One of the key challenges for AfSIS is how to measure soil functional properties on tens of thousands of georeferenced soil samples. Spectral diagnostics – the use of low cost, high throughput analytical techniques based on light reflectance – solves this problem. Infrared and x-ray spectroscopic analytical techniques that required minimal sample preparation are used as a front line screening tool. More expensive and time-consuming measurements of soil functional properties are then calibrated to the spectral measurements on a subset of samples. The calibrations, or pedotransfer functions, are then used to predict soil functional properties on the full set of samples. 

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Soil functional properties

The AfSIS soil analytical procedures emphasize the measurement of soil functional properties that determine soil health – the capacity of soil the capacity of land to sustain delivery of essential functions or ecosystem services, such as hydrological regulation, nutrient supply to plants, and nutrient retention. Three classes of properties can be defined according to the degree to which they show dynamic properties and to which they are influenced by management:

1. Slow, management insensitive. Intrinsic properties of soils that change only slowly with time, primarily in relation to soil forming factors. They are key determinant of intrinsic soil functional properties and therefore important to measure to describe spatial variation in soil functional capacity. Examples are mineralogy and particle size distribution. Of course in extreme cases, all propeties can be affected by management, e.g. severe human-induced soil erosion.

2. Slow, management sensitive. Key indicators or determinants of soil functions that are responsive to management over periods of several years. Soil organic matter content is a good example. These are the most useful variables for long-term monitoring of soil functional capacity as a reliable estimate can be obtained from a single measurement point in time.

3. Fast, management sensitive. These properties may be important for some soil functions but fluctuate rapidly (e.g. within a year) in response to climatic, hydrological and management conditions. These variables require frequent monitoring to develop an understanding of their behaviour and to obtain reliable estimates of average values. It is generally to difficult and expensive to conduct such measurements in large area surveys. Examples are microbial activity, topsoil macro-aggregation. Few fast variables are management insensitive.