Grow Organic: Soil Amendments to Avoid

Cannabis demands high amounts of nitrogen, and though your organic fertilizers and compost teas supply more than enough, losses occur at every step of the way. Don’t throw away your nitrogen; learn to protect this relatively volatile nutrient for unhindered growth during both the vegetative and flowering cycles.

Both outdoor growers and indoor growers that mix their own soil must use some kind of soil amendment to improve water drainage and other soil properties. It’s tempting to add fibrous materials such as leaves, straw, peat manure and saw dust, but these materials come with an unexpected consequence. The raw cellulose in these materials is perfect bacteria fodder, and as soon as soil bacteria come into contact with it they begin the slow process of decomposition. The thriving bacterial colony will consume precious nitrate for their own metabolic processes, leaving little for your plants.

Fibrous materials are great for aeration, but they need to be made into stable, fermented composts before they’re ready to harbor and promote healthy, vigorous plant growth. If you grow outdoors and harvest once per year, you can compost leaves, straw or peat during the growth season so the compost is ready for amending the soil in the spring. The composting process takes time, but by the time the bacteria have finished they turn your kitchen wastes, grass clippings and fall leaves into pristine humus that’s ideal for organic cannabis growing.

Applying straw or mulch to the top of the soil does not lock up nitrogen because they stay away from the root zone. These ground covers hold onto water and reduce water loss through evaporation, but are detrimental if mixed into the soil. On the other hand, never add sawdust to the top or any part of the soil. Sawdust has such a high ratio of carbon to nitrogen (C/N ratio of 50:1 or more) it will lock up nitrogen no matter what. Wood chips, twigs, branches and sawdust alike all need long amounts of time (two to three years) to decompose enough so they don’t lock up nitrogen.

We’ve seen how soil amendments with a C/N ratio in excess 25:1 can hurt nitrogen availability because bacteria tie up nitrogen in the process of decomposing cellulose, but what else can cause unexpected nitrogen deficiencies?

Plants take up nitrogen mainly in the form of nitrate, NO₃^–, and not so much in the form of ammonium, NH₄⁺. The nitrate ion is soluble in water and readily leaches out of soil, and plants have adapted to be able to uptake nitrate when it’s present in very small amounts. Plants can't use organic forms of nitrogen in the soil, but a three-fold process exists to transform it into nitrate. First bacteria mineralize organic nitrogen from dead fungus or plants turning it into ammonium. In the second step, Nitrosomonas bacteria oxidize ammonium to nitrite (NO₂^–), a form of nitrogen that’s toxic to plants. In healthy soils nitrite never lingers around enough to cause damage because the bacteria Nitrobacter rapidly turn it into nitrate.

This process happens naturally in well-aerated soil that is abundant in essential micronutrients, but it may suffer imbalances with improper fertilizer applications. Applying too much fertilizer with ammonium salts overloads the Nitrobacter, and most of the nitrogen gets stuck in the toxic nitrite form, a process called “denitrification” that means bad news for your plants. Adding too much nitrogen in the form of urea can have similar effects because urea is readily transformed into ammonium.

Not only will excess ammonium lead to denitrification, the ammonium is readily transformed to ammonia (NH₃) which evaporates into the air for one big foul-smelling waste of money. Adding consistent and modest amounts of fully fermented compost teas or fertilizers with nitrates are a better bet for cannabis because they deliver nitrogen in the correct form so you don’t waste it all in the runoff.

Photo credit: Terry K from Ontario