The effect of fertiliser and soil nitrogen on the overall uptake of nitrogen in the plant, and the grain N content of spring-sown malting barley

Abstract

1. Work was carried out over a 4 year period from 1987 to 1991, to study the effect of nitrogen on the yield and grain nitrogen concentration of spring barley grown for malting. The research undertaken can be divided into two main areas:

(a) The effect of the rate, timing of application and the form in which fertiliser nitrogen was applied on nitrogen uptake and grain nitrogen content of spring barley; calcium nitrate, ammonium sulphate and ammonium nitrate fertilisers were applied at rates up to 150 kg N/ha at sowing, with some treatments split between sowing and emergence, or between sowing and tillering.

(b) The assessment of two chemical extraction techniques used to determine the potentially mineralisable native soil nitrogen, and its likely uptake into the crop.

2. The form of fertiliser nitrogen applied had little effect on grain nitrogen concentrations, except under dry soil conditions when concentrations were higher using calcium nitrate fertiliser.

3. Fertiliser applications in the form of calcium nitrate improved grain yields at low fertiliser rates, but at rates nearer recommended levels there was little difference in yield between fertiliser forms.

4. Split or late applications of 120 kg/ha fertiliser nitrogen only improved yields when applied as calcium nitrate, and then only when early applications had been followed by heavy rain, increasing the risk of leaching losses.

5. In 1989 split applications at the lower rate of 90 kg N/ha improved yields to the equivalent of those obtained with 120 kg N/ha applied at sowing. There was no increase in grain nitrogen concentrations.

6. At low fertiliser rates, the efficiency of recovery of fertiliser nitrogen (15N) in the plant shoots was greater when applied as calcium nitrate than when applied as ammonium sulphate or ammonium nitrate. Efficiency of recovery fell at higher rates in the calcium nitrate treatments, but rose in the ammonium sulphate treatments. Under the dry soil conditions in 1989 the efficiency of recovery was significantly increased in all fertiliser treatments.

7. Uptake of fertiliser nitrogen was rapid in the calcium nitrate and ammonium nitrate treatments, usually reaching a maximum by flowering. There was evidence of losses between flowering and harvest of up to 26 kg N/ha of fertiliser nitrogen previously taken up by the crop. This could have been due to gaseous losses from old plant tissue or through root exudation. There was little evidence of losses in the ammonium sulphate treatments in which plant uptake of fertiliser nitrogen was slower, but continued several weeks longer than the other fertiliser forms.

8. The uptake of soil nitrogen (unlabelled N) in the calcium nitrate treatments remained constant over the range of rates and timings of fertiliser application. There was evidence of increasing uptake of soil nitrogen with increased rates of ammonium sulphate fertiliser at several sites. This appeared to be due to 'pool substitution' of 15N-labelled fertiliser nitrogen with unlabelled soil nitrogen by soil micro-organisms. This required complete mixing of fertiliser and soil nitrogen which only occurred at sites with more moist soil conditions. Therefore there was no evidence of a real priming effect of fertiliser nitrogen applications increasing the rate of mineralisation of soil organic matter.

9. Uptake of soil nitrogen was less rapid than fertiliser nitrogen before flowering, but continued right up to harvest in most treatments. Prior to flowering uptake was often greater in the ammonium sulphate treatments which again indicated the probable occurrence of pool substitution of 15N-labelled fertiliser.

10. Late uptake of soil nitrogen occurred despite the amount of mineral nitrogen in the soil having fallen to pre-fertilisation levels. This suggested the occurrence of net mineralisation of soil organic matter up to harvest.

11. The most significant factor in determining total nitrogen uptake in the crop was the soil on which the barley was grown, rather than any of the fertiliser management treatments studied. Soil nitrogen uptake was significantly more variable between sites than fertiliser nitrogen uptake, despite the similar cropping histories at most sites.

12. The variation in soil nitrogen uptake was derived mainly from differences in the mineralisation of soil organic matter over the growing season, rather than from the amount of mineral nitrogen in the soil at sowing.

13. Good correlations were found on all ADAS N-Index zero soils, between soil nitrogen taken up in the plant and values obtained using the chemical extraction techniques for measuring potentially mineralisable nitrogen.

14. Soil organic matter was not a good predictor of potentially mineralisable nitrogen, but when used in conjunction with estimates from the chemical extraction techniques correlations were better than using chemical extraction predictions alone.

15. The correlation between soil mineral nitrogen at sowing and soil nitrogen uptake was even better than that with the chemical extraction techniques; soil mineral nitrogen at sowing and extractable nitrogen were also highly correlated, but this was not so for mineral nitrogen measured in January.

16. Suggestions for future work are as follows:

(a) Validation of the initial results from the chemical extraction techniques over a wide range of soil types.

(b) Investigation of whether the time of sampling will alter the estimated potentially mineralisable nitrogen. Earlier times for sampling will be necessary if the technique is to be used to advise farmers on modifications to standard recommended rates of fertiliser application.