Fertilizer Calculations for Organic Farmers

Sometimes it can be tricky determining the optimal application rates for organic fertilizers. This is especially true for a crop like cannabis since there is not much concrete data about what the true nutrient requirements are for the plant. For example, if you want to know how much nitrogen, or phosphorous, or potassium (NPK) to apply to a field of corn to get a decent yield, you can turn to University publications to get that info. That data just doesn't exist in the same easy-to-find way yet for cannabis. You certainly won't be able to get basic nutrient requirement data for cannabis from your local extension agent! But this doesn't stop hundreds of state licensed cannabis farmers across the country from growing high quality and high yielding cannabis using only organic soil amendments. Many of the most successful state licensed cannabis farmers have had years or even decades of experience growing medical cannabis before getting licensed in the new adult-use, "recreational" cannabis sector.  These growers understand how to grow a great crop based of their experience with the plant and years of honing and refining their methods. But not many cannabis growers that were part of a state-sanctioned medical cannabis program have experience growing hundreds or even thousands of plants that are commonly allowed for licensed adult-use cannabis farmers in states like WA, OR, and CO. If folks have a property where native farm soils are decent enough, a cannabis farmer can avoid the extra expense of pots and potting soil by planting directly into the ground. A common practice in farming is to get a soil test of the area you intend to plant into and then base your fertility strategy on the nutrients that the soil tests tells you are lacking or insufficient in your soil. Once you determine what nutrients you want to add to the soil, the trick is finding quality, yet affordable sources of organic soil amendments and then applying them at the correct time and in the correct manner prior to planting. Cannabis farmers eager to work up the soil with tillage equipment would be better off waiting until the soil has dried out enough that clods of soil do not form while tilling. If large clumps of wet clay soil are forming as you are tilling, this is a sign that it is too wet for tractor work. Repeated tillage of wet soils destroys the soil's physical structure and it also impacts your soil biology. If the soil is wet, chill out. Resist the urge to use your tractor or rototiller. If you need to till up beds, wait for a dry spell. Getting back to organic fertilizers, you will need to figure out the correct application rates. For most cannabis farmers in the adult-use market, this means you need to accurately determine how many pounds of an organic fertilizer to apply over a specified square footage of area. For example, if you have decided that you want to apply the equivalent of 300 pounds/acre of organic nitrogen to a row that is 400 feet long and 10 feet wide you could decide to use 11-0-0 Feather Meal. You would calculate the area of your row to be 4000 square feet (400 ft long x 10 ft wide). Over that area, to apply the equivalent of 300 lbs of N per acre you would divide 300 lbs of N by total number of square feet in an acre, 300 lbs N/43,560 square feet and get an application rate of 0.00689 lbs per square foot. Then you take this value of 0.00689 lbs per square foot and multiply it by the total area in the row where you are making the application which we determined to be 4000 square foot, so you have 0.00689 lbs square foot x 4000, which equals roughly 27.548 lbs of Nitrogen. How much of the actual feather meal do you need to apply? To calculate that, you have to understand that 11-0-0 Feather Meal is 11% nitrogen and 0% phosphate and 0% potash. So you would ask yourself 11% of what number equals 27.548 lbs N and to solve for that winds up being 27.448 lbs N/0.11 = 250.43 pounds of feather meal. To summarize this exercise, to get the equivalent of 300 pounds of Nitrogen into a 4000 square foot bed you would need to apply roughly 250 pounds of feather meal. If you were applying that over 40,000 square feet of area designated as your cannabis flowering canopy space, you would need roughly 2500 pounds or 1.25 tons of feather meal. If you are farming cannabis at the largest scale, you might want to find a compost spreader to help you easily apply these organic soil amendments. If you want to compare the cost of different fertilizers based on the amount of nutrients they actually provide over the growing season, then check out the fertilizer calculator created by our colleagues at Oregon Tilth and Oregon State Small Farms extension, the calculator is not designed for cannabis growers but should work to help you determine rates of application as outlined in the above exercise and help you compare the costs of various organic soil amendments. 

andrew black