Fertilizing Hemp: The Basics
Are you growing hemp for the first time or considering adding it to your crops? Since growing Cannabis was illegal for decades, researchers and farmers are still newly developing best practices for fertilizing hemp. Research has picked up since growing hemp became legal, but you’re probably still wondering what to do as you plan your new crop.
In this article, we’ll go over fertilization requirements for outdoor and indoor-grown hemp.
Fertilizing Outdoor-Grown Hemp Crops
Hemp grown for fiber or grain requires 100-200 lbs of nitrogen per acre, 45-70 lbs of phosphorus, and as much as 120 lbs of potassium. Amend the soil two weeks or less before planting so the added nutrients don’t wash out.
Different soil types need different amendments since macro and micronutrients are present at varying rates. Macronutrients include nitrogen, potassium, phosphorus, sulfur, magnesium, and calcium. Micronutrients include boron, chloride, copper, iron, manganese, molybdenum, nickel, and zinc. Depending on the land, farmers may need to amend the soil to increase or decrease rates of certain nutrients.
For example, soil too high in magnesium will need to be amended with gypsum to bring the calcium and sulfur rates into balance. Sulfur binds to magnesium, leaching it out of the soil and increasing calcium.
NC State research trials found that for floral biomass, nitrogen needs were between 125-170 pounds per acre, depending on the original soil. They suggest split-applying 150 pounds per acre during vegetative development. Fewer nutrients leach out of the soil for farmers using plastic mulch, so less nitrogen amendment may be needed. Potassium doesn’t limit biomass production, and growers should aim, after soil testing, for a total potassium content of 110 lb K2O/ac.
Generally, nitrogen drives floral hemp biomass production in NC rather than potassium. If the soil is sandy or coarse, it will leach nutrients more quickly and require more nitrogen input. Clay soils retain nitrogen more effectively and require less input. The threshold for best effects on hemp from nitrogen applications is about 170 lbs/acre in sandy soil or 120 lbs/acre in clay soil. Amendments at higher rates didn’t change biomass production. The threshold for potassium is about 100 lbs/acre, regardless of soil type.
Before you plant hemp or amend the soil, test the soil to create a finely tuned fertilizing program for your land.
Split apply any nitrogen, especially in coarse or sandy soil, to prevent leaching after heavy rainfall. There’s no point in pouring nutrients into the soil if the plants can’t get them.
Fertilizing Indoor-Grown Hemp Crops
Greenhouse-grown hemp requires more varied feeding over the course of its life. Rather than amending the soil once or twice during the season, you’ll apply fertilizer at different rates during the different growth stages.
Basic NPK rates for fiber or grain Hemp crops are as follows. Feed seedlings when they are three or four weeks old, once they’ve begun vegetative growth. Use a fertilizer with an NPK ratio of 2:1:2. At six weeks from planting, increase fertilizer nutrients with a ratio of 10:5:7. For about a week before the plants begin blooming, as the vegetative phase ends, lower nutrient levels to 7:7:7.
Floral hemp for CBD or other uses requires additional potassium. Use a fertilizer with an NPK ratio of 5:7:10 for the first two weeks of flowering, then increase to 6:10:15 and decrease to 4:7:10 for the last few weeks before harvest. Some growers flush the plants with plain water just before harvest.
Researchers are still determining the best target level for phosphorus for greenhouse-grown hemp, but generally, a constant feed program of 15-20 ppm P is acceptable. Indoor hemp crops will need higher quantities of potassium, calcium, and magnesium at a ratio of 4:2:1. This ratio will prevent antagonism. (When one element is deficient because another is too high.) A formula of 200 ppm K, 100 ppm CA, and 50 ppm Mg will suffice.
The ideal pH for Hemp crops indoors is 5.8-6.2, but they will tolerate anywhere from 5-7 without chlorosis.
For more general information on growing hemp, check out our blog, and feel free to reach out and talk with someone in our Superior Hemp division. To read more about fertilizing hemp crops, check out NC State’s research for yourself:
