How soil information impacts the model

The best way to understand the soils on a farm is by getting a local soil map of the farm and using this to set up the blocks. 

S-map provides soil maps in various parts of the country and so where they are available these should be used to better understand the soil properties. The soil map is made up of polygons (areas of land) that contain one or more soils (siblings). A sibling is a member of a soil family. Each sibling has a defined set of properties that are used to determine things like their profile available water (PAW). Siblings are given names such as Glen_4a.1. You will find more information about S-map and soil properties on the Landcare (Manaaki Whenua) web site.

If S-maps is not available, soils can be defined by their order and applied to blocks. This will use very general soil properties. Where possible soil properties should be entered if available (refer Soil profile section below).

The properties of each soil can be overridden if required. These options should only be used if you have measured data to support these values, e.g. farm specific soil map, produced by a trained soil pedologist.

It is important to block the farm in such a way that it helps to analyse changes in management. For example, you may recognise a big change in soil composition and separate that area into its own block. This would enable easier analysis of results and then run scenarios for some sort of change in that block.

If using OverseerFM to compare/monitor change, the soils must be consistent and so past analyses may need to be updated if new information about the soils is discovered. The compare tool in OverseerFM provides a way to compare two analyses and see whether there are any significant differences in climate or soil. Access to the compare tool is on the farm home screen against each analysis.

The underlying Overseer science model supports one soil and irrigation system per block. So, blocks with more than one soil are broken down before calling the model. If more than one irrigator is applied to the block, it is broken down by irrigator as well. The results are combined into a single overall block result. The limit of 3 soils was put in place to prevent very small percentages being entered and therefore creating a lot of relatively small blocks that have little significance on the results and can lead to the model struggling to allocate feed to animals.

Results can be viewed by soil and irrigator under the block details or farm details reports.

The following lists the soil characteristics OverseerFM uses in the model. The major driver of drainage and hence potential leaching is the calculation of profile available water (PAW) in each layer of the soil. If soils are entered using soil order, PAW is estimated based on the order and modifiers listed under the soil profile. This is not as accurate as using the soil water content values supplied by S-map or providing your own values from a qualified soil specialist.

Soil profile

If using an S-map sibling these values are set and so there is no reason to change them unless you have your own soil data that overrides these values.

If using soil order only, it is recommended that these profile values are set based on expert observation of the soil in the block. This is because without them the model will use a very generic approach to defining soil holding capacity that may not be appropriate for the soil.

• Profile drainage class: Profile drainage class defines how well the soil is drained in its natural state i.e. without artificial drainage. It is not a measure of the rate of drainage or permeability of the soil. It is combined with the propensity for pugging (runoff characteristics) to estimate saturated conductivity used in the hydrology model to estimate leaching. It is also used to estimate pasture sulphur content and in the wetland model. Nitrogen denitrification rates, runoff propensity in the P runoff sub-model, and wetland sub-model also use the natural drainage class and propensity for pugging as inputs.

• Topsoil texture: Topsoil texture can affect runoff, particularly for heavier textured soils.

• Is stony: Greater than 35% stones in topsoil (10cm) layer. If have not entered soil water contents, this has a reasonably significant effect on the calculation of soil water and hence drainage of the soil.

• Maximum rooting depth: This is the maximum depth (cm) that roots can penetrate the profile, typically due to a chemical layer, such as a layer that is aluminium or manganese toxic, or anoxic.  It is not the depth that roots naturally grow to and must be less than the impeded layer depth. Drainage of water can still occur below this depth.

• Depth to impeded layer: Depth that an impervious layer starts, and hence there is negligible vertical drainage of water past this depth. Roots are also assumed not to penetrate this layer, and hence this factor is included in the N leaching sub-model.

• Non-standard layer: Used to improve estimation of water content for stony soils.

• Depth to non-standard layer: If non-standard layer is set this is the depth at which it starts.

• Soil texture group: Can be set for brown and recent soil orders as they are likely to have variations in sub soil texture. If set is used to adjust soil water content below 10 cm.

Soil properties

These values are set for the sibling or soil order selected and should only be overridden if have your own soil data as a result of specialist soil testing and analysis.

• K leaching potential: Potassium (K) leaching potential describes the potential for a site to lose K via leaching, with a value between 0 and 6.  It is a multiplicative factor for estimating K leaching.

• Soil water contents: The user can enter soil water contents at wilting point, field capacity, and saturation (mm/10 cm) for three depths (0-30 cm, 30-60 cm, and > 60 cm).If entered these are used to set water content at each layer.

• Soil chemical and physical properties of topsoil (to 10cm): The user can enter the following properties for the 0-10 cm soil layer:

  • Bulk density (kg/m3)

  • Structural integrity

  • Topsoil carbon content (%).

  • Topsoil clay content (%)

• Soil chemical and physical properties of subsoil (to 10-60cm):  The user can enter the following properties for the 10-60 cm soil layer:

  • Sub soil clay content (%).

  • Saturated conductivity (mm/day).

• Soil carbon levels: The change in soil carbon is used to estimate maintenance lime requirements and to provide a minimum level of nitrogen immobilisation for the pastoral block. Selecting default uses the N immobilisation potential to provide an estimate. The option to enter soil pH is also available. Soil pH only affects the extent to which changing soil carbon levels affect acidification rates, and hence maintenance lime requirements.