The pH of tap water and rain water

Somehow, it just seems instinctively ‘right’ that rain water is the best water for the garden. And, by and large, that is completely true. Keeping to an instinctual level, it’s easy to comprehend that the soil and all its microorganisms are ‘used’ to rain water. Plants are used to it too, and provided nothing gets badly out of kilter things go along just fine.

One of the major differences between rain and tap water is (usually) their pH. Now, despite having studied soil science for years, I still find pH, acidity, alkalinity, basicity, conductivity and the various other terms relating to a soil’s chemistry pretty confusing. But sticking to pH, I was interested to know how the rain water here in north Wales compared to the tap water. So I checked them out with some pH papers.

pH paper test of rain water and tap water

It’s not proper science, but it’s still interesting to see that while the tap water is showing somewhere between pH7 and pH8*, the rain water is somewhere between pH5 and pH6. In a way, that seems like hardly any difference at all, but because the pH scale is logarithmic, each one point difference in the pH value is the same as a ten fold increase (or decrease) in the amount of hydrogen ions (which could be called acidity or alkalinity for simplicity, but that would upset a lot of people). Two points difference in pH is 100 times more (or fewer) H ions and so on. And to plants and soil organisms, that matters a lot.

During the recent dry spell, my collected rain water soon ran out and for nearly three weeks I’ve been watering with tap water. Mainly that hasn’t affected the plants (not visibly anyway) but some, particularly the blueberries (which love a low pH) show yellowing of the leaves very quickly when watered with tap water (not least because they’re really not at all well suited to the soil and weather here – but that’s a whole other story). That chlorosis can be  quickly remedied with a liquid feed aimed at ericaceous (low pH loving) plants applied to the leaves as well as the soil around the bushes: that will tide them over until the rains come back again.

pH strips can be bought for a few pennies on the internet – they’re great fun to play with and you can soon find yourself marvelling at the pH values of all kinds of household liquids:)

*Dwr Cymru – the local water supply company – provides full analyses for the water they supply: they’re freely available on their website. The actual average value for the water’s pH here is 7.622 – so that seems to indicate that the cheap little pH papers are giving a fair estimation.



Soot is not ash

Nope. It’s not. No matter what people have been saying on the radio, in their gardening columns or on gardening forums. Soot and ash are not the same thing.

Soot is black and, for the most part, carbon. But the other parts can contain all kinds of nasties. Soot is one of the earliest documented carcinogens: back in the 18th century it was associated with scrotal cancer – an unusual cancer strongly linked with working as a chimney sweep. Soot is listed by the International Agency for Research on Cancer as a known carcinogen. More specifically, it is the compounds within soot that are mutagens, but as they are intrinsic to the soot, soot as a whole is classified as carcinogenic.

Soot particles are so tiny, it’s hard to imagine. They’re classed as being around 2.5 picometers small – you could fit twenty soot particles on a cross section of human hair. The particles are so small they can travel deep into your lungs and your blood.

Soot might bring some mineral nutrients to your garden. But it might also bring all kinds of other unwanted consequences. We would recommend avoiding it.

It’s true that soot did used to be widely used in gardens, and especially for rhubarb, because rhubarb benefits from the sulphur in it. However, it also used to be common to use arsenic, mercury and lead in a wide range of pesticides. The old ways are not always necessarily the best ways.

Ash, in the way we commonly refer to it, is the residue from a fire that remains in the hearth or stove – it’s usually pale-ish in colour. Soot is the black stuff that goes up into the chimney and out in the smoke. Anybody who’s experienced a bad chimney sweep will know just how pervasive soot can be – settling for weeks after the sweep has left.

Is tea good for plants?

In the UK, the dregs from the teapot were traditionally poured onto plants – often a hydrangea by the front door. With the rise of the teabag and the demise of brewing loose leaf tea in a pot, the habit of sharing tea with plants has largely died out. But is it something we should think of doing again?

From a plant (and human) nutrition viewpoint, the principal agents of interest in brewed black tea (Camellia sinensis, aka PG, Tetley, Typhoo, etc) are the minerals. And of the minerals, tea is particularly rich in potassium and manganese, with appreciable fluoride levels too.

Potassium is familiar to growers – it is the K in NPK (nitrogen, phosphorous, potassium). The reason it has the chemical letter K is because it used to be known as kalium.  Incidentally, in fertilizers it is generally in the form K2O – potassium oxide. For plants, potassium is of tremendous importance. It is classed as an essential macro-nutrient. Essential,because without it the plant will not thrive and macro because it is needed in relatively large quantities. Growers associate potassium with fruits and flowers – that is why, for example,
tomato plant food is high in potassium. However, it is fundamentally involved in many processes within the plant all year round. Deficiency of potassium makes the plant generally more susceptible to damage from pests and from the environment (frosts, drought, etc).

The amount of potassium in the average cup of tea will depend on many things, including where the tea plants were grown, how they were processed, and what the mineral content of the water used to make the cup of tea was. The USDA gives a value of around about 80mg of potassium in an 8oz (237mg) cup of black tea made with tap water. This is about the same amount of potassium as is in a couple of ripe cherry tomatoes.

