Deep Soil Sampling

Applications of organic manure on arable land, particularly in Nitrate Vulnerable Zones (NVZs), will increase the need to improve the accuracy of nitrogen recommendations. According to East Anglian based independent consultant Iain Turner, one way of doing this would be by soil testing: “The new NVZ rules require growers to follow the Codes of Good Agricultural Practice which refer to DEFRA fertiliser recommendations in Bulletin RB209. These recommend that soil should be sampled to a depth of 90cm - the Soil Mineral Nitrogen (SMN) analysis method - where high or uncertain amounts of soil nitrogen can be expected.”

These recommendations state that direct measurement and estimation of the key components of Soil Nitrogen Supply (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 fertilizer decisions. “Soil sampling company Envirofield, who work in close cooperation with Anglian Water operate Geonor deep sampling equipment mounted on six-wheel ATVs and can test fields quickly and representatively to give an accurate measure of SMN, usually the largest contributor to SNS.

By using data collected in the autumn and analyzing the crop appearance in the spring, Envirofield are also able to estimate the amount of N taken up in the autumn,” says Iain. “Where the organic matter content of the soil is low to average, the DEFRA recommendations suggest 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 fertilizer nitrogen requirements can be calculated.”

The cost of the complete service, from sampling to report, would be covered by a saving of only 10kgN/ha or an additional wheat yield of 44 kg/ha on a 15ha (37 acre) field or indeed by an improvement in crop quality. “Samples taken in the spring of 2004 showed
variations in soil mineral nitrogen levels between 25 kg/ha and 400 kg/ha of available N. Where organic manures had been used in the recent past the levels were usually at the higher end of the range and more difficult to predict with any accuracy.

The location in the soil profile was also more variable which would influence a crop’s ability to utilize the nutrient as the spring growth begins. The high level of lodging in the summer of 2004 emphasises the need for accuracy in nitrogen inputs. Deep soil sampling is integral with this” concludes Iain.