Many people ask us if lime and gypsum can be used interchangeably. This is not surprising since they are both labelled as soil amendments and recommended for improving soil conditions. However, while they do share a number of other similarities, they have very different uses.
Lime and gypsum are both soil amendments but lime is used to reduce soil acidity while gypsum is used to improve the physical condition of the soil.
Similar origins
Lime and gypsum are evaporites, formed millions of years ago by the evaporation of sea water high in calcium into layers of sedimentary rock. Today, they are both mined, although Gypsum is also sourced from coal burning power stations as a byproduct of flue gas desulphurisation.
Similar forms
Lime and gypsum are powders in their natural forms, and lime can be ground very finely to increase its effectiveness. But they can both be granulated. This reduces application issues such as wind-drift.
Similar nutrients
Being formed from calcium rich waters, lime and gypsum contain calcium as seen in their scientific names and formulae: calcium carbonate (CaCO₃) for lime and calcium sulphate dihydrate (CaSO₄.2H₂O) for gypsum. So they are a useful source of calcium when added to the soil, although the calcium in gypsum is thought to be 200 times more soluble than the calcium in lime, and therefore more available to plants.
Differences
Gypsum differs from lime in that it is also a source of sulphate (SO₄), useful since these nutrients are found less often in phosphate fertilisers nowadays. Lime differs from gypsum because it contains carbonate (CO₃), which is not a nutrient as such, but is key to the way that lime works in the soil.
Lime for soil acidity
The main use for lime is to reduce soil acidity which increases over time from natural processes like weathering, plant and microbial activity, and nutrients leaching from the soil. Soil acidity can also be increased by the use of some fertilisers like elemental sulphur.
An acid soil is a problem for many plants because they grow best where pH is around 5.8 - 6.0. Below this and some nutrients like phosphates become less available. On the other hand, micronutrients such as iron, copper, manganese, and zinc increase in availability, but this can make them toxic to plants. Also, once the pH dips below 5.4, the level of aluminium ions in the soil solution rises, and plant roots can suffer deformity. Additionally, clover’s ability to fix nitrogen can be compromised because rhizobia can’t produce the enzyme nitrogenase which converts nitrogen to ammonia, the form of nitrogen that plants take up.
The traditional way of managing soil acidity is to apply lime. And it’s the carbonate in the lime that reduces the acidity – not the calcium.
Gypsum for soil aggregates
Gypsum does not contain carbonate and has absolutely no affect on soil pH and soil acidity; this is not why we use it.
The main use for gypsum is to improve physical properties in the soil like better drainage, more aeration, and less surface crusting. It does this by flocculation, a process which causes small soil particles to clump together to form larger ones. This process loosens up hard, compacted soils such as clay soils, acid subsoils, and soils damaged from heavy machinery or livestock treading.
A more open, aerated soil which allows water to pass through improves other soil properties. For example, earthworm and microbe activities are increased, and plant roots can penetrate deeper into the soil to access water and nutrients, so improving resilience in times of drought or other stresses.
Another benefit from gypsum is that due to aggregation, the soil becomes more chemically stable. This means that it is less prone to nutrient loss which in turn enhances fertiliser efficiency.
Different uses
So in sum, lime and gypsum are not interchangeable. Yes, they are both added to amend the soil, but for very different reasons. Lime with its carbonate is for reducing acidity, while gypsum with its ability to form aggregates is for opening up the soil structure allowing better drainage, aeration, and surface structure.
This article was published in the Coast & Country News.
