Most plants prefer a neutral soil around pH 6.5 to 7.5, but many will grow favourably in the broader pH range of 5.5 to 8. When soils become too acidic or too alkaline, some essential plant nutrient will become unavailable to plants, negatively impacting plant growth and yields, so it’s desirable to correct the soil pH and bring it closer to the ideal range which plants prefer.
The Importance of the Soil Food Web
Before we mess around with soil pH too much though, we need to keep in mind that the soil is not just dirt, but a very complex living ecosystem, better understood as the soil-food web, an extensive community of living organisms that live all or part of their lives in the soil, which exist in complex relationship to one another. The soil-food web breaks down organic matter to build soil and make the nutrients available to plants in the process.
The organisms which comprise the soil-food web include organisms such as bacteria, fungi, nematodes, protozoa, earthworms, arthropods, small animals and living plant and tree roots. It has been estimated that there are about 50 billion soil organisms in 1 tablespoon of healthy soil, whereas the human population numbers just over 7 billion currently. All of these organisms play an important role in the soil-food web, which functions to keep plants alive.
Shown below is a diagram from the United States Department of Agriculture (USDA) which shows a simplified overview of how the soil-food web works. The arrows indicate the paths of nutrient and energy flow, or point to which organisms eat them!
Since soil is a living ecosystems, where everything affects everything else, the health of the soil directly affects the health of the plants growing in it. Plant health in turn determines a plant’s resistance to pests and diseases, as well as it’s capacity for growth and production.
The Effect of Soil pH on the Soil Food Web
At this point you may be wondering what the soil-food web has got to do with changing soil pH?
Changing the soil pH will affect beneficial soil microbe activity since soil microorganisms prefer a soil pH close to neutral (pH 7.0).
If we make any drastic changes to the soil pH, making it either too acidic or too alkaline, we can disrupt the delicate ecological balance and adversely affect soil health.
Healthy plants don’t grow in unhealthy soils!
In the previous article Soil Chemistry Fundamentals, Part 1 – Understanding Soil pH and How it Affects Plant Nutrient Availability we discussed how changing the soil pH can affect soil fertility by making certain essential nutrients less available or more available. Nutrient availability is only one aspect of soil health, the other is beneficial microbe activity.
If we must change the soil pH, we need to do so in an environmentally responsible way. Some of the methods employed by conventional agriculture and gardening are ecologically unsound because they damage the soil and harm soil organisms, which ultimately affects plant health. They’re also not allowed in certified organic gardening practises, and for good reason.
How to Change Soil pH
Soil chemistry is a very complicated subject, but altering soil chemistry needn’t be so.
To change the soil pH in a way that is compatible with organic gardening, we can do the following:
- add sulphur to the soil to reduce pH and make the soil more acidic.
- add garden lime or dolomite lime to the soil to increase pH and make the soil more alkaline.
- add lots of organic matter to the soil, as this will break down to produce humus, which buffers the soil pH to keep it stable and help maintain the desired pH levels.
Here is the basic procedure for altering soil pH:
- Start by adding a small amount of the required soil amendment (sulphur or lime) to the soil.
- Make sure that the soil amendment is worked (mixed) into the soil well and watered in after application.
- Give the soil amendment time to act in the soil.
- Test each year to determine if more soil amendment is needed.
Adding too much sulphur or lime all at once can create imbalance of the soil pH and disrupt soil ecology.
When changing soil pH, which soil amendment we use to change the soil pH and how much we add can make all the difference.
How to Decrease Soil pH to Make Soil More Acidic
If soil is too alkaline, reduce pH and make the soil more acidic by adding elemental sulphur.
It’s important to note that changing the soil pH using sulphur is a slow biological process, not a fast chemical process, so it can take a few months to a year to change the soil pH to the desired level. The process is slow because it’s dependent on soil bacteria which oxidise the sulphur (S) into sulphate (SO₄²⁻), and convert it into sulphuric acid (H₂SO₄), and the process releases hydrogen (H+) ions, which acidify soil. Incidentally, when sulphur is oxidized into sulphate (SO₄²⁻), this is the form available for uptake plants.
