You might double check your source. Lawn fertilizer is a $5B industry, but it is usually priced at a dollar a pound, and a 50 lb bag might have 5-10 lb of N and 5-10 lb of P. USDA numbers for fertilizer use by crop are available, and based on those it looks to me like lawns account for less than 10% of the national fertilizer consumption. Corn uses 50%.

    http://www.epa.gov/greenacres/wildones/handbk/wo8.html
    http://www.lowes.com/cd_Fertilize+Your+Lawn_624094472_
    http://www.ers.usda.gov/data-products/fertilizer-use-and-price.aspx#26720