CHAPTER 3: SUGARCANE RESPONSE TO POTASSIUM FERTILIZATION ON
3.4 DISCUSSION
42
43 exchangeable and reserve K (Appendix 3.1). Nonetheless, sucrose yield at the second ratoon did increase with increasing K application indicating that application of K can improve yields in later ratoons on a soil with low reserve K. This finding is consistent with earlier reports of higher responses to applied K in ratoons compared to plant crops (Chapman 1980; Wood and Meyer 1986). In this study, the delayed response on the umbric Acrisol is most likely associated with a decline in exchangeable K with time.
Chapman (1980) also reported that the observed delayed response to K application is influenced by depletion of K reserves in control treatments. In fact, K requirements in the umbric Acrisol also increased with time (Appendix 3.2). Hence, positive sucrose yield responses to K application on the umbric Acrisol are expected to persist for succeeding ratoons, while the lack of crop response on the cutanic Acrisol is also expected to persist for more ratoons.
The effects of K applications on exchangeable K levels in the two soils is most likely influenced by K reserves and fixation (the latter reflected in the KRF values in Table 3.1). Exchangeable K in the control of the cutanic Acrisol, which had very high reserve-K, did not decrease below the initial levels, but in the umbric Acrisol it did decrease below the initial levels. Likewise, in the cutanic Acrisol, which had a high K fixation capacity there was no response to K application, but exchangeable K on umbric Acrisol increased with all K applications after harvest of second ratoon and at 240 kg K ha-1 after harvest of plant crop and first ratoon. The lack of exchangeable K response to K applications could be caused by either the release of K from the non- exchangeable reserves, K removal being equal to the amount of K applied, or K fixation, or the combination thereof. However, since there was no stalk yield response to K, it is unlikely that the lack of response was caused by K removal equalling the amount of K applied. Inasmuch as there was an apparent release of reserve-K to exchangeable K in the control, the amount of K released was always below the amount of K applied. Hence, it is suggested that the lack of effect on exchangeable K levels in this study was caused predominantly by high K fixation capacity of the cutanic Acrisol.
The increase in exchangeable K in the zero K treatment of the cutanic Acrisol over the years, despite K removal, is most likely explained by the release of reserve-K. Similar
44 results were found by Bar-Tal et al. (1991), where applied K was fixed by smectitic soils but K was released from the reserves at zero K treatment. It is proposed then that high K fixation and high reserve-K serve as buffers for soil K and regulate exchangeable K concentrations. Another aspect for consideration is that the increase in exchangeable K in zero K treatments, despite K removal by the crop, occurred only in the cutanic Acrisol whereas there was a decrease in exchangeable K in the umbric Acrisol. This observation is at variance with the reports of Khan et al. (2014). These authors observed an increase in exchangeable K in zero K treatments in Morrow plots (University of Illinois) despite 51 years of crop K removal and concluded that testing soils for exchangeable K was unnecessary. The results of the current study highlight how in zero K treatments exchangeable K may increase or decrease with cropping, depending on levels of reserve-K. It would appear, therefore, that instead of discarding soil testing using exchangeable K, there is a need to supplement this parameter with measurements of soil K fixation and levels of reserve-K. Clearly, in our study the changes in exchangeable K in both the umbric Acrisol and cutanic Acrisol following K application or removal could be explained by considering soil K fixation and levels of reserve-K.
Increasing leaf K concentration with increasing K application is to be expected and is widely reported for crops (Simonis et al. 1998; Ogaard et al. 2001; Oborn et al. 2010) and this was expected for the umbric Acrisol because it had low levels of reserve-K.
However, leaf K concentration results should be interpreted with caution because of the phenomenon of luxury consumption of K, where the plants have sufficient K but continue to absorb K without any effect on yields (Ogaard et al. 2001; Staines et al.
2014). Three critical sugarcane leaf K concentrations are reported in the literature: the first is 1.05 %, the threshold below which the crop will be deficient in K; the second is 1.25%, beyond which yield response to K is unlikely; and lastly 1.5%, above which luxury consumption is considered to occur (Wood and Meyer 1986; Meyer et al. 1989).
Leaf K in the umbric Acrisol control was below 1.25% and significantly lower than that at 240 kg K ha-1 for the second ratoon. Interestingly though, sucrose yield responses were also observed only in the second ratoon on that soil, indicating that leaf K concentrations could also be used as indicators of the likelihood of response to K fertilization.
45 The findings of this study indicated that K reserves and fixation does influence the response of exchangeable K, sugarcane stalk and sucrose yields to K application. It is thus argued that these should be incorporated in routine soil testing and formulation of K recommendations. It is also recognised that subsoil K and release rate on K reserves can influence the response of sugarcane to K application. However, for several reasons, their incorporation in routine soil testing and formulation of K recommendations remains a challenge. Routine soil testing is based solely on the topsoil and does not include subsoils and furthermore, subsoil exchangeable K of the soils used was lower than that of topsoil which is in agreement with Miles (2012) who reported depletion of subsoil K in the soils of the South African sugar industry. While cutanic Acrisol had subsoil reserve-K, the main cause on the lack of response to K application is believed to be topsoil reserve-K. Further studies are required to investigate the contribution of reserve-K and subsoil K reserves on K uptake. Release rate of K reserves, on the other hand, is influenced by a number of temporal variable factors such soil solution K concentration, temperatures, and wetting and drying (Sparks and Huang 1985; Wood and Meyer 1986). It is presumed that levels of reserve-K and K fixation control release rate but this requires further investigations.
Combinations of reserve-K and K fixation will be investigated in Chapter 4.