The oxidation of sulphur occurs more rapidly in warm, moist soils with high organic matter contents. This is because the bacteria are active when the soil is moist and warm.The soil temperature needs to be above 12.8°C (55°F). The soil must not be saturated or waterlogged, as this will exclude oxygen, and make it anaerobic, whereas the oxidation of sulphur is an aerobic process and requires oxygen. If no oxygen is available, the sulphur is instead converted by anaerobic bacteria to hydrogen sulphide (H₂S), better known as rotten egg gas, which kills plant roots.
Applying sulphur to the soil in autumn and winter is not advisable as the soil bacteria won’t be active at that time. It can be done, but no changes in soil pH will occur until the soil warms up enough in spring. If the soil is irrigated to maintain moisture levels throughout the year, the oxidation reaction will be maximised. Cultivating the soil to increase aeration can also help speed the process. It is fortunate that the oxidation of sulphur occurs faster in alkaline soils than in acidic soils, as these will be the soils where we will more commonly seek to reduce the pH.
Elemental sulphur should be incorporated (dug into the soil) to increase the speed of oxidation, because the soil bacteria will have better access to it. Additionally, the sulphur is oxidised at a greater rate by the soil bacteria when the sulphur particle size is very fine, as oxidation of sulphur is a surface-based process, and smaller particles provide a larger surface area. This is one of the most important attributes affecting oxidation.
When is the Best Time to Apply Sulphur
The recommendation is to dig the sulphur into the soil a year before planting, due to it’s slow reaction with the soil. If sulphur cannot be easily incorporated to the soil because the soil is already planted up, then it can be applied to the surface of the soil, and will lower the soil pH to the same degree, but will take much longer to do so. The best time of the year to apply sulphur to the soil is in spring and summer when the soil temperature is above 12.8°C (55°F) and the soil bacteria are active. It can be added at any time but won’t begin to take effect until the warmer weather sets in.
Sulphur Application Rate
How much sulphur should you add to lower the soil pH?
Remember, lowering the soil pH by one point makes the soil 10x more acidic, 2 points makes it 100x more acidic.
In established plantings, to avoid plant injury, it is important not to apply too much sulphur all at once. Some sources advise do not apply more than 180kg (400 lb) per acre or 44.8g per square metre at a time, while others suggest a greater maximum amount of 2 pounds per 100 square feet, or 97.6g per square metre at a time.
If more is required, spread the application out over several years, and wait at least 3 months to make another application.
The application rates for unplanted soil are provided below in both metric and imperial measurements.
Grams of Sulphur per Square Metre to Lower the Soil pH to the Recommended Level
6.5 6.0 5.5 5.0 4.5
8.0 146 195 244 293 342
7.5 98 146 195 244 293
7.0 49 98 146 195 244
6.5 – 49 98 146 195
6.0 – – 49 98 146
Pounds of Sulphur per 10 Square Feet to Lower the Soil pH to the Recommended Level
6.5 6.0 5.5 5.0 4.5
8.0 0.3 0.4 0.5 0.6 0.7
7.5 0.2 0.3 0.4 0.5 0.6
7.0 0.1 0.2 0.3 0.4 0.5
6.5 – 0.1 0.2 0.3 0.4
6.0 – – 0.1 0.2 0.3
Conversion: 1 lb = 0.4535924 kg, 10ft²= 0.9290304m² ,100ft²= 9.290304m², 1 acre = 4046.856m²
Other Methods of Decreasing Soil pH
The soil pH can also be lowered by adding sphagnum peat moss to the soil, but this method is only recommended if this resource is obtained sustainably. Most often, extensive environmental damage is caused when it is harvested commercially. The pH of sphagnum peat generally ranges from 3.0 to 4.5, it’s very acidic. Due to its high cost, it is only viable to use in small gardens. To amend the soil, add a 2.5 – 5cm (1 – 2”) layer of sphagnum peat and incorporate it into the top 20 – 30cm (8 – 12”) of soil before planting.
Using organic mulches over the soil slowly reduces the sol pH as the products of slow composting are slightly acidic.
Even better for creating an acidic soil is a pine needle mulch. Pine needles take a very long time to break down, acidify the soil much better, and can be laid over the soil in thick layers. The fallen pine needles under pine trees can be gathered to use as mulch.
