The best fertilizer for your plants depends on the source of your water supply, the type of potting mix, and the temperature of your growing area.
Almost all fertilizers contain the “Macro Nutrients” of nitrogen (for foliage), phosphorus (for blooming and root development) and potassium (for overall plant health). Some will also contain calcium and magnesium. Most city water sources contain calcium and magnesium in sufficient amounts. However, if you use distilled, rain or Reverse Osmosis water, you will have to add a supplement of Cal/Mag to supply these 2 necessary nutrients.
Many fertilizers also contain “Micro Nutrients” in very small amounts, which may or may not be listed on the label. An important one is sulfur which tends to lower the pH in the root zone. If your growing environment is already too acidic (below 6), then sulfur may make it worse.
The source of nitrogen is important to consider when choosing a fertilizer: ammoniacal, nitrate and urea are the 3 sources. Plants can only use nitrogen in its nitrate form. Ammoniacal nitrogen is easily converted to the nitrate form by the potting mix. Urea must be broken down into ammonium carbonate and then into nitrate. This requires the work of microorganisms. If you use soilless potting mix which lacks microorganisms, or grow in very cool conditions when soil microorganisms are inactive, excess urea can build up and cause fertilizer burn of the roots.
Depending on the type of nitrogen, the pH of the growing medium may be lowered or raised.
- · Ammoniacal nitrogen tends to lower pH
- · Nitrate nitrogen tends to raise pH
- · Urea has little effect on pH it is readily absorbed by the roots
Some brands of fertilizer list “Potential Acidity” on the label. This refers to how acidic the product is and is defined as” how many pounds of calcium carbonate it takes to neutralize the acidic effect of one ton of fertilizer”. The higher the number, the more acidic. For example, one brand of 20-20-20 has a potential acidity of 597, but their 15-16-17 product has a potential acidity of 215. Your choice may be influenced by the type of plants you are growing – acid loving or a more alkaline environment.
It is recommended to use a balanced fertilizer, which means almost equal amount of each macro nutrient. These nutrients are delivered in the form of a salt. Using large percentage numbers can cause salt build up, so it is recommended to periodically leach the soil by pouring distilled water through the pot until it runs clear. Or repot at least every 6 months.
Remember, it is best to fertilize “weakly, weekly” – small amount of fertilizer each time you water. And then use plain water without fertilizer every 4th watering. Some growers like to rotate kinds of fertilizer. Do what works best for you and your plants.
