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From the Milorganite web site:

Warm-Season (Southern) Grasses
For grasses such as Bermuda, St. Augustine, Centipedegrass, Bahia, & Zoysia fertilize 3 times per year.
Instructions
Avoid applying fertilizers with nitrogen to warm-season grasses in the fall less than one month prior to dormancy or the average first killing frost date in your area. Several additional fertilizations can be made for winter overseedings, but only as needed to keep the overseeding green. Centipedegrass and Bahia grass prefer spring and summer feedings. Avoid late fall fertilization of these species to prevent possible winter kill.


Holiday Season

Application




Easter (after turf breaks dormancy)

36 lbs. per 2,500 ft2



Memorial Day

36 lbs. per 2,500 ft2



Labor Day

36 lbs. per 2,500 ft2



Early October (overseeding)*

36 lbs. per 2,500 ft2



*In regions where killing frosts can be expected, avoid nitrogen applications to warm-season grasses in the fall less than one month prior to the average date of the first killing frost. For best results, fertilize when over-seeding.

Twice a month may be too much.
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More on fertilizing Zoysia;

When and how do I fertilize my zoysia grass lawn?


Providing proper nutrition is one of the most important maintenance practices that you can do to keep your zoysia grass (warm-season) lawn healthy and attractive.

Proper fertilization provides the nutrients needed to enable grasses to survive the stresses they may incur from mowing, weeds, insects, diseases, and drought. Most importantly, it also provides that nice green color we love to see.

Improper fertilization, however, can do harm to the lawn and the environment. Too little fertilizer often results in a yellowish green, thin stand of grasses that have limited ability to recover from the stresses above.

Over fertilization of zoysia grass, especially with quickly available (water soluble) nitrogen, increases the need for mowing and can contribute to the build up of thatch.

Over fertilization may also lead to lower drought tolerance, greater susceptibility to various leaf diseases, insect activity, and leaching of excess nutrients into the groundwater.

To avoid over fertilizing, start your lawn fertility program with a soil test. It will provide the baseline information you need to do a proper job of fertilizing. It not only tells you the soil's pH, but also the levels of phosphorous, potassium, and organic matter in the soil. See �How do I test my soil?�

Established zoysia grass lawns need a higher percentage of nitrogen and very little additional phosphorous and potassium.

All bags of lawn fertilizer contain a set of three numbers that represent the percentage of primary nutrients (by weight) in each bag. The first number represents nitrogen (N), the second phosphorous (P), and the third potassium (K). Additionally, the back of the bag also lists whether the available nitrogen is slow release (water insoluble), quick release (water soluble) or a combination of the two.

The majority of synthetic lawn fertilizers contain a combination of the two types. Slow release types of nitrogen in these products include isobutylidene diurea (IBDU), sulfur-coated urea, and urea formaldehyde (a.k.a., urea form). Quick-release types include ammonium nitrate, ammonium sulfate and urea.

Organic lawn fertilizers that are formulated from sewage sludge (such as Milorganite), composted manures, soybean meal, and other natural materials are excellent sources of slow release nitrogen.

Try to choose a lawn fertilizer with a good percentage of slow release nitrogen. The nitrogen in these formulations is released over an extended period of time. This provides more uniform growth, grass color, and reduces the pitfalls of over fertilization.

So when should I apply it and how much should I use?

Turfgrasses should be fertilized in their most active periods of growth. For warm-season grasses such as zoysia grass, in the St. Louis area, May through August represents the period of most active growth.

Zoysia grass requires about 1� - 2 pounds of actual nitrogen per one thousand square feet per year. Calculate the amount of actual nitrogen in a bag by first multiplying the percentage of N (the first number) by the total pounds of product in the bag. As an example, a 45 pound bag of a 30-0-4 would contain 13 � pounds of actual nitrogen. This product, applied at the recommend rate on the bag of 3 lbs per 1,000 square feet, results in a little less than 1 pound of actual N per 1,000 square. It�s best to apply no more than 1 pound of actual nitrogen per thousand square feet in one application, especially if the nitrogen is all in the quick release (water soluble) formulation.

Organic lawn fertilizers, with their slow release attributes, make an excellent choice for summer applications to turfgrasses.

For warm season grasses, divide the total recommended amount of fertilizer into 2-3 applications. One method is to apply � of the total amount in early June and mid to late July. Another method is to apply 1/3 of the amount in early June, July and August.

Zoysia grass should not be fertilized in early spring or fall. Early spring applications (before May) can promote the growth of weeds in the lawn and contribute to premature top growth before the root system is actively growing. Fall fertilizer (after August) can delay the natural dormancy period of warm-season grasses and make them more susceptible to winter injury.
If crabgrass cotrol in the spring of the year is needed or desired for your zoysia grass lawn, a product without fertilizer should be selected. See "Turf Crabgrass