The most acidic soils will be those that include a lot of decomposed or decomposing organic matter. When anything rots, it tends to generate acids. The same thing happens when we make compost. There are things we can add when composting to make it less acidic and possibly take it to the ideal neutral point where compost can be made efficiently.
Soil would need the same treatment to make it less acidic. You may think that all soils will be acidic to a degree and most of them are. Take a sample of soil from anywhere on the planet and there will be a test reading that shows an acidic element.
This shouldn’t be a surprise. All soils can support plant life, of any type, to take root and grow. All plant life will shed organic material that will fall to the ground and become part of the soil. Eventually the plant itself will succumb, fall to the ground, decompose and become part of the soil.
Looking at acidic soil
The soil surface becomes a collection point for a whole range of organic material that will inevitably decompose. Acids will be produced. It may be at a low level but it will be enough to make the soil acidic.
The acidic level of any specific soil will simply depend on just how much vegetation has occupied the ground in which the soil resides. This we can’t quantify just by looking at it. The history of how soil is formed into what we see at this moment in time will be a mystery to us.
One indication of acidic soil will be the appearance of the vegetation on it. It’s always the plants that we want to thrive that object to acidic soil and demonstrate this by appearing spindly and feeble. Most weeds have an irritating habit of coping with acidic soil. Some weeds prefer acidic soil. There’s a list of weeds which, when you see them thriving well, can be taken as an indicator that they are on acidic soil.
We can’t look at the subject of acidic soil without acknowledging that there are some soil-types that aren’t and most likely never will be acidic. There are favoured areas of land that have a high level of chalk mixed in the soil.
Vegetation that decomposes on this soil will attempt to make the soil acidic but the chalk being calcium carbonate, will react with any acids as they’re generated. This will prevent this particular soil from becoming acidic.
How do I know if soil is acidic?
There are a few ways that you can establish whether your soil is acidic. You can tell a lot from just looking at the type of weed plants that will inevitably show up on the soil. I’ve been looking around to see what plants there are that can be taken as an indicator that the soil beneath them is acidic. There’s quite a list, you may recognise some of them.
- Bracken (ferns)
- Silvery Cinquefoil
The visual indicator will be a guide and will do for a start. But you will probably want to know just how acidic your soil is. There are a couple of ways that you can do this. One is rudimentary, the other is much more precise and will give you a measure of just how acidic your soil is.
The rudimentary way involves a basic test that can be carried out on the kitchen table. The procedure will tell you if you have acidic soil, alkaline soil or neutral soil.
For the test to establish whether a soil sample is acidic, you need a small container that will hold a sample of soil. 1 or 2 tablespoons of soil should do. Then add to it half a cup of bicarbonate of soda (sodium bicarbonate). Look closely for signs of bubbles. This is known, in the trade, as an effervescence. If this happens, then it can be taken as an indication that the soil sample is acidic. It won’t tell you just how acidic the soil sample is.
There’s a similar rudimentary test to see if a soil sample is alkaline. To do this, you need a small container that will hold a sample of soil just like the test for acidic soil. Again, 1 or 2 tablespoons of soil should do. Then pour in a cup of vinegar. Look closely for signs of bubbling. If this happens, then you can take it that the soil sample is not acidic but alkaline.
If you do both tests and find that neither deliver a result of bubbles being generated, then this would suggest that the soil sample is neither acidic or alkaline.
If you want a much more refined test that will give you a specific reading, then one option is to purchase a soil testing kit. These will present an accurate result that will display a pH number. A pH of 7 will tell you that the soil sample is completely neutral.
Any measure below pH7 will indicate that the soil sample is acidic. The lower the number is, the higher the acidity will be. Any measure above pH 7 will tell you that the soil sample is alkaline. The higher the number is, the higher the alkalinity will be.
There is another option. You could send a soil sample to a professional testing laboratory, for a fee. The advantage in doing this is that you will get a full report of everything about the sample. This will include the pH reading and the measurements of all identifiable minerals, including levels of phosphate and potash. All of this will be useful information about the soil that you are working with.
It’s worth noting that whichever system you use to assess the acidic or alkaline level of your soil, that there may be variations over a large area. The result from a soil sample taken in one place can’t be taken as an accurate measure for the entire garden plot.
To get a complete picture of the state of your garden’s soil, you may need to take a number of samples, taken from spread-out locations. This could be expensive if you take the lab test option but it would depend on how important the knowledge of your soil is to you.
What can you add to soil to make it more acidic?
