TL;DR: Check the water you are watering your plants with, add organic matter, pine needles, compost, lime, red algae… And keep on monitoring.
Before we get to the heart of the subject there are a few things you need to know about the pH scale you can encounter in different soils.
What exactly is pH ?
In water, a small proportion of H2O molecules break up to become hydroxide ions (OH-), the remaining hydrogen atom being left as H+. This hydrogen ion binds to a water molecule to form H3O+ (also called hydronium). In a neutral solution (such as pure water), these proportions are the same, i.e. they cancel each other out. In an acidic solution, there are more H+ ions than OH- ions, and these extra ions are responsible for the active nature of an acid or a base.
The acidity or alkalinity of a substance is measured in pH units, a scale running from 0 to 14. “pH” is the abbreviation of “potential Hydrogen” and refers to the capacity of a solution to donate hydrogen ions (H+), which is equal to the concentration of H+ ions.
A pH of 7 is neutral, this means it is neither acid nor alkaline (or basic). As numbers decrease from 7, the acidity gets higher. As numbers increase from 7 and onwards so does the alkalinity. This scale is logarithmic in relation to the concentration of H+ ions (not linear): pH=-log[H+] (in chemistry, square brackets mean concentration). This simply means that a step of 1 unit on the pH scale indicates a 10-fold increase in H+ ion concentration! In other words, a pH of 5.5 is 10 times more acidic than a pH of 6.5. Conversely, a pH of 8.5 is 10 times more alkaline than a pH of 7.5.
There are ways to determine the exact pH of your soil with simple tests. You can find pH meters for a small amount of money on the internet or in gardening stores. We recommend you purchase or borrow a simple digital pH meter like one of these:
You can also use pH paper if you prefer to use this medium. There are also simple items to test phosphorus or potash levels for example.
Why do pH levels vary?
pH levels are different around the world, from continent to continent, from country to country, from region to region and sometimes even from garden to garden. There are many causes for these changes, including historic ones. The soil’s pH may have changed over time depending on the plants or elements that have been deliberately introduced into these spaces, but there are other factors in action.
Here are some example you might relate to (for French or American readers).
pH levels in the US state by state (Source: IPNI)
Of course these are median levels so they don’t mean that if you live in Kentucky your garden’s pH level is necessarily 6.2.
pH levels in France (Source: AgroParisTech)
Variation from rainfall
For example rainfall contributes to a soil’s acidity. Water (H2O) combines with carbon dioxide (CO2) to form a weak acid — carbonic acid (H2CO3). The weak acid then ionizes, releasing hydrogen (H+) and bicarbonate (HCO3). The released hydrogen ions replace the calcium ions held by soil particles, causing the soil to become acidic. The displaced calcium (Ca++) ions combine with the bicarbonate ions to form calcium bicarbonate, which, being soluble, is leached from the soil. The effect is increased soil acidity. We hope this doesn’t sound like Chinese to you. Don’t worry this isn’t a scientific paper, it is totally in your reach and we’ll do our best to explain in easy and illustrated terms.
Soils become acidic when basic elements such as calcium, magnesium, sodium and potassium held by soil particles are replaced by hydrogen ions. Soils created under conditions of high annual rainfall tend to become more acidic.
Variation from season to season
Similarly, soil pH may also vary from season to season. Indeed, pH levels in soils tend to lower in the winter in areas with large amounts of winter rainfall. Rain leaches alkaline elements including calcium, magnesium and potassium from the soil into runoff water, leaving acidic elements like hydrogen, aluminum and manganese to replace the bases.
On the other hand, soils subjected to low rainfall conditions tend to be basic with soil pH readings that range around 7.0 or more.
Variation from agriculture
Another factor that may affect soil pH is agriculture. Intensive farming over a long number of years with nitrogen fertilizers or manures can also result in soil acidification. The plants ideal for an area may change over time if you do not amend to the soil.
Variation from parent materials
The continuing contributions to soil pH by parent materials (the existing soil and bedrock) and other environmental sources like rain, water runoff and existing plant life always try to get the soil slowly but steadily back to its “normal” levels. This is why changing and maintaining pH requires more of a constant effort rather than a single amendment once in a while when your see your plants health deteriorating.
