What is going on with our soils and why is there a perception amongst farmers that they have to use higher rates of fertilisers to get the same results they used to get 40 years ago?

The answer to both these questions is intrinsically linked and lies in the soils – specifically soil chemistry and soil biology. Clearly, soils are not performing like they used to despite a shift to reduced tillage, better rotations, controlled traffic, and so on. To understand what is going on, we have been looking at the science behind two key issues, namely nitrogen and organic matter in the soil.

Organic Farm

Image: Left Field - Compost vs. Right Field - No Compost

Long term application of Nitrogen fertilizer is reducing organic matter (OM) and the availability of soil Nitrogen (N) to crops and pastures

This is not what you would expect. How can adding nitrogen fertiliser deplete soil nitrogen? We should qualify this by saying soluble soil nitrogen. Most fertilizer N is soluble. So adding more fertiliser nitrogen stimulates soil biology to burn off more OM. OM stores soil N and soil biology makes it available to plants. Crops can only take up so much N at any one time. The surplus N can be lost in leaching, run-off, and to the air in gaseous forms.

The global use of nitrogen fertiliser has increased from 12 to 104 million metric tonnes per annum over the last 40 odd years to support the increase in cereal yield. Now Australian farmers, and farmers around the world, are saying they need to put on more and more nitrogen fertiliser to achieve the same yield.

A global review by researchers at the University of Illinois in the USA has examined this problem in more detail (Mulvaney et al 2009). The key points from their paper include the following:

  • Synthetic nitrogen fertilisers are depleting soil nitrogen at a global level. The decline is in agreement with numerous long-term baseline data sets from chemical-based cropping systems involving a wide variety of soils, geographic regions and tillage practices.

  • Such an effect is consistent with a net loss of soil organic matter. The loss of soil nitrogen is decreasing agronomic efficiency (kg grain per kg N) and has been implicated in widespread reports of yield stagnation and sometimes decline.

Other conventional farming practices that reduce soil Organic Matter (OM)

In natural ecosystems OM provides plants with all their nutrients, stored moisture, soil aeration and many other benefits. Over the past 200 odd years Australian farmers have used up soil OM to produce wheat, meat, wool, milk, butter, and other produce. Just as well fertilizers came to the rescue. Unfortunately, soil OM has continued to decline and now we have problems with nutrient availability in general.

There is a prevailing view that global food and fibre production will continue to expand because of modern agricultural management systems with improved cultivars and intensive chemical inputs dominated by fertilizers. The use of fertilisers has led to some concerns regarding water and air pollution but is generally perceived to play an essential role for maintaining agricultural productivity.

There is now an assumption fertilisers will increase the quantity of crop residues for building soil organic matter. The scientific soundness of the build-up concept has yet to be substantiated using baseline data sets from long-term cropping experiments. In fact, the science supports the view that fertilizer N depletes organic matter by promoting microbial breakdown of organic matter and N mineralization.

The second assumption was that switching from tillage to no-till, stubble retention, and better rotations would increase soil organic matter. Unfortunately, this has not happened with mixed results showing slight increases, no increases or actual decreases. So obviously the processes of organic matter accumulation are not fully understood nor are they working.

Image: No Compost vs. Compost

Building soil organic matter requires a new way of thinking

Australian farmers have done a remarkable job in a very tough environment. They currently feed 60 million people (domestic and export). However, the conventional farming methods used are very hard on our soils and we are starting to see the consequences. As the US economist Herman Daly once put it, with regard to calculation of national GDP: “There is something fundamentally wrong in treating the Earth as if it were a business in liquidation.” This means that running down our soil quality, including organic matter, shows up as a gain for Australia’s national GDP.

So we need to make some changes to how we manage farms as businesses and how we manage the soil and nutrition for crops and pastures. Farm case studies are emerging that show the smart way to go is to manage your farm to make nature work for you instead of the usual man vs nature approach. There is plenty of evidence to show that it can be done. Finding the best way for you and your farm is the first step.

What to do on your farm

If we look at soil biology, all organic matter production starts when plants, in this case crops or pastures, take CO2 from the air and combine it with water to capture and store sunlight energy. The process is called photosynthesis. All food and fibre production begins here.

What does this mean on your farm? It means as long as your crops and pastures are growing, the amount of OM is increasing on your farm. The other way is to bring OM in from off-farm as mulch, manure or compost.

Think of it as your farm OM account. It works in a very similar way to your bank account. The idea is to put more in than you take out over the whole year. There will be ups and downs during the year but a net gain is what you need.

Crops and pastures are generating OM on your farm (called production) at the same time as organic matter is leaving your farm. Simply put, losses from paddocks occur when plants stop growing in the paddock. So a perennial pasture with timed grazing will deliver one of the best results. Annual crops with fallow periods will make this more difficult and the result will depend on your management practices.

Results suggesting compost and fertilisers may not be compatible

It is important to understand that the three major nutrients for crops and pastures are carbon, hydrogen and oxygen. Carbon comes from carbon dioxide in the air, hydrogen comes from water and oxygen from water and air. Combined, they make up 96 % of plant weight.

For legume crops and pastures, nitrogen can come from bacteria in root nodules. This only happens if available soil nitrogen is low, say less than 20mg/kg (20ppm), and the correct strain of rhizobia is available at the right time.

All other plant nutrients are taken up by roots from the soil, unless you are topping up some nutrients with foliar applications. A deficiency in one of the essential nutrients will reduce growth and production, even if the others are in plentiful supply.

Synthetic fertilizers and lime or gypsum supply a limited range of nutrients, usually nitrogen, phosphorous, potassium, calcium or sulphur. If you are bringing organic matter in from off-farm you will be bringing a full range of nutrients as well. The best quality compost has about 40% organic matter plus nitrogen, phosphorous, potassium, sulphur, calcium, magnesium, chlorine, iron, zinc, copper, boron, manganese and molybdenum.

It follows from the work of Mulvaney et al (2009) that the organic matter present in compost will be at risk if soluble nitrogen is also added in fertiliser forms. Nitrogen and other essential nutrients are already in compost so there should be no need to add fertilisers as well. And adding both is also too expensive

ASM field work on farms across southern NSW is supporting these points. Our best results for compost come from farms with a history of no use of synthetic fertilisers. Benefits are also seen when we combine compost with synthetic fertilizers, however they are less dramatic and often complex, suggesting other interactions are involved.

Changing to better practices may be more affordable than you think

Our next newsletter will look as the economics of conversion to better practices on broad-acre cropping and grazing enterprises. It can be more profitable than your current practice.

References and further reading

Mulvaney RL, Khan SA and Ellsworth TR. 2009. Synthetic nitrogen fertilizers deplete soil nitrogen: A global dilemma for sustainable cereal production. Journal of Environmental Quality. Volume 38, pages 2295 – 2314. Freely available on-line.