Which Methods of Decreasing Soil pH Are Not Compatible with Organic Gardening?
These chemical methods lower the soil pH very quickly, but are damaging to plants and the soil ecology, and are therefore not recommended.
Iron sulphate (FeSO₄) has an immediate effect on soil pH, but it’s also important to consider that this is sold as a herbicide to kill broad leaved weeds in lawns, and also a moss killer for paths.
Aluminium sulphate (Al2(SO4)3) is also used to acidify soils quickly but should not be used as aluminium availability increases greatly at soil pH below 5.5, it can limit the ability of plants to take up phosphorus by reducing phosphorus solubility, and aluminium may reach high levels where it can lead to aluminium toxicity in plants, as it can be extremely toxic to plant roots, including blueberries.
Even though aluminium sulphate and iron sulphate react more quickly with the soil than elemental sulphur, they must be applied at a 5 to 6 times greater rate than sulphur. Do not apply either of these at more than 5 pounds per 100 square feet or 244g per square metre, as they can cause injury to plants.
Acidifying nitrogen fertilizers that contain the ammonium (NH₄) form of nitrogen, such as ammonium sulphate, diammonium phosphate, monoammonium phosphate, urea, and ammonium nitrate will acidify soil. Soil bacteria convert the ammonium (NH₄) form of nitrogen to the nitrate (NO3–) form, and the process releases hydrogen (H+) ions, which acidify soil. Ammonium sulphate is more acidifying then ammonium nitrate or urea, which are equal in effect, as it supplies twice as much acidity as they do. Ammonium phosphate’s ability to acidify soil is slightly less than ammonium nitrate or urea.
One of the problems with acidifying fertilisers is that they decrease soil pH more gradually than elemental sulphur, and it can take more than 2 years to decrease the soil pH by 0.4 pH units. Additionally, being synthetic nitrogen fertilisers, they’re very harmful to the environment and to the soil ecology. If they’re used in autumn or on unplanted ground, the winter rains will carry nitrate down where it will leach through the the soil to contaminate groundwater.
How to Increase Soil pH to Make Soil More Alkaline
Garden lime (calcium carbonate) is added to highly acidic soils to increase soil pH and make them more alkaline or basic. It’s also a source of calcium, an essential plant nutrient, and is added to the soil to prevent deficiency diseases such as blossom end rot in tomatoes, and to support fruiting trees, as tree fruiting is very sensitive to calcium shortages.
Lime should be incorporated (dug into the soil) to work properly. Increasing contact with the soil increases its effectiveness because most liming materials are only slightly soluble in water. Soil moisture is required for the lime-soil reaction to occur, and lime will have little effect on the soil pH if the soil is dry, even if it is mixed in really well.
Changing the soil pH with lime is a slow process, and several repeat applications may be required. Don’t add too much lime all at once as it can throw the soil chemistry out of balance.
Don’t add any form of lime or wood ashes to alkaline soils as this will create excessively alkaline conditions which will make soil nutrients unavailable to plants and cause nutrient deficiencies. It is unnecessary to increase the soil pH of acidic soils above 6.5, as this may cause deficiencies of trace elements such as manganese.
Different Forms of Lime Used in Gardens
Garden lime is calcium carbonate (CaCO₃), and it’s exactly the same thing as ground limestone or agricultural lime. It’s also the same material that seashells (shell lime) and eggshells are made of. This is the cheapest form of lime, but it’s the least reactive, though the premium grades are more finely ground.
Dolomite lime is calcium magnesium carbonate (CaMg(CO3)2) and contains a mixture of calcium carbonate and magnesium carbonate, so it also adds large amounts of magnesium to the soil, an important plant nutrient. The disadvantage of dolomite lime is that it is less reactive than calcium carbonate, and won’t increase the pH as well because the magnesium carbonate doesn’t work quite as effectively even though it’s alkaline just like calcium carbonate. Use dolomite lime when magnesium is required, it’s similar to adding Epsom salts (magnesium sulphate) to correct magnesium deficiency, otherwise excesses of any nutrient can cause imbalances. Magnesium deficiency is much more common in fast draining soils, such as sandy soils. Clay soils and clay loams rarely suffer from magnesium deficiency, but excessive magnesium can make soil compaction worse, harming denser clay soils. Also, it’s harder to correct over-application of dolomite lime compared to garden lime, as it takes more work to bring the magnesium levels down.