Now back to the core of our subject.
How to change pH for small quantities of soil
First of all this article is only about changing the pH levels in small quantities of soil (e.g. plant pots or small garden beds). If your are searching for ways to adjust the pH in your entire garden or on a large piece of land you might want to check out these other resources:
You may wish to apply the ideas given in this article on a larger scale. If you have raw material in larger quantities and easily accessible this may be a great idea, but unfortunately that is not the case for everyone.
Different pH levels for different plants
Secondly, the most important thing to know about plants is the following : every plant has its preferred range of soil acidity, and when the pH level is out of that range, a long list of ills may and probably will, follow. What does this mean? Well, basically if your plant is ill, sometimes you shouldn’t look any further than the soil it sits in. Your plant may not be adapted to the earth it’s growing in or your soil pH may have changed over time (yes that does happen, especially for plants in small pots that need constant watering, like bonsais) making it a hostile environment for your plant to grow in. The soil pH is important because it affects the availability of nutrients in the soil. Many plant nutrients are not readily available to plants in highly alkaline or acidic soils. These essential nutrients are most available to the majority of plants at a pH level between 6 and 7.5. This is unfortunately not a general rule and some plants like Blueberry bushes are exceptions.
Getting to know a few things about pH levels
Knowing a few things about pH levels may not only help you keep your plant healthy but it may also help you save face when shit hits the fan. For example have you ever tried to grow blueberries only to have them turn yellow, then brown and eventually die? If you have, chances are you planted them in an alkaline soil. Blueberry bushes are one of the plants that thrive in acidic soil. Of course you can plant them in neutral soils but they will not flourish and chances are your harvest will be pretty meager.
In some places the soil is naturally acidic. If you take the US for example you will be more likely to find acidic soils in Minnesota than in Iowa where the earth in more alkaline. Places where many pine trees grow are usually made of rather acidic soil because pine needles add acidity to the soil. Another tip is that in general, soils in climates with high rainfall tend to have soil that is more acidic. Most garden soils have a pH between 5.5 and 8.0 (unless you regularly suffer from acid rain for example, which probably won’t be your case this century, well, hopefully). This number helps you determine when and how to adjust your garden soil’s pH level.
The original range of acidity or alkalinity in soils is a result of many factors, some more obvious than others. These can include a soil’s parent material and the amount of yearly rainfall an area receives (as we already mentioned earlier) for example, temperature or even soil physical structure (sandy / clay / organic conditions) can also influence pH to change.
What exactly does pH do and why is it important?
Previously we’ve already mentioned that pH is important in order to determine whether or not nutrients are accessible to plants but this is not the only reason why an adequate pH is vital to a plant’s state. Under alkaline conditions in your potted plant’s soil, vital elements such as Iron, Zinc & Manganese are trapped in the soil and cannot be absorbed properly by the plant even though the soil may be anything but lacking in these elements. This is one of the reasons why alkaline soils we commonly see as yellowing (deficiency) of the newest leaves due to lack of availability of these elements (if one of your plants has the proper pH but still has its leaves turning yellow you should read this guide on why your plant’s leaves are turning yellow). Need an example? Yellowing between the veins of fresh leaves or sprouts may indicate an iron deficiency, a condition arising not from a lack of iron in your plant’s soil but from insufficient soil acidity to put iron into a form that a plant can absorb. This is a common ailment in some vegetables, fruit and flowers but can also be easily noticed in different varieties of Citrus trees.
Affects availability of plant nutrients
This chart might make it a little easier for you to understand the concept. Of course different plants need different levels of each resource in order to thrive. That’s why plants usually grow better in the region from which they originate.
Plants in pots have very little soil to work with and these conditions can slowly lead to their demise. You can see in the chart above which elements are available in the proper quantities at the recommended pH level. Some elements are also toxic to plants and an inadequate pH level can force a plant to absorb too much of these elements, making them ill. This is the flip side of coin : plant poisoning. Too low a pH level can make manganese, which is in the best conditions an important plant nutrient, available at toxic levels; geraniums are particularly sensitive to this. You can easily spot the plants that are being poisoned by looking for yellowed, brown-flecked, or dead leaves. Another example is a pH level that is acid liberates aluminum, which is never a plant nutrient, in amounts that can stunt or completely stop root growth and interfere with a plant’s absorption of nutrients. Lastly, at a high pH level, the plant nutrient molybdenum, too, becomes available in toxic amounts.
