Spring is here and the grass is growing. After all the rain this winter, you have probably noticed that the weeds are growing too.

If your lawn is soft and spongy due to heavy thatch buildup, your first spring job is to remove that excess underlying growth by verticutting, followed by soil core aeration to improve air and water penetration (see Aug. 5, 2005, column).

Early spring is also the time for fertilizer, which helps control new weeds by encouraging a thick, healthy lawn. In addition, if herbicides are sprayed, healthy grass is less likely to be damaged and existing weeds are more susceptible to herbicides when they are young.

When it comes to fertilizer, lawns have different needs than trees, shrubs and many ground covers. Because the leafy portion of grass is constantly being mowed off, turf grasses need a lot of nitrogen. A good source of nitrogen is grass clippings — so don't bag them, just leaving the clippings on the lawn.

But you still need to use additional fertilizer, which comes in many different formulations. In most cases it is best to use one that contains the three major nutrients: nitrogen, phosphorus and potassium. The percentage of these nutrients is on the package label, such as 27-5-10, representing nitrogen, phosphorous and potassium, in that order. Fertilizer formulations for turf always contain more nitrogen than the other two, usually a ratio of at least 3:1:1.

The chemical form of the nitrogen is an important consideration. Some forms of nitrogen are very water-soluble and can burn the lawn if over-applied or not watered in well. These types of nitrogen enter the soil quickly and result in a rapid burst of green leafy top growth, which will require more frequent mowing for a while. In four or five weeks, the nitrogen levels will decrease and the grass growth will slow down, lose some of its color and you need to fertilize again. This is common when the nitrogen is only in the very soluble forms of ammonium sulfate or urea.

some turf fertilizers contain time-release or slow-release nitrogen sources. These longer-lasting fertilizers can be applied at a higher rate without burning the grass, and produce a more even growth rate.

There are several forms of slow-release nitrogen. The most commonly used is sulfur-coated urea or SCU. Others are urea-formaldehydes such as MU and UF, and IBDU. The ingredients are always listed on the bag. Look for the section called "Analysis." Many formulations contain a combination of water-soluble nitrogen such as ammonium sulfate and slow-release forms. Look for those that have a high percentage of slow-release nitrogen, usually in the form of SCU.

Each species of turf requires its own type of fertilizer application. Bermuda grasses have the highest demand for nitrogen; zoysia grasses, St. Augustine grass and seashore paspalum are intermediate, while centipede grass needs the least. Each application should contain one pound of nitrogen for every 1,000 square feet of lawn. The actual amount of fertilizer will depend on the percentage of nitrogen and the total area of lawn. If using 25-5-5 on 2,000 square feet, for example, you will need 8 pounds of fertilizer. Slow-release forms can be applied at more than one pound of nitrogen per 1,000 square feet and less often.

The main purpose of fertilizing your lawn is to maintain a slow, steady growth and nice green color. Over-fertilizing with nitrogen will require more frequent mowing and can lead to rapid thatch buildup.

A few other hints: Always use a spreader to apply the fertilizer. Hand spreading on grass results in uneven application — some areas will burn and others won't get covered at all. Water the grass well immediately after application, particularly if using a water-soluble type of nitrogen. For best results, apply fertilizer one day after mowing. If you just want to boost your grass' color, apply iron, available in granular and liquid forms.

For more lawn and garden information, visit www.ctahr.hawaii.edu and click on free publications.