Soil nitrogen testing yields benefits

Soil testing could allow some growers to cut nitrogen fertiliser rates by up to one-third, with no detrimental effect on yield, say experts.

With rising fertiliser prices, this represents potentially big financial savings, as well as benefits to the environment, says former Rothamsted scientist, Laurence Blake, from Hill Court Farm Research in Gloucestershire.

“There is still a lot of ‘slack’ in the system based on current fertiliser N recommendations, as people tend to want slightly more N in the soil than is needed, rather than too little.

But as fertiliser costs and restrictions (eg Nitrate Vulnerable Zones, Soil Management Plans) become tighter it will be increasingly important to improve the accuracy of soil nutrition.”

About 10% of all samples (including those outside NVZs) tested last season were at more than 170kg/ha of nitrogen (the NVZ limit) based on the determination of soil mineral N (SMN) and potentially mineralisable N (PMN).

Many growers may not realise how high levels of residual N are until they get soil tested, adds colleague Mechteld Blake-Kalff. “For those who have been able to cut N rates, it appears to have paid off.”

Field trials found that where feed wheat is being grown at fertiliser rates of 180-250kg/ha, the nitrogen balance can be up to 100kg/ha above what is required by the crop, says Dr Blake.

For higher input milling wheats (up to 300kg/ha), the balance between crop requirements and soil N is closer, but there may be scope to cut rates depending on residual N levels, he adds.

But he warns growers first to ensure levels of other soil nutrients, particularly potassium and phosphorus (copper on light organic soils), are sufficient to get the best from soil or fertiliser N.

“Generally, the balance for potassium across the country has fallen and is now negative.

This may mean less efficient crop use of available nitrogen in some locations.”

N levels higher

Commenting on analysis from last season, Dr Blake says that in contrast to many views, mineral nitrogen levels in lighter soils were higher than in heavier clay soils.

“This confirms what a lot of agronomists have been saying.

It is likely the relatively mild winter and spring, caused light soils to warm up quicker than heavy, wetter, clay, increasing mineralisation and making more N available earlier.”

Many lighter soils, such as those around the Fens, also contain more organic matter and, therefore, more residual nitrogen, he notes.

Last year’s results also highlight the possible drawback of estimating soil N availability based on the total N value of the previous season’s crop or grain N, he says.

“There is a tendency to assume this [soil/crop balance] behaves the same each year, but this year clearly proves it is not the case.”

There is some debate about whether soil cores need to be taken 30, 60 or 90cm deep.

“Measurements suggest that crop N is mostly derived from the 0-60cm soil layer throughout the growing season,” says Dr Blake-Kalff.

“But in NVZs you may need to go down to 90cm to account for the majority of available N and to assess potential losses from the 60-90cm layer”.

From an agronomic point of view this is less important and shallower samples may be more practical, she notes.

It is possible to measure the N attributed to microbial mineralisation (PMN) from the 0-30cm layer where organic residues are concentrated and estimate the potential for additionally available N (PAN), she says.

The Kemira GrowHow yearly calibration allows such estimates to be made from 0-30cm cores, she says.

“We are constantly trying to fine-tune the accuracy of residual nitrogen determination and crop N supply, as there are so many variables involved, like climate and soil type.

There appear to be agronomic and environmental benefits from determining mineral N or mineral N plus PAN.”

For more information, email Mechteld Blake-Kalff,

Upcoming webinar

What does the future of farming look like post Covid-19 and Brexit?

Register now