Outdoor Cannabis Leaf Discoloration? Check Your pH Levels

During the outdoor cannabis season, most growers will have to deal with some issues – from minor deficiencies to dealing with pests and diseases. The leaves of the plant offer an early indication that something might be wrong. One common issue is highlighted by leaves turning yellow at the base of the plant. Often this is an indication of nutrient lock caused by an imbalance in cannabis plant pH levels.

While getting the cannabis pH levels correct is vital for any cannabis grow, it can be an Achilles heel for the outdoor grower due to the nature of outdoor growing. With bigger spaces to play with, many growers mix their own soil, often combining indigenous soil to bulk out the mix, and indeed some potting soils have varying pH levels.  

What is the best pH level for growing cannabis?

The pH scale runs from 1 – 14 and measures the balance of acid to alkaline. At one end of the spectrum ‘1’ represents ultimate acidity, while ‘14’ represents ultimate alkalinity, with ‘7.0’ as neutral. Cannabis grown in soil prefers a pH level of 6.0 – 6.5 – in other words slightly acidic. 

At first glance, especially to those not familiar with the cannabis pH aspect of growing, those points may not seem significant. “Hey I’m one point out, is that so bad?”. However, each point change in pH represents a ‘x 10’ increase or decrease in alkaline or acid levels… and that makes a big difference.

The Importance of Getting the Cannabis pH Level Right

Without the right pH levels a cannabis plant cannot absorb nutrients efficiently even if they are present (ie in the soil or from added nutrients) and this will lead to deficiencies. This is often referred to as nutrient lock. 

Of course the grower can’t see this until later in the plant’s growth, when those deficiencies start to appear in the form of discoloured leaves or slow growth.  By this time it is often too late to fix. The best way to combat this issue is to make sure to test the soil at the beginning of the process. Ideally soil mixes should be prepared in advance, to let the soil ‘settle’ before plants grown from cannabis seeds start developing. 

However, this advice is no good if your plants are already growing, and if you are reading this, chances are you are looking for a quick fix. Unfortunately there is no magic bullet, but there are some steps you can take to try and deal with the issue. There are many products on the market offering to fix your pH issue (‘pH up’ and ‘pH down’ aids), but as most outdoor hobby growers prefer to take a more natural route, here are some tips to correct that balance.   

How to Fix Cannabis Plant Soil With Acidic pH?

If cannabis plant soil is too acidic, then a reliable way to raise the pH levels is to add lime to the mix. A common garden product such as pulverized limestone, which can be added to the soil, will achieve this purpose. However, this is not a speedy process and will take a few months before the effects are notable, so this is more of a long term solution. 

Quicker fixes include wood ash (make sure it does not come from treated wood) and bicarbonate of soda. Wood ash can be mixed into the soil and also laid on the surface. Bicarbonate of soda should be mixed with water (at a ratio of 1 tablespoon of bicarb to 4.5 liters water – adjust accordingly) and applied to the soil.

*** As with all these processes (for raising and for lowering cannabis plant pH levels), it is better to be conservative with dosage. The best advice is to apply modestly and keep testing the soil as the weeks progress and adjust accordingly.     

How To Fix Cannabis Plant Soil With Alkaline pH?

Natural ways to lower cannabis plant pH levels, reducing the alkalinity of soil include good home-made garden compost (well rotted), the regular application of a good compost tea, and sprinkling coffee grounds into the soil. Off the shelf solutions which are effective include ferrous (iron) sulfate and elemental garden sulphur. For both of these products, it is important to follow the guidelines on the packet.  

For outdoor cannabis growers experiencing an issue due to pH level imbalance, this season may be all about damage limitation. However, the key to gardening is to learn from negative experiences and apply the lessons to the next season. To avoid a repeat, start preparing those outdoor plot pH levels for next year way in advance!