Lime, Sulphur and Gypsum – When and where to use these in your garden. 

Lime and Sulphur for pH Adjustment

Acid and Alkali pH

Most plants prefer soil in the pH range from 5.5 to 7.0. If soils become too acidic (below 5.5) or too alkaline (above 7.0) many plants become unable to absorb nutrients from the soil and suffer nutrient deficiencies.

Phosphorous (P) availability is the most affected by pH because phosphorous reacts with other minerals in the soil at varying pH levels. At high pH, P reacts with calcium, while at low pH, P reacts with aluminium and iron. When P reacts with calcium, aluminium, or iron, it forms insoluble compounds that plants cannot easily use.

Many weeds can thrive in acidic soils and become more prevalent by outcompeting the weakened desirable plants.

Testing pH

Before adjusting soil pH you should test it to know what the current pH is. There are many home garden pH metres and test kits available in garden centres and hardware stores. Test the soils in various parts of your garden, do not assume it is the same everywhere.

Here are a couple of simple home tests that will indicate if your soil is too acidic or alkaline:

• Add a few drops of vinegar to a tablespoon of dry garden soil. If it fizzes, your soil's pH is greater than 7.5.
• Mix a pinch of baking soda to a tablespoon of moist soil. If it fizzes, your soil's pH is less than 5.0.

Soil Texture

Also, assess your soil’s texture, be it clay, sand, or something in between. More material is needed to change the pH level of a clay soil than for sandy soil.

Do you have sand, loam, or clay soil?

All soils are largely made up of tiny mineral particles. The size of mineral most abundant in your soil determines whether you have sand, loam, or clay. This mineral composition of the soil is called the soil's "texture."

Sand particles are small but compared to other soil particles they are largest. The other two main categories of soil particles are silt, which is smaller than sand, and clay, which is still smaller.

• Sand - Water and nutrients move through sand soils fast meaning both need more frequent replenishing. A handful of sandy soil won't clump or hold a shape when you press it.
• Loam - These soils have a beneficial mix of all the mineral particle sizes.
• Clay - Microscopic clay particles pack together tightly. Water and nutrients move into and through clay soils slowly. Clay soils are heavy, and you can easily form a ball or ribbon with a clump of moist clay soil in your hand.

Most soils are mixtures of all three particle sizes but in varying proportions. A predominance of sand particles makes a lighter, more open soil with good drainage and aeration; lots of channels for air and water circulation. Minute clay particles pack together tightly making a clay soil heavier, denser, and with less favourable air and water circulation. Because of the greater density of clay soils, more lime or sulphur is required to change their pH.

Plant Preference

Check the pH levels that your plants prefer. Plants such as azaleas, rhododendrons, strawberries and blueberries prefer low pH acidic conditions of around 5.5 while honeysuckles, clematis, poppies, geranium and others prefer higher pH conditions of 6.5 to 7.0. Lawn grasses prefer pH between 6 and 7. Autumn or cool-season green, leafy vegetables, these green, leafy vegetables prefer soils with a higher pH, between 6.8 and 7.5.

Lime

Generally, lime is used to raise soil pH level, and sulphur is used to lower it. Neither lime nor sulphur is water-soluble, so for quick effects, it is necessary to mix them thoroughly into the top 15 cm of soil. Otherwise, just spread the material on top of the soil, and let it gradually work its way down.

Lime (calcium carbonate, CaCO₃), is a naturally occurring mineral used to ‘sweeten’ an acid soil.  When the soil pH is less than 7, the soil is acid. Acidity is an accumulation of hydrogen (H) ions, and their presence is measured by a pH test.

When lime is applied to acid soil, the carbonate from the lime and hydrogen combine forming water (H₂O) and carbon dioxide (CO₂).  Because the carbonate liberates the hydrogen, the soil pH will ‘improve’ and become less acidic, this is known as sweetening the soil.

Table 1. Kilogrammes of lime needed for raising soil pH of 10 m² to a depth of 15 cm.

Sulphur

The cheapest way to lower the soil pH is to add elemental sulphur (sulfur) to the soil. Soil bacteria change the sulphur to sulfuric acid, lowering the soil pH. In general, if the soil pH is greater than 7.0, apply elemental sulphur (S). Apply the following total amounts in 2-3 applications 6-8 weeks apart. Do not apply more than 1 kg per 10 m2 on any one application.

Table 2. Kilograms of elemental sulphur needed to treat 10 m² of loam* soil to a depth of 15 cm.

*For Clay Soil, INCREASE amounts by half (50%). For Sandy Soil DECREASE amounts by one-third (33%).

Gypsum

Gypsum, or calcium sulphate (CaSO₄), is a naturally occurring mineral.  It provides the essential nutrients calcium and sulphur but does not affect soil pH; it is neutral (pH is ~6.7). Gypsum is typically 20-22% calcium and 16-18% sulphur.  The sulphur is in a plant available sulphate (SO₄) form.  In addition to providing needed nutrients, the calcium in gypsum is soluble, even at a pH that is greater than 7.  This can be very helpful in improving soil tilth, reducing soil compaction and improving aeration.

Gypsum does not work in sandy soils and it can cause problems due to an excess of calcium in regions where the calcium is already abundant in the soil.

Generally, using gypsum in your garden soil will not harm your plants, but it is often not necessary. Adding organic matter such as compost dug into the soil to a depth of 20-30 cm will improve aeration and drainage to a similar level. Soils with 10% organic matter, or more, do not benefit from the addition of gypsum. It also has little effect on soil fertility, soil structure or pH, while generous amounts of compost will do all that and more.

Lawn soils can benefit from the addition of gypsum if they are clay and poorly draining because it is not easy to dig in organic matter to a lawn.

Gypsum is an ingredient in Kiwicare LawnPro Lawn Thickener where it helps break down the surface of compacted lawn soils allowing the roots of new grass seed to more easily penetrate and establish.