FERTILIZER NITROGEN ADDITION TO WINTER-WHEAT CROPS IN ENGLAND - COMPARISON OF FARM PRACTICES WITH RECOMMENDATIONS ALLOWING FOR SOIL-NITROGEN SUPPLY

Measurements were made during 1987/88 on 20 winter wheat crops grown i n Essex, four on each of five soil types - sandy loam, sandy silt loam , silt loam, calcareous clay loam and clay loam - where winter wheat, dried peas, winter faba beans or winter oilseed rape was the previous crop in 1986/87. The sites had a wide range of soil mineral N (SNS), 4 0-198 kg/ha N, as NH4 plus NO3, in the 0-90 cm soil profile. Optimum g rain yield and fertilizer N could not be estimated but yields measured in the absence of fertilizer N enabled evaluation of the use of SNS o n its own and the response to fertilizer N (187 +/- 32 kg/ha). For cro ps receiving fertilizer N, each tonne of grain was associated with 35 kg of total available N in the soil profile (SNS + fertilizer N); a li ttle over 63% of this N was found in the above-ground parts of the cro ps at harvest. Fertilizer N requirement was predicted as: Fertilizer N needed (kg/ha) = [35 x predicted grain yield (t/ha)] - SNS (kg/ha) An arbitrary assumption of 8 t/ha grain for every site (fortuitously the same as the average of 8.07 t/ha measured in crops given fertilizer N ) showed that differences between added and predicted amounts of N dif fered by c. 30 kg/ha for only seven of the 20 sites, mostly because of large SNS or yields much less than 8 t/ha. Measuring SNS and using th e formula would be justified in the majority of winter wheat crops, pr ovided reliable yield estimates could be made in time.