Improve profits with soil-specific nitrogen

A soil-specific nitrogen strategy will provide the best fertiliser returns, with the highest profits coming from prioritising sandy soils over heavier soils in the low-rainfall Mallee environment.

Six years of trials in the Mallee have analysed the profitability and risk of different nitrogen strategies on sandy dunes, mid-slopes and heavy swale soils.

CSIRO farming systems scientist Dr Therese McBeath says the trials were performed in the Mallee but the findings are likely to be relevant in many areas where variable soil types are present.

She recommends growers zone their paddocks, including soil testing to characterise their soils, and then, based on their total nitrogen budget, allocate nitrogen with priority given to sandy zones where return on investment is likely to be highest and most reliable.

Soil nitrogen supply

Dr McBeath says a key factor affecting the profitability of nitrogen applications on different soil types is the amount of nitrogen the soil can supply.

"Measurements by Dr Vadakattu Gupta indicate that mineralisation rates are linked to microbial activity, organic matter and soil moisture," she says. "This means that in Mallee systems, highly fertile swales will have the highest nitrogen supply, with light, non-wetting sands providing the lowest nitrogen to crops."

Data collected from Karoonda by CSIRO researchers have confirmed this relationship over five years (Table 1).

Table 1: Nitrogen supply potential (kg/ha/cropping season in a decile 5 year) of continuous cereal cropping swale, mid-slope and dune soils at Karoonda, South Australia (McBeath et al. 2015).

An additional factor that drives nitrogen supply is crop sequencing, with break crops fixing nitrogen to boost soil nitrogen reserves.

CSIRO researcher Mark Peoples analysed pre-season measures of soil mineral nitrogen from Clare, Maitland and Laura, SA, between 2002 and 2014 and found average concentrations of soil mineral nitrogen after legumes were between 25 kilograms of nitrogen per hectare to 35kg N/ha higher than following cereals.

"Growers should consider doing soil tests before sowing to better understand the nitrogen situation in their different soil types," Dr McBeath says.

"A few soil tests are a very cheap way to learn whether it's possible to reduce fertiliser rates and improve profitability."

Soil nitrogen response

A useful rule of thumb is that one tonne of wheat yield requires about 40 kilograms of nitrogen in the form of soil reserves, in-crop mineralisation and added fertiliser.

But this does not mean higher yielding crops always require more fertiliser.

"Fertile soil types with high water holding capacity will have the highest yield potential and therefore require the most total nitrogen, however these soils can also potentially have higher soil nitrogen reserves, so will not necessarily require higher fertiliser rates," Dr McBeath says.

Fertiliser efficiency is often assumed as 50 per cent. That is, if 10kg of nitrogen is added as urea, it is assumed 5kg of nitrogen reaches the plant. However, this factor can vary depending on soil type, with trials at Karoonda in the SA Mallee from 2010 to 2012 finding sandy dunes and mid-slopes provided higher crop responsiveness to nitrogen fertiliser compared with heavier swale soils.

"The range of factors that drive fertiliser response in different soil types, from soil nitrogen supply, to crop yield potential and fertiliser efficiency, mean that calculating crop fertiliser response can be a complex matter," Dr McBeath says.

"Software such as Yield Prophet can help growers assess their likely fertiliser response for each soil type, which can help determine how to split up a fertiliser budget."

Karoonda trial results

Trials were conducted at Karoonda from 2010 to 2015 comparing yield and profit response to different nitrogen rates (9kg/ha and 40kg/ha) applied at different times (up-front or split) on three different soil types — sandy dunes, mid-slopes and heavy swale soils.

The results (Table 2) show that while the absolute gross margins are higher for the swales, the largest benefit from higher nitrogen is on the dune, with a $1150/ha benefit in going from 9kg/ha to 40kg/ha, compared to only $230/ha on the swale.

No difference was seen in an up-front application compared to a split application in this series of trials.

"This means that on Mallee soils, there appears to be little benefit to splitting nitrogen applications throughout the season based on our experiences using rates up to 40kg/ha of nitrogen," Dr McBeath says. "Therefore, it may be more cost-effective to apply the full expected rates of nitrogen at the start of the season."

"I would expect this result may be different in other regions with different soils and higher rainfall, so I would only advise full up-front N applications to growers with similar rainfall and soil types to Mallee conditions.

"While splitting nitrogen does allow more responsiveness to the season, Mallee farmers have also reported a risk of not having adequate opportunities to top dress nitrogen across large areas."

Table 2: Cumulative gross margin (2010-2015 $/ha) from continuous wheat production comparing 9kg/ha and 40kg N/ha applied at sowing to key Mallee soil types at Karoonda. Gross margin input costs and commodity prices derived from Rural Solutions Gross Margin Guide.

A farm-wide model was developed from the trial results, comparing the different strategies over the five years from 2006 to 2010, chosen to cover a range of deciles (Figure 1). Over the five years, soil-specific nitrogen, or variable rate nitrogen, had increased net worth by $548,000, compared to $80,000 and $419,000 over the same period for low and high nitrogen strategies.

"These results show that zoning farms and paddocks by soil type and using a soil-specific strategy can significantly improve profits compared to low nitrogen strategies," Dr McBeath says.

"Soil-specific strategies can also give higher profits than high nitrogen strategies, while reducing the risk associated with high fertiliser applications in low-rainfall regions."

Figure 1: On-farm net worth across a selected run of seasons for low N (30kg/ha on all soils), high N (80kg/ha on all soils) and soil specific N (80kg/ha on dunes, 40kg on mid-slope and 20kg on swales) on a 2400ha farm at Karoonda. Growing season rainfall deciles were 2 (2006), 5 (2007), 3 (2008), 5 (2009) and 10 (2010).

The grower: Peter Maynard

• Owners: Peter, Susan, Fred and Claire Maynard
• Location: Lameroo, SA
• Area: 2000 hectares
• Annual average rainfall: 380 millimetres
• Soil types: Sandy loam
• Enterprises: Cropping
• Crops: wheat, barley, hay/lupins

Lameroo farmer Peter Maynard has used variable rate technology (VRT) to apply fertiliser for eight years and is very pleased with the results he has seen.

Since adopting VRT, Peter has noticed paddocks are a lot more even and growth more consistent despite the paddocks being split into separate zones.

Peter determines his fertiliser application rates by starting at a farm level, then breaking it down into paddocks and then zones.

"In an ideal world I would set the rates based on nutrient replacement considering what the previous crop used, but that way I could end up blowing my budget," he says. "Instead I set a fixed fertiliser budget and then after I decide my rotations I break that budget down to each paddock based on which crop I'm going to grow, then split down to each zone based on the soil type."

Peter participated in a Natural Resources SA Murray Darling Basin trial known as the Mallee Challenge in 2015. In the challenge, one quarter of a paddock was managed using Peter's VRT maps and rates, with three-quarters subject to varying treatments based on an EM map.

Peter's VRT strategy improved profits of $18/ha compared to the best single-rate treatment. The 'ideal' strategy, combining the best results on each different soil type, could increase profits by a further $27/ha.