WithTheGrain: Long-term thinking key to nitrogen supply challenge

Author: | Date: 13 Jul 2018

Heavy application of fertiliser nitrogen can leave reserves in the soil for the next season, but that is often not desirable from an economic or environmental standpoint.

Maintaining the nitrogen supply capacity of soil can be imperative in ensuring long-term productivity and profit for grain producers.

This, combined with the ability to maintain and predict nitrogen provision from your soil, can lead to more profitable grain production due to a reduction in fertiliser costs.

CSIRO research scientist Dr Jeff Baldock says studies have shown that, on average, cropping systems are mining soil organic matter and nitrogen reserves, and reducing nitrogen capital in soils.

“As nitrogen capital declines, the ability of a soil to continue to supply the nutrient will be reduced and it creates a greater reliance on fertiliser application to achieve rainfall-limited potential yields,” he says.

“As fertiliser nitrogen requirements increase, growers will have a greater dependency on an input with increasing costs.

“Additionally, application of fertiliser nitrogen can contribute to soil acidification. Increasing fertiliser nitrogen application rates will require increased lime additions for neutral to acid soils.”

Factors which influence the availability of soil nitrogen to crops include soil properties, climate and on-farm practices.

Agricultural management practices such as previous fertiliser nitrogen application rates, crop and pasture types, and residue loads can influence the nitrogen supply capacity of soils.

Heavy application of fertiliser nitrogen can leave reserves in the soil for the next season, but that is often not desirable from an economic or environmental standpoint.

Crop types are also a factor, according to Dr Baldock.

“We know that different grain legume crops leave different amounts of nitrogen behind, depending upon the crop type, the amounts of biomass produced and grain removed” he says.

“Also, there seems to be more nitrogen available in the system the year after growing a canola crop than found after cereals.

“We’re not exactly sure why that happens but I’ve seen it enough times to think that a flush of nitrogen can be provided to the next grain crop after canola.”

High stubble loads after a bumper year and decomposing roots and stubble can also alter nitrogen availability.

For soil properties, the content and composition of organic matter, soil texture, soil depth and soil biology are all factors which can alter the amount of available nitrogen.

The vast majority of nitrogen in the soil is part of the organic matter, and as this decomposes, mineral nitrogen is released, making it available for uptake by plant roots. Therefore, the soil’s organic matter content is a good indicator of its ability to supply nitrogen to plants.

Figure 1: Forms, fluxes and transformations of nitrogen in soil. (Source: Baldock et al. 2018)

Dr Baldock says the main reason organic matter and nitrogen stocks in Australian farming soils were declining was because more nitrogen is extracted from the system via harvested agricultural products than what is added by legumes and fertilisers.

“Fertiliser nitrogen tends to be applied to meet crop demand, not replace all that nitrogen harvested in grain that leaves the farm, and this approach often leaves the soil in deficit,” he says.

“Contributions from legume pastures have also reduced, mainly due to a drop in the frequency of their incorporation into rotations, especially in low rainfall areas.”

Other processes such as leaching of available nitrogen after large rainfall events, denitrification of nitrate due to prolonged periods of wet soil, and volatilisation of granular urea on high pH soils can reduce the effectiveness of the soil nitrogen.

Dr Baldock says the result of progressive mining of nutrient stocks will be an increasing gap between the amount of nitrogen required by grain crops and the quantity that can be supplied by the soil.

That causes a reliance on increased fertiliser application, leading to increasing costs to achieve desirable yields and less profitability.

According to Dr Baldock, assuming all other variable costs remain the same, the economic optimum yield (where marginal benefit from extra yield equals marginal cost of the fertiliser - Figure 2a) will decline as the ability of a soil to supply nitrogen decreases (point A versus point B in Figure 2b), as will profits (Figure 2c).

