pH and whatnot in soil.

PeeJay;2141168 said:

I’ve written about pH, ion transport and such in soil many times here – usually in bits and pieces. I’m going to make this post as complete as I can and then blog it so I can just use it as a pointer instead of writing out the same answers over and over.

First, when we discuss pH it is important to remember that the pH scale is logarithmic. The difference between pH 7.0 and pH 6.9, or pH 6.3 and pH 6.5 is really, really small. Because the scale is logarithmic the difference between 5.9 and 6.0 is equal to the total difference between and 6.0 and 7.0 The difference between 5.0 and 4.9 is equal to the total difference between 5.0 and 6.0, and 100 times the difference between 6.9 and 7.0… In order to accurately alter pH in the 6.0-7.0 range you need to do a careful titration with a very sensitive indicator or use a very expensive laboratory grade meter.

pH meters are more useful for hydro growers because the scale is more friendly and less precise between 5.0 and 6.0. I’m not going to delve into why hydo requires a lower pH here. This post will be lengthy enough without doing so. The point I want to make is that readings between 6.0 and 7.0 are rarely accurate to one decimal point. Heck, they are rarely accurate within three or four tenths! So long as the water you use is in the 6.0 to 7.0 range in soil there is no need to be concerned.

If you are using tap water that is out of the 6.0 to 7.0 range it is going to hurt you in the long run. If you are pH adjusting out of range water adjust a large volume of water at a time. Adjusting one gallon is much more difficult than adjusting six. You only have so much control over drop size of pH up and down additives. A larger volume of water is more forgiving to work with.

Bottled nutrients typically have a pH in the 5.0 to 5.5 range. The reason for this is that the manufacturers need to preserve the product so they add acid. Most bacteria can’t live in that range. If they didn’t add the acid bacteria would grow in the nutrient bottles resulting in spoilage. Therefore, when you add bottled nutrients to the water it drops the pH. We don’t add much volume of nutrients to the water. The change in pH is negligible when we do so. As mentioned before we are splitting hairs between pH 6.0 and 7.0 anyway.

Almost all commercial soils contain added buffers. Garden lime is the most common. You can add some extra garden lime to soil if you want. It is very cheap. A buffer contains significant amounts of both a weak acid and it’s conjugate base. The weak acid neutralizes added base, and the conjugate base neutralizes added acid. Since the differences in pH between 6.0 and 7.0 are so very, very small; a little buffer goes a long way. Additionally the plant itself and the bacteria, fungi, and protozoans in the soil also work to buffer the pH. We will discuss that next.

When a soil grower checks the pH of runoff or the soil they make an assumption that pH is homogenous throughout the medium. In fact, the pH is far from homogenous. It varies across literally millions of tiny micro environments in the soil/root complex. The plant manufactures sugars and signaling molecules (exudates) that are secreted through the roots and encourage certain types of bacteria and fungus to thrive in the immediate proximity of miniscule root fibers. We have all seen the pH/nutrient availability charts that are always posted here on the forum. I do not know who created these tables. They are not referenced. They do not take into account that soil pH is not homogenous to begin with – that the plant itself and the soil biota work symbiotically to create micro environments that facilitate nutrient uptake. pH is only one factor in nutrient uptake. The charts also don’t take that into account.

The nutrients a plant takes out of the soil are all parts of salts. When most folks hear the word salt, they think of salt as sodium chloride (table salt.) In reality salts are ionic compounds – a molecule or atom with positive charge bound to a molecule or atom with a negative charge; that will generally come apart (disassociate) in water. Most (but not all) of the nutrients a plant needs are the parts of ionic compounds that have a positive charge. K+ (potassium,) Ca2+ (calcium,) Mg 2+ (magnesium), etc.

These positive ions do not just “soak” into the roots. They need help getting moved from the soil into the plant. There are helpers which are proteins embedded in the cell walls of the roots. We have learned a ton about how these protein helpers move ions in both plants and animals in the last ten years. You would think that there would be separate proteins helpers for each nutrient. That isn’t the case. The same helper moves multiple charged nutrients. Things get complicated here… For our purposes suffice it to say that there are many factors that influence what gets moved by a helper.