Before you take any direct action to attempt to raise the acidic level of your soil, you need to establish whether your soil is acidic or alkaline. If we take it that your test shows a low level of acidity and we want to make an area of soil more acidic, there are substances and materials that we can add to that soil.
Almost any organic mulch will make soil more acidic. In some parts of the world, you may have access to Sphagnum peat. This is known to be a highly acidic medium. Any organic material will bring the additional advantage of improving soil structure.
Then there are a range of substances which will make soil acidic. There are sulphurous compounds that can affect a lowering of the pH level and make soil more acidic. Among these are ammonium sulphate, aluminium sulphate and iron sulphate.
Ammonium sulphate is used as a basic artificial fertilizer that’s used by farmers around the world. It promotes plant growth due to the available nitrates that become instantly mobilised when ammonium nitrate dissolves in water. The down side from the farmer’s perspective is that using ammonium sulphate will generate acidity which they don’t want. The more ammonium sulphate is used, the more acidic the soil becomes.
To make a small area of soil more acidic there are two choices. We can either use the chemical or sphagnum peat, organic matter approach. It comes down to cost and practicalities.
The granular sulphur is the simplest and cheapest way of achieving increased soil acidity but it’s slow acting. The organic matter approach will make soil more acidic. It will improve the soil structure, although the structure benefit will only be temporary as organic material will eventually break down completely and become lost in the soil.
Can you use vinegar to acidify soil?
If using chemicals isn’t your thing and you can’t get hold of enough organic material, then you could consider vinegar. Vinegar is known to be acidic and its effects will be instant. Handling vinegar is very safe, there are no side effects that we need to worry about.
I’ve never tried using vinegar for anything other than putting it on my fish & chips. Vinegar is acidic but it’s a very weak acid. I would question whether using vinegar to make soil acidic is worth the effort when considering the low level of acidity.
But vinegar is acidic and it’s safe to use. You can handle vinegar without fear of losing your skin. You can feed vinegar into an irrigation scheme. Finding out from the experience of others, it appears that you need to dilute vinegar before applying it to the soil. A basic way of doing this is to add a cup full of vinegar to 1 gallon of water. The vinegar solution can then be poured from a ‘rose’ watering-can, onto the patch of soil that you want to treat.
With vinegar being a weak acid, most would wonder why the need to dilute it when we are trying to acidify soil. But I then go on to discover that vinegar, in its concentrated form, has the ability to act as a weed killer.
It’s been shown that vinegar can be a very effective weed killer. Spraying concentrated vinegar over grass or a weed-infested area will kill everything from the soil surface upwards.
Vinegar won’t react in the same way as some chemical weed killers do. It will only kill off vegetation that’s on or above the soil surface. The roots of grasses and weeds will live to regrow.
There are no harmful residues generated from the use of vinegar as a weed killer. Vinegar sprayed in the air around plants won’t upset the bees or any other wildlife. Looking around to learn more, it isn’t clear what type of vinegar works best. There’s nothing to suggest that white vinegar is better than brown vinegar or that there’s any specific need for malt vinegar.
There is a suggestion, to secure the best results, that adding salt will make a difference. This surely shouldn’t be advised. The salt will help to kill off the unwanted plants but you run the risk of permanently contaminating the soil and preventing anything else from growing.
The only area where a salt and vinegar combination may work, could be on a gravel drive where you’re unlikely to want anything to grow.
While we are on the subject of vinegar, there are other uses for it. Some people find vinegar useful for washing hydrated lime from their hands. Lime does tend to stick in the cracks in the skin on hands. Vinegar, being acidic, will react, at a low level with the lime and remove it. This is followed by rinsing the vinegar and lime residue using clean water.
I’ve also heard of people reaching for a bottle of vinegar if they have spilt any caustic (alkaline) substances on their skin. The vinegar will react immediately with anything that’s alkaline and neutralise it before it does any lasting damage.
There’s also a suggestion that vinegar can be used for killing slugs. This is apparently used as a spray. This may be a little tricky. We’ve looked at how applying vinegar to vegetation will kill it, the idea of spraying vinegar onto foliage to kill slugs, would surely risk the plant that we’re trying to protect from the slug.
I can see the attraction of using vinegar to deal with slugs rather than using less desirable chemicals but this would have to be handled very carefully.
But then we could take it a stage further. Most people know that slugs curl up when they’re touched with salt. So, may be, here, there’s a case for using a salt and vinegar solution for slugs but paint it on.
Do coffee grounds acidify soil?
