The logarithmic measure of the acid-alkaline balance in water or soil is called pH. The most acidic solution is pH 1. A level of pH of 7 is neutral and a pH of 14 is the most alkaline solution. For several reasons the pH has a profound effect on the cannabis plants. Various plants live under different pH levels, they have adapted to this. In hydroponic solutions marijuana grows with a low pH of 5.5. Marijuana grows best in water or soil with a pH level between 6.0-6.5 (slightly acidic). Good garden soils consist of this pH level. In this range the plants nutrients are water soluble. This means that the nutrients are available to the marijuana plants. When the pH level is outside of this range, the nutrients become less available because the nutrients won't dissolve as well.
There is only one accurate way to adjust the pH. This is using pH test papers or a pH meter. When the pH level is outside the range, the nutrients are less available to the marijuana plant. This means that the roots don't have access to them and now the plant indicates deficiencies, even though the nutrients may be present. If the pH level is outside the proper pH range, marijuana plants have small dark-green leafs and grow very slowly, when growing in water or soil. Check the pH (by using test strips or a pH meter) before you plant them in soil or planting mix.
To check the pH use collected runoff water. If it's to alkaline, use soil sulfur to adjust the pH. If it's too acidic, use lime. You can check with an agricultural extension agent familiar with local soils or a knowledgeable local nursery man. Ask them for advice on proportions because the soils have various reactions to adjustments. Soils are mostly not indoor planting media. The media use as main ingredient peat moss, coir or bark. To adjust porosity and water retention, other materials are added. You can consider the mixes pest-free and disease-free.
If you want already pH balanced soils, use commercial potting soils and topsoil. When the plants are in the ground and the pH levels is out of range, you can still adjust the pH level. To raise alkalinity use pH-up. For the increase of acidity use pH-down. Keep checking the pH level in the soil every now and then. That's the best way to judge if you have to stop these treatments.
Use pH-up or pH-down to keep in the range of 6.0-6.5 while using hydroponic solutions. The pH levels vary a little bit per strain. The ingredients of soluble fertilizers also affect the water pH. So adjust the pH after you added the soluble fertilizers.
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