Influences living organisms
Soil pH also influences soil-dwelling organisms (the small ones and the really really small ones), whose well-being, in turn, affects soil conditions and plant health. The slightly acidic conditions enjoyed by most plants are also what most earthworms like. This is also the case for a large part of microorganisms (eg: denitrifying bacteria) that convert nitrogen (nitrates) into forms that plants can use. If you want to learn more about the Nitrogen Cycle we invite you to read these resources: https://bioh.wikispaces.com/More+Elemental+Cycles.
You can now see why it is important to be approximately at the right spot in the spectrum in order to keep your plants healthy. If you are not too bothered about having the best possible conditions for your plant’s growth but just want to keep it relatively healthy we recommend you keep the pH levels of your potted plants and plant beds around neutral on the slightly acidic part of the spectrum (around 6.5). This also goes for your garden on a larger scale.
Another solution to avoid any pH problems is exclusively grow plants that are adapted to the soil available in your garden or in your area. You can identify this by looking at this list of plants and their ideal pH levels which we will try to make available on the blog as soon as we can. Of course we can’t list all plants, but you’ll find the ones we grow, the ones we’d like to grow and even some more!
What can you do about it?
Before trying to change your soil’s pH you should note that soil pH varies by up to half a point over the course of each year. Soil pH tends to be higher (more alkaline) when the soil is cool, and lower (more acidic) in summer, when increased bacterial activity in warmer weather has an acidifying effect on soil. This is an important thing to factor in and at least take into account when considering changing soil pH. It is also important to note that this does not apply to your plants if they are always kept at room temperature inside a house or an apartment.
If you notice your pH varies more than this (it is possible in bonsai pots or in small flower pots, in you are watering every day with tap water) there are solutions to try and buffer these changes. The most obvious one would be to put your plant in a larger pot. Unfortunately this is not always an available option.
Add organic matter
Adding organic matter is another way to adjust your pH level. Organic matter buffers soil, especially sandy soil (you must make sure rich soils suit your plant type before doing this). You should also know what soil you have in your garden. If you don’t already know this you should definitely read up on how to identify the type of soil you have in your garden on our blog.
The more organic matter is contained in your soil, the more lime it takes to raise the soil pH 1 point, and the more sulfur it takes to lower the soil pH 1 point.
In this article we are looking for natural and quick ways to change your soil’s pH in small quantities.
Use sulfur or lime
For large quantities of soil there are faster and more effective ways of changing pH. The best known way of changing this acidity of your loam or other type of ground is to use sulfur or lime. These two chemicals found in raw form can be included in natural soil amendments and are a measurable, effective way to change pH. Powdered elemental sulfur, sometimes called flowers of sulfur, is dug and mixed into gardens by gardeners in order to lower their soil’s pH. To be effective, sulfur needs to be spread evenly, perhaps in several applications, and given several months to interact with soil (you wouldn’t want big lumps of sulfur in your garden would you?). The microorganisms, bacteria, earthworms and other small friends will happily take care of this for you. Dolomite lime, or ground limestone, also needs time to decompose and bond with soil in order to raise the pH.
Try alternative methods
These are good solutions but we would like to offer you alternatives for several reasons. These methods aren’t particularly adapted for soils in which plants are already planted, after all it does require to dig up your garden and mix in these elements at the bottom of your garden bed. These processes also require interaction between these elements and your current soil in order for them to mix. These interactions are facilitated by earthworms, bacteria and other microorganisms that sometimes have a hard time thriving in flower pots, therefore making the process longer. Too much of one of these elements can also lead to your plants death so you have to be sure to dose them perfectly. This is pretty hard for a small flower pot. You don’t necessarily want to buy huge quantities of these soil amendments either. So why don’t you try these alternatives?
