Nitrogen options post-drought

Fert Options
with Robin Boom
Agronomic Advisory Services

I’m generally not a great artificial nitrogen advocate on pasture, particularly when there are other glaring soil deficiencies, as money spent on nitrogen fertilisers often eats into the budget of the other elements required for pasture growth. Addressing known soil deficiencies of P, K, S, lime and trace elements will give a much better financial return in terms of the cost per kg of dry matter grown long-term compared to nitrogen. However, artificial nitrogen can give a temporary ‘get out of jail’ card when feed demand is greater than supply, and seems to have a kind of antifreeze effect in reducing frost burn.

As a result of drought, many farms are going into winter with a lot less cover than ideal. Research institutions and farm advisors are quick to point to nitrogen as the cheapest form of dry matter to fill this feed gap, so the question is what are the best products to consider? Straight urea is the cheapest form of nitrogen at about $560 for 46 per cent N, which works out to be about $1.20/kg N. SustaiN is 10 per cent more expensive, but has half the volatilisation rates of Urea. If 20 per cent of the N in urea volatilises, then the extra cost of SustaiN is justified. But this is unlikely to happen between late-autumn and late-spring. Volatilisation is mainly an issue during drier sunny periods. To avoid volatilisation, apply urea either early morning or in evenings, or just before, during or after rain.

Gibberellic

Dissolving urea in water and spraying it on with gibberellic from late-autumn to mid-spring can be the cheap source of dry matter, but with gibberellic acid it should ideally be applied within five days of grazing and grazed again within 45 days to get the benefit. During late-spring, summer and autumn, plants produce plenty of gibberellins naturally, so its benefit at these times is questionable.

DAP (Di Ammonium Phosphate), CAN (Calcium Ammonium Nitrate) and SOA (Sulphate of Ammonia) products lose very little N through volatilisation, so are not so weather-dependent on when they should be applied. During the summer-autumn drought there will have been an accumulation of sulphate sulphur from the oxidisation of organic matter, so sulphate of ammonia will be of little value going into winter compared to urea. Coming out of winter and during spring is the best time to consider sulphate of ammonia as soil sulphur levels are normally at their lowest then. Phased N from Ballance is a good autumn option on soils with known sulphur deficiencies, as sulphur is in elemental form and not subject to leaching like sulphate sulphur, so should be working during late-winter and spring when sulphur levels are lowest. As straight nitrogen sources, SOA and Phased N work out to be between $1.90-$2.35/kg depending on supplier. But when sulphur is considered they can work out to be $1.20-$1.70/kg DM depending on supplier.

DAP

DAP should really only be considered when phosphate and N are required. DAP has 20 per cent phosphate, which at $3/kg is worth $600, so when being sold for $700/tonne the N only costs $0.55/kg N of if being sold for $800/tonne costs $1.10/kg. And being high analysis can give significant savings on cartage and spreading compared to some low analysis phosphate fertilisers. MAP (Mono Ammonium Phosphate – sometimes sold as Ammophos) has 22 per cent phosphate, which is worth $660/tonne, so when being sold for $700/tonne, the N cost is $0.40/kg. But if being sold for $900/tonne the N cost is $2.40/kg N. MAP is the most common fertiliser used in parts of Australia, but has been rarely used here, mainly because of its high price from the co-ops. During the last few years there have been several private importers bringing in SOA and DAP for around $100/tonne cheaper than the two big fertiliser co-ops, whereas MAP is up to $200/tonne cheaper.

CAN and ASN (Ammonium Sulphur Nitrate) both have nitrate nitrogen, which is very susceptible to leaching but can be of value in cooler conditions when ammonium nitrogen uptake is slowed. Because of this, ASN has historically mainly been sold in the South Island where its elemental sulphur has particular value. CAN is popular in horticulture with its two forms of nitrogen and calcium make-up, but as straight nitrogen sources, CAN and ASN are quite expensive at $2.40/kg N, double the cost of urea.

Horticultural crops

For horticultural crops, cereals and vegetable growing the use of artificial nitrogen is generally a must if you want high production. Vegetable growers are often very high N users, and a good crop of maize and crops like avocados are very N responsive. However N leaching rates are very high for vegetable and avocado growers, and with increasing awareness of environmental degradation from high N inputs some of these practices are coming under more public scrutiny. Similarly, for dairy farmers with high stocking rates and N inputs, there is increasing regulatory pressure to lower these, so water quality is improved. Artificial nitrogen does nothing positive for the soil. It increases acidity, lowers soil organic matter, lowers moisture retention, removes positive cations from soil, increases shallow rooting and pasture pulling problems and produces greenhouse gas, nitrous oxide. But it can be a short-term ‘get out of jail’ card.

Robin Boom, CPAg, member of the Institute of Professional Soil Scientists. Phone: 0274448764.

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