Managing Alkaline Soils

Alkaline soils in Western Australia are generally found along the limestone coastline where more carbonates can be abundant in the soil. According to GRDC, about 24% of Australian land is affected by alkaline soils making this a fairly common problem. Unfortunately there is no hard and fast rule to adjust the pH of alkaline soils like there is for acidic soils. There has been some ideas of reducing the pH of soils and we will touch base on them to understand why they may not be the best choice to go with.

Why is Alkaline soils a problem?

The pH of the soil affects the solubility of various nutrients in the soil solution and since plants need nutrients to be dissolved this can affect their growth. Unless the soil pH is higher than 9 there is no toxicity in the soil from my experience that will impact on the yield of your crops, only from the lacking availability of mostly micronutrients such as iron, zinc, manganese, and copper.

Managing alkaline soils

The best practices for managing alkaline soils is monitoring through soil and leaf analysis. By keeping track of nutrient quantities you can predict where your crop may fall short of nutritional requirements, particularly the micronutrients mentioned above. Once the crop is past the seedling stage, applying foliar micronutrients that are in a chelate form or EDTA can be the most efficient way of dealing with deficiencies as this is not reliant on root uptake through the soil. Be sure to read the label recommendations of the product you are using as applying high concentrated solutions can cause foliage burning.

Adjust your cations to the required levels, it may seem crazy but adding lime to alkaline soil to increase the calcium content has been something that I have practiced with many growers with great success. Calcium is very important to horticulture crops and is responsible for cell wall development and shelf life. As long as your soil is not above a pH of 9, adding lime is a great way to increase the calcium cation exchange capacity above 60%. While you may be tempted to apply gypsum to achieve the same calcium increase, gypsum will not be as available nutritionally as lime will be. Plants exudate acids in the rhizosphere which will react with lime (calcium carbonate) making the calcium more available, but acids will not assist in dissolving gypsum (calcium sulphate).

Often alkaline soils will have a high amount of sodium in the soil as well. The sodium is sometimes present as sodium hydroxide and sodium carbonate which will buffer the soil, this means when acid applied to a soil the pH will not change very much. To remove the sodium you will be required to have adequate calcium in the soil (above 60% CEC) and to monitor the magnesium in the soil to ensure it is not inhibiting the downward movement of the sodium (magnesium less than 18% CEC).  If you irrigate your crops, you will want to also achieve a small amount of leeching through your soil, you can do this by monitoring your soil moisture at various depths to ensure you're reaching further than the crops effective root zone.

These above strategies are very similar to taking care of acidic soils. Even when your soil is not overly acidic or alkaline it is important to be monitoring your soil and plants through analysis to ensure you are applying the correct amendments and getting the best efficiency from your crop. You'll also see that there are no strategies to actually reduce the pH and that is because reducing the pH of your soil has so far been proven to be unsuccessful and expensive as I will show now.

Lowering the pH of the soil

There has been attempts to decrease the pH of alkaline soils which have unfortunately been found to be unsuccessful or too expensive. One practice is to apply elemental sulfur to the soil which undergoes microbial metabolism and is turned into sulfuric acid. However, studies have shown that to decrease the pH from 8 to just 7.5 takes a considerable amount of sulfur. Numerous research groups have trialed this sulfur theory only to have the soil return to its original pH due to the buffering capacity of the soil. From agriculture retail outlets elemental sulfur costs anywhere around $3/kg in bulk and trials have used rates of up to 11 tonnes per hectare! only to achieve a pH change from 8 to 7.5 and for it to return a few years later. It goes to show that you're better of not using elemental sulfur as a strategy to decrease your soil's pH.

Due to the idea of using elemental sulfur to decrease the pH of the soil, this can confuse growers into thinking any sulfur will achieve this but in fact sulfur can come in different forms. The sulfur used in the trials to decrease the soil's pH is elemental sulfur (7S) meaning there is nothing else but the element sulfur. There is also sulfur in products such as fertilizer but this is not in the form of elemental sulfur but as sulfate (SO4-) and will not be metabolized by microbes in the soil to produce sulfuric acid. This means products such as epsom salts, gypsum, potassium sulfate, and iron sulfate. Ammonium sulfate does decrease the pH but that is because of the ammonium and no the sulfate.

Key points

To finish off, here are a few things to keep in mind when managing alkaline soils:

1. don't focus on reducing the pH of your soil, that will happen over time.
2. Monitor your trace elements such as iron, zinc, copper, manganese, and molybdenum in your soil and plant tissue. Apply chelated trace elements when needed (such as EDTA).
3. Monitor and adjust your cations in the soil appropriately. Magnesium is often found to be high in alkaline soils as well as sodium, a good way to reduce them is by applying lime, only if you're soil is not above 9.

It is relatively easy to look after a crop on alkaline soils and it is not much different to any other soil. I have seen huge yields come off heavy clay soils that had a pH of 9 and we did not focus on reducing the pH but on taking care of the crops nutrition, protecting from pest's and diseases, and persistently monitoring the soil through laboratory analysis.

Happy Growing!