Plants That Produce

The Scoop On Fertilizers, Part III

By Richard Frost

I believe all of you are savvy shoppers when it comes to purchasing household supplies.  For example, when buying paper towels you know to check the cost per sheet instead of the cost per roll. To achieve this with fertilizers, you need to be aware of one wrinkle: nutrients are measured by percentage of weight. This means that to compute the unit cost you must first compute the net weight of the primary nutrient.

Example 1 – Computing the cost per net weight. There is a popular natural fertilizer product called AlgoFlash™ that currently sells for about $40 per 5-liter bottle. Its N-P-K rating is 6-6-6, and it also contains a nice array of micronutrients. According to the label, the net weight of the liquid product is about 13.5 lbs. Since it is 6% Nitrogen (N), the net weight of N in the product is 0.06 x 13.5 lbs = 0.81 lbs. So the unit cost of this product is $40 divided by 0.81 lbs, or $49.38 per net pound of Nitrogen. By the way, you can make 5 liters of this product at home for about $10. Just add 4 pounds of Scotts or Grow More water-soluble 20-20-20 to about 4 liters of distilled water!

Example 2 – Outdoor Leafy Vegetables and Herbs, plus Non-blooming Ornamentals. Consider these non-synthetic fertilizers, all of which contain a balanced set of micronutrients (see 12/08 column for dosages‡):

Product

lbs

cost

$ / lb-N

Scotts 30-5-5

5

$10

$6.67

GroPower+ 5-3-1

40

$30

$15

Neem Seed Meal 5-1-2

5

$6

$24

Liquid Fish Fertilizer 4-1-1

2.25

$5

$55.55

Worm castings♦

40

$15

$75

Worm castings ♦(0.5–0.4–0.5) contain micronutrients plus important non-plant food ingredients and I use them along with humic acids in my garden.  But as you see, they are an expensive choice as a sole source of nitrogen for plants.

Example 3 – Outdoor Subtropical Fruits and Ornamentals. Citrus, Palms, Avocado, Guavas, Myrtles, Pineapples, Sapotes, Cycads, etc. The following products all have similar costs, although the horse manure requires enormous quantities (see 12/08 column for dosages‡):

Product

lbs

cost

$ / lb-N

Horse manure 0.9-0.3-0.6

100

$4

$4.44

Vigoro 12-5-8

20

$13

$5.41

Grow More 28-8-18

25

$40

$5.71

Example 4 – Outdoor Non-tropical Fruits & Berries, Fruiting Veggies, Flowering Herbs, Ornamentals, Bulbs. Here the primary nutrient is potassium (K). It is hard to find products with the proper N-P-K ratios, but here are a few to consider:

Product

lbs

cost

$ / lb-K

Grow More 20-5-30

25

$40

$5.33

Scotts 10-5-15

25

$35

$9.33

íK-Mag 0-0-22

15

$15

$4.54

íwhen used as a supplement to a Nitrogen source, such as those in Example 2.

 

Member Richard Frost is also a member of the California Rare Fruit Growers.

‡For copies of past articles and more information, please see www.plantsthatproduce.com.

 

Reprinted with permission from February 2009 "Let's Talk Plants".  © San Diego Horticultural Society, www.sdhortsoc.org