Calcium and magnesium are a great example. It turns out that they are both dependent, for the most part, on the same protein helper to move them into the plant. If you understand the periodic table and look at it you’ll see that both are in the same column of the table. Mg is right above Ca and both have a charge of +2. It makes sense that the same helper protein would move both. The plant needs more Ca than Mg. At the same time for various reasons we don’t need to discuss in detail, the helper protein has a greater affinity for moving Ca than for moving Mg.

There is rarely not adequate Magnesium in the soil for the plant, but when there is too much Ca it outcompetes the Mg for transport. Think of it like this: The helper has an easier time grabbing calcium and moving it than it does grabbing and moving magnesium in the first place – it has a more convenient “handle” to grab onto. When there is lots of calcium around it will rarely grab magnesium and move it into the plant just because there is lots of calcium around. The problem isn’t that there is not enough magnesium in the soil. The problem is that there are too many other + charged ions like calcium around out-competing the magnesium for transport into the plant. There are too many cars on the freeway to begin with…

A magnesium deficiency or lockout is the most common problem experienced by soil growers. Once you have one you’ll get multiple opinions expressed on how to deal with it. One, you can add Epsom salts to the feeding regimen. By doing so you’ll increase the ratio of magnesium available for transport. But, since there are already too many cars on the freeway that is problematic. If the magnesium levels get too high then it will out-compete the calcium for transport and you’ll end up with a calcium deficiency. Interestingly, once too much magnesium is transported into the plant it also screws up the way potassium (K+) is moved around in the plant, too…

Two, you can supplement your feeding regimen with a Cal/Mag supplement with the appropriate calcium magnesium ratio. Again, the big problem is too many cars (charged ions) on the freeway. The best thing you can do is flush the crap out of the plant and resume feeding as usual. Less traffic in general increases the chance of helpers grabbing magnesium. As a side note, the protein transporters in plants and animals are remarkably similar. One of the common problems for people who supplement lots of calcium to combat osteoporosis is they end up with a magnesium deficiency….

The folks who manufacture nutrients are no dummies. If you are using any sort of reputable nutrient line and soil everything you need is there. These folks also have a vested interest in encouraging you to use as much of their product as possible, and in you having great results. A periodic flush is advisable for anyone who is paying for and using their products, however. Let’s look at an analogy. Growing in containers is not the same as growing in the ground where there is an infinite leach field for what goes unused…

Someone feeds me breakfast every day. Usually it's the same thing, a big bowl of Total breakfast flakes fortified with eleven different vitamins and minerals. They give me a nice big portion. I don't finish it all. The next day I'm given another nice big portion of Total. The leftover uneaten Total is still in the bowl from the day before. I eat roughly the same amount of Total as I did the day before. The amount of uneaten total in the bowl doubles. The next day I get another big serving of total dumped on top of two days-worth of left-over Total... You can, I'm sure, see where we're headed... What happens is not instantaneous.

When we mix up our nutes and feed plants growing in a container, we are dumping "Total" into a bowl. All of the eleven essential Vitamins and Minerals that aren't used by the plant stay in the bowl. Over time the concentration of some or all of the Vitamins and Minerals increases and increases. It can get quite toxic. An animal pees the extra out. Plants can't really pee. When we pee, we flush the unnecessary extra Vitamins and Minerals away. *Imagine the sound of a toilet flush*

In a container plant environment there are a bazillion bacteria and fungi living. They are breaking down organic material. Just like humans they poop and pee. Large microbiological populations commonly die off because they end up drowning in their own piss and shit.

Striving to give a container plant exactly what it needs, no more and no less, is a lofty and unrealistic goal. Some things are going to build up. Many of them are "salts." These "salts" tend to end up in the lower root zone. Salts are water soluble. They are washed to the lower root zone of our bowl containers. If you water and feed and never let any water drain from the bottom of the pot it only compounds the problem. No poop, pee, or salts get washed out at all.

Flushing is not something to do only when there is an obvious problem. It is something that should be done periodically as a prophylactic (preventative) measure.

When you also take into account the way that ions compete against each other for transport…. New soil growers rarely underdo anything. When in doubt, flush! Don’t get too caught up in pH readings – you’ll only get a migrane. I hope that this lengthy post is helpful for new growers.

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