PH and nutrient availability

The pH level in the soil influences the availability of each nutrient used by plant.  Some nutrients are more available in acid soils, others in less acid soil. Metallic micronutrients are particularly adversely affected by alkalinity. Iron, which is necessary for the formation of chlorophyll and for the process of photosynthesis, is virtually unavailable at pH 8.0.

Target pH

We have seen University papers suggesting AV soil pH levels down to pH 5.5. Most commonly you see recommendations targeting 6.0-6.5.  We suggest a long-term target of 6.5 just because it seems the most safe. However, if you have been watering with very alkaline water (i.e. pH 8.0) you may want to use pH 6.0 water for the first few waterings in order to help correct the previous imbalance.

If you are just starting to check pH
we suggest the following:

1. Check your basic tap water pH

2. Check your water after you have added all fertilizer and additives that you use

3. Check the run-off water from a few pots

4. Separately test runoff from fresh potting soil

This process will give you a much better idea of what is going on with your pH, and what might be causing problems

Measuring Soil pH

Soil pH generally moves toward the pH of water that is used. If you measure the pH of your watering water, and then the pH of the water run-off, you will get a good idea of soil pH.  A more accurate way to do it would be to start with pH 7.0 water (neutral pH). Then create slightly wet slurry with you soil. Let this mixture soak for 10 minutes. Wrap this mixture in some paper towels and squeeze out some clear water (only ¼ tsp is required). You can test as any normal water and it will give you soil pH

Potting mix, Peat and Lime

Most AV growers use a potting mix that has some percentage of peat moss. Peat moss is naturally acidic and decays rapidly. As it decays, the acidity can change. Most growers add lime to the mix to correct the acidity of the peat moss. This lime leaches out of the soil over time. In the end you have a potting mix with an unstable pH level. We prefer coconut coir instead of peat moss because it has naturally correct pH and decays very slowly.

Commercial and peat based potting mixes may present additional hidden problems for AV growers. We have heard reports that commercial potting mixes have very variable and inconsistent pH levels. This is probably because of the difficulty of mixing up large commercial batches that distribute the added lime evenly.

Fertilizer

We were extremely shocked to learn that fertilizers often have a significant effect on pH.  Very few fertilizers are neural pH. Even different fertilizers within the same brand line will have different pH. Often the fertilizer will have buffering agents that can resist pH correction. So, it is extremely important to test all of your fertilizer and additives to see if they are causing hidden changes in your pH level.

Chemicals and additives

If you carefully read the instructions for fungicides, insecticides, etc… you will often see that they suggest a certain pH level for best effect. Most commonly this level is 5.5-6.5… but you need to check each one to be sure. The manufacturers web site can be the best site to get this information since it may not be on the label.

PH Correction and rebound

We have seen many reports of pH rebound. This is when you try to correct water pH level, and then find it has moved back towards the original pH level over time. This phenomenon may be because the water company is adding buffers to keep pH in a certain range. We "thought" that we were having this problem, but discovered that our "rebound effect" was linked to a certain fertilizer we were using.

Water and water companies

Many of us have hard water. Hard water has lots of minerals and is typically very alkaline. Our water is about pH 8.0 (pH 7.0 is neutral:  AVs prefer  pH 6.0-6.5).

Even if your water starts out acid, many water companies add chemical to make it alkaline (acid water can leach unhealthy metals from pipes). In some cases, your tap water quality may shift through the year:  our water company uses rain runoff water in winter and spring, but shifts to well water later in the season. Your water company may have a web page or an information number that you can call to find out more details about your tap water.

Chloramines

Some water companies are replacing chlorine with chloramine. Chlorine evaporates quickly from the water, while chloramines lingers. Water companies use chloramine to insure a constant level of chlorine in the water. Unfortunately, this product can accumulate in soils and there have been reports that some violets are particularly sensitive to it. Pet or aquarium supply store sell chemicals to correct this problem if you think it is an issue.

Reverse osmosis water

Reverse osmosis is a way of creating "pure water." Much of the water that you buy in the store or from vending machines is simply tap water that has been purified through reverse osmosis. This water is very similar to distilled water… that is to say it has minimal minerals. This may be good, or bad in some circumstances.  For AV growing, reverse osmosis water is mostly good since it is neutral pH. If you have hard water, you may notice that mineral salts build up in the soil (particularly during hot dry summers). Since RO water has essentially no minerals, it corrects the problem of mineral salt build up.

On the other hand, most AV growers use soilless mixes that have no minerals in the soil. If you use RO water, you may have a shortage of minerals for your plants. You can correct this problem by adding ¼ tap water back to your RO water.

Water softeners

Water softeners DO NOT remove minerals from hard water. They perform an exchange in which salt  ions replace the mineral ions. The result is softened water that has salt in it. This is important to know because plants do not like salt and it may cause problems for them.

PH correction

Most people suffer from alkaline water. We prefer our pH down because it has buffers to stabilize pH and adds a little phosphorus to your water. Sulfuric acid (available for swimming pool treatment) is also good to lower pH.

Some people say you can use vinegar to lower pH, this link says you shouldn't use vinegar; you decide: http://forums.gardenweb.com/forums/load/hydro/msg060920042111.html

Links

http://www.avsa.org/AVMFiles/pH RxForHealthyPlants.pdf; http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex6607?opendocument