Fitting compost into grazing systems

Spreading compost at a SWCC trial site in Gelorup

Demonstrations on dairy farms in the Harvey Dardanup region have helped to clarify how compost might best fit into grazing systems. The demonstrations have generated discussions between industry and other stakeholders that have highlighted the need to clarify the purpose of using compost, the rates that typically fit the purpose, and whether composting is effective on soils with high soil organic matter (SOM).

The dairy industry in Western Australia has been looking into compost for several years following uptake by some Victorian farmers. However, many farmers in WA have tended to look at compost primarily as a source of nutrients, and less so for its effect on biological and physical soil properties. But according to Emeritus Professor Lyn Abbott, who researches soil biology at the University of Western Australia, farmers shouldn’t think that adding compost is the same as adding a fertiliser.

“Adding compost won’t lead to immediate effects. If you measure in terms of plant production, compost will give you a slower return.”

Local vegetable and beef farmer Brett Kirkpatrick has been using compost for 20 years.

“It’s certainly a challenge to get the growth that a lot of farmers are attached to in the conventional system. I can have lush pasture growth but only after spreading compost on vegetables and then converting the area to pasture. It’s not something I do to permanent pasture because it’s so expensive.”

Brett applies compost on vegetable crops at rates ranging between 40-120 tonnes per hectare. Lower rates are applied in warm and moist conditions when microbes are actively recycling nutrients. Higher rates are used when he needs a more direct source of nutrients because the microbes are less active, e.g. when the weather is cold. The return from his vegetables justifies this investment, but not his permanent pastures, which receive just 2.5 tonnes per hectare.

The purpose of compost in a pasture context isn’t to directly supply all nutrients, although he does mix other inputs such as rock dust into his compost. His main purpose is to provide a nutrient source to soil microbes so they can function efficiently and mineralise nutrients into plant available forms, and fix nitrogen from the atmosphere.

The relevance to the dairy industry is that compost could be used as a ‘bolt-on’ to existing nutrient management systems to improve fertiliser use efficiency. While cost-effectiveness is yet to be established, E/Professor Abbott thinks this could be the best fit with a productive dairy system.

“Anything we can do to help reduce inputs is the driver, so I think we should be looking at using compost to balance out a reduction in fertiliser. The thing with high fertiliser inputs is that you actually shut down some of the beneficial microbial activity. To balance microbial activity you don’t really want to hit it hard with soluble phosphorus and nitrogen. If you keep using high levels of nutrients you are not going to get the most efficient nutrient cycling. So it’s good to minimise those inputs and get more benefit from microbes.”

A functioning community of soil microbes not only helps with nutrient cycling but also with the formation of soil aggregates, which help air, water and plant roots to move freely in the soil.

“Work was done a long time ago looking at the role of microbes in aggregate formation. Aggregates don’t form unless SOM and microbes are there. Certain bacteria and fungi produce sticky substances that hold particles together. This can happen even in a sandy soil.”

Using compost to improve microbial activity, nutrient cycling and soil structure rather than as a direct source of nutrients means that large rates may not be required.

“Some of the measures we have taken suggest a better effect of lower rates on microbial activity (not production) than with higher rates of compost. This isn’t a recommendation but it’s something we want to look at more closely.”

The primary function of compost is to replenish SOM, which is considered critical to soil function and fertility. But if dairy farms already have high levels of SOM, which is often the case, do they need to worry about adding more? Perhaps not, but it’s also worth considering why SOM is high. Irrigated pastures are expected to have relatively high SOM, but excessive levels could indicate another underlying soil constraint.

“If conditions aren’t suitable for microbial activity, SOM can build up in the soil. In a waterlogged or compacted soil, the microbes are less active and mineralise less SOM. So adding compost may be less effective because of these other constraints.

“As a general principle some soil disturbance from time to time might be necessary. I don’t mean dramatic disturbance but some way of breaking up compaction (or dealing with waterlogging) if that is a constraint. You might end up with less SOM as it gets consumed by microbes, but it might be better if the soil has more microbial activity. You need to strike a balance between retaining enough SOM and having conditions suitable for microbes.”

So all constraints to soil health need to be managed to balance microbial activity, including SOM, and compost is just one tool to help manage the balance between SOM and microbial activity. But compost may play other roles too, such as providing a suitable environment, not just a food source, for microbes.

“Everything comes down to the physical conditions that microbes are experiencing. People are experimenting with various forms of compost, and this is where the science is lacking, but a change in the physical environment appears to be beneficial to microbes. A good compost mixed with clay or biochar might provide a better environment for microbial activity.”

This article was originally published in Farm West News in February 2017. 

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