Interestingly, tea drinking is a major source of potassium for many people. The recommended daily intake for potassium is 3500mg for an adult in the UK, or 4700mg if you’re an American… I guess that’s just the vagaries of national science policies, not some biological difference between the two populations.

Manganese (Mn) is less well known (and not to be confused with Magnesium – chemical symbol Mg).  It is also an essential component in plant nutrition, but is classed as a micronutrient as it is only required in very small amounts. Manganese plays a crucial role in photosynthesis and also in many other processes within a plant. Its role is complex and it often works in conjunction with, or sometimes instead of, other minerals. Nevertheless, it is essential.

As with potassium, the manganese content of your average cuppa will vary widely. However, the USDA provides a value of 0.5mg per 8oz cup of black tea made with tap water.

There is no established figure for the recommended daily amount or intake of manganese in human diets, therefore guideline figures are provided instead. The guideline amount for adults is in the range of 2mg – 5mg per day. In the US, intakes of more than 11mg are considered to be “possibly unsafe”. Again, in human nutrition, tea often plays a major role in providing manganese.

Tea is also somewhat acidic – again, depending on many other factors, particularly the water used to make the brew and the length of time the tea steeps. This probably explains the tradition for “feeding” tea to acid loving plants.

On balance, sharing your cup of tea (once it’s cooled, obviously) with your plants and garden is unlikely to do any harm and is quite likely to be beneficial, particularly to plants that have a high demand for potassium (e.g. tomatoes, strawberries, fruits in general) and those which prefer an acid (low pH) soil.

The curious case of cobalt…

In the late 19th and early 20th Century, sheep farmers across the world became increasingly aware of a devastating disease that caused their sheep to literally waste away and die.

Initial symptoms of the disease were a general failure of the animals to thrive, they also developed an abnormally coarse fleece. 

The disease went by many names specific to the locality where it occurred.  In the UK it was most commonly called pine, or pining disease – because the animals just seemed to give up the will to live, as though they were pining.  Other names included bush sickness and coast disease.

Stockmen had soon realised that the disease was associated with specific soils and geographic areas.  They also found that if they could move the stock to different pasture, they would sometimes recover with no further intervention. 

Initially, scientists thought the disease was caused by a deficiency of iron.  However, in the 1930s this theory was disproved and cobalt was identified as being the deficient nutrient. 

That’s all a long time ago, but I was reminded of it when the vet at a farm I was working on last year recommended giving the sheep B12 injections.  Coincidentally, at the same time a surprisingly large number of my friends, and their friends and families, had been given vitamin B12 injections by their doctors. 

Few farms bother testing their soils at all, and those that do generally only test for pH (to see if they need to lime) and maybe Phosphorous (P) and Potassium (K).  Hence, few if any would know whether their soils were deficient in cobalt (or any other micronutrient).

And what does that have to do with B12?  Well, cobalt is an essential constituent of vitamin B12; another name of B12 is cobalamin.  In ruminant animals, like sheep, cobalt is needed by the bacteria that live in the animal’s rumen and carry out most of their digestion.  These bacteria also synthesise vitamin B12 which is absorbed by the animal.  So, a deficiency of cobalt is thought to adversely affect ruminants through reducing digestive efficiency and causing B12 deficiency.*

In humans, B12 is essential for red blood cell formation and energy production (amongst lots of other things).  Humans need to obtain an adequate amount of vitamin B12 in their diet and the best natural sources are red meat, eggs, fish and dairy products.  A lack of B12 causes a type of anaemia.

So, maybe if the soil is deficient in cobalt meaning the lambs are needing an injection of B12, perhaps their meat, and the milk products from the dairy herd, are also going to be lower in B12 than might be expected.  And maybe that’s why so many people have been needing B12 injections lately…

The role of cobalt in plant nutrition is poorly understood, except for that in nitrogen fixing plants it plays a critical role for the nitrogen fixing bacteria.  Research is still on-going to find out more about its role in non-nitrogen fixing plants.

The take home message of this story is that the interactions between soil, plants/crops, livestock and human nutrition are incredibly complex and fascinating.  There are at least 15 essential mineral elements involved in plant nutrition – and that number tends to keep on increasing as scientists develop a better understanding of plant nutrition.

* for useful information on cobalt deficiency in sheep see the Teagasc factsheet at

Green waste compost

Few would dispute that the concept of composting green waste to create soil improvers, growing media etc. is a good thing. 

However, the experience of utilising such products is not necessarily so good. 

Contamination of the compost, particularly with plastics, but also things like hypodermic needles, engine parts, oil filters, mdf of unknown composition, rat carcases, etc. can make it off-putting to use. 

Contamination by things not so easily seen can be just as bad, if not so quickly apparent.  Weed seed burdens continue to be at unacceptably high levels, even in many PAS100 certified green waste composts.  There also remain continuing reports of herbicide residues causing problems – particularly, but not exclusively, aminopyralids.

A new and unlikely champion of quality control and improvement in green waste composting has entered the arena: the National Council for Metal Detecting (NCMD).  Apparently the amount of metal residue in the waste is causing them problems and concerns…

It is probably good to have input from such an unlikely perspective.  Anything that helps to improve the quality and safety of composted green waste has got to be a good thing.