Figure 2. Changes in (a) fertiliser nitrogen use efficiency (b) grain yield and the profit optima and (c) profitability of grain production with increasing fertiliser N application rates for soil with a low (solid line) or high (dashed line) nitrogen supply capacity. (Source: Baldock et al. 2018)

“If we keep relying on more and more fertiliser, declines in fertiliser nitrogen use efficiency with increasing inputs is going to move the profit optimisation point to a lower yield,” Dr Baldock says.

“Being forced to target a lower yield might mean net profitability could drop, as well as productivity.”

Maintaining soil organic matter and nitrogen supply capacity can help ensure long-term productivity, which is ultimately better than the short-term approach of using high applications of fertiliser nitrogen to satisfy yield targets and boost annual profits.

Working out the supply capacity of the soil and therefore knowing how much fertiliser nitrogen to apply is important for grain growers, though it is not easy due to different paddocks experiencing different annual inputs, extractions and losses of nitrogen.

Dr Baldock recommends growers complete annual net nitrogen balance calculations to define how stocks are changing due to on-farm practices and climate variations.

Net nitrogen balance models and rules of thumb exist but it is an area which needs more work to get right, according to Dr Baldock.

“The models and rules vary significantly in complexity and are good for guidance,” he says.

“Models are likely to provide the best estimates, however they should define risk – particularly that associated with climate.

“From a research point of view, predicting how much nitrogen comes from a soil during a growing season remains a big question.”

Dr Baldock says although increasing soil organic matter and nitrogen stocks is encouraged, temporary periods of mining nutrient stocks are acceptable, as long as it is followed by a rebuilding phase.

Several studies have shown that the inclusion of grain legume or pulse crops helps reduce the decline in soil organic matter in cropping systems, but an extended phase of pasture legumes is required to build soil organic matter.

He says growers should think about the future, saying a growing reliance on external factors like the cost of fertiliser nitrogen could leave producers in a vulnerable position.

“The economics of grain production tend to be completed on an annual basis – growers and advisers need to additionally start thinking of longer term economics,” he says. “What will the impact be in 10 or 20 years if the soil nitrogen stock is run down? What is the future value of maintaining soil nitrogen stocks?

“We need to be really careful we’re not robbing the future to get higher productivity and profit now.

“In an annual economic analysis, the solution will likely be to maximise the extraction of nitrogen from the soil, because that’s going to give growers the best profitability in that year.

“It is important for growers not to erode something they have some control over – that is, soil nitrogen status – to becoming, in the extreme, dependent upon fertiliser nitrogen which is one of their biggest costs.”

There are many management options for growers looking to halt the downward trend of soil organic matter and nitrogen stocks, including adding more fertiliser and using biological fixation to a greater extent.

Dr Baldock says incorporation of legume pastures or pulses as break crops and fertilising to a higher level than just for crop yield were such options.

“Growers tend to supply the minimum amount of fertiliser nitrogen required to grow a crop in order to maximise profit in that year,” he says.

“Fertiliser trials have done a good job at defining such values.

“But if you turn this around, it also means that growers are maximising the amount of nitrogen that is extracted from the soil.”

Dr Baldock says the grain industry should not just think about agronomic efficiency of fertiliser nitrogen, but also consider the efficiency of retaining fertiliser nitrogen in the system.

“If fertiliser retention efficiency is high, the nitrogen not used by a crop in a given year will contribute to building nitrogen capital in the soil and become available in future years,” he says.

“It is likely to be retained in the form of organic nitrogen and then, in essence, growers will be converting added fertiliser into a slow release form that can be supplied to future crops in a manner that is usually more synchronised to crop demand.”

More information:

Jeff Baldock, 08 8303 8537, jeff.baldock@csiro.au

Useful resources:

Fertiliser expenditure in the Australian Grains Industry

Nitrogen – crop nutrition and application

Budgeting crop nitrogen supply better estimation better decisions

Nitrogen dynamics in modern cropping systems

Factors influencing nitrogen supply from soils and stubbles