More Profit from Crop Nutrition: Micronutrient Survey

Summary:
The approach taken in this report was to assess each Australian Soil Classification soil order for its properties related to the primary risk of micronutrient deficiency. This risk was then assessed against reports in the literature, a survey of soil test data and a survey of grain nutrient concentration. A summary is shown in the table below, with green indicating low risk of deficiency, yellow moderate risk and pink high risk. Some parts of the matrix are uncertain.

Soil Order	B	Cu	Mn	Mo	Zn
Calcarosol	Low	Mod	High+	Low	High
Chromosol	Mod	Uncertain	Low	Mod (C)***	Mod
Dermosol	Low	Mod	Low	Low	High
Ferrosol	Uncertain**	Low	Low	Low	Mod
Kandosol	High	Mod	Mod	Mod (C)	Mod
Kurasol	Uncertain	Mod	Uncertain	Uncertain	Uncertain
Podosol	Uncertain	Uncertain	Uncertain	Uncertain	Uncertain
Sodosol	Mod	Mod	Low	Low	Mod
Tenosol	High	Low	Low	High (C)	Mod
Vertosol	Low	Low	Mod	Low	Mod
VertosolAlk	Low	Uncertain	High+	Low	High

* Where free calcium carbonate is present; ** Probably low. *** For Canola

It is clear that the major risk is from Zinc deficiency and while Copper deficiency is not widely indicated, there are uncertainties about the risks associated with that micronutrient. The major investments should still focus on these two micronutrients

Summary risk tables for each GRDC region were developed based on the relative area of each soil class and modified these could be further modified environment. These were used for reference and discussion, and identified areas of uncertainty.

For the Northern Region the main issues appear to be with Zinc (Zn) on Kandosols, Vertosols and Sodosols. There is uncertainty about Copper (Cu) generally.

For the Southern Region, the main soil types of Calcarosols, Sodosols and Vertosols have high risk of Zn deficiency, while Manganese (Mn) is likely to be a significant risk on these soils if they contain more than about 60% free calcium carbonate.

For the Western Region, Kandosols and Tenosols are more significant than the eastern states, although Sodosols is are the major soil order, and low Zn is seen on these soils. The acid soil types such as more strongly acidic Tenosols are likely to be at risk of Molybdenum (Mo) deficiency, while Mn deficiency is moderately likely Kandosols and Tenosols.

Additional tables for each agroecological zone could be developed as communication tools.

Limitations:
In general, the use of soil tests and grain testing to assess micronutrient supply are not conclusive guides to the risk of deficiency, so the assessments made are somewhat subjective based on the weight of the soil tests, grain tests, soil properties and the literature.

The Australian Soil Classification (ASC) Orders assigned in this report are based on imperfect information about the true ASC Order, and so groupings based on ASC Order are not definitive for the soil test or grain nutrient concentration data. The digital mapping used to link a NVT site to ASC Order was indicative and the expected ASC order was based on that indication plus soil properties known from the soil test (texture, pH, EC).

The NVT sites used in the survey may not be representative of the whole regions characterised, as the NVT sites and the Western Australia (WA) Farm Focus paddocks were not selected as average paddocks.

Conclusions from the survey:

Even under good fertiliser management, low Zn, Mn and B grain concentration are common in many cropping regions. Further research aims to link these deficiencies to particular soil types so that growers can make a reasoned assessment of the risk of micronutrient deficiency for their production system.

A collation of field and laboratory experiments on micronutrient response has supported the general view of risk of micronutrient deficiency as:
· B responses for canola on Tenosols.
· Cu responses for wheat on Chromosols and Sodosols.
· Mn response for wheat on Calcarosols, and for pulses on Chromosols, Sodosols and Tenosols.
· Mo responses on very acid soils generally (Tenosols, Kandosols)
· Zn responses for wheat on Chromosols and Calcarosols.

Soil properties can be used to infer a prima facie risk of micronutrient deficiency.

Maps of the ASC Soil Orders are already available through digital platforms, so the opportunity is to link current mapping the risk assessments based on soil order rather than remapping risk.

Sodosols and Kandosols are significant cropping soil types in all regions, while Kurasols and Tenosols are important in WA and Kurasols also in northwest New South Wales and southwest Queensland. Vertosols and Calcarosols generally are more important in the eastern states.

The soil types assessed as important from the Australian Soil Resource Information System (ASRIS) were consistent with the distribution of soil types estimated from the NVT database, although data from WA is scarce.

Based on the soil properties, crop are most likely to be at risk of deficiency on particular soils are;
· B deficiency on Kurasols, Podosols, and Tenosols,
· Cu deficiency on Kurasols
· Managenese deficiency on Calcarosols.
· Mo deficiency on Kurasols, Podosols, and Tenosols,
· Zn deficiency on Calcarosols,

Soil test values from the NVT database indicate that:
· 6% of B soil test values were below critical and they are lowest on Kurosols and Tenosols and highest on Ferrosols and Calcarosols.
· 34% of soil B tests were less than 0.5 mg kg^-1 and 11% were less than 0.25 mg kg^-1 in the Western Australia data set particularly on Chromosols and Tenosols.
· 2% of Cu soil test values were below critical, and they were lowest on Chromosols and Sodosols and highest on Tenosols and Vertosols.
· 21% of Mn soil test values (24% WA) were less than 5 mg kg^-1, and re below critical, but Mn soil test values are extremely variable on all soil types. Values were lowest on Caclarosols, Tensols and alkaline Vertosols. The high variability is likely a reflection of the low reliability of this soil test.
· 15% of Zn (1% in WA) soil test values were below critical, and they were lowest on Dermosols and Tenosols, and also low on alkaline Vertosols.

