Carrots are Picky Eaters April 12, 2013 00:38

I end up using Gaia Green 4-4-4 organic fertilizer for all kinds of stuff, but it's not always ideal.  In particular, root vegetables, especially carrots, tend to be phosphorus-hungry and nitrogen sensitive.  West Coast Seeds recommends a carrot-specific fertilizer blend of kelp meal, rock phosphate, and greensand, with a 1-9-3 ratio. These numbers, by the way, refer to percent composition by weight of useable nitrogen, phosphorus, and potassium respectively, OR the weight of these three elements relative to one another in an arbitrary amount of fertilizer.  So while all-purpose 4-4-4 fertilizer contains 4% by mass each of nitrogen, phosphorus, and potassium, the fertilizer we want just needs to have the right ratio.  So if a certain volume of our Carrot Fertilizer has 1 gram nitrogen (N), it should have 9 grams phosphorus (P) and 3 grams potassium (K). This is available as a bagged pre-mix (which I'm sure is excellent), but since I happen to have all those ingredients on hand it's cheaper to just whip it up. Luckily, the ingredients come with NPK macronutrient analysis, as follows: Kelp meal 1-0-2 Greensand 0-0-3 Rock Phosphate 0-3-0 From this, we can make a little chart to tell us how much of each to add to get the desired ratio of 1-9-3.
Carrot fertilizer N P K Parts Mass (g) mass per kg
Target 1 9 3
Kelp meal 1 0 2 1 900 231
Greensand 0 0 3 0.333 300 77
Rock Phosphate 0 3 0 3 2,700 692
Total 1 9 3 4.333 3,900 1000

Since we know the ingredients, and there are only three, we can now fiddle with the numbers to get the ratios right.  For example, the only ingredient with a non-zero P column is Rock phosphate, so we need 3 parts rock phosphate because 3x3 is 9, the target P. Then we see that Kelp Meal is the only one with a non-zero N entry, so we need 1 part kelp because 1x1 is 1, the target N. This puts us at 2 for K, one short of the target 3, so we need 1/3 part greensand to make up the difference.  More complex recipes can be solved as a system of equations, or using linear algebra.  Maybe one day I'll write a little software program to do it (does anyone know of something like this that exists?) The mass per kg is probably the most useful column here as it tells you how much of each ingredient to add per kg desired fertilizer. The mass column is based on an arbitrary 1 part = 900g, which is what I mixed here:

 2013-04-11 12.04.27
Kelp meal, rock phosphate, and greensand measured out and ready to be mixed.

 Stir it up and there you go! Carrot-specific fertilizer. Recommended usage is 1 cup (approx 260 grams) per 8 feet of row.