Powdered pre-mixed plant fertilizers are often a cheaper and easier way to grow for the hobby horticulturist.
When using premixed powdered fertilizers, it is very difficult to achieve a homogeneous mixture of fertilizers when hundred of pounds of individual compounds are mechanically mixed together and will not mix evenly due to the different size, weights and volumes of the individual components. Many of the compounds are in powder or fine grain form which often gets clumped together during shipping and storage. Experience with such powdered pre-mixes has often revealed shortages in magnesium and iron and excess of manganese.
This nutrient imbalance is why many commercial grows use pre-made or custom made liquid plant fertilizers in order to avoid any nutrient imbalances, deficiencies and toxicities.
Prior to dispensing any premixed liquid plant fertilizers, the container should always be slightly agitated to ensure an even distribution of nutrients in the liquid solution prior to use.