Managing acidic soils

You have just bought a piece of farm land, started up a veggie garden, or leased your neighbors paddock to start growing your own produce. Like the organized grower, you sample the dirt and send it of to a lab to be analyzed and when the results return you realize that you soils are acidic with a pH ranging between 4-5 in the top 30cm of your soil.

Once your soils pH drops below 5 it can be considered as acidic soil and can be losing you yields without you even realizing but don't feel ashamed because it is extremely common around the world. 50% of farmland in Australia is affected by soil acidity causing the agriculture industry massive losses according to the Australian State of the Environment report done in 2016, (there is a new report being finished every few years). Majority of soil acidification has been occurring in Western Australia and New south Wales but if left unchecked has the ability to occur wherever agriculture operates.

Why?

Before getting into how to manage acidic soils it is important to understand the costs and reasons behind it. Acidic soils are an issue because they reduce the potential yields of crops and harvest on your land. How? when soils become more acidic (lower pH) certain nutrients and metals become more available in the soil solution one of them is aluminium which causes root damage once it enters the plants roots. When you go to an area that is heavily affected by acidic soils you will find plants with incredibly small roots. Another issue with acidic soils is when the soil gets low enough it will start dissolving the clay particles in your soil which is extremely bad, causing irreversible damage unless you apply more clay to your soil...which can be expensive on large scales of operation.

Sometimes it isn't just your topsoil than can be acidic, more often than not farmers are dealing with subsoil acidity which isn't as detrimental to yields but can cause micronutrient deficiencies as that is where most nutrients such as iron, copper and zinc are held by the clay.

Hydroponic setups can be different, the crop is sometimes in coir peat (grinded up coconut husk) and not in soil. which I believe does not contain aluminium. A crop in coir peat may be able to experience a lower pH soil/medium environment without the same yield penalties experienced with acidic soil environments that contain aluminium.

Managing and ameliorating acidic soils

Apply Lime
Calcium carbonate, more commonly known as lime, is a mineral mined out of the ground and can be applied straight onto the soil. Often it is put through a grinding process to a sand consistency so that it reacts with the soil faster. Depending on your soil's pH and texture will determine how much lime you'll need. The Grains Research and Development Corporation (GRDC) have a very simple equation to determine what rate of lime should be applied to the soil.

100% lime rate (T/Ha) = pH increase desired / soil texture factor
Clay: 0.26
Clay loam: 0.37
Loam: 0.47
Sandy Loam: 0.57
Sand: 0.67

Credit: GRDC, Feb 2019, Lime and Liming - managing soil health. https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2019/02/lime-and-liming-managing-soil-health

This will provide you a lower rate than is required but if you wish to go further then you will need to account for the impurities in the lime you're purchasing and the organic content of your soil. Below is an example of the equation for a clay soil:

Soil factor: Clay = 0.26
pH increase desired = 4 to 5.6 = 1.6

100% lime rate = 1.6 / 0.26 = 6.15T of lime per Hectare.

6.15T of lime is a lot, I would only recommend a maximum of 2T/Ha of lime per year. This allows you to crop or grow for a season and then do more soil sampling to monitor how the soil pH is adjusting. This liming process is slow but it avoid toxic environments which 6T spread over the soil surface can do. It is possible to cultivate 6T if lime into the soil via rotary hoe, discs, deep ripping ect but why run the risk unless you're dealing with a pH of less than 4.

Trigger points for adding lime
A good time to add lime is when your top soil gets to or below 5.5 and this is because once soils reach a pH of 5.5 and above it allows for better movement of carbonates down the soil profile, this is important especially if you use urea which causes soil acidity mostly in the 7-12cm depth.

Urea
For broadacre farming for crops canola, corn, barley, oats, wheat and lupins, you can expect to be liming every five years of 2T/Ha to keep your topsoil above 5.5. Once again, this will depend on your soil texture and how how much you demand out of your farm, the more you harvest from your land, the quicker it can acidify. Urea is a main supplement that growers will use on their farms to increase their yields but the sandier the soil the more likely the nitrogen from urea can leech from the soil and speed up acidification. Urea acidification works by urea breaking down via microbes into ammonium and then into nitrate which is the most plant available. However, when nitrate and ammonium are formed there is hydrogen ions released into the soil as well which results in a temporary decrease in the soil's pH. Once nitrate is taken up by the plant and the plant releases exudates or dies the plant will increase the soil's pH temporarily. But, nitrate is very mobile and so if there is a rain event or over irrigation occurs then that nitrate will move deeper into the soil where the roots of the plant can't access. When nitrate leeches out, the once  temporary decrease in the soil's pH becomes a permanent one, which can only be fixed through the application of alkaline salts like lime.

There are other products than just lime that you can use to increase the pH of acidic soils, these are dolomite (magnesium carbonate) and burnt lime/bricklayers lime (calcium hydroxide) which is very reactive but risky. The reason lime is so popular is because soils have a large requirement for calcium and generally lime is cheap compared to dolomite or burnt lime.

It is possible that soils may originally be acidic and not the result of agriculture but for growers it is important to monitor your soils as there is a lot of yield that is currently being lost in just Australia because soils are too acidic. Until the soil is severely acidic, it is very hard to identify visually if the soil is too acidic and if it is affecting your yields. Liming, in my opinion, is the biggest return on investment any farmer can make on their soils.

Happy growing!