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UNDERSTANDING HOW TO GET THE MOST OUT OF SOIL N TESTING IN 2019

Establishing correct fertiliser applications through accurate assessment of soil Nitrogen reserves particularly important in 2019.

* Challenging growing conditions in 2018 have created a legacy of variable soil reserves

* Mild autumn and winter to date resulting in likely ongoing mineralisation of soil N

* Additionally Available Nitrogen (AAN) must be factored into fertiliser N application equation

Accurate assessment of soil Nitrogen reserves will be particularly important in establishing correct fertiliser applications in 2019 following last year’s challenging growing conditions, warns CF Fertilisers’ arable agronomist Allison Grundy.

The legacy of 2018 is likely to be high levels of variation in soil N supply that will be difficult to categorise without soil N testing, she says.

“If ever there was a time to invest in good quality soil N testing then spring 2019 is it.

“Many growers were unable to apply their full amount of fertiliser last year due to the very wet conditions in the spring and in these cases, it is likely soil reserves will be low.

“Conversely, for those who did manage to get their full application on, subsequent dry weather and lower yields are likely to have resulted in less N being recovered and utilised by the crop.”

Much of this could have remained in the soil after harvest, she explains.

“N reserves are likely to be higher than typical, especially since rainfall this autumn and the winter so far have been low to average.

“Growers who have chosen to apply organic manures to the land before establishing their autumn crops also need to understand how much organic N is likely to become available to crops in the spring."

Accurate picture of soil N reserves essential

Simple analyses of Soil Mineral Nitrogen (SMN) will be insufficient to give the true picture of total soil N supply [SNS] this year, Allison Grundy believes.

“SMN gives you a snapshot of what N is available at the time the soil sample is taken but fails to account for the amount of N that will be mineralised in the soil and taken up by the crop as the growing season progresses. This is termed Additionally Available Nitrogen [AAN].

“Accounting for AAN is always important but after a difficult season, such as 2018, it’s more important to include AAN in fertiliser calculations because it can be a sizeable quantity.

“If you don’t you could end up applying more N from the bag than necessary which has a cost financially and potential environmental implications.

According to Dr. Mechteld Blake-Kalff of Hill Court Farm Research Ltd who carry out over 7000 soil Nitrogen tests a year, AAN is particularly beneficial in fields where mineralisation is expected to be high.

“These include fields where last year’s yields were disappointing due to the drought, but also, for example, where cover crops are grown.

“SMN is a useful indication of soil Nitrogen, but including AAN in the analysis provides a lot more detail about what the true picture is.

“When soil measurements of SMN and AAN are combined with a spring crop N estimate, the overall SNS can be determined with greater accuracy.

“It is particularly useful in those soils with higher organic matter content, having manure inputs or following leafy crops like vegetables.”

But weather conditions in the next few weeks will also be important with soil type also playing a role, she says.

“Generally a cold spring means that soil mineralisation begins later and AAN will be delayed until later in the season.

“Warmer spring conditions result in earlier soil N mineralisation so plants have the opportunity to get away quicker, particularly in lighter soils that warm up faster. However, these lighter soils are also more prone to N leaching, making them at risk of nutrient loss.

“Heavier land is more able to hold on to soil Nitrogen but warm up more slowly, particularly if it is wet, so mineralisation of AAN will be later compared to lighter soils.

“But if soil conditions are too dry, then Nitrogen cannot be taken up by the plant no matter what type of soil it is.”

Return on investment - ROI

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

“The CF N-Min test is the only commercially available soil N test that measures SMN and AAN which in conjunction with an estimate of the amount of N already taken up by the plant gives the best picture of SNS.

“When you review CF Fertiliser trials in wheat which have shown yield improvements of 1.0t/ha and improved farm profitability from using the N-Min service, it is very difficult to argue against it - especially taking into account the low cost of the test.”

With growing concern over agriculture’s contribution to greenhouse gases and the developing Clean Air Act singling out urea-based fertilisers as contributing to overall ammonia emissions from agriculture, the correct product choice and fertiliser management will be critical in the years ahead, she says.

“There’s a genuine case for annual N-Min testing from a production point of view, but in the future it will become even more important as the industry strives to improve how Nitrogen fertilisers are used by the crop.

“Nitrogen fertiliser Use Efficiency (NfUE) is a measure of understanding how well crops recover Nitrogen to prevent leaching and environmental problems and this is coming increasingly under the spotlight.

“One of the best ways of maximising it is to use a high-quality source of ammonium nitrate (AN) such as CF Nitram (34.5%N) or an AN-based true granular compound where the N contained is directly available to the crop and utilised with optimum levels of efficiency."

Understanding the terms

SMN - Soil Mineral Nitrogen. This analysis gives a reflection of what has been mineralised in the soil up until the date of sampling and is a measure of crop-available soil Nitrogen.

AAN – Additionally Available Nitrogen. This is the N that will be mineralised in the soil between the time of sampling and harvest and taken up by the crop. Gives a better indication of true SNS when used in conjunction with SMN.

SNS – Soil Nitrogen Supply. SMN and AAN supplied by the soil combined with an estimate of N already taken up by the crop at the time of sampling. SNS is then used as a basis for calculating fertiliser application rates based on target yield.

NfUE – Nitrogen fertiliser Use Efficiency. The proportion of fertiliser Nitrogen recovered by the crop at harvest. This typically varies between 50% and 80% in arable crops.

 

How to test soil N accurately

The best way to get an accurate picture of Soil Nitrogen Supply (SNS) is to use a CF N-Min analysis which includes Soil Mineral Nitrogen (SMN), Additionally Available Nitrogen (AAN) and an estimate of N taken up by the plant by the spring says Allison Grundy.

“The best time to sample is in the spring before the first fertiliser application and at least six weeks after the last manure application.

“The quality of the soil sample is critical, so planning always pays dividends. Take 10 to 15 cores - up to 20 cores for fields greater than 20 ha – using a W-shape pattern across the field and avoid areas such as gateways, headlands and old manure heaps.”

It is important not to pick up any crop residue or manure and soil cores should be taken to a depth of 60cm or to the maximum rooting depth of the crop if it is shallower, she advises

“Push the auger in firmly and measure how deep it goes. If the field varies, test in a few places and take an average but always make sure you record the actual sample depth

“Once collected, the cores should be combined together, gently mixed and sub-sampled. Samples should be kept cold but not frozen and sent for analysis within 1-2 days.”

Before leaving the field, an estimate of the amount of Nitrogen already in the crop should be made.

“This can be done by counting the tillers in cereals or measuring the Green Area Index (GAI) in oilseed rape.

“Once the results are returned – usually within two weeks - the results can be fed into CF Fertilisers Nitrogen Calculator N-Calc alongside target yield to obtain the optimum amount of Nitrogen fertiliser to apply and timing guidance on applications.”