Sulphur deficiency (S)
Diffuse yellowing on youngest leaves (cf. N deficiency starting on older leaves).
Treatment: Foliar application of EPSO Top®. Korn-Kali® with 5% S and ESTA® Kieserit gran. with 20% S are excellent choices for preventive applications, as the contained sulphur is fully water-soluble and is therefore immediately available to plants.