Instead of adding synthetic fertilizers to your grow year after year, try out this organic method that will save you money and water in the short and long terms. Take advantage of one of nature’s best sources of free nitrogen fertilizer: the legume-Rhizobia symbiotic relationship.
As consumers increasingly push for organic and sustainable cannabis, growers must keep up with the times without cutting into their profit margins. Little-known fact, growing cannabis organically does not have to be more expensive than any other “traditional” method, and it may actually be cheaper. Good organic cultivation focuses on creating a healthy soil, which in turn harbors healthy plants. Besides using adding composts and mulches, how else can an organic farmer enrich their fields with the levels of nitrogen cannabis demands for explosive growth?
Meet the Rhizobia. The roots of legume plants such as the common clover somehow recognize these bacteria and allow them to “infect” them at the roots. Rhizobium grows and reproduces forming nodules along the taproot. A specialized enzyme they produce pulls nitrogen out of the air and fixates it into organic forms such as amino acids and proteins.
In order to get the most nitrogen out of your legume cover crop, you will need to plant a cool-season legume (such as hairy vetch, crimson clover or medium red clover) into your plot after harvesting the cannabis. Clover will germinate very quickly, likely within one week or less, but won’t start fixating a lot of nitrogen until the following spring. Two to three weeks before planting your cannabis simply till over the cover crop for “green mulch.” Legumes decompose much quicker than grasses due to their high nitrogen content, so if you plant grasses along with the legumes make sure to give yourself more time for them to decompose into the soil.
For this symbiotic relationship to flourish, the pH of the soil needs to rest between 6.5 and 7, luckily the same range that cannabis requires for optimum growth. In addition, the bacteria require the presence of molybdenum in the soil, a micronutrient that cannabis also desires.
If you plant clover or any other legume into soil that does not already have any legumes growing in it, you may have to inoculate the seeds before planting to ensure rhizobium colonization. To inoculate, follow this guideline from Penn State University. Even though that guide recommends seeding the legumes using a seed drill, you can still get away with using a broadcast seeder. Seeders can run anywhere between $3,000 and $5,000, and a cannabis gardener may not have any other use for them. Alternatively, a broadcast seeder costs about $60, and you can also use it to broadcast fertilizer (if you must use it), inoculation pellets, or even salt for de-icing your driveway. Read this additional literature about broadcast seeding legumes.
You may notice clover already grows in your garden, especially if you live in the Midwestern or Northeastern parts of the United States. In this case, you may not need to inoculate your new legume seeds with rhizobia because it may already be present in the soil.
To verify the presence of a rhizobia “infection,” carefully dig up the taproot of a clover. You’ll notice tiny nodules along the taproot that are about half the size of sesame seed or smaller. Take the root and rinse it under water to remove dirt, the hold it under a lamp. You may notice that some of the nodules have a very light pinkish hue on the outside; take a small pair of scissors and cut one of these nodules in half to reveal the interior.
Here you can see the taproot of common Alsike Clover that grew in a yard in Ohio. Cutting the nodule in half revealed a blood red interior indicating a strong presence of rhizobia that’s hard at work fixating nitrogen making this lawn healthy and fertile without having ever used fertilizers.
Why is the inside of the nodule blood red? The enzyme that turns nitrogen from the air into amino acids cannot operate in the presence of oxygen. To remove the unwanted oxygen from the inside of the nodule, the bacteria also produce a protein called leghemoglobin that scavenges oxygen before it can do any damage. Leghemoglobin is similar to the hemoglobin in our blood, and it stains the inside of the nodule pink or blood red just like in our bodies.