Category: Lawn and Garden

A productive and lush lawn requires some fertilizer every now and then. The major plant nutrients required for grass lawn are nitrogen (N), phosphorus (P), and potassium (K). Nitrogen is the nutrient required in the largest amount, although too much nitrogen can create other problems. A general rate of one (1) pound nitrogen per 1,000 square feet is adequate for most grass lawns, but some more intensively managed lawns may require more nitrogen. The total annual nitrogen budget should be split through the year according to season (Table 1). Common cool-season grasses in lawn mixtures include Kentucky bluegrass, ryegrass, and fescues.

Table 1. Nitrogen fertilizer guidelines for established cool-season grass lawn.
Maintenance Intensity	Early Spring Mar – Apr	Spring May – June	Summer July – Aug	Early Autumn Sept	Total Annual N
	————————- lb nitrogen per 1000 square feet ————————-
Low, no irrigation	0.5	0.5	0	0.5	1.5
Medium, with irrigation	0.5	1.0	0.5	1.0	2.0
High, with irrigation	0.5	1.0	1.0	2.0	4.5
Source: Bigelow, C. A., J. J. Camberato, and A. J. Patton. 2013. Fertilizing established cool-season lawns: Maximizing turf health with environmentally responsible programs. Purdue Univ. Ext. Circ. AY-22-W. Purdue Univ., West Lafayette, IN.

The nutrient application rates given in Table 1 are the actual nutrient rates. To calculate how much fertilizer product you require, you will convert the nutrient rate to fertilizer rate, using the labelled fertilizer analysis. The fertilizer analysis label reports the nitrogen-phosphorus-potassium concentration of the fertilizer product. A product with 12-4-8 analysis contains 12% N, 4% P₂O₅, and 8% K₂O. To convert 1.0 lb N/1000 sq. ft, you divide the nutrient requirement by the fertilizer analysis (12% N), thus 1.0/0.12 equals 8.3 lb fertilizer/1000 sq. ft. The application rate of 12-4-8 fertilizer is 8.3 lb/1000 sq. ft.

A soil containing ample nitrogen may require less nitrogen fertilizer. If soil test nitrogen is more than 50 lb/acre nitrate-N (0 to 6 inch soil depth), the next nitrogen fertilization may be skipped. The soil test nitrogen value of 50 lb/acre nitrate-N is equal to 1.0 lb/1000 sq. ft nitrate-N.

Late fall is an optimal time to fertilize lawn, when grass growth has nearly stopped but before winter dormancy. Avoid fertilizing during hot summer months (July and August), unless you have ample irrigation. Controlled-release nitrogen fertilizer products applied in May and September help prolong nitrogen release to grass during critical growth periods in spring and fall.