Arable News

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Improving measurement of soil nitrogen supply boosts profitability

Accuratelydetermining soil nitrogen supply (SNS) this spring could enable nitrogen (N)applications on crops of oilseed rape and winter wheat to be reduced by anaverage of 60kg N/ha, representing a saving of 46/ha at current prices, onfields where a high level of additional available nitrogen (AAN) is expected,such as where organic matter is high or where organic manures have been used.Across all fields the average saving would be 30kg N/ha or 23/ha. Nitrogenlosses to the environment could also be greatly reduced, according to a leadingresearcher in this field.

Dr MechteldBlake-Kalff of Hill Court Farm Research Ltd says that recent trials comparingthe estimated N requirements for winter wheat against the optimum N levelidentified post-harvest showed that predicted N requirements were within50kg/ha of this level on only 25 of the 50 fields surveyed, primarily due toinaccurate estimation of SNS. Although economic losses resulting fromsub-optimal crop performance were small in cases where N applications werewithin 50kg/ha of optimal levels, larger errors were more serious in terms oftheir adverse impact on yield and profitability, with under-fertilising andover-fertilising causing similar losses. Dr Blake-Kalff states:

The standard soil mineral N (SMN) sampling method only includes an estimate of additional Nmineralisation, so a more accurate determination of AAN in the soil has thepotential to provide farmers with considerable cost saving and environmentalbenefits. However, experiments to determine the Potentially MineralisableNitrogen (PMN) fraction in the soil have often shown a poor relationshipbetween the mineralised pool and crop N offtake. This is because the incubationmethod used results in relatively high N values which do not reflect fieldconditions in a temperate climate.

Determining theamount of N which is mineralised from organic matter from the time when soilsampling is carried out in the spring through to harvest provides scope toimprove the accuracy with which the level of AAN is assessed. To quantify this,Dr Blake-Kalff and Dr Laurence Blake examined data from three research projectsin the UK. These comprised long-term field trial sites used by GrowHow UK for Nfertiliser monitoring, a three-year project conducted by GrowHow and Agrii tomonitor N changes throughout the growing season in oilseed rape, together withthe three-year HGCA SNS Project which was managed by ADAS.

Their study foundthat including data for AAN, which is a feature of the GrowHow N-Min nitrogenmanagement service, improved the prediction of SNS or fertiliser N requirementwhen measured against N offtake at harvest, such that for all soil types SNSexplained about 62% of the variation in crop N uptake.

The projectconcluded that where the AAN is relatively high, namely where the soil organic matter was greater than 5% and the carbon:nitrogen (C:N) ratio was 10-15,considerable reductions in N fertiliser applications could be made without anysignificant reduction in crop yield. This was particularly evident in crops ofoilseed rape and wheat. For example, by accounting for AAN in wheat, fertiliserapplications could be reduced by an average of 60kg N/ha,equivalent to a 46/ha saving at current fertiliser prices, although maximumsavings could exceed 100kg N/ha with correspondingly greater financial savings.

GrowHow arable agronomist Allison Grundy states: This work confirms that the GrowHow N-Minservice, which incorporates the GrowHow Nitrogen Calculator (N-Calc), enablesfarmers to accurately determine fertiliser N requirements, optimise cropperformance and improve their environmental credentials. N-Min is the onlyNitrogen test to measure both SMN and AAN, providing farmers with an accurateprediction of the total nitrogen that will become available to crops throughoutthe growing season. It is this dual approach which makes it uniquely accurate.


  • Written by: Farmers Guide
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