CHAPTER 6: GENERAL DISCUSSION AND CONCLUSIONS
6.2 MAIN FINDINGS
Potassium reserves and fixation capacity had an influence on the response of sugarcane to K fertilization. Sugarcane depleted K in the control plots (zero K application) of a soil with low K reserves whereas on a soil high in K reserves there was no K depletion. Furthermore, there were sugarcane sucrose yields and leaf K responses to K application on the soil with low K reserves but on the soil with high K reserves there was no response to K application. The lack of response on soil high in K reserves indicate that there was no need for K fertilization in this soil which is in agreement with the proposed changes to calculating K requirements. Based on the findings of the field K response trials, it was clear that fertilizer recommendations must be modified to account for K reserves and fixation capacity. The soil with high K fixation capacity regulated exchangeable K concentration and it was postulated that the combination of K reserves and fixation capacity provide a measure of soils K buffering capacity. The cutanic Acrisol which had high K reserves and fixation capacity buffered exchangeable K but umbric Acrisol with low K reserves and medium K fixation capacity did not. Combination of high K reserves and fixation capacity accounted for 15% of samples analysed in this study while those with low K reserves and medium K fixation
81 capacity accounted for 31%. There is thus a need to assess how the various combination of K reserves and fixation capacity and modified fertilizer K requirement influence crop response to K application.
In terms of variations, the current study revealed wide variations in K reserves and fixation capacity. In addition, there were also variations in the way levels of reserve-K related to K fixation capacity (which was postulated in the K response trial) to represent soil K buffering capacity. Lastly, K requirements were modified using K reserves and fixation based modifiers and the modified K requirements were often appreciably different from the ‘original’ K requirements. Introduction of K reserve modifiers resulted in reduced K requirements whereas K fixation modifiers resulted in both reductions and increases in K requirements depending on the K fixing capacity of the soil.
Fertilizer requirements resulting from the introduction of both K reserves and fixation modifiers were, however, not different from those obtained when only K reserves modifiers were introduced, with the exception of few soils (about 10 % of soils used in this study). This finding coupled with the fact that only reserve-K could be predicted well with MIR and MLR, suggest that it might be necessary to include modifiers based on MIR predicted reserve-K alone. It is envisaged that the modified fertilizer K requirement would result in significant savings in terms of fertilizer costs and what is more, yields will be improved on soils with high K fixation capacity when the MIR calibration for KRF has been improved. Thus, it is recommended that K reserves and fixation based modifiers are introduced when formulating K requirements and this would require routine measurement of K reserves and fixation capacity.
The measurements of K reserves and fixation capacity are, however, laborious and time consuming, but they could be estimated using either MLR models or MIR. The success of estimating K reserves and fixation with these secondary techniques will be of huge value considering the wide variation of K reserves and fixation in soils and their influence on crop response to K application. This implies that if estimation of K reserves and fixation with MLR and MIR is successful then K responsive soils can be discriminated from non-responsive soils. Estimation of K reserves with MLR, particularly when ‘routine-plus’ soil properties were included, was satisfactory, but that
82 of K fixation capacity was unsuccessful. The K reserves obtained from MLR could at this stage be used only for the screening of soils that will respond to K fertilization from non-responsive soils.
Estimation of K reserves and KRF (fixation capacity) using MIR was better than that of MLR. A successful calibration was obtained for K reserves using 1 mm samples and thus can be implemented in routine soil testing, albeit with caution. The calibration for KRF, however, was not as good but could be used for screening K fixing soils from non-fixing soils. The outcome from the MIR calibrations and predictions can be approached from two possible angles. The first would be to include K reserves, estimated from MIR, when formulating fertilizer K requirements, and to continue using a KRF of 3.0 across all soils. However, modifiers based on reserve-K would need to be adjusted to accommodate the SEP obtained when predicting K reserves. This would be justified because for about 90% of the samples, K requirements formulated by introducing K reserve modifiers only were no different than when both K reserves and fixation modifiers were introduced. The second approach would be to use both K reserves and fixation modifiers, but adjust both K reserves and KRF modifiers so as to accommodate the SEP obtained when predicting K reserves and KRF. The implication is that the criteria for K reserves (predicted from MIR) would be higher than that used by Haysom (1971) and a KRF value of 3.0 would be used for non-fixing soils and a value of 4.5 for K fixing soils. Changes in K requirements as a result of introducing MIR K reserves and fixation modifiers would be an improvement from the current approach and would be closer to what would have been from modifiers based on traditional ‘wet chemistry’ K reserves and fixation measurements (Figure 6.2).
83 Figure 6.2 Changes in K requirements (%) for twenty soils used as validation set in Chapter 5 as a result of accounting for K reserves and fixation capacity measured using traditional ‘wet chemistry’ and mid-infrared (MIR) analysis.