There’s plenty of things being said about coffee grounds and what a useful bi-product it is. This is very convenient because there’s a tremendous amount of coffee being drunk every day. There’s a whole lot of coffee grounds out there that need to be used for something.
So, do coffee grounds make soil acidic? It will make soil acidic but it appears that any acidity that coffee grounds generate in soil, will be short-term. It doesn’t stick around in the same way as other materials do.
If you apply other organic materials like sphagnum moss or pine needles, these will raise the acidity of soil and maintain a measure of acidity for a useful length of time. This is probably because there are larger volumes of moss and pine needles available at any single point in time than coffee grounds.
It’s worth pointing out that there are a number of varieties of coffee bean that will produce variations in the acidity of coffee grounds. Then, there is scope for further variation due to the methods of brewing the coffee. We can’t say that all coffee grounds will have the same level of acidity.
If you have a limited supply of coffee grounds, they may have to be spread over a wider area. The effect will be diluted unless you are lucky enough to secure a vast quantity of coffee grounds from a coffee bar.
The short-lived effect of coffee grounds may be because they tend to break down in soil. They’re high in nitrates. This is the first part that will disappear. Either used by the growing plants or vaporized away by organic material breaking down.
Coffee grounds have very little fibrous structure but the carbon to nitrogen ratio appears to be quite favourable. The nitrates, to be found in coffee grounds, are quite high, believed to be about 10%. Regardless of whether we want to achieve acidic soil or not, the nitrates available in coffee grounds will have a direct effect on plant growth.
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Applying coffee grounds, straight from the kitchen, will provide the available nitrates for instant use. This would be categorised as using coffee grounds as a mulch. Coffee grounds have been shown to be acidic. We could wonder about what it is in coffee grounds that causes this acidity. If it helps we could list all the compounds and oils that survive the boiling-water-moment that delivers us our daily coffee.
We won’t do that here but we could look at the factors in coffee grounds that lean it towards acidity. Here there is speak of structural lignin, which may mean something to someone but the ingredient that caught my eye was protective phenolics. Anything that contains phenolic compounds will have the ability to disturb the progress of anything microbial.
You will see the name ‘phenol’ show up on some household cleaners. You may want to think about this when applying coffee grounds to soil around plants. This is probably irrelevant. You will be putting coffee grounds down as a mulch, in part or as a whole, around plants that prefer acidic soil. Levels of any phenolic compounds found in coffee grounds will be so low that they will have very little effect on much in the soil.
To increase acidity by applying coffee grounds to soil, you need to apply it when it’s relatively fresh. You need to apply it without anything else unless you’re adding materials like pine needles or sphagnum peat.
Putting coffee grounds into compost and expecting it to come through as a finished compost that has a high level of acidity because of the coffee grounds, may not work.
Compost tends to be acidic in the early stages of its development. Adding coffee grounds or anything else that has an acidic leaning, will maintain that acidity. But if other ingredients are included, throughout the process, that take it in an alkaline direction, then the acidic value of any coffee grounds is likely to be reduced.
See what others say about coffee grounds
Looking around to see what others have to say on the subject of coffee grounds, there are a number of interesting comments.
There is the view that using coffee grounds as a mulch, will help to contain temperature and moisture levels in the soil. Don’t put down a layer of coffee grounds that’s so thick that the ground becomes ‘capped’. The nature of coffee grounds is such that a thick layer can become compacted and solidify.
This will stop water both water and air from getting through. The best way to avoid this is to mix the coffee grounds with other organic material, e.g. woodchip, pine needles or dead leaves. This will keep the mulch layer open without compromising the acidity factor that we are attempting to achieve from the coffee grounds.
There is a suggestion that adding coffee grounds to soil will lock down some pesticide residues. It apparently has the ability to contain heavy metals, that are generally toxic, limiting their movement, thus hindering the risk of travelling to waterways.
It isn’t just nitrates that come with coffee grounds. There are other plant-useful elements that are available. Among these are phosphorus, zinc and iron.
When any organic material is introduced to soil, the combined effect of fungi and bacteria will break it down. Apparently, for coffee grounds there are specialised bacteria and fungi that will do this. You may wonder what such specialised bacteria and fungi do when there are no coffee grounds for them to work on.
Following the fungal and bacterial performance, normal ground-rules apply and as with any other organic material the residue from the breakdown of coffee grounds are ingested by worms and taken down to be released into the soil.
