18 January 2002

…and a stimulus to soil testing…

ANY extension to the Nitrate Vulnerable Zones will increase interest in ways of improving the accuracy of fertiliser N recommendations, not least by way of soil testing.

That is the belief of East Anglian-based independent consultant Iain Turner.

NVZ rules require growers to follow the Codes of Good Agricultural Practice for the Protection of Air, Soil and Water, which refer to DEFRA fertiliser recommendations in Bulletin RB209, he notes.

"RB209 recommends soil sampling to 90cm (the SMN analysis method) where high or uncertain amounts of soil nitrogen can be expected.

"It states Direct measurement and estimation of the key components of SNS (SMN, total crop N and mineralisable N) will usually result in the most accurate assessment of the amount of soil nitrogen available for the crop, and, therefore, the most accurate nitrogen fertiliser decision."

Envirofield operates Geonor deep sampling equipment mounted on a six-wheel ATV and can test fields quickly and representatively to give an accurate measure of SMN, usually the largest contributor to SNS, says Mr Turner.

The next largest contribution for autumn sown crops will be the N taken up over autumn and winter. Envirofield can estimate this using a large amount of data collected relating crop appearance in spring to the amount of N already taken up."

The smallest contribution will be from soil organic N that mineralises in late spring/early summer, he says.

"Where the organic matter content of the soil is low to average, RB209 suggests that this amount of N can be ignored. For higher organic matter soils, Envirofield use a typical value derived from trial work.

"From this data we can produce an SNS index which relates directly to the tables in Bulletin RB209 and from which crop fertiliser nitrogen requirements can be calculated."

Cost of the complete service from sampling to report would be covered by a saving of only 10 kg/ha of nitrogen or an additional wheat yield of 44 kg/ha on a 15ha (37-acre) field, he estimates.

"Benefits of accurate N application to crop quality could be added to this." &#42

Deep N sampling

&#8226 NVZ extensions likely spur.

&#8226 RB209 advocating SNS.

&#8226 Accurate SMN from soil testing.

&#8226 N take-up & organic release back-up.

&#8226 NVZ extensions likely spur.

&#8226 RB209 advocating SNS.

&#8226 Accurate SMN from soil testing.

&#8226 N take-up & organic release back-up.

Soil testing before top-dressing could be given a boost by the NVZ proposals. It helps show how much N is available so rate adjustments can be made.