The grain micronutrient survey indicates that there are:
· 38% of wheat grain samples had B < 1.0 mg kg^-1 and lower values were recorded on Ferrosols.
· 3% of wheat grain samples had Cu <0.2 mg kg^-1 and the lowest values were recorded on Tenosols.
· Wheat grain Mn levels was less than 10 mg kg^-1 in 1% of samples, but grain Mn is an unreliable indicator of soil Mn status.
· Wheat grain Mo levels were less tha 0.02 mg kg^-1 on 23% of wheat samples, mainly from Kandosols and some Chromosols.
· Low wheat grain Zn levels are seen on Calcarosols, Dermosols, Kandosols and Sodosols, but not on Vertosols. 15% of the wheat tested had levels less than 15 mg kg^-1.

Canola has a a much higher B and Mo concentration than wheat, as well as a higher Zn concentration, although it also appears more efficient than wheat at accessing Zn in particular in all soils. Canola grain Cu levels were about 30% lower than Cu levels in wheat.


Summary of Research Areas Identified
Boron
Because B is mobile in the soil and does not have a long persistence in the soil (residual value), decisions on management of B will be likely made on an annual basis so either soil or tissue tests will be an important management tool for areas at risk of B deficiency.

There is a need to address B deficiency for canola on Tenosols mainly around delivery strategies including timing and products. Low solubility B sources such as B-phosphates, B-frits or B-silicates could be considered.

In terms of developing better risk profiles, the mapping initiated in WA (Wong et al. 2005) could be extended to the Southern and Northern GRDC zones.

There are tissue tests developed for wheat and canola that are used diagnostically, but the data to support the critical levels of 5-10 mg kg^-1 for wheat is based on a NSW DPI AgFact Sheet (Dear and Weir 2004). There are no critical values for canola although they are likely to be higher than those for wheat.

Copper
It would seem that Cu is widely used by growers, in anticipation of deficiencies. However, the true extent of Cu deficiency is yet to be defined within cropping systems. Field agronomists using visual symptoms often diagnose Cu deficiency, but better use of tissue tests could be made in diagnosis and validation of the critical values for tissue tests would increase diagnostic confidence.

The long residual nature of applied Cu on soils in eastern Australia should be established, as this will largely determine if management approaches should be tactical or strategic. Options for growers are to use either base fertilizer supplemented with Cu or to use in-crop foliar applications. There are few published experimental comparisons of these management strategies from the eastern states and well-documented case studies could assist growers with this decision. It is likely that soil type could have a major effect on the most appropriate approach, but there are few guidelines on which this assessment could be made.

Unresolved issues around Cu (similar to Zn) relate to higher yielding environments and an improved understanding of the interaction of N and Cu with modern high yielding cultivars. Under high N conditions, Cu is less able to be remobilized so late Cu deficiency may occur (Hill et al. 1978). This may have a genetic aspect, but the timing suggests that applications of Cu before flowering may be more important than earlier applications under high yielding conditions. The combined Zn and Cu demand could be considered as a single project.

Manganese
The use of foliar Mn sources, or of better formulations of Mn, such as with new or novel chelating agents, would seem to be opportunities to more efficiently address this micronutrient. The focus of the research on Mn would be in the VicSA Mallee, and in WA, particularly on pulse and canola crops, with a view to comparing strategic and tactical approaches to Mn management.

Molybdenum
Declining soil pH due to product removal suggests that there is an increased risk of Mo deficiency on soils that were once near neutral in pH. Increasing lime use will reduce some of these risks, but for soils on the pH “brink” (4.8?) there is a need to evaluate options for product delivery and formulation. At these pH values, molybdenum will be a cheaper option than liming but liming will provide a longer term solution to the deficiency.

Zinc
Comparisons of strategic with tactical application of Zn may require investigation, and to aid in this decision better information on the residual values of different formulations and presentations may be needed.

Zinc nutrition has been shown to interact with liming, root pruning herbicides and high soil P concentration, and the impacts of these factors require further consideration, which may include the development of alternative delivery strategies or product formulations.

Unresolved issues around Zn (simular to Cu) relate to in higher yielding environments and an improved understanding of the interaction of N and Zn with modern high yielding cultivars. The combined Zn and Cu demand could be considered as a single project.

Other Issues
There are good images available to growers to identify micronutrient deficiency (eg IPNI, Nutrient Deficiency Images App) and these should be incorporated into materials currently being developed as part of the diagnostic agronomy project. It would be useful to link the risk assessments made here based on ASC Soil Order to the SoilMapp App, to enable growers to check if the observations are likely to be micronutrient deficiency on their soil type. A second part to this identification is a strategy to confirm the diagnosis using tissue testing, and the data on tissue tests in Reuter and Robinson, along with sampling protocols should be linked to the images. This then becomes part of an extension package, linking risk (ASC Soil Order) to symptoms (Image database) to diagnosis (tissue tests).

The quality of seed retained on-farm or from commercial seed production can be affected by low Zn, Cu and Mn and late sprays to enhance seed vigour may be a good option, even if there is no particular yield advantage gained. Grain nutrient content (i.e. content per seed) has been found to be more important than than grain nutrient concentration.

The levels of Zn in Australian wheat grain are low on world standards, and are significantly below the Harvest Plus target of >30 mg kg^-1 which aims to meet human health needs for those on grains based diets. There is no price premium for high Zn grain, but supplying such grain is a significant humanitarian issue.

Selenium (Se) is important for human health and although not an objective of this project, a summary of grain Se content can be made available to GRDC.