Soil structure and when to use calcium amendments

/ by

Soil structure is often said to be the key to soil productivity. A fertile soil, with desirable soil structure and adequate moisture, constitutes a productive soil.  Soil structure affects plant growth through its influence on infiltration, percolation, water retention and runoff, aeration, and it’s mechanical impedance to root growth¹.

Healthy soils have good structure and porosity to allow nutrients, air and water to circulate freely. Poor soil structure has coarse, firm clods, and few visible cracks or holes.³

 

Soil aggregate formation and stability is the primary feature of soil structure,  and are two of the most important manageable soil physical properties.²  Clays, organic matter and materials excreted by soil organisms bind the soil particles together to form soil aggregates.

 

The spaces between the aggregates are called pores. The pores allow air, water and organisms to move within the soil and provide spaces for roots to grow. 

Calcium can help stabilize aggregate structure of some soils. Negatively charged soil clay particles can be bound together into clumps or aggregates by positively charged molecules (called cations). The formation of stable soil aggregates, a process called flocculation, encourages water infiltration and drainage and prevents surface soil crusting.²

 It is important to be familiar with the properties of the various calcium materials available and to understand the chemical processes that occur when amendments are applied to soil.² 

The most widely used calcium additives are agricultural lime (calcium carbonate), dolomite ( calcium magnesium carbonate) and gypsum, (calcium sulphate dihydrate). 

Lime is used to raise soil pH to reduce soil acidity, its solubility decreases as soil pH increases towards pH of 8.0. When soil pH is above 8.2 lime becomes very insoluble.²  Dolomite is used when there is a need to address soil pH in an acidic soil as well as deficiencies of calcium and magnesium.

Gypsum will work across the soil pH range  (acid to alkaline) as it doesn’t change soil pH. Gypsum is often used when soil pH is in the alkaline range which limits the use of lime. Gypsum works best in the soil when the base saturation for calcium is greater than 60%.

Gypsum can be blended with agricultural lime, up to 40% of the blend, to ensure a source of short to medium term available calcium, supplied from the gypsum. Whereas agricultural lime provides a long term source of calcium which releases calcium as it weathers in the soil.

Talk to your fertiliser advisor about your soil type and whether your farm would benefit from an application of gypsum. 

Gypsum is a naturally occurring mineral. Winstone Gypsum is BioGro certified.

 

Reference