Soil pH and Plant Health
In the world of science, pH measures the activity and concentration of the hydrogen ion. Pure water has a pH very close to 7 at a temperature of 25° C.
How does this relate to your soil and plants? Well, different plants require different levels of various nutrients in order to flourish; the major of which are Nitrogen (N), Phosphorus (P) and Potassium (K) often referred to on the side of a bag of fertiliser as the NPK levels. Other elements high up on the requirement list are calcium, sulphur and iron and of course smaller quantities of trace elements such as copper, zinc and magnesium.
These levels can be modified in the soil in a variety of ways, the simplest of which is adding a chemical or organic based fertiliser such as Organic Xtra. You can also modify these levels with natural compost, either your own home produced product or a manufactured compost such as Soil Conditioners or Enviroganics Premium Compost, or organic mulches such as Sugar Cane, Lucerne or any number of organic mulches available.
This is where soil pH comes into play. The ability of your plants to draw the available nutrients from the soil can be greatly influenced by the pH levels. Soil pH is measured on a scale of 1-14 where the lower the number the more acidic or sour the soil is and the higher the number the more alkaline or sweet the soil. A pH measure of soil at 7 is said to be neutral.
As a general rule most plants require a pH of somewhere between 6 and 7.5 as this is when most nutrients they require are readily available. There are always going to be some exceptions to this rule and you must always research what plants you intend to plant and purchase the right soil for the right job! This is also important to keep in mind when working with your existing soil. Throwing in a heap of fertiliser, compost or manure just to “enrich” the soil may not necessarily be required for your intended usage and could actually be detrimental in the long run.
You can buy small testing kits from most hardware stores, these will give you a rough indication of your soil pH. While these testing kits are useful, they can lack in accuracy, so be sure to test in multiple areas and follow the manufacturers instructions precisely. It may be wise to first test something with a known pH level like a bagged potting mix where the pH is stated. This isn’t always possible, so alternatively test a range of products with known varied pH levels. Like a mushroom compost or cow manure, which should be quite alkali and will give you an indication of the accuracy of your kit (if it reads as acidic, you might need a new one!)
So where to now? You have tested your soil in numerous locations and identified a level that is not desirable. Fortunately it is not the end of the world! Soil pH levels, as mentioned above can be modified fairly simply. But remember, although your pH level might be slightly low or high, this does not necessarily mean your plants won’t survive, it just mean they are not growing in their optimum soil conditions.
If your soil pH is elevated you can lower it (make it more acidic or sour) by:
- Add composted sawdust
- Add some composts or manures
- Add composted pine needles
- Add powdered sulphur
- Add green leaf litter or mulch
If your soil pH is low you can raise it (make it more alkaline or sweet) by:
- Add Dolomite or Garden Lime
- Add poultry manure like Organic Booster
Most Australian Top Soil is low or acidic as standard. Manufactured soils are generally made to be as close to neutral as possible but can vary depending on the manufacturing process and raw ingredients used.
It is not just pH that matters
Remember there is more to plant health then soil pH, although the pH level can have a big impact on the way a plant can draw the required nutrients from the soil, if those nutrients are not present at all, regardless of your pH levels, your plants may struggle.
Soil Nutrient Deficiency
| Deficiency | Sign | How-to Remedy |
Nitrogen | Slow or poor growth, yellow leaves and premature or no flowering. | Add seaweed solutions, fish fertiliser, compost or Urea |
Phosphorus | Can be caused by higher rainfall on acidic, clay or chalk soils. Leaves turning blue/green but not yellow (do not confuse with Nitrogen deficiency) | Check pH levels, if required add rock phosphate or high-phosphorus fish fertiliser. |
Potassium | Older leaves yellowing, leaf scorching and brown spots. | Add sulphate of potash or a potash-rich compost. |
Magnesium | Yellowing between the veins of older leaves | Water with Epsom Salts |
Calcium | Cupping of the foliage, Blossom-end rot in tomatoes, Bitter pit in apples, black heart in celery. | Add gypsum to the soil and work in well. Also Lime (calcium carbonate) can be used especially on acid soils. |
Iron | Yellowing between the veins of the youngest leaves, while older remain green. | Check your soil pH levels. Iron sulphate can be used. |
Remember:
- Excess calcium can reduce the uptake of potassium and magnesium
- Excess magnesium can reduce the uptake of potassium and calcium
- Excess potassium can reduce the uptake of magnesium
- Excess nitrogen can reduce the uptake of phosphorus
So it is extremely important to know what to look for and how to remedy the problem as excess of one can cause problems with another. For this reason, keeping a pH level suitable for the plants you choose and using a balance of fertiliser regularly is extremely important.
Some Popular Fertiliser Choices
Organic Xtra – An organic based fertiliser with a good balance of Nitrogen and Potassium with lower phosphorus levels
Eco 88 – An organic and chemical based fertiliser with a good balance of Nitrogen and Potassium with lower phosphorus levels.
Blood & Bone – High and balanced levels of Nitrogen and Phosphorus.
Urea – Very high Nitrogen.
Gypsum – High levels of Calcium.
Seaweed – It is worth noting that Seaweed solutions varies greatly between brands. Seasol’s Powerfeed is by far one of the best available on the market. A liquid plant tonic with high available levels of Nitrogen and Potassium and a good balance of other nutrients including magnesium, iron, and calcium.