Had a ‘hard lesson’ recently – thought I’d share it – since the impact on productivity of our veg growing was quite alarming. We (and our Garden Towers) have recently moved house and whilst we previously lived in a hard water zone, the new abode appears to be suffering from an even harder zone. However a bit of learning by the experience here, means I now better understand the impact on the plants….
We know the impact on our kettle of the hard water, because we regularly have to de-calcify the white deposits out of it, other wise they end up in the tea! For keeping plants and Garden Towers watered, we have usually relied on rainwater from the barrel collected off the greenhouse roof. For some reason we decided to clean out the barrel, and to stop it blowing away, put some tap water in as ballast. I must qualify the tap water here, it is not mains supplied but comes from a borehole. In this part of the world that is likely to be straight out of the chalk and heavy with dissolved limestone. Anyhow I subsequently forgot that the barrel had tap water in it and we drew on it as usual to water the plants.
I put a quick crop of radish into the top of the indoor tower at the end of February as the temperature in the conservatory was providing a soil temperature of around 12°C and warming. I also put in some loose-leaf lettuce and some pak-choi. They took ages to grow. By the end of March they weren’t even a couple of inches high. Something was amiss. Nothing thrived. Then we noticed aphids, and thrips. Pests honing in on the leaves that were looking chlorotic and spindly with cupped leaves. Definitely something amiss, and the problem wasn’t the pests. They were secondary symptoms of the real issue – alkaline-induced chlorosis.
I also noticed that if the compost dried a little on the surface there was a faint hint of white. This was interesting because it looked like the effect you get with salt crystallisation. We go to some effort to avoid anything that might bring salts into the Garden Tower. Artificial fertilisers are laden with them and worms and plants don’t like them in excess. Particularly important in the Garden Tower because the process of recycling your ‘nutrient tea’ water, combined with ‘pure water’ loss from evaporation and transpiration from plants, salt will become increasingly concentrated in a closed loop system. The whiteness however was not salts, but a chalky deposit and this was apparently concentrating out of the tap water for similar reasons.
Having re-calibrated my trusty pH meter I tested the tap water – pH 7.5. Although pH 7 is “neutral” (not acidic or alkaline), and 0.5 extra is regarded as mildly alkaline, it is not the optimum pH for watering container crops. (The recommended range of pH for irrigation water should be around 5.4 – 6.8.) This pH measurement however is not the real problem, but is a good indicator of it. Whilst water can be “alkaline” in pH, it also has the potential to have a high “alkalinity”, which must not be confused as the same thing. “Alkalinity” specifically is about the quantity of carbonates in the water. High levels of calcium carbonate in particular increase the “buffering” capacity of the water, in other words, its ability to neutralise acid. And this can have a cumulative effect as the carbonates are filtered through the potting substrate.
In my case the continued use of hard water whilst watering the Garden Towers was causing this accumulation of calcium carbonate in the potting soil. These were the chalky looking deposits. The pH can be a good indicator, and the best way to do this is to measure 1) your input water, and 2) the water from the sump drawer. My watering water was 7.5 but my sump water was reading pH 8.4 indicating that the cumulative buffering capacity of the substrate was high.
Ideally your Garden Tower potting mix/soil (and thus the drawer water) should test to pH 6.4 – 6.7 for most crops. Amazingly this is the “perfect soil pH” not only for organic plant growth, but is also the ideal pH of sap in a healthy plant, and the pH of saliva and urine in a healthy human.
The reason for this is simply that most plant nutrients are optimally available to plants within a 6.5 to 7.4 pH range, and this range of pH is generally very compatible to plant root growth. Above pH 7.5 and higher the availability of a lot of many nutrients decrease. Generally, there are 16 basic elements that need to be available within the soil for successful plant growth. The harder the water, the more calcium and magnesium is available relative these other elements, and the less available those elements become, so they are effectively locked out. The availability of the macro-nutient phosphorus (P) particularly, is directly affected. Low P levels in the soil can be responsible for stunted growth of plants.
There is also a decrease in availability to the plant of the micro- nutrients iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and boron (B) as soil pH increases, (although molybdenum (Mo) behaves counter to this trend). Iron (Fe) particularly, is needed to produce chlorophyll, hence its deficiency causes chlorosis, or pale, yellowing or whitening of the leaves. This means the plant cannot produce enough carbohydrates through photosynthesis. This is one of the first signs to look out for.
The table pictured gives an idea of the availability of the most important nutrients with change in pH. (with acknowledgment to pda.org.uk)
Beyond just mineral nutrients and equally importantly in the Garden Tower, soil microbes are also pH sensitive and many can’t survive in above pH 7.5. This compromises many important processes not least the the ability of the soil-life to assist the conversion of inorganic minerals into plant available forms.
The lesson here is simple. If you only have access to tap water do a pH test on it. Occasionally also on your substrate fluids out the drawer of the Tower. If you notice pH any higher than 7.5 be concerned. Also watch for chlorosis in the leaves of plants or plants simply not responding.
The best solution is to use rainwater if at all possible. Another good reason for doing so is that in some parts, tap water is heavily chlorinated. Chlorine is a bactericide. Your Tower organisms won’t appreciate it. If you have to use tap water, leave it to stand overnight in a bucket for the chlorine to volatilise away first.