cheaper brands.
Trace elements will not be listed in the ratios on bags of fertiliser, however, they will still be listed, and if a quality fertiliser is what you're after, it may be wise to turn the bag over and look for the entire makeup of the fertiliser. If Trace Elements are present, they will be listed on the back of the bag.