Soil modification

Sandy textured soils have inherently low chemical fertility and limited water holding capacity, which limits productive potential. They are also highly prone to wind erosion. 

Many sandy soils are also water repellent, due to natural waxes that make soil particles hydrophobic. Water repellence causes uneven wetting of the soil, resulting in reduced soil water uptake by plants that leads to poor or patchy plant germination, decreased growth and yield and therefore poor water use efficiency. These problems are more severe in dry seasons.

The incorporation of clay into sandy soil can help address the issues outlined above.

There is approximately 2.4 million hectares (24%) of ‘sand’ textured soils in the agricultural zone of SA that could benefit from the addition of clay. Increasing the clay content of sandy soils has potential benefits including:

• overcoming soil water repellence
• sequestration of atmospheric carbon
• less nutrient leaching and offsite pollution
• increased soil chemical fertility
• increased waterholding capacity
• increased resistance to erosion
• decreased deep drainage which contributes to rising watertables and salinization
• increased productive potential.

This is done by one or more of the following methods:

• Clay spreading - subsoil clay usually excavated from broad pits then spread onto the soil and incorporated into the topsoil.
• Delving - deep ripping with specialised tynes that lift clay from the subsoil into the surface soil layer) then incorporation by tillage or spading.
• Spading - deep rotary cultivation that mixes sub-surface clay through the topsoil.
• Other innovative methods e.g. Plozza plough.

Poorly structured subsoils are those affected by compaction and/or impaired drainage that can limit soil productive potential. This is often associated with sodic clays and can lead to the following problems:

• Physical restrictions on plant root growth – lack of pores for root growth, and presence of dense layers that cannot be penetrated by roots.
• Reduced soil aeration – lack of structural pores can cause oxygen deficiency for root respiration.
• Low infiltration - higher risk for soil erosion from water run-off.

There is approximately 1.2 million hectares (12%) of agricultural soils with moderate or higher subsoil structural limitations.

These physical constraints can generally be treated or overcome by deep ripping. This mechanical soil amelioration process breaks up compacted layers below the normal cultivation depth, turning over the soils and reinstating macro soil pores. This helps improve water infiltration, gas exchange and support better root growth.

Gypsum can be added to soils to address the issue of sodicity and soil dispersion (clay particles pushing away from each other). The calcium in the gypsum can displace the sodium from cation exchange sites on the clay and promote sodium leaching down the profile. The calcium also promotes flocculation, where clay particles combine together into clumps, improving soil porosity.

The addition of organic materials such as compost, manure and biochar can improve soil structure. Organic matter reduces the bulk density of the soil, improves water holding capacity and helps stabilise soil aggregates (e.g. root exudates and growing fungi hyphae in response to nutrients help clump together soil particles).