Calcium hydroxide (Ca(OH)2) is also known as slaked lime or hydrated lime, is better known as builders’ lime which is sold under the product name of ‘Limil’. It’s much more reactive and will raise the soil pH faster than calcium carbonate, but it’s much easier to over-lime the soil with this product. Being a building construction material, it’s a bit too expensive for agricultural use. This is a building product, garden lime is a better choice.
Calcium oxide (CaO) is also known as quicklime or burnt lime, and is the most reactive form of lime. It reacts with water to become Calcium hydroxide, but produces lots of heat in the process, and also swells considerably, so it must be stored dry to prevent it absorbing moisture. It should not be allowed to come into contact with young trees as it can burn tender bark and roots. This is a building product, garden lime is a better choice.
Additional precautions with slaked lime (calcium hydroxide) and quicklime (calcium oxide) – do not let either of these products make contact with animal manures or nitrogen fertilisers, as this will cause the release of ammonia gas, which will be partly partly lost to the atmosphere, but in the soil is toxic to young trees.
When is the Best Time to Apply Garden Lime?
Because of its low solubility lime should be cultivated into the soil so that it can reach the root zone of fruit trees. Ideally lime should be incorporated into the soil before planting. It is also more economical to apply fairly large quantities every five years or so once trees are established, if soil pH tests show that liming is needed. Generally, orchard soils need liming only where the top 200 mm of soil is more acidic (has a lower pH) than pH 5.6.
Garden Lime Application Rate
How much lime (calcium carbonate) should you add to increase the soil pH?
Remember, increasing the soil pH by one point makes the soil 10x more alkaline, 2 points makes it 100x more alkaline.
The amount of lime that needs to be applied to correct overly acidic soils is determined by two factors:
- The acidity of the soil
- The soil texture – a heavy clay soil will need more lime than a sandy soil.
Suggested amounts of lime (tonnes per hectare)
Soil texture 4.0-4.5 4.6-5.0 5.0-5.5
Sands 4.0 2.5 1.5
Loams 5.5 3.25 2.0
Clay sand 9.5 6.25 3.5
Suggested amounts of lime (grams per square metre)
Soil texture 4.0-4.5 4.6-5.0 5.0-5.5
Sands 400 250 150
Loams 550 325 200
Clay sand 950 625 350
Conversion – 1 t/ha = 0.1 kg/m2
Pounds of ground limestone needed per 100 square feet to raise the pH to 6.5 in the top 6 inches of soil.
Soil pH Sandy loam Loam Clay loam
5.0 8 10 15
5.5 6 8 10
6.0 3 4 6
Conversion: 1 lb = 0.4535924 kg, 100ft²= 9.290304m²
If using dolomite lime, to increase the soil pH by one point, apply 100g/m2 in sandy soils, and up to 250g/m2 in clay. The change in soil pH will be observed after two to three months.
Using Wood Ash to Increase Soil pH to Make Soil More Alkaline
Wood ashes can also be used to raise the soil pH and make the soil more alkaline. Compared to garden lime (calcium carbonate), wood ash is only around 25 to 59 percent as effective in raising soil pH. Calcium is the most abundant element in wood ash, almost 20%, with potassium at around 3%, magnesium around 2% and phosphorus at around 1%. In terms of fertiliser ratings, the average N:P:K ratio wood ash would be about 0-1-3.
Avoid using large amounts or repeated use of wood ashes as they can raise the pH value of a soil very high levels and cause plant nutrient deficiencies, especially in sandy soils. They can also cause damage If they come in contact with germinating seedlings or plant roots.
The best way to use wood ashes is very sparingly, by spreading a very thin layer and incorporating (digging) it into the soil in the spring. Don’t use coal ashes or barbeque briquettes as that don’t alter the soil pH but contain lots of toxic chemicals which will cause soil contamination.
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