Question
we recently laid sod (centipede) about 3 months ago. the past 2 weeks it has started turning yellow. we have an irrigation system on a well and it gets plenty of water. we fertilized it and the green spots got greener and the yellow seems to have spread. HELP!!

Answer
Is there a pattern to this yellowing? If it has a curved or circular outline, then it is likely to have a biological cause like insects or fungi. If it has straight (more or less) boundaries then it is likely to be environmental (abiotic, as we say). If the pattern is circular and fertilizer (the nitrogen part) has caused the yellowing to spread, that could point to fungus activity (Rhizoctonia or brown patch). If the boundaries are straight and fertilizer has caused the patch to spread, that could mean a nutrient deficiency. I am thinking iron. You can verify this by applying liquid iron to the yellow patch(es) and look for a greening up within days. Do not over apply the product or the grass will turn black. Also, the product stains concrete, wood etc. an ugly brown color. It is comparatively inexpensive.