The Response of Crops to Potassium Placement Depth and Band Spacing

Summary points and observations to date:

· At Gindee in 2013, supplying both K and P gave a 51% chickpea grain yield increase even under very low rainfall growing conditions.
· The value of the extra response to fertilizer over the two years were $310/ha for P only, or $380/ha for P+K at Capella, or $160/ha for P only, or $320/ha for P+K at Gindee.
· Responses to K depend on adequate supplies of other nutrients, especially P in this situation.
· Deep placement has shown responses 2 years after application.
· Chickpeas are more responsive to K than wheat, which is more responsive to narrow row sorghum, which is more responsive than wide row sorghum.
· Seasonal rainfall patterns have a large influence on response, determining both crop K demand and the volume of soil exploited by the crop to access K.
· Soil tests for K provide limited intelligence for determining site responsiveness.

Project Overview:
The overall research undertaken by Dr Bell and his team aims to assess the size of P and K reserves in the main cropping regions, and develop diagnostic criteria to predict the amount of plant available nutrients, and so the need for added P and K fertilizers. Dr Bell and his colleagues have around 20 field sites in the whole project with 4 of these supported by Canptoex. The sites are all in farmers paddocks and they follow the host farmers crop selections. In this northern grains region, winter or summer crops are selected mainly on stored water in the soil at the potential planting times, and as a result crop sequences can involve summer crops (cotton or sorghum) or winter crops (wheat or chickpea).

This report presents the results of the 2013 winter crop program – which was wheat grown at Capella (chickpea in 2012) and chickpea grown at Gindee (sorghum in 2012) in central Queensland. The sites in southeastern Queensland reported previously have been terminated.

2013 Field Experiments
The Capella site was sown to wheat in May 2013 following a chickpea crop in 2012. The crop was established over the base treatments laid down before the 2012 crop and no further fertilizer was added. The seasonal conditions at both Capella and Gindee were very dry, with May to October rainfall at Emerald of 76 mm compared to a long-term average of 181 mm.

The Gindee site was sown to wheat in winter, 2013 following a failed sorghum crop sown in January 2013. This site showed that in biomass (Figure 1) unless K was present, minimal responses to P were evident. Fixing K with out fixing P gave 35% growth improvements, but the combination of P and K gave 75%. The Capella site showed the opposite, where the first response was to P, then there was an additive effect of K with P. These soil K levels are the lower at Gindee and while the soil P level is the lower at Capellla, so the order of P and K responsiveness in a responsive crop like chickpea picks up those differences in relative availability.

Figure 1. Grain yield response to chickpea (2012) and wheat (2013) to P, K and S alone and in combination at Gindee, central Queensland.


Summary from Gindee:

¥ Main response to deep P, with a slight trend for an additional benefit of K once P adequate
¥ Lack of in-crop rain (2^o roots and tillers) limiting responses in 2013 wheat?
¥ Additional crop production (2012 chickpea @$550/t; 2013 wheat at $275) worth $310/ha for P only, or $380/ha for P+K.
¥ Compared to commercial practice in 2012, deep P provided 900 kg yield benefit (deep tillage + P), lifting combined benefits to $600/ha
¥ Residual value excellent so far. How long will it keep performing?

Figure 2. Grain yield response to sorghum (2012) and chickpea (2013) to P, K and S alone and in combination at Capella, central Queensland.


Summary from Capella:

¥ Response varied with crop - only P in sorghum, but (K + P) in chickpea
¥ Unless K present there was no P response in chickpea
¥ Lack of in-crop rainfall enhancing the value of deep K in 2013?
¥ Additional crop production (2012 sorghum @$200/t; 2013 chickpea at $375) worth $160/ha for P only, or $320/ha for P+K.
¥ Residual value excellent so far. How long will it keep performing?

