Autumn soil sampling for N levels possible, says ADAS

Sampling for soil mineral nitrogen (SMN) in autumn is as accurate as spring sampling on medium to heavy soils, according to initial results of an HGCA-funded project. But getting the sample to the lab quickly is crucial to obtaining an accurate idea of soil nitrogen supply (SNS).

Soil nitrogen supply is the most important factor influencing fertiliser N requirement,” ADAS research scientist Daniel Kindred said at a recent soil organic matter workshop hosted by HGCA and Rothamsted Research Association. “The trouble is there are major issues in predicting just how much N you have in the soil and how much can be used by the growing crop.”

Typically there will be about 100kg/ha N at the start of the spring, supplied by SMN, that will contribute to crop growth. Gauge this figure accurately and you can optimise your N input for maximum yield, safe in the knowledge that little is going down the drains. But SNS varies according to soil type, over-winter leaching, cropping history and a number of other factors.

That’s why ADAS researchers, along with others from The Arable Group (TAG) and SAC, have embarked on a four-year levy-funded project to establish best practice for estimating SNS.

The project focuses on sampling soils for SMN, which gives the most accurate estimate of SNS. Having started in November 2007, the project is now starting its third and final season. Good progress has been made in standardising lab-testing procedures.

“Major labs are very committed to ensuring consistent SMN results. Some quality control issues have been identified and remedied and testing procedures are now working well,” reported Dr Kindred.

But how a sample is stored between field and lab, and how long it takes to get there, can have a major influence on the result you get back. A study in the project sampled four different soils, which were then stored for different durations and temperatures before analysing. Generally, SMN increased with storage time, even where samples had been kept cool.

“Samples should be kept cool, but it’s critical to minimise the time between sampling and extraction – we should probably be aiming for a maximum of five days.” Work in the coming year should confirm the importance of storage duration and temperature, allowing guidance to be given.

Progress has also been made on autumn versus spring sampling. Initial data suggest autumn sampling is as good as spring on medium to heavy soils and in low rainfall areas. “Given that most sampling is currently done in spring, this could be welcome news for growers, samplers, advisers and the labs to avoid the spring rush,” noted Dr Kindred. Sampling from 0 to 60cm seems to be enough in autumn, to discount over-winter leaching, while 0-90cm gives a more accurate figure in spring.

Accounting for crop N, especially in oilseed rape, is another issue under investigation. “The OSR crop can contain more than 100kg/ha N and can be judged quite easily from measuring green area index (GAI). It’s likely that crop N is used as effectively in delivering final N uptake as soil N, but data so far is not conclusive.”

A separate HGCA-funded project has looked at crop sensing for SNS using a tractor-mounted sensor or satellite technology. “SMN sampling is notoriously laborious. So if we can accurately relate crop-sensing data to soil N supply, this could be of real value to growers,” enthused Dr Kindred.