Soil health - back to basics

Soil physical condition - commonly referred to as 'soil structure' - has an important influence on farm economics and environment.

Soil structure determines the porosity, strength and stability of a soil.

For all practical purposes, these are the most important soil physical properties because they influence the movement and storage of water in the soil, the ability of roots to penetrate and grow in the soil and withdraw water and nutrients, and finally the ability of a soil to resist erosion.

A well-structured soil has many stable aggregates with a wide range of sizes. It contains a large number of pores, both within and between those aggregates, which maintain the correct balance of air and water in the soil and also allows easy emergence of seedlings and growth of plant roots.

Poorly structured soil either does not have aggregates of many different sizes or the aggregates are packed tightly together with few pores. Poor structure in a light textured soil results in unstable aggregates and the soil appears fine and powdery. Poor soil structure can markedly increase the risk of wind erosion.

Soil porosity and water holding capacity

Over half the total soil volume consists of pores in a well-structured soil and about 40% of these pores are very small and nearly always filled with water held so tightly that plant roots cannot withdraw it.

The proportions of sand, silt, clay and organic matter determine volumes of these small pores.

Medium size pores retain water, which can be taken up by the roots. The volume of these pores determines the water holding capacity of a soil and therefore its susceptibility to drought.

Large pores (macropores) are the structural soil pores, and are important for infiltration of water into the soil, drainage of excess water from the soil, aeration of the root zone, and as passages for root growth in the soil.

Damage to soil structure

The two main forms of soil structural damage are aggregate breakdown and soil compaction.

Aggregate is caused by some natural processes, such as wet/dry and freeze/thaw cycles. Soils comprising mostly of silt and sand, with only a small proportion of clay, are most susceptible because these soils are only loosely bound.

Soil compactiion is the compression or squeezing of a soil. Soil compaction reduces the number and volume of large soil pores (macropores) and increases soil density. Large pores are important for the movement of water and gases in the soil. Small pores are responsible for water and mineral storage.

A soil health and biodiversity field day is being held at Paterangi, near Te Awamutu, on Wednesday, 19 October.

Please visit waikatoregion. govt.nz/publicevents for the details and to RSVP.

This article is part one of a two-part series by Bala Tikkisetty, a sustainable agriculture advisor technical with Waikato Regional Council.