A biological method for evaluating N availability and forecasting N fertilizer needs of wheat (Triticum aestivum L.) is proposed.

Spring wheat plants were grown in containers with 8 kg of soil from the field under consideration. Total N uptake, as determined 60 days after sowing (at the beginning of heading), indicated the potential response of the wheat crop in the field. The test of the method's suitability for practical use was based on the response of the wheat crop to N application in the field.

The total N (in milligrams) taken up by the test plants was designated as the nitrogen index of the soil (NI). On the mineral loess soils of the semi-arid Israeli Negev, wheat responded significantly to N fertilization only in fields with NI lower than 100.