Background to Omission Trials

Seasonal Conditions:
2015 - The rainfall across the HRZ was at Decile 1 for the year (Table 1), with very hot conditions (40oC plus) during early grain filling.
To date, 2016 has been one of the wettest on record.

Table 1. Growing season rainfall (April-November inclusive) at Naracoorte, Lake Bolac and Winchelsea during the omission trial experiment (2015) and deciles relative to the 1900-2015 period.

Location	Relevant to sites at…	Year	Rainfall (mm/yr)	Decile
Naracoorte	Bool Lagoon/Frances	Average	408
		2015	262	1
		2016
Lake Bolac	Chatsworth	Average	399	5
		2015	260	1
Hamilton	Tarrington/Hamiton	Average
		2016
Rutherglen	Rutherglen	Average
		2016
Winchelsea	Inverleigh	Average	466	5
		2015	260	1
		2016

Soil Samples:
The results of the soil tests taken prior to sowing for each of the sites are shown in Table 2.

Pre-sowing soil tests indicate that the 2015 sites all exceeded the currently accepted critical value for all nutrients tested for topsoil (0-10 cm ) criteria, except S for canola at Frances and Inverleigh (Table 5). Samples of the deeper soil indicated S and K tended to increase down the profile at many sites. The relatively high K and S levels at depth are likely to reduce the likelihood of crop responses to those nutrients at most of these sites.

Table 2 Initial soil mineral N (nitrate + ammonium) to a depth of 90 cm, and topsoil (0-10 cm) available P (Colwell), K (ammonium acetate, calculated as equivalent to a Colwell K extract) and S (KCl-40), and critical values at which 95% of maximum yield from the Better Fertilizer Decisions database (Bell et al. 2013a, Brennan and Bell 2013, 2013b, Anderson et al. 2013).

Site	N (kg/ha)	P (mg/kg)	K (mg/kg)	S (mg/kg)
2015
Bool Lagoon (wheat)	55*	27	1017	8.8
Chatsworth (wheat)	124	53	112	29.5
Frances (canola)	76	32	155	6.7
Inverleigh (canola)	84	58	254	6.8
2016
Bool Lagoon (wheat)	43*	24	1050	7.2
Inverleigh (wheat)	199	84	290	12.5
Bool Lagoon (canola)	28*	24	1250	6.2
Tarrington (canola)	276	62	210	16.8
Rutherglen (canola)	100	19	280	7.2
Hamilton (canola)	829	90	150	32.0
Critical value (wheat)		22	40	3.1
Critical value (canola)		22	46	7.1

Nutrient Addition Rates
Rates of nutrient supplied in the omission trials are high enough so that that nutrient is not limiting. The exception is nitrogen which was supplied at rates equal to 60% or 100% of the estimated water-limited yield potential.

Table 3. Rates of nutrient (kg/ha) applied in the omission experiments in 2015 and 2016. Rates of N, P, K, S, Cu and Zn at sowing were consistent across all sites, whereas in-crop N varied from site and year.

Nutrient/site (2015)	2015	Site (2016)	2016
Sowing	kg/ha		kg/ha
N	18,18, 30		30, 30, 30
P	0, 25		0, 50
K	0, 50		0, 50
S	0, 24		0, 20
Zn	0, 1.1		0, 1.1
Cu	0, 2		0, 2
In-crop N
Bool Lagoon (wheat)	0, 46, 110	Bool Lagoon (wheat)	0, 134, 247
Glenthompson (wheat)	0, 0, 39	Inverleigh (wheat)	0, 63, 236
Frances (canola)	0, 57, 106	Bool Lagoon (canola)	0, 64, 157
Inverleigh (canola)	0, 57, 106	Tarrington (canola)	0, 49,185
		Rutherglen (canola)	0, 229, 366