All change for nitrogen rules

Under the Essential Freshwater package, the amount of synthetic nitrogen fertiliser applied to pastoral land is now capped at 190kg/ha/year as a permitted activity

Molybdenum is required by all plants to turn the nitrate they have absorbed into amino acids and proteins.

By Karen Trebilcock

July 1 marked the start of the Government’s new rules on nitrogen.

Under the Essential Freshwater package, the amount of synthetic nitrogen fertiliser applied to pastoral land is now capped at 190kg/ha/year as a permitted activity (requiring no resource consent).

The cap applies to the average amount of fertiliser applied across a farm but also to every hectare of pasture and cannot be offset by non-grazed land.

You can apply more nitrogen on forage crops but only if you offset it by applying lower amounts on pasture. The cap does not apply to arable or horticultural crops.

The changes have been a long time coming and many dairy farmers already use way less nitrogen than the cap.

However, for those who have become reliant on nitrogen, not just to fill feed gaps but for year-round production, there must be a change in farming practices or get ready to apply for costly consents.

And changing from high-N to low-N use is not just a matter of using less nitrogen fertiliser. Nitrogen-fixing clover needs time to re-establish, as much as two to three years, and pasture growth will suffer until that happens.

Strategic use of nitrogen fertiliser has always been important but now more than ever.

Apply it only when the soil temperatures are between 6C and 16C which is when pastures will be actively growing. N is only taken up by growing plants and if not utilised quickly it leaches through soils or is lost into the atmosphere.

Little and often, especially in high rainfall areas, is best.

If there are no growing plants don’t apply it, waiting for seed strike and then putting it on is the best option.

Don’t put it on areas in a paddock that you know are already high – where stock camp, gateways and around water troughs. And look at skipping paddocks which are sprayed with effluent.

Use coated nitrogen products as they are less likely to be leached or lost to the atmosphere before they’re absorbed by plants.

Gibberellic acid (GA) can also be used. It’s not a fertiliser but a growth hormone found in plants. When applied with N, if conditions are right, it can increase the response rate.

But there are other things you can do as well and soil and herbage tests are the way to start. Yellowing, slow-growing pasture can be due to lots of reasons – not just a lack of nitrogen.


Nitrogen is held in soil in two ways – mineral N and organic N. Mineral N is the stuff plants can use and includes nitrate and ammonium.

Organic N can’t be taken up by plants (with a few, small exceptions) and it’s what makes up 98% of nitrogen in soils. Soil microbes, when moisture levels and soils temperatures are right, convert organic forms of nitrogen to mineral forms when they decompose organic matter and plant residues in a process called mineralisation.

These soil microbes are your livestock under the ground and looking after them should be as important as looking after your livestock above the ground.

Building organic N should be the aim of every farmer as it will grow more grass year on year.

A total nitrogen soil test, down to 75mm, shows the amount of mineral and organic N. As organic matter changes are usually slow, it only needs to be done every four to five years.

Once you know the areas low in organic N, and so potentially mineral N, they can be targeted for nitrogen fertiliser applications.

But nitrogen does not work on its own, sulphur is one element that helps it make a difference. Good growth rates can happen with low-rate nitrogen applications if sulphur levels are where they should be.

Sulphur is often overlooked and is usually lacking in New Zealand soils. Pastures deficient in sulphur appear pale green or yellow and it’s worse in spring when winter’s wet and cold conditions have leached it out of soils.

Sulphate S is the form of sulphur that plants can use but it’s also the one that leaches the easiest so the type of sulphur you put on and when is important.

Use elemental S in autumn so it stays in the soil during winter and when soil temperatures increase and bacteria becomes more active they will convert it to sulphate S providing a slow release over the growing season.

Otherwise, apply sulphate S in spring or, where there are high spring rainfalls, use a mix of both.

Another element often forgotten about is molybdenum (Mo). Molybdenum is required by all plants to turn the nitrate they have absorbed into amino acids and proteins.

Legumes, such as clovers, also need it to complete the process of nitrogen fixation from the atmosphere by the nodules on their plant roots.

About 70 years ago it was found to be deficient in NZ, especially sedimentary soils with a low pH. However, other soil types including ash, pumice and organic soils can also be low in Mo.

But farmers are usually wary of using it as high levels of molybdenum causes copper deficiency in animals, with disastrous results in the past.

Soil tests do not accurately show Mo levels so herbage tests must be done. However, if there is no record of molybdenum being applied in recent years, and clovers are small-leafed or non-existent in pasture which has not had excessive nitrogen fertiliser then it could be the answer.

Raising pH to the optimum 5.8 – 6.0 also helps the availability of Mo to growing plants.

Also remember it is only synthetic nitrogen fertiliser which is capped under the July 1 rules – don’t forget about non synthetic forms.

Effluent, compost, chicken manure and other forms of organic matter all contain nitrogen. They need different thinking and different ways of application but they still make grass grow.