Soil sensors for nitrogen availability

DM Wells & AJ Miller

The amount of nitrogen (N) available to plant roots is the main factor limiting the yield of crops. Depending on the soil type, climate and crops grown during previous years the fertiliser requirements vary within a field and throughout the year. Profitable and efficient agriculture needs to match crop N supply to the demands throughout the season. Soil mineral N is currently measured using costly labour intensive field sampling methods that require later laboratory analysis. The aim of this work was to produce robust sensors that can be used to continuously monitor the available soil water N in the field.

New soil N sensors have been developed and extensively field tested. A range of different chemical sensor cocktails have been tested in order to develop an optimal system for use in the soil. These sensors measured both the nitrate and ammonium present in the soil water and they have proven to be robust, recording continuously and reliably for up to 3 months. The measurements provided a daily measure of the N concentration that is available to crop roots but also give a measure of potential nitrate leachate. The probes can be used with standard data-logging equipment and a prototype hand-held logging system has been produced.

The sensors have been field tested on two plots of the Broadbalk winter wheat N experiment at Rothamsted with readings taken every 10 minutes throughout a year. One of these plots has had no added fertiliser since the experiment began while the other receives a high application of manure in the autumn. These measurements showed five-fold higher levels of soil available nitrate at 30 cm depth on the treated plot in comparison with the untreated plot during October and November. During the remaining winter months and throughout the spring these large differences between the two plots had disappeared. Parallel measurements of soil temperature at 30 cm showed that on both plots there was an increase in soil available nitrate that accompanied a rise in temperature from 4 to 16 C. Comparison of rainfall data and soil moisture measurements in December on the high N plot suggested that leaching rather than dilution by rainwater was responsible for some of the daily fluctuations in soil water nitrate.

The soil N probes were also used to test if they can measure the effects of changes in temperature on mineralisation rates. The temperature dependence of changes in soil water ammonium and nitrate concentration ratios provided good evidence that the probes can be used in this way.

During the project the soil N sensors have been demonstrated at several public events and to various potential end users including groups of farmers. At all these occasions the work has generated considerable interest. The sensors can provide a useful tool for farmers but they will also give a measure of nitrate leaching into drainage waters around a field. The measurements provide a single point measure within the soil profile and so they can be potentially used to provide temporal and spatial mapping of N availability in soils. More research is now needed to measure how much variation there can be in any soil and the factors that can affect this heterogeneity require identification. Furthermore, we need to identify the key depths for monitoring under any crop.

DEFRA Final Report