Fertilizing Potted Plants
Early to bed, early to rise, Work like hell: fertilize. - Emily Whaley
Whether you are growing indoors or out, fertilizer is essential to the success of container gardens. The easiest way to go about fertilizing potted plants is by preparing a nutrient solution and pouring it over the soil mix. The fertilizer is absorbed by the roots and quickly adds what is missing from the existing soil. Even if your potting mix is perfect from the get-go, it will soon become depleted of nutrients as they are constantly used up by plants and leached out by watering. The faster a plant grows the more fertilizer and water it will require. Consequently, as watering is increased so is leaching and nutrient loss.
Once you’ve selected a fertilizer (make sure you use an organic one!), you’ll need to apply it about once every two weeks for container grown plants. This assumes that you’re growing in a high quality, compost rich potting mix that will help retain nutrients. With that said, some gardeners prefer to fertilize with a weak nutrient solution every other time they water. If this is your preference, make sure to use about 1/5 the amount called for on the label.
Tip: When adding fertilizer to potted plants use organic blends (derived from many nutrient sources). Organic fertilizers are just as effective as chemicals, will not burn, and supply the required macronutrients (N-P-K) as well as a large selection of minerals, micronutrients, amino acids and vitamins.
When you fertilize is also very important. Plant nutrient needs change with different stages of growth. For example, to get the most from annual bedding plants, start them on a high nitrogen fertilizer to promote growth and leaf development, then switch to a low nitrogen, high phosphorous solution to encourage blooming.
If you aren’t familiar with how to buy fertilizer here is a quick tutorial on what to look for:
There are three numbers on the front of every bag or bottle of fertilizer. The numbers are always in the same order and stand for the percentage by weight of the N-P-K or Nitrogen, Phosphorous and Potassium contained in the fertilizer. Each of these is necessary for healthy plant growth and development. Read What Do the Numbers Mean? for more information.
For example, a bag of all-purpose organic fertilizer might carry these numbers, 5-5-5. The first number indicates nitrogen, which promotes lush, green growth and overall plant health. A fertilizer high in nitrogen is a good choice for leafy vegetables grown in containers, such as lettuce and Swiss chard. Natural sources high in this nutrient include blood meal and liquid fish emulsion.
The second number indicates phosphorous, which is used to develop healthy roots and is necessary for fruit and flower development. Blooming plants, like tomatoes, peppers and most ornamentals, benefit from nutrient solutions rich in phosphorous, especially when they are ready to set buds. Organic fertilizers known for their phosphorous content include bone meal, seabird guano and many liquid blends.
The third number is potassium, which is necessary for stem growth and overall plant vigor. Potassium is used especially on root crops such as carrots, beets and potatoes, but also on young trees during the fall to encourage strong roots during the winter months. Sources of organic potassium include sul-po-mag (sulfate of potash magnesia), greensand, and many liquid fertilizers.
Note: Plants can absorb nutrients through both their roots and through leaf pores. Foliar feeding (applying fertilizer solutions through leaf pores) can supply nutrients to your plants immediately. It is especially effective for giving fast growing plants, like vegetables, an extra boost during the growing season.