Your crop is Sulphur deficient.

• Sulphur is essential for synthesis of sulphur-containing amino acids and is therefore vital for protein synthesis.
• Sulphur aids in seed production and producing the chlorophyll necessary for plants to carry out photosynthesis.
• Sulphur aids in increased nitrogen use efficiency.
• Sulphur is a component of vitamin B1 (particularly for cereals and legumes).
• Important for production of innate plant defence substances (phytoalexines, glutathione).
• Sulphur fertilization is increasingly common because higher yielding crops are taking up and removing more S from soil as harvested products.
• Form used by plants: SO4+2 (sulphate)
• Both solution- and adsorbed- sulphate are the forms available for plant use.