Grid Soil Mapping V NDVI


Todd Buck


Precision Agriculture uses grid soil mapping to understand and manage surface soil characteristics.  Based on many years of experience, we believe this is the gold standard. This method uses proven soil science for testing soil properties across a paddock and replicates this multiple times to allow for Variable Rate solutions.

NDVI and yield maps provide a measure of the variation in crop growth at a point of time and overall production in a season. This variation is the result of many variables including surface and sub-surface soil properties, soil type, elevation, management, pest, disease, and frost.  They do not identify the cause of the soil variability.

A study by Brigette Snell from Monash University and Kirsten Barlow shows that in some paddocks (<10%) the NDVI-derived management zones could identify areas with significantly different surface soil properties and correlate low NDVI with soil constraints. Even when zoning worked, it still created an average soil property for the zone, limiting the ability to manage the high and low soil test values.

However, in over 90% of the paddocks, these zones did not identify soil constraints or the variation in surface soil properties (Case Study). In addition, the variation in soil properties within management zones (coefficient of variation) was similar to the variation across the full paddock.

The results showed that management zones based on NDVI and yield could not identify soil constraints and the variation in surface soil characteristics in over 90% of the paddocks. Therefore, they have limited value in developing VR solutions to address surface soil constraints.

This study continues to strengthen our argument that grid soil mapping is the best-measured evidence base for Variable Rate management of surface soil properties. 

Image 1. Case Study – Paddock M6
Surface Soil pH V NDVI management zones

  • Soil pH was not significantly different between the 3 management zones (paddock mean = 4.7, zone means =4.71,4.72 and 4.69)
  • The CoV was 9.2 for the paddock and ranged from 5.6-12.4 for the three zones.

When we look at the correlation between August/September NDVI and soil pH over multiple years we can then see we start to get a high degree of variability. This highlights that while pH will be affecting yields in parts of the paddock there are a number of other factors affecting yield. In this paddock, we know that some areas of the paddock have high ESP and other areas have low Mg values. Both of which may affect yield. There is no relationship in space between the different soil constraints.

75% of paddocks had an average pH of 5.2 or below.

50% of paddocks had an average  pH of <4.8

This is just one example of many where grid soil sampling was seen as best practice to obtain the full soil properties across a paddock and replicates this multiple times to allow for Variable Rate solutions. To read the full study click here:

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