Question
Strolling through Home Depot I saw a very toll pile of bags of "Scott's Lawn Pro" Super Turf Builder on sale. While my wife was shopping I took myself time to read the instructions and then I decided to buy a bag after all it was written in big letters "Lawn fertilizer w/2% iron" and also "Apply anytime on any grass."
When I got home I spread it by hand over the lawn. Few days later my lawn was burned in the shape of the motions of my spreading that is, only at certain parts, probably where the fertilizer contacted the grass, I guess. Can you tell me if I did something wrong and if yes, what?

Answer
You do not mention where you are located, but lawns can generally be divided into two different types:

a) cool season lawns typically found in northern half of USA and elevated areas of the South. The grass growing here stays green longer into fall and winter, but handles summer relatively poorly. The grass types are fescue, perennial ryegrass and kentucky bluegrass. Most of USA grow these grass types. These grass types are not grown where the summers are very hot and last a long time.

b) warm season lawns are typically found in southern half of USA and southern California including the dessert. The areas these grass types grow are usually those with very hot weather year round and little or no frost. These areas may still have winters, but the winters are short and mild. These grasses include bermuda, st. augustine, bahia, zoysia and centipede. These grasses will go dormant in late fall through late spring when soil temperatures are below 60F. If grown in the north, they would stay brown for most of the year whereas in the south they stay green for 8-10 months of the year. These are used in areas with very hot summer temperatures (consistenly 90's and above for long periods) as the cool season grasses would die in these conditions.

If you have a cool season lawn, the lawn should be fertilized in spring and fall but never in summer. It does not matter what the fertilizer bag says. If you fertilize cool season grass when daytime temperatures are much above 80F then you are asking for trouble.

Warm season lawns can be fertilized in summer, in fact they should be fertilized regularily through this period. However, it is still advisable to avoid the hottest days of the year.

When you apply fertilizer, it is much recommended that you buy a spreader. A drop spreader is a "bucket on wheels" with a cable release handle. You push it like a small shopping cart over the lawn and it will drop fertilizer evenly below the spreader when the cable is engaged by your hand. You can also buy a rotary spreader for larger lawns. These will "throw" the material in an arch and are a bit more expensive and tricky to use.

Finally, fertilizer should be watered in immediately. You should never apply to a draught or heat stressed lawns (which is another reason not to apply in summer in the north) but the grass blades should be dry. Once the product is applied, you should water immediately for 1-2 hours to wash the fertilizer off the grass into the ground.

I.e.
In northern areas, never ever fertilize in summer. never.
in souther areas, you can fertilize in summer, but still watch out.

Never apply fertilizer to a draught stressed lawn (or a lawn stressed by heat such as a lawn in the northern half of USA when daytime temperatures are above 80F).

When applying, always apply to a dry leaved lawn. Then water immediately to activate the product and wash it from the leaves into the ground (soil).

The only exception to the above is FERTILIZER W/BROADLEAF WEED CONTROL (also called Weed-n-feed products). They must be applied to a WET lawn and allowed to stay on the leaves of the grass / weeds for 24-48 hours (read lable) and then be washed in. I never recommend using these unless you do so in spring or fall when temperatures are modest and make sure the lawn is not draught stressed.

Fertilizer producers may make products which they describe as "safe to use anytime on any grass" but in my own opinion this is a truth with modification.

1) A cool season lawn should not be fertilized in summer. Cool season grasses can not metabolize the fertilizer in summer in hot temperatures so it is not even a matter of whether it is safe to use (i.e. will it burn the grass ??) .. On these grass types in summer, you can not get anything positive out of fertilizing.

2) If you would like the biological explanation of what happens to the individual grass plants I will gladly give it to you.

3) cool season grass are biologically very different from warm season grass. The fertilizer may be suitable for both types and so while technically the fertilizer can be applied "any time" (summer on warm season grass, late fall on cool season grass) it is not the same as you should. The risks if used improperly or at the wrong times for the specific lawn grasses are high.

4) if you apply fertilizer too late (dormant) the fertilizer is washed through the root zone and into our ground water supply which is another reason not to apply "any time".

The correct times to apply fertilizer is:
in the north, apply fertilizer to cool season grass late spring, early fall and late fall. Never in summer. Apply to dry grass but not a draught stressed lawn. Then water in well with plenty of water.

In the south, apply fertilizer to warm season grass from late spring through early fall. follow same applications as above.

It is always preferred to use a spreader for distribution to avoid uneven applications.