Q. I applied the dry fertilizer/micro nutrient mix by hand. I did notice that there
seems to always be undissolved fertilizer, even after several waterings. This didn’t seem to bother the more mature plants, but I’m wondering if it could “burn” the younger plants, and also wondering if I should modify the fertilizer routine for the younger plants.
A. The Weekly Feed Mix is about 45% actual mineral salts, and the other 55% of the material is NOT fertilizer, but just filler material. The only time you would not see residue is if you were to purchase the expensive totally water-soluble fertilizers that the hydroponic growers use in the greenhouse.
Therefore, what you are seeing as residue on the surface of the soil after a few days of watering is not fertilizer and will not harm your plants.
Do not modify the feeding routine for smaller plants. Sixty years of experience and millions of lab (garden) experiments have established the importance of the fertilizer formula, the amounts, and the frequency of feeding.
What happens is that the fertilizer dissolves, then some of it quickly adheres to the soil particles and becomes “fixed” or unavailable to the plants. This is particularly true of phosphate and potash. Over time, those adhered particles are released and once again become available, and meanwhile the continued application of fertilizers each week feeds the plant. When you stop applying fertilizers, there is a residual effect, and the plant continues to receive the benefit of the previous build-up for a few weeks.
Fertilizer will not burn or harm your small plants if used properly. It is supposed to be placed in the center of the bed between two rows of plants, and so will be several inches distant from plant stems.
Leaves should not come in contact with the fertilizer, because you should always be pruning any leaves that touch the ground.