Communication Activities
1. K was a feature topic at the four Incitec Pivot Agronomy Community meetings held in July 2013. Dr Bell spoke at Kingaroy, Dr Mark Conyers at Wagga, Mr Andrew Speirs at Bendigo and Dr Nigel Wilhelm at Adelaide. Dr Bell will also speak at a meeting in Townsville on K convened by Incitec Pivot in March 2014.2. In November 2013, Dr Bell spoke about nutrition issues – including this work – at eight workshops in central Queensland, contacting around 150 growers and agronomists.
3. The research was part of a GRDC Groundcover supplement, distributed to grain producers in the Northern Region. It was also reported in the “Pulse Growers Newsletter” distributed Australia wide.
4. Dr Norton spoke at the GRDC Advisor Update meeting in Adelaide in February on K issues in that region.
5. A one-page summary of the 2013 winter crop results was produced and supplied to Agrow for their use.
6. Summaries of all experiments and the results to date has been posted on the IPNI ANZ website. See http://research.ipni.net/project/IPNI-2010-AUS-13

Plans for 2014
A new site exploring the rate response to applied KCl has been established on a K responsive site at Chelmsford, north of Kingaroy. The site has been fertilized to ensure adequate background N, P and S, and rates of K from 25-200 kg/ha have been applied in 50 cm bands at 20 cm depth. The site was to be sown to sorghum or mungbeans depending on planting rains, but drought meant there was nothing sown in December or January. The plan is to sow this site to wheat or chickpea in the 2014 winter with nutrient levels plots. Should the drought not break, the area will be sown to sorghum as a summer crop late spring. The information from this experiment will be used to test the hypothesis about that chickpea is more responsive than wheat to applied K. This is the fourth site for the final year of the project.

In 2014, the two existing sites at Gindee and Capella will be sown to chickpea and wheat respectively. This will assist in determining the residual value of the K applied in 2012. These CQ sites have a reasonably full profile of water, and it is likely that growers will sow under these conditions. Sowing of winter crop is usually in April or May, and the plans for these sites still depend on the rains between now and sowing. Measuring the residual value of P and K across crop cycles like this is hugely valuable, as it gives growers the confidence to apply reasonably high K rates at opportune times for deep K placement, and will complement two new trial sites established in the GRDC program (at Dysart and Gindie). The combined data will help improve our confidence in using soil testing to indicate K fertiliser response.

Relative crop responsiveness
The results from the 20013 winter crops have confirmed that chickpeas respond more strongly to K than the cereal crops like wheat. The relative responsiveness of sorghum is less clear, given it is typically grown in much wider rows under rainfed conditons (100-150cm v's 30-50cm in chickpea). These wider rows allow plants access to larger soil volumes, so even if K supply is very limited the crop can get by as long as roots find 'new' (unexploited) soil. If this holds true, we should expect to see more consistent K responses on the same soil types in crops grown in narrow rows (typically the winter crops).

The difference in response between wheat versus chickpea was seen quite early in chickpea. This may be because the large tap root of chickpea is less capable of thoroughly exploiting K from a given soil volume than roots of cereals like wheat which have more fibrous root system, lots more fine roots etc. The early K deficiency seen in chickpea may be more pronounced when most of the K is in the top 10cm of the soil profile, as surface root proliferation is not noted in chickpea.

Cotton is one of the main high value crops, and it is know to have almost double the soil K requirement of crops compared to wheat, sorghum, maize, soybean and peanut. The work to date suggests that cotton, like chickpea, is not very efficient at exploiting low K in a given soil volume. The cotton planting at Brookstead last summer was curtailed due to a lack of soil moisture in the planting window. The adjoining field was sown, but was sacrificed for an insect refuge area. The irrigated cotton situation is less clear, but it appears that K placement is important. Under irrigation a large volume of soil is available for the crop to exploit and so even if the concentration is low, the volume enables enough K to be accessed. Some additional research, using Rb tracers can help understand where in the profile cotton is taking up most of its K, and if the current fertiliser practice of banding or dispersing K in the hills is an effective way of getting fertiliser K into the crop.