I don't believe it is nute burn. Nute burn will affect older leaves and usually just the tips will turn brown. Your PH might be off and you may be lacking nitrogen. With nitrogen deficiency plants will exhibit lack of vigor, slow growth and will be weak and stunted. Quality and yield will be significantly reduced. Older leaves become yellow (chlorotic) from lack of chlorophyll. Deficient plants will exhibit uniform light green to yellow on older leaves, these leaves may die and drop. Leaf margins will not be curled up noticeably. Chlorosis will eventually spread throughout the plant. Stems, petioles and lower leaf surfaces may turn purple. I hope this helps... Happy growing!!