Newly laid sod needs a fertilizer with an adequate amount of phosphorus to help it establish roots quickly and encourage growth. Phosphorus is an essential nutrient for plants, and it helps to promote cell division and root development. A fertilizer with too little phosphorus will result in a stunted, unhealthy lawn, while one with too much phosphorus can actually be toxic to plants. For best results, use a fertilizer that contains between 5% and 8% phosphorus. This will ensure that your newly laid sod has the nutrients it needs to thrive.