The development of Infrared (IR) techniques has been revolutionary in the rapidity and number of soil samples that can be analyzed. Soil spectra are used for prediction of specific soil properties such as nitrogen and organic carbon with good coefficients of determination. The predicted soil properties are then used to relate with crop production. The question remains to what extent soil spectra can describe and predict crop response directly. In AfSIS Objective 4, we explore this unfamiliar route of linking spectra directly with crop response (see Figure 18).
Fig 18. Different routes relating spectra with crop responses
The variability in responses observed in diagnostic trials is related to soil properties that are largely captured in soil spectra. We established the relationship between crop response and soil NIR and MIR spectra, separately and in combination. Mid infra-red is usually preferred to near infra-red due to better predictions of soil properties. Preliminary results, relating principal components from the spectra and the crop response data, indicate that there is a relationship worth investigating further (see Figure 19). This is ongoing work and has presently employed local spectra dataset from diagnostic trial fields as opposed to generation of PCs from a more global spectra library. We are exploring a robust way of determining strength of model fit, and also finding explanations for the observed dispersion.
Fig 19. Relationship between observed and fitted maize grain yield of the control (unfertilized) treatments based on combined soil NIR and MIR spectra ranges. Fitted line is 1:1 line
The available set of diagnostic trials is providing a first step in developing the spectra-crop response linkage. However, in order to generate more accuracy in prediction, there will be a need for more diagnostic trials to increase the data density and assess the robustness of the test. As a follow-up, AfSIS II should further explore the scientific principles and evidence based on which such a spectra-crop response tool would be designed and developed and guide further product development.