Soil information for plantation managers
Soil provides the nutrients and water essential for tree growth. Soil properties like fertility, structure and texture affect the availability of nutrients and water to trees. It is not surprising, then, that plantation productivity is closely related to soil properties.
Understanding some key soil properties or attributes will help you make important management decisions when establishing a plantation.
Classification and examination of soils
Soils are classified and named using features such as depth, colour and texture of the various layers. Soil classification systems can be found in 'The Handbook of Australian Soils' (Stace et al. 1968) or 'The Australian Soil Classification' (Isbell 1996).
You can examine soil features by digging a hole with a hand auger or when exposed in a pit or road cutting. These have to be 'faced back' to expose fresh soil.
Discreet soil layers (soil horizons) have various depths and different features. The surface soil is frequently darker in colour, reflecting the accumulation of organic matter over time. The subsoil often contains much more clay, as seen in the figure below.
Key soil attributes
Soil depth is the distance to an impenetrable layer, such as massive clay or rock, indicating the amount of soil potentially available for exploitation by plant roots. The amount of water held in the soil that is available to plants (plant available water) is largely determined by soil depth as well as soil texture and structure.
The deeper the soil, the more available moisture it can hold. Soils less than 1m deep in a low rainfall belt (600mm per annum or less) are unlikely to be able to hold sufficient moisture and are therefore unsuitable for plantation forestry. Where the annual rainfall is more than 1,000mm, soils less than a metre deep may be quite suitable for productive tree growth.
Soil colour should be assessed for each horizon. Colour is determined by the soil's parent material, drainage, aeration, leaching, and the amount of organic matter present.
- Red and yellow colourings are usually related to the presence of iron.
- Red soils are formed from parent materials with a high iron content and are better drained than yellow soils.
- Grey colours and mottles (red or yellow streaks within a paler background colour) in subsoils often indicate periodic waterlogging, and colour often changes with depth.
Soil colour can affect management options. For example:
- Red soils can 'fix' phosphorus, making it less available to tree roots, so higher fertiliser rates may be needed.
- Grey colouration in the subsoil indicates waterlogged soil, which means improved drainage might be needed for some tree species.
Soil texture is related to the proportions of clay, silt and sand in the soil and can be determined by moulding moist soil in your hand. The main soil texture classes are described in the table below.
Texture has an important influence on the soil's:
- water-holding capacity
- nutrient supply
- root penetration
- susceptibility to erosion.
Changes in texture between the different soil layers (down the profile) influence internal drainage and nutrient supply. Heavy clays in the soil profile may impede drainage enough to cause waterlogging. Texture is likely to change down the profile and should be assessed for each horizon.
|Texture||Soil behaviour when moist|
|Sand||The soil will feel very sandy and will fall apart easily.|
|Sandy loam||The soil will feel sandy and slightly sticky. It will hold together just enough to be handled.|
|Loam||The soil will feel spongy and possibly greasy. It may also feel slightly sandy. When moulded into a ball, it holds together.|
The soil is strongly coherent and can be moulded and shaped.
The soil is easy to mould and shape and behaves like soft plasticine.
The soil will behave like hard plasticine.
Table 1: Key soil textured (adapted from McDonald et al. 1990).
Soil structure describes both a cluster of soil particles as a single unit (ped) and the amount of peds evident. Structure ranges from strongly pedal (more than 2/3 of the profile is structured) to weakly pedal (less than 1/3 of the profile is structured). The soil is said to be structureless where no peds are evident (apedal).
Increased structure is generally related to texture and enhances the availability of water and oxygen to roots and facilitates root penetration.
It is important to know how susceptible the soil is to erosion before planning deep ripping or other cultivation. Soils vary in their susceptibility to erosion and eroded soils are often seen in the landscape.
Special precautions have to be taken when cultivating erodible soils. Where erosion is likely, strip cultivation should always be done on the contour. If the slope is greater than 15o, spot cultivation is recommended. On soils with an erosion-prone B horizon and a slope that allows water run-off, special care must be taken when ripping to avoid tunnel erosion along the rip line. In these cases, a shallow rip or no ripping may be most appropriate.
Soil pH is a measure of acidity (acidity increases with decreasing pH) which affects nutrient availability and ultimately tree growth. Most trees perform best in a soil with a pH between 5.0 and 7.0.
Soil pH can be assessed in the field using a field pH kit. It simply involves taking a small sample of soil, adding an indicator solution, coating this with a powder and comparing the colour that develops with the calibrated colours provided.
For excessively acid soils, incorporating lime or dolomite into the top layer of soil will increase the pH. Applying sulphate of ammonia to highly alkaline soils will reduce the pH in the top soil layer.
Influence of topography on soil
Topography plays an important part in soil-forming processes. Soils on the lower slopes are often formed from materials derived from upper slope positions. This results in deeper, more fertile soils in the gullies and shallower, 'droughty', less fertile soils on the slopes.
Position in the landscape also influences site temperature, drainage and moisture availability. Frost may be an issue in low-lying areas and where the overland flow of cold air is impeded. North-westerly aspects are generally hotter and drier than south-easterly aspects.
Effects of land use history on soil
Land use history affects the physical and chemical properties of soil, and knowing something of the past history of the site will help you determine management options. Soils under undisturbed native vegetation are relatively stable, but when the area is cleared for agricultural or pastoral use soil compaction and erosion often cause soil degradation. The past use of fertilisers often has a long-lasting effect on soil chemical properties.
Learn more about soils in Queensland.
Plantation forestry rules of thumb
The following guidelines can give help when making soil-related decisions in plantation forestry:
- Choose a species that suits the soil and rainfall available.
- Cultivate the planting rows to provide optimum soil conditions and to aid planting and provide good growing conditions for the young seedling.
- Deep rip if the site has been used for intensive agriculture in the past and a plough layer has developed. This will facilitate root penetration into deeper soil layers and improve tree growth. Cracking clays, sands and soils are unlikely to have a compacted layer and may not require ripping.
- Plant the seedlings deep into the soil. Cover the potting mix with 3-5 cm of soil. If the soil is dry, water the seedlings in.
- Maintain a weed-free environment by using both knockdown and residual herbicides. This reduces competition for soil water and nutrients. A bare earth perimeter around the tree (1m radius) for the first 12 months will greatly enhance growth and survival.
- Make sure you remain on top of weed control at all times.