The Response of Crops to Potassium Placement Depth and Band Spacing

Field sites have been established in the northern grains zone at Brookstead for grain cropping and at Warra for irrigated cotton. Two further trials are in the rainfed grain producing areas in central Queensland at Gindie, south of Emerald and Capella, north of Emerald.

Brookstead wheat experiment (southwest of Toowoomba)

This site was sown to wheat in May 2011. The experiment aims to investigate K placement depth and band spacing for rainfed grains. The whole site was treated at sowing with 40 kg P (as mono-ammonium phosphate), 110 kg N and 15 kg S (as ammonium thiosulphate) as well as additional zinc. The treatments imposed at Brookstead are:

· Nil K,
· Surface spread K
· Shallow K placement (7.5 cm), 25 cm seeding rows
· Shallow K placement (7.5 cm), 50 cm seeding rows
· Shallow K placement (7.5 cm), 100 cm seeding rows
· Deep K placement (20 cm), 25 cm seeding rows
· Deep K placement (20 cm), 50 cm seeding rows
· Deep K placement (20 cm), 100 cm seeding rows
· Split Depths (7.5/20 cm), 25 cm seeding rows
· Split Depths (7.5/20 cm), 50 cm seeding rows
· Split Depths (7.5/20 cm), 100 cm seeding rows

The rate of applied K is 200 kg K as muriate of potash. These rates are very high, but are designed to last a number of years, and also (in high CEC clay soils) to overcome any lag in the response surface caused by strongly buffered soil solution K concentrations. The average rates of K removal in chickpea (11.0 kg K/t) are at least twice that of the nearest grain crop, with average removal rates in barley (5.1 kg K/t) > wheat (4.1 kg K/t) > sorghum (3.1 kg K/t). So the K application rates may be best targeted as the heaviest K users in the rotation, and because K is likely to be well held on the soil colloids, higher rates are not likely to be lost but will be used later in the crop rotation.

I inspected this site in August with Dr Bell and Mr David Lester (QDPI). At that time, the crop was only establishing and seasonal conditions were dry so there were no visual effects. In early October, Dr Bell reported that there are strong phosphorus and sulphur responses in the wheat crop, a consequence of low background fertility. Unfortunately root lesion nematode damage is impacting on the likelihood of getting good K responses this crop. Biomass sampling was done at anthesis in mid-October to assess K uptake from the different application strategies. Preliminary results indicate that the average K effect on biomass at anthesis was 10% over the basal phosphorus and sulphur treatments, while the best K placement (shallow) was 16%. . There will be K responses in this study - it is just that the additional variability generated by the nematodes may make some of the more subtle effects of band spacing and placement less significant.

These results from the wheat on fallow experiment were consistent with results for double cropped wheat (wheat on sorghum) in an adjacent experiment. Harvest of the site is expected by the end of November and a full report provided in February.

Root lesion nematode (Pratylenchus thornei and P. negluctus) is a common pathogen on wheat and chickpea crops in the northern grain belt. The management option for the site in 2012 is to avoid susceptible cultivars of these two crops (such as Jimbour chickpea or EGA Wylie wheat), or to sow summer crops such as sorghum, millet or cotton which are resistant to the nematode (Thompson et al., 2008). The option adopted will depend on seasonal moisture patterns over the coming year and the decisions of the host farmers participating in the trials. The area will now be fallowed to cotton or sown to late maize or sunflower - all of which are also resistant

Warra cotton experiment (northwest of Toowoomba)

This site was sown to cotton in late October 2011. The planting window for the Bollguard cotton varieties opens in mid October, and soil moisture on the site is good. The experiment at this site has the same aims and treatments as the Brookstead wheat site. This is an irrigated site.

Central Queensland sites

The CQ sites are likely to be sown to sorghum (Bendee, near Gindie south of Emerald) and chickpea (Stranraer, near Capella north of Emerald), but a lot will depend on rain in the next month or two. The experiments at these two sites aim to investigate the interacting effects of P, K and S in dryland grains systems.

The treatments applied at these two sites are:

	Trt	Id	N	P	K	S	Micro
With micro-nutrients	1	Ctrl	100	-	-	-	y
	2	P	100	40	-	-	y
	3	K	100	-	200	-	y
	4	S	100	-	-	40	y
	5	P:K	100	40	200	-	y
	6	P:S	100	40	-	40	y
	7	K:S	100	-	200	40	y
	8	P:K:S	100	40	200	40	y
Without micro-nutrients	9	Ctrl-TE	100	-	-	-	n
	10	P:K:S-TE	100	40	200	40	n

The micronutrients have been added into this design based on comments from local agronomists. The P, K and S treatments were in band applied 50 cm apart, with nutrients split between shallow (7.5cm) and deep (20cm).

Reference:

Thompson J, KJ Owen, GR Sterling, MJ Bell (2008) Root lesion nematodes (Pratylenchus thornei and P. neglectus): a review of recent progress in managing a significant pest of grain crops in northern Australia. Australasian Plant Pathology, 37, 235-242.