Incorporate peat moss, pine needles, egg shells, wood ashes, …
Using natural materials to change the pH balance of your soil has many upsides and also accomplishes several goals for your garden. Natural materials are the basis for soil are they not? This means that additional materials are just as easily absorbed (unless in excessive quantities). Dirt is created by the deterioration of dead leaves, branches and other plant detritus, combined with the rocks that originally contributed to the chemical basis of soil (and still do). Another neat advantage is that materials like peat moss, pine needles, egg shells, oak leaves, manure or wood ashes deteriorate more slowly and steadily than applications of chemical fertilizer. They are also more enjoyable to to the small organisms (and mycelium) in your garden and their happiness is very important to you. After all they are responsible for the well being of your plants.
How to lower your soil’s pH?
Find free and easily accessible material
Organic matter frequently used to reduce soil pH include peat moss, rotted manure and rotted leaf compost. There are some very effective organic substances like Canadian sphagnum peat moss, with a pH of between 3.0 and 4.5, that can be used, but these often come at a high price if you want to incorporate them on a large scale. The best thing you can find for your garden is organic matter that you can easily access and get for free (for example leaves from a nearby forest). Shredded evergreen bark, pine needles, home-composted leaf and vegetable wastes and even coffee grounds can also increase soil acidity. Try a mix of these in small quantities every time you think about it and your soil will slowly change. It is important to monitor these changes for the future. This is why you should test soil pH before application, then at monthly or quarterly intervals to determine change. It is also important to improve water drainage and prevent water evaporation by adding compost or a mulch of shredded leaves (which are very acidic) to slowly incorporate these into the soil.
As it decomposes, the organic matter you have introduced into your soil, will acidify your soil. The thing is you have to add enough organic matter to your soil for it to make a change and this might take some time. What you want in the long run is a great quality isn’t it? Here are a few examples of organic matter that you can use:
- Composted oak leaves
- Coffee seeds or ground coffee
- Peat Moss
- Pine bark
- Cottonseed meal
Please don’t add vinegar to your soil. While it might seem like a good idea at first it will damage your soil over time. It will immediately lower your pH, but even if you measure it correctly the shock will be too brutal for the soil’s microorganisms. These will die and your soil’s pH level you rise again, leaving you with the same soil without all the goodness (small organisms). Your plants might die from the shock as well.
Think about the long term benefits
If you wish for your soil pH levels to remain stable you should add important quantities of fertilizers or organic matter such as cottonseed meal or even composted oaked leaves from the nearest forest. If you don’t have time to compost it you may directly use the leaves as mulch. You can also much with pine bark or pine needles. Cotton seeds are a great byproduct of the cotton industry that work perfectly for plants that love acid soils like azaleas.
How to raise your soil’s pH?
As with lowering the pH, there are several easy and natural ways to raise the pH of your soil.
Mix in woodash
For example, ash from a woodfire will change the pH pretty quickly. Be careful not to add ash in too large quantities, because this can induce root-burns in your plants:start small and move your way up if needed! However, the effect on large quantities of soil will not be long lasting and you will need to renew the action every few years.
For flower pots and small garden beds adding wood ashes is a healthy solution (if the wood burnt was not treated with chemicals of course). There are also hidden advantages to these ashes. They will protect your outdoors plants from snails and slugs, and ash contains some very useful minerals for plants and acts as a fertilizer.
Add oyster shells
In a similar fashion, shells from oysters can be finely ground into lime and added in small quantities to plant pots. Note that this is also useful to correct calcium deficiencies as these shells contain high levels of calcium.
These natural solutions provide easy fixes to raise pH for some alkaline-loving plants, or in acid soil regions. They have the additional benefit of being beneficial to soil microorganisms and root function, and potentially less aggressive for the plants than chemical solutions.
Use elements with care
Ground limestone can also be used, as well as powder obtained from some calcareous red algae (Phymatolithon calcareum). Please consider that pure lime (used for mortar) will change soil properties too quickly and cause damage to roots and soil flora and fauna.
