Farmers have a significant problem with soil vulnerable to structural collapse, particularly when there is a combination of prolonged heavy rains and vulnerable soils. As part of the Topoclimate South Soil Mapping Project, we devised a simple index from the mapping data that we were gathering that gave a quick guide to farmers of the relative vulnerability of their soil to structural collapse. This Index is known as the Structural Vulnerability Index or SVI.
The scale is shown as a range from 0 to 1;
if your soil has a SVI tending towards zero, then it is less prone to structural collapse,
if it tended towards 1 it is very prone to this problem.
The tool is a very simple but effective mechanism based on some quality science and objective measurements that farmers could use to improve their understanding of their soils.
However, to prove the usefulness of the Index in practical terms to the farmers, we needed some real situation examples of the SVI in action. After three weeks of solid rain in the middle of a Southland winter, I dispatched my Soil Pedologist staff into the field armed with a camera to try and capture a good example of the SVI in action. Luckily the weather had finally cleared and paddocks were just starting to dry after the prolonged rainfall although the ground was still very sodden.
They came across two adjoining paddocks just south of Mataura where they captured these remarkable images. In the first paddock on Otaraia Soils (SVI 0.42) cattle were happily grazing at a break-feed of a winter feed crop and there was little evidence of pugging or structural collapse of the soil, even with the constant pressure of the heavy dairy cows.
However, on the adjoining paddock on Waikoikoi Soils (SVI 7.6) where the same strip grazing activity was taking place, the heavy cows had completely pugged up the area and the topsoil had completely collapsed structurally. It would take a long time for this soil to recover from this collapse (if ever) which clearly illustrates the unsustainability of strip grazing of crops on high SVI soils, particularly after prolonged rain.
This is a particularly dramatic example and, being photographs that were taken on the same day, shows the importance of farmers understanding the issue of soil structure and its relevance to their farming system. See article Why maintain good Soil Structure?
It is also a graphic illustration of the value of objective measurement and how powerful the value of quality information and relatively simple information tools is to farmers.
Observing the effects of soil structure on your paddocks.
This is a further example illustrating the importance of soil structure to farm management. This ploughed paddock was spotted near Tapanui in Southland (NZ) on one of my frequent trips to the area during the Topoclimate South Project. The only farm operation that has occurred on this paddock is ploughing of the soil. Notice the interesting and distinct line across the middle of the paddock where there is clearly a soil change.
Notice too how the soil has come off the plough across the two parts of the paddock separated by the soil boundary. In the foreground the soil has stayed in plough ridges and would require further work to turn it into a friable seedbed for a crop. In the background, the soil has broken down easily off the plough and would require much less further working to produce a good seedbed for a crop.
When we surveyed the soils in this paddock, the soil in the foreground was mapped as a Mataura Silt Loam. This is a relatively recent soil formed by the infill of a depression in the paddock by flood deposition. Because the soil is recent, it has not had much time for soil structure to develop but also, because of its recent deposition from floodwaters from other areas, it is considerably more fertile than the soil in the background.
The soil in the background was mapped as a Crookston Silt Loam. This is a much older soil on a higher level which is hardly ever refreshed from floodwaters. This soil has had time to develop a much better structure but is less fertile because of the leaching impact of the heavy annual rainfall in the area.
So how did the two soils rate using the SVI index?
The Mataura Silt Loam rated 0.69 on the scale which indicated that it was more vulnerable to soil structural collapse.
The Crookston Silt Loam rated 0.59 on the scale which showed that it was less vulnerable to soil structural collapse.
So, how could the farmer benefit from all of this information?
In this case, the farmer was advised to build a fence close to the soil boundary in this photo and to manage the two areas completely differently in recognition of the different qualities of each soil. The area in the foreground would probably grow more pasture because of greater inherent fertility in the younger soil but shouldn’t be grazed during wet spells because of the risk of pugging and soil structural collapse. The soil in the background would require more fertilizer to maintain production levels but would be capable of being grazed during wetter periods because of better structural soundness. This has proved to be valuable advice for the farmer concerned and has added considerable value to his farming operation from a better understanding of his soils.
