Understanding soil pH
Potential hydrogen or more commonly known as pH, is a unit the measures how acidic (low pH) or alkaline (high pH) a solution is. When it comes to farming or your plants soil pH it is important to know because it can influence how available nutrients in the soil can be. The pH of soil can also determine if metals such as aluminum, which is caused by a pH level below 4.8, are present in the soil which can cause root damage to plants which is not ideal.
How to find out your soil's pH
Testing the pH of your soil is pretty easy and straight forward. Any home hardware store will stock kits that can test your pH, they can be a little rough but can give you the general idea of what the pH of your soil is. Usually all you need is a small amount of dirt, water to mix your dirt into and a solution called an 'indicator' which changes color based on the pH of the soil solution you have created.
More advanced equipment are also available where you can mix a solution of 1 part soil to 5 parts water and you place a probe into the solution where it can accurately measure the pH of your soil to a decimal. If you're a farmer you can send samples of your soil and get a full analysis of your soil, including the pH, nutrients, salts, organic matter and the texture! laboratory tests usually only cost $60-$100 depending on the tests done and where you go but can provide peace of mind for growers because it can give you a window into what your soil may be lacking for your future crop or to identify a nutritional deficiency when there are obvious symptoms of something going wrong on the surface.
The ideal pH
A good range for most plants is between 5.5 -6.8 but through my experience I find that plants can grow in ranges from 5-8.4 without any significant yield losses or plant stress especially if you're growing plants for yourself then I wouldn't be going too far down the rabbit hole of the pH of your soil unless it is in the extremes of below 4.8 and above 9.
Amending acidic soils
A common problem in Australian Agriculture is acidic soils, it is very rare that you'll find soils that have a high pH especially in Western Australia, unless you're from Carnarvon. An acidic soil is one that has a pH of less than 5. If you're re-reading this you've probably just found out that you have acidic soils and need to a solution. The most common and probably the cheapest option is "Lime". Lime is calcium carbonate and is sometimes called a weak alkaline salt which is great because we don't want the pH to go to far the other way! How much should you add? This will depend on your soil texture but a good rule of thumb is 200g per square meter for gardeners or 2T per hectare for the farmers and then measure the pH of your soil a year or two after applying the lime to allow it to react with the "Hydrogen" in the soil and increase the pH. Growers with sandy soils can get away with using less while those with heavier, clay soils can benefit from adding more, if you're a farmer you should have someone from the lab or an agronomist to give you a good idea on how much you'll need.
How do soils become acidic?
Acidification in the soil occurs when there is a build up of acids in the soil that haven't been neutralized by alkaline materials such as your harvest. Plants will absorb nutrients from the soil solution and in doing so will exchange the acid "hydrogen" which is positively charged for a nutrient that is also positively charged such as Calcium, Magnesium, Ammonium and potassium. Usually this cycle is completed once the plant dies and returns to the soil and soil acidification is avoided, but if you're a farmer then you are going to be removing some of the plant as your harvest, whether that be tonnes of wheat, a tomato from your vegie garden or roses from the flower bed.
Leeching fertilizer is another way that causes soil acidification, and can be much faster. This occurs when you apply nitrogen fertilizer to your soils and this is followed by a large rainfall or irrigation. If the nitrogen fertilizer is made up of ammonium or urea then it will bring more hydrogen to the soil, this is fine until large quantities wash through the soil resulting in the nitrogen being leeched down deep into the soil and leaving all of the hydrogen in the top 10-30cm of the soil.
Amending alkaline soils
Have you got soils with a high pH? then I have the answer for you....do nothing! If you're a gardener you could just get more potting soil to bulk up your garden beds to decrease the pH but if you have large areas of soil with a high pH particularly if you're a farmer there is no hard and fast way to drop you pH. I have seen trials done with Phosphoric acid, organic acids and large quantities of elemental sulfur to make the soil more acidic but all this results is a soil that has a lower pH for a few years before returning to its original pH. All of the above mentioned trials were extremely costly.
High soil pH of 8-9 is manageable and should not cause too much concern especially as over time the soil will acidify with any harvest you take away with you and especially more so if you're using nitrogen fertilizers.
How do soils become alkaline?
The most common reason is because that is how the soil was naturally. Usually the reason is caused by a large amount of carbonates in the soil which are found in lime and they're preventing the soil from becoming more acidic. Carnarvon in Western Australia has naturally occurring alkaline soils but this does not stop the horticulture region from achieving high yields in the heavy clay soils.
The best practice guide
It is always a good idea to keep tabs on your soils pH, I like to analyze soils every two years to give me a good idea on how my soil health is tracking and how changes have come about from what I have been doing. If you're a farmer, sometimes the biggest profit gains can be from simply sampling your dirt and giving it a bit of lime where it is needed.