Grass turning yellow

We have fertilized and put iron on our lawn, but it gets yellower each day. The grasses are bermuda grass and fescue. We fertilized late in the spring and added iron last week. What else can we do?

A mixed cool season and warm season lawn creates some difficulties. The cool season fescue should receive less fertilization and be allowed go dormant in the heat of summer, while the bermuda grass needs to be fertilized and grows best in the hot weather. Keeping both healthy is difficult. Fertilizer needed by the bermuda creates increased disease susceptibility in the fescue when the weather is hot. Water management becomes especially critical.

As far as fertilization, apply a light application of fertilizer to the bermuda grass once or twice a month through the summer. The most important constituent of the fertilizer for growing grass is nitrogen. Other nutrients are needed as well, but the nitrogen is needed in the greatest amounts. Apply fertilizer so that one-half pound of nitrogen is applied per 1000 square feet with each fertilization. If the fertilizer contains 10 percent nitrogen, you will need to apply 5 pounds of the fertilizer per 1000 square feet to achieve the rate of one-half pound nitrogen over this area. If the fertilizer contains 21 percent nitrogen, ammonium sulfate, then only a little less than two and one-half pounds of fertilizer will be needed for each 1000 square feet. That is calculated by dividing the one-half pound nitrogen needed by the percentage nitrogen in the fertilizer.

Too little water will limit growth and health of the bermuda grass; too much water will favor fungal infection of the fescue. Irrigations which promote deep rooting of the grasses and drying of the grass crowns between irrigation will help prevent fungal attack while providing adequate moisture. The frequency of irrigation will depend on soil type - sandy soils hold less water and need more frequent irrigation. It will also depend on the permeability of the soil and the depth to which moisture can penetrate. If the soil is compacted or has an impermeable caliche layer near the surface, it will not be possible to irrigate deeply enough to develop a deep root system to allow drying of the soil surface between irrigations. Soil preparation to provide a deep root zone is important. If the soil is not compacted, irrigate to moisten the soil to a depth of eight to twelve inches. Observe the grass to determine when irrigation is needed again. Rolling of the leaves, development of a grayish cast to the leaves, and persistence of footprints in the lawn are indicators that irrigation is necessary. Initially the grass may not have roots established in the full depth of soil which you are moistening, but the grass roots will develop to greater depth within a week or two, so continue modifying your irrigation regimen while the grass is developing its root system.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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