Specialty Gardening: Container Soils - Water Movement and Retention III , 0 by tapla

tapla wrote:
NPK % tell you how much Nitrogen (N), Phosphorous (P), and Potassium (K) are in the fertilizer by weight. The fertilizer RATIO is much more important than the NPK %s, and describes the presence of the 3 elements in relation to each other. For example, Miracle-Gro makes 24-8-16 in a granular soluble fertilizer. Those 3 numbers are the % of NPK in the product. They also make 12-4-8 in a liquid fertilizer (yellow jug). Both fertilizers are 3:1:2 RATIO fertilizers. For every 3 parts of N, there are 1 part of P and 2 parts of K, thus the 3:1:2 ratio. It's a little like reducing fractions.

I use another 3:1:2 ratio fertilizer, Foliage-Pro 9-3-6. If you reduce the NPK %s to their lowest whole number form by dividing by 3, that too is a 3:1:2 ratio.

All plants use the same elements, in surprisingly close to the same ratio. They use abut 6X as much N as P, and about 3/5 as much K as N, so it's very difficult to build a solid case for using 1:1:1 ratio fertilizers like the popular 20-20-20 or 14-14-14 controlled release. 1:1:1 ratio fertilizers supply more than 2.5X the P plants want or can use, and the "bloom booster" fertilizers are an extremely poor choice for containerized plants because P they supply in great excess is usually detrimental. Remember, anything dissolved in the soil solution that is not needed has the potential to be limiting.

Unless we're intentionally using fertilizer in a way that limits growth, the perfect goal would be to ensure that all the nutrients plants normally take from the soil are present in the soil and available for uptake at all times, in the same ratio at which the plant uses the nutrients, and at a level concentrated enough that it ensures no deficiencies yet low enough it ensures the plant can easily take up water and the nutrients dissolved in the water. As hobby growers, the closest we can come to that would be by using a fertilizer that has all the essential nutrients in a 3:1:2 ratio.

MG 24-8-16 and 12-4-8 have almost all the essential nutrients in a favorable ratio, but they lack Ca & Mg. This isn't usually an issue if you're using a commercially prepared soil because it will almost certainly have been pH adjusted with dolomitic lime, which raises pH ans supplies the missing Ca/Mg. If you make your own soils, you need to add the lime or supply the Ca/Mg by using other amendments, usually gypsum (CaSO4) and Epsom salts (MgSO4) ...... but now I've drifted well beyond the scope of your question.

Al