Fertilizer Facts

 In our Southern California gardens, March is a great time to fertilize. Plants that are beginning to grow can really use the boost in nutrients: Lawns turn greener. Flowers almost immediately bloom. Veggies grow quickly. Fruit trees set more fruit. Citrus green up and look happy.

To many, especially to those new to gardening, figuring out fertilizers can be a challenge. There are so many different kinds on the market: General purpose, multipurpose,  specialty fertilizers for lawns, shrubs, flowers, for citrus, acid-loving plants, tomatoes, azaleas, and so on--add to this the choices of slow-release, quick release, organic, non-organic, granules, liquids, soluble powders, spikes, stakes, beads, gels, concentrates...yikes! So many choices. We just want to fertilize our plants.

By law, the fertilizer label must show the percentage of nitrogen (N), phosphorus (P; phosphoric acid), and potassium (K; potash). These percentages are listed in order (N-P-K) on the label in large numbers, such as 10-10-10, or 12-4-2. So a 5 pound bag of a leading brand's 24-8-16 fertilizer will deliver 1.2 pounds of nitrogen (24% of 5), 0.4 pounds of phosphorus (8% of 5), and 0.8 pounds of potassium to your garden soil. So that's 2.5 pounds of N, P, and K -- what is the rest of the stuff in the bag? Basically, it is filler, so figure the cost per pound of nitrogen when choosing your store-bought fertilizer.  For example, suppose five pounds of 10-10-10 bag and a 12-4-2 fertilizer costs the same. The 12-4-2 is a better buy, especially if you are adding the fertilizer to the surface of your garden soil (more on this below).

There is persistent garden folklore that says that nitrogen encourages green growth, while phosphorus and potassium encourage flowering and fruit growth. The fact is that all three are necessary for all growth. Also, there are at least a couple dozen other minerals that are necessary in trace amounts for healthy plant growth.

Thirty years ago, Sunset magazine asked researchers at California Polytechnic State University at San Luis Obispo to test various fertilizer formulas on various plants. It turns out that in California's soils, only nitrogen makes a difference in the size and amount of fruit.

Because of the alkalinity of our soil, phosphorus and potassium when added from above the ground do not migrate to the root zone of most plants. Nitrogen, however, moves quite easily with rain and irrigation. Therefore, nitrogen needs to be added a few to perhaps several times during the growing season (depending on rain and watering practices). Phosphorus and potassium should be added at planting time in the bottom of the planting hole, or dug deeply in to the garden bed.

Since potting soil often contains little or no natural soil, plants in containers are more dependent on a balanced fertilizer providing all three main minerals, N-P-K, along with other minerals, added throughout the growing season.


 All that having been said, here is what we do:

First of all, we don't use chemical fertilizers--only organic. We prefer to use our own homemade compost. 

At planting time we add a bit of 10-10-10 or 5-5-5 organic fertilizer (usually this one) and/or compost. We also add trace minerals in the form of Sea-90 at planting time and once during the growing season.

During the growing season we add compost around the plants and mix it into the top layer of soil. For extra nitrogen, we add coffee grounds (N-P-K: 2.0 - 0.3 - 0.3) or tea grounds (4.0 - 0.6 - 0.4). You could instead add blood meal (15 - 1.3 - 0.7) which is a quick release fertilizer or cotton seed meal (7 - 2.5 - 1.5) which releases over time. Cotton seed meal offers us the added benefit of acidifying our alkaline soil, perhaps helping P and K to migrate down into the root zone more effectively.

We also rotate our crops. Legumes (peas, beans, clover) fix nitrogen in the soil as they grow and release more of it into the soil as they decompose, so we plant these in each raised bed every three years or so.

Here is the bottom line from the Sunset article: Since phosphorus, potassium and micro nutrients don't need to be added to the soil frequently, the only element we need to provide with regular fertilizing is nitrogen. For us that means getting the best deal on high-nitrogen fertilizers. We have a constant supply of coffee and tea grounds, and grass clippings, and compost, all for free! Pretty good deal. If you have to buy your fertilizer, get the most nitrogen for the money and follow the label directions for application rates and frequency--too much nitrogen at one time can "burn" your plants.

Additional resource:
52 Weeks in the California Garden by Robert Smaus, pp.137-139.