Changeable weather creating wide variations in soil nitrogen levels

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Careful assessment of soil Nitrogen will be particularly important in establishing correct fertiliser applications in 2017 following considerable variability in weather conditions over recent months, says Dr. Mechteld Blake-Kalff of Hill Court Farm Research Ltd.

Early indications suggest Soil Mineral Nitrogen (SMN) levels will be higher than expected for the time of year due to warmer than average temperatures in the autumn allowing greater mineralisation, she says.

“While this has resulted in forward crops in many parts of the country, recent colder weather has slowed down the take-up of this by plants and relatively low rainfall has prevented it from leaching so it is probably higher than average – particularly in Nitrogen retentive soils.

“But because conditions have been extremely variable across the country, particularly with regards to localised heavy rain in late autumn, there is likely to be considerable regional differences in SMN this year.”

Allowance for individual climatic conditions as well as for soil type, crop uptake and condition will also need to be made, she believes.

“Heavier land is more able to hold on to its Nitrogen but lighter soil leaches Nitrogen more readily making any significant rain a problem with regard to loss of nutrients.

“Last year, the persistently wet conditions led to many saying extra Nitrogen would be required but this was largely untrue.

“Mineralisation continued throughout the much milder than average winter so spring crop N was high and only the lightest free-draining soils were low in overall Nitrogen supply.”

Hill Court Farm Research carries out around 7000 soil Nitrogen tests a year of which up to 3000 are CF N-Min which include an estimate of Additional Available Nitrogen (AAN) as well as SMN.

In fields where mineralisation would be expected to be high, the analysis of AAN can improve the picture for many growers, Dr. Mechteld Blake-Kalff says.

“Standard SMN analysis gives a reflection of what has been mineralised in the soil up until the date of sampling and is indicative of the soil Nitrogen available to plants – or that could be prone to leaching – at that moment in time.

“AAN is the Nitrogen that will be mineralised in the soil between the time of sampling and harvest.

“When measurement of SMN and AAN are combined, this allows the overall soil crop N balance to be determined with greater accuracy, particularly in those soils with higher organic matter content, having manure inputs or following leafy crops.”

CF Fertilisers’ arable agronomist Allison Grundy believes the AAN estimate in the company’s patented N-Min test is essential in building up the true picture of fertiliser need.

“In years where weather conditions are difficult, on land new to combinable crops or where high levels of organic matter are used it’s the best way of working out exactly what Nitrogen you have available and, therefore, what the most economic applications of Nitrogen are subsequently.

“Only by knowing what is currently available in the soil, what is likely to become available and what the plant already has taken up can you make the right decision about how much extra Nitrogen to apply and these factors are always dependent on individual circumstances.”

Weather and soil conditions have a profound effect on AAN, she says.

“Generally a cold spring means AAN will not be available until later in the season whilst warmer conditions make it available much earlier so plants will get away quicker.

“A light soil heats up much quicker than a heavier soil one – particularly if it is wet – making AAN available earlier in the year. But if conditions are too dry, the Nitrogen cannot be taken up by the plant no matter what type of soil it is.”

Regardless of how the spring develops, there are now sufficient indications that a clear understanding of AAN levels will be essential this year, Allison Grundy concludes.

“The mild autumn and relatively dry winter so far could be leading to an accumulation of soil Nitrogen in some locations and the levels of mineralisation at the time of SMN testing could be highly variable.

“When you consider CF Fertiliser trials in wheat during 2016 showed yield improvements of 1.0t/ha and a 20% reduction in fertiliser costs from using CF N-Min, it is very difficult to argue against it – especially taking into account the low cost of performing the test.”

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