There’s a warning out there about some plants not being receptive to the acidic nature of coffee grounds. To some plants coffee grounds are poisonous. You need to know the requirements of the plants that you have. You must avoid allowing coffee grounds from coming into contact with the roots or stems of some plants.
There’s another warning that highlights the risk of applying coffee grounds where there are seeds germinating. There are many plants that will be inhibited in the early stages of growth. The understanding is that there are toxins that are released from coffee grounds as they decompose. This may explain why coffee grounds have been seen to depress some weeds.
There is advice about using coffee grounds to apply to soil through composting systems. When making compost, set a limit of no more than 20% of it being coffee grounds. This level of coffee grounds inclusion will avoid the acidifying of soil, if that is your aim.
Does epsom salt make soil more acidic?
Epsom salt, otherwise known as magnesium sulphate, is neutral pH 7. All salts are neutral. Salt is the result of the reaction that occurs when acid meets alkaline. The term ‘salt’ could almost define neutrality and epsom salt is no exception.
Looking at putting epsom salt on soil
So, when asked if epsom salt makes soil more acidic, we can only say no. Epsom salt won’t make soil acidic or alkaline. You may ask what use is it to add epsom salt to the soil? We need to look at what epsom salt has that’s of use in other ways.
Epsom salt is magnesium sulphate. Consisting of approximately 10% magnesium and 13% sulphur the magnesium in epsom salt will become mobilised in solution but it will be a slow release. The sulphur in the sulphate portion and magnesium is of use to plants. So, these are the direct benefits of adding epsom salt to soil.
Epsom salt is a rock mineral
Acidic soil often carries a magnesium deficiency. It’s interesting to note that the adding of epsom salt doesn’t rectify the acidity. It will merely add magnesium, countering the deficiency that’s there. As for the sulphur factor, adding just sulphur to soil tends to make soil more acidic. However, the sulphur in epsom salt doesn’t lead to soil acidity.
Do you need to use epsom salt?
You need to think before applying epsom salt. Do a soil test to see if there’s a deficiency of both magnesium and sulphur. Epsom salt is spoken of as being ‘non-persistent’. This is the description used for anything that resides in the environment without making any protracted changes to it.
But then we are told that, unless the plants in the area where epsom salt is applied, make use of the compounds within, any excesses of magnesium and sulphur will stay in the soil. With the compounds in epsom salt being slowly dissolved, there is a risk that, over time, they will be leached out of the soil.
These compounds are not identified as toxic but there is a concern that they may interact with other compounds, in the wider environment, leading to a possible adverse effect. It’s seen as unwise to apply epsom salt to soil unless there is a clear indication that you have plants that would directly benefit from both magnesium and sulphur.
How do we apply epsom salt?
Epsom salt can be applied as a dry powdered form. Apply at a rate of 1 to 2 pounds per 100 square feet. This would be done on an open area of soil with nothing growing on it.
To apply epsom salt on soil where there are plants growing, you can make up a solution. Use a concentration of 5 ounces of epsom salt to 5 gallons of water. Apply this solution at a rate of about 2 pints to every square foot.
While on the subject of making solutions of epsom salt, there is an effective foliar spray option which can help to rectify any obvious signs of magnesium deficiency in the plant. To make the solution, dissolve 1 tablespoon of epsom salt into 1 gallon of water.
This can then be sprayed onto the leaves and is known to be effective on plants like tomatoes, peppers and roses. Be careful with this. Avoid spraying an epsom salt solution on any foliage during hot weather when there’s bright sun.
Does sawdust make soil acidic?
Sawdust will make soil acidic but only slightly. It won’t make soil as acidic as other materials like sphagnum peat or pine needles. If you have sawdust available to you, then you can use it effectively as a mulch around plants that require acidic soil. Sawdust will help to maintain soil that’s already acidic to remain acidic.
There is a downside to using sawdust to improve or maintain acidic soil. Sawdust is a carbon ’brown’ in the compost and general decomposition world. When it decomposes it will draw nitrates from wherever it’s available in any surrounding material.
Sawdust will make soil acidic
If sawdust is used as a mulch, it will ultimately decompose and in so doing, it will take nitrates from the soil around the plants that need nitrates for themselves. You can compensate for this by adding nitrates from another source.
One option that will have an immediate effect is the application of ammonium nitrate, otherwise known as artificial fertilizer. This can be mixed in with the sawdust before spreading it around as a mulch.
The acidic factor of sawdust will be short-lived. When the decomposition process has completed, sawdust will become part of the soil and add to the soil structure.