Soil chemistry is complex and would probably require a full course to discuss fully. However, some simple notions can be explained here. Trace elements in the soil can be available to plants if they are dissolved in the soil water, or weakly attached to the surface of some soil particles. Elements that are enclosed in minerals in more complex structures are unavailable to plants. On particular structure that is highly important to plants are what we call the clay-humus complex, which are simply clay particles and organic matter that are mixed together largely thanks to the work of soil workers such as earthworms. These particles are tiny, and are capable of attracting both water molecules (adding to the soil water content that plant roots look for) and positively charged ions (cations such as magnesium Mg2+, calcium Ca2+ or aluminium Al3+). Naturally, these these positive charges slowly get replaced by H+ ions , thus lowering the soil’s pH. This process is slow however and in a healthy soil with plenty of worms and other beneficial creepy-crawlies, it’s not a big issue.
The buffer effect
In a very active soil (biologically and chemically), these structures build up to high levels, and allow important cation exchanges, which forms a chemical barrier to pH variation: a larger stockpile of positive cations makes any incoming H+ much less significant. This is called the buffer effect of the clay-humus complex. A soil that naturally contains calcium carbonates (CaCO3) will also be buffered against pH variation: in acidic conditions, this molecules degrades into water and carbon dioxide (H2O+CO2). – this is why adding lime to your soil will raise pH, as it neutralises H+ ions! These buffer effects are largely beneficial, but need to be considered when trying to artificially change your soil pH: a buffered soil will take longer and more effort to change, i.e. a larger quantity of lime to raise pH, or sulphur to lower it!
Why is it important to keep monitoring your pH levels and re-adjusting them regularly to optimal conditions?
Soil pH is an essential parameter to monitor and adjust for your plants’ health. A minimum is knowing (even approximately) what the baseline pH is for your region/locality, or garden even. Repotting or planting a plant is already a relatively traumatic experience, and a misjudgment of optimal soil pH can easily push a weakened plant towards illness or death. Furthermore, like many soil parameters (e.g. water content or nutrient levels), pH is dynamic and varies depending many factors as detailed previously. If you’re worried about some of your plants (based on your knowledge of their tolerance) or maybe after an unusually wet period, don’t hesitate to check for any rapid changes in soil pH.
Planting the ideal plants
Another solution for gardeners is to favor plants that grow naturally in the soil your garden is offering you and to grow plants with special needs such as pine, oak, gardenia, blueberry, azalea, and rhododendron (which are among the plants that demand a very acidic pH of 4.5 to 5.5) in containers. This way, you will only having to modify small quantities of soil.
Going with the flow is sometimes the best strategy for growing plants successfully in alkaline soils (or in any soil). Grow plants that naturally favor alkaline soils so that you won’t have to work very hard at improving soil pH – here are a few examples:
- Trees, shrubs and bushes: Butterfly bush, Judas tree, beauty bush, black mulberry, Indian bean tree, yew, rosemary, black maple, northern catalpa, ginkgo biloba, common hackberry, Kentucky coffee tree, Japanese barberry, shore juniper, creeping juniper.
- Vegetables and herbs: Asparagus, okra, parsley, yams, sweet pea, baby’s breath, beets, cabbage, celery, cauliflower.
- Flowers: Evening primrose, hellebore, flax, pink carnation, red valerian, yarrow, common lilac, delphinium, clematis, Madonna lily, purple coneflower, phlox, and candytuft.
On the other hand if you soil in naturally acidic here are some ideas. The quickest way to get plants thriving in acidic soils is to grow plants that love these soils. Here are a few to get you started:
- Trees, shrubs and bushes: Balsam fir, china fir, larch, magnolia, crabapple, gardenia, anise, Norway spruce, Japanese snowbell, weeping willow
- Vegetables: Tomatoes, carrots, beans, peas, peppers, Irish potatoes
- Fruits: Blueberries, cranberries, strawberries, apples, pears, bananas
- Flowers: Golden lights azalea, amelia chrysanthemum, autumn blush coreopsis, Echinacea, blue haze euphorbia, ferns, hosta
If you are looking for a list of plants and their favoured pH level you are welcome to look at our non-exhaustive list of commonly (and less commonly grown) plants.
If you want to know more about types of soil we recommend you do a little research starting here : https://en.wikipedia.org/wiki/Alkali_soil
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