Warra site - 2011/12 cotton results

Warra site - Cotton 2011/2012 results

This site was sown to cotton in late October 2011. The site was chosen after an extensive site characterisation (see soil test information below), with existing industry guidelines indicating a strong K fertilizer requirement as critical K soil test values are around 0.3 cmol(+)/kg (Cotton CRC NUTRIpak). The farming company who manage the paddock regularly apply K fertilizer at rates of 50-100 kg K/ha, depending on field.

Table 1. Site soil test for the 0-30cm layer in the raised beds of this irrigated farming system.

	pH	Exch Ca cmol(+)/kg	Exch K cmol(+)/kg	Exch Mg cmol(+)/kg	Exch Na cmol(+)/kg	CEC
0-30cm	8.1	14.5	0.31	8.9	1.7	25.4

The experiment at this site had the same aims and treatments as the Brookstead wheat site, and was designed to explore the responses to K fertilizer applied in bands (deep or shallow or split between depths, that were 100, 50 or 25cm apart) or broadcast onto the hills and incorporated with tillage during final bed preparation. The site was flood irrigated, and also supported trials (funded by the cotton industry) to look at similar placement issues for P, and at the interaction between P, K and S fertilizers. The trial was harvested in May and was high yielding, but disappointingly showed only minimal response to K fertilizer (Table 2).

Table 2. Yields or raw cotton (seed and lint) measured as t/ha and also approximated in terms of industry standard bales/ha (the final bales/ha yields cannot be determined until samples are ginned).

	raw cotton (seed + lint)
	t/ha	bales/ha
Control	5.78	13.1
Deep bands	5.87	13.3
Shallow bands	5.82	13.2
Split bands (deep and shallow)	5.96	13.5
Mixed through bed with tillage	6.11	13.9
25cm bands	5.88	13.4
50cm bands	5.85	13.3
100cm bands	5.91	13.4

The control (farmer standard) treatments without any K fertilizer produced 5.78 t/ha or approx 13.1 bales/ha. There were no significant interactions between K band spacing and depth, and no significant differences in band spacings as a main effect. However there was a trend for yields to increase as the volume of soil enriched with K increased (i.e. Deep K bands or Shallow K bands alone < K split between deep and shallow bands < K mixed through the bed using tillage). The differences the standard practice and the K mixed treatment was <5% in absolute terms. However, at a cotton price of $450/bale the extra yield from the mixed K treatment would have provided an additional $360/ha gross return.
It was also interesting to observe the lack of any ‘premature senescence’ symptoms during boll filling, which are traditionally used as an indicator of K deficiency. The biomass samples to determine K uptake by the cotton crop are still being processed, and these will provide a clearer picture of what has happened in terms of K fertilizer recovery.

The site was resown to cotton in 2012/2013.