Organic gardeners should focus on two elements with regards to fertilizers: phosphorous and nitrogen. Potassium is also an important element, though a bit less so than the other two. There is really no single organic fertilizer source that provides ample quantities of both phosphate and nitrogen. As a result of this, it is important to blend multiple fertilizer sources to give your plants everything they need to grow.

There is a difference in the way soil reacts to phosphorous and nitrogen. Excess phosphorous is stored in soil where nitrogen evaporates. For this reason, organic gardeners will always need to supplement their crops with nitrogen, but phosphorous will not always be necessary if enough have built up in the soil. In order to fertilizer your organic garden, you’re going to need to do a little math. For example, a common organic gardening fertilizer recommendation could is 100 pounds of each nitrogen and phosphorous per acre. Let’s say organic gardener Jim has a quarter of an acre of plants he needs to fertilize. This means Jim needs 25 pounds of nitrogen and 25 pounds of phosphorous to put into his soil. If Jim is using a the fertilizer blend fish meal, he’ll be putting about 9% nitrogen and 2.5% phosphorous for each pound he puts into his soil. So if Jim puts 250 pounds of fish meal on his crops, he’ll get about 22.5 pounds of nitrogen and 6.25 pounds of phosphorous.

So as you can see, if Jim applies this fish meal fertilizer, Jim’s job is not done. He still needs 2.5 pounds of nitrogen and 18.75 pounds of phosphorous. To achieve this, Jim can supplement his fish meal fertilizer with bone meal, a fertilizer high in phosphorous but fairly low in nitrogen. Bone meal is 22% phosphorous and 3.5% nitrogen. If Jim adds 80 pounds of bone meal to his crops, he’ll add 17.6 pounds of phosphorous and 2.8 pounds of nitrogen. When added to his fish meal fertilizer, this brings his total phosphorous to 23.85 pounds and his total nitrogen to 25.3 pounds. While that’s not exactly 25 and 25 pounds of each, it’s close enough that Jim’s crops will thrive beautifully.

Bone meal is really the only organic fertilizer high in phosphorous (it has 22% while most others have no more than 2 or 3%). For this reason, all organic gardeners will need to use bone meal on their crops. Fertilizers high in nitrogen in addition to fish meal include dried blood (13%), and animal hoofs and horns (also 13%).