Merriden nutrient trials

24 Dec, 2001 10:00 PM


MOST WA farmers would consider that producing European-type wheat and barley yields is beyond reach in the basically dryland farming environment that characterises most of WA's Wheatbelt.

And many farmers who have improved yields over the last decade to around the 3-4t/ha mark believe they may have reached yield limit on their land.

This is borne out mainly by decreasing gross margins caused by higher cost inputs to "pump up" crops with all sorts of nutrients.

While some farmers are reporting yields between 6-9t/ha on better performing paddocks the general trend is more between 2.5t and 3.5t/ha.

But in some shires in the eastern Wheatbelt, for example, yields have gone lower even in the wettest years.

So why have yields hit the ceiling, or perhaps more accurately in the eastern Wheatbelt, the basement?

According to Agriculture Department senior agronomist Mohammed Hamza, soil compaction is a major reason, at least in his patch of research in the Merredin district.

But anecdotal evidence from farmers in other areas of the state, with heavy and light soils, also point to compaction or lack of soil structure as the major inhibitor of not only crop yields but also pasture growth.

Hamza has been involved in a GRDC-funded research project over the past five years at the Merredin Dryland Research Institute to find a solution to the problem and results are confirming that an amelioration package is the best approach, based on four years of trial data.

Year 2000 trial results, for example, showed the "package" increased wheat yields on a clay soil at Merredin by more than 150pc (0.63t/ha to 1.63t/ha) in a very dry season, with a total of 113mm rainfall.

This package consists of an application of 2.5t/ha of gypsum, deep tilling to 40cm (16in), incorporating stubble and applying nutrients based on soil chemical analysis.

Too hard? Well don't be put off because Hamza's trials at Merredin (heavy soil), Nungarin (medium soil) and Tammin (sandy soil) have shown that this package is successful in improving soil physical and chemical fertility.

The package scenario sees gypsum applied as early as possible to take advantage of summer and autumn rains, so gypsum is washed into the soil profile (at least 15mm of rain is required to achieve this).

If this is achieved a deep tilling operation can be done, ensuring soil moisture is optimal -- if soil can be formed between the palms of the hands in an unbroken ribbon 3-5cm long then soil moisture is suitable.

Land should then be deep tilled before seeding with nutrients application with or after seeding.

Before the mind takes off at the multiple workings and the cost of gypsum and deep tilling, remember the exercise only needs to be done every four or five years or even longer, depending on the soil type. If soil structure is maintained through stubble retention the residual effect of the package can last much longer. So costs should be amortised over at least a four year period.

The application of nutrients after seeding, generally 10 days, is done specifically to maximise the timing of when the germinating plants are looking for nutrients.

"Each seed carries its own nutrients for germination, so it is best to wait 10 days before applying other nutrients," Hamza said.

The nutrient package is probably one of the most vexing issues in agriculture, akin to the plethora of "magic" nutrient packages available for humans.

This is where you need a very good understanding of soil science and soil and plant testing.

You also will need a good understanding of cation exchange capacity (CEC), base saturation and calcium-magnesium ratios for a good soil structure. (Information on these topics is available from Hamza).

Anectdotal evidence throughout WA shows that done properly, it can produce dramatic changes to the soil and consequently to plant health and yield.

There is basic agreement that the quality of the soil structure is maintained by a balanced calcium:magnesium ratio, around 60-70: 10-20 depending on soil texture.

According to Hamza, if you maintain this ratio, you create a good environment for water and oxygen infiltration.

The calcium:magnesium ratio is the starting block but you also will need to refer to your own paddock soil test analysis sheets to determine the application rates of macronutrients and micronutrients.

And you must understand the two types of fertility in the soil -- physical soil fertility which is related to soil structure and CEC and chemical fertility relating to the concentrations of nutrients.

The status of soil structure must be understood to assess whether application of the package is warranted -- waterlogging is a tell-tale sign along with soil that is very hard when dry and muddy when wet (especially heavy soil).

According to Hamza, stubble retention is an integral part of the package.

"The effect of gypsum will remain in the soil for several years because it is moderately soluble, but eventually it will disappear and soil structure will deteriorate," he said.

"To make the soil system more stable, plant residues must be returned to the soil after each harvest."

"The best stubble management is to chop it into small pieces to expose as much surface area as possible for bacterial activities and facilitate cultivation and seeding operations."

Another strategy is to wait for summer rain then top dress ammonia phosphate at between 5-10kg/ha to assist in encouraging bacteria to multiply and break down the stubble so the nutrients are returned to the soil.



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