Why wash fertilizer off the grass?

If you use fertilizer containing weed killer they want you to put it on wet grass and do not wash it off the grass.
But if you use fertilizer without weed killer they want it washed off of the grass. Why? Is this fertilizer different chemically?
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On Sat, 6 Oct 2007 17:33:49 -0400, "Academia"

The broadleaf killer granulars need to stick to the leaves (of the weeds) to work.

Some fertilizers can be harsh and may burn the grass. Fertilizer applied to a damp ground, then watered in are much less likely to burn.
BTW, in most US areas this is a poor time of the year to be using lawn weed killer.

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Thanks
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Academia wrote:

'weed & feed' is a contradiction.
herbicides need to stay in contact with the plant they are intended to kill long enough for the plant to absorb the chemical doing the killing.
fertilizers of the granular type need to go on the ground so they can be absorbed into the soil and made available to the root systems they are intended to feed.
most types of granular nitrogen in contact with plant foliage do more harm than good.
products known as 'weed & feed' were invented as some kind of great time saver and are mostly used by the unfortunate unknowing unaware types of people looking for a quick solution with the least amount of time away from their beer and watching their sports-guy heroes on video.
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thanks

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On 10/6/2007 2:33 PM, Academia wrote:

I used to use a "weed & feed" combination where the herbicide (monuron) worked through the weed roots. It was for dichondra lawns (D. micrantha, a broadleaf ground cover sometimes called "leaf lawn"). This had to be rinsed off the foliage and into the soil. It was very effective, killing grasses, oxalis, spurge, dandelions, and even seedlings from my ash tree. Unfortunately, it's no longer available in the U.S. The manufacturer declined to renew the EPA permits, claiming the market was too small to bother with the permitting process. (D. micrantha only grows in non-desert, mild-winter climates.) It's very difficult to keep oxalis and spurge out of a dichondra lawn. Thus, when monuron was no longer available, I had my garden redone with a lawn of red fescue (Festuca rubra), an ornamental grass that needs mowing only once or twice a year.
For a dry product, I can't imagine a "weed & feed" combination working by contact.
On the other hand, dry fertilizer in the open will eventually absorb moisture from the air and slowly dissolve (too slowly for an herbicide to be effective). (Ever notice how dry fertilizer sometimes even cakes while in the bag?) Some dry products (e.g., ammonium sulfate) will even extract moisture from foliage that it contacts. In contact with foliage, this will easily burn. Thus, you want to rinse the dry product off the grass and into the soil. Not only does that prevent leaf-burn but it starts moving the nutrients into the soil in a solution much more dilute than what is created by absorbing moisture from the air.
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thanks

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No. The week killer in question attacks weeds through the leaves so if you wash it off it does no good (which is harm to the weeds). The fertilizer fertilizes through the roots but it is not in any particular hurry so you can let the weed killer do its thing and then wait until the fertilizer gets washed into the ground to do its thing.
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Why do I have to wash off the non-weed killer fertilizer if it is the same as the weed killer fertilizer that is allowed to stay on wet leaves.
Forget about the weed killer. It's the fertilizer that I'm wondering about.
No one answered the question I asked except you. You say they are the same. If they are it seems to me that no harm will come if I don't wash it off.
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On Tue, 9 Oct 2007 16:39:45 -0400, "Academia"

It may or may not harm the grass. Most fertilizers today are slow-release and coated so it doesn't matter. However, if you apply fertilizer during a draught situation to young grass it may burn it. If you can, it is best to apply fertilizer when there is a good chance of rain in the forecast. Easier on the roots too.
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thanks a lot that help understanding
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Let's not make this complicated.
The "weed" part like Jim said needs to be in physical contact with the target plant. This is usually a broadleaf. Lawn is not a broad leaf and is not harmed. You must turn the irrigation on to simply wet the foliage. 5 seconds will do.
The "feed" comes mostly via the roots some is taken in via the foliage but nothing significant. The next morning when your lawn is watered the fertilizer will wash to the ground and be picked up by the roots.
It is best to apply a weed and feed on a sunny day in the morning. Not over 95. This will allow the target weed to absorb as much of the kill juice as possible.
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thanks
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I was wondering if there is more than one kind of fertilizer chemical.

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On 10/8/2007 9:33 AM, Academia wrote:

I shop with a calculator. I buy the fertilizer that has the most nitrogen per dollar.
Nitrogen is the first number of the three numbers. It's the percentage of nitrogen in the total weight the the fertilizer. Thus, a 20-10-10 fertilizer is 20% nitrogen. A 20 lbb sack of 20-10-10 contains 4 lbb of nitrogen. (The second number is the percentage of phosphorus; the third is the percentage of potassium.)
I take the weight of the bag and multiply by the percentage of nitrogen and divide by the price. Whichever lawn food gives the highest result, I buy. Generally, it's the house brand of the store where I'm shopping.
I buy the lawn food WITHOUT weed killer. Thus, I can also use it on my shrubs and in my flower beds.
For a special jolt (e.g., for roses and trees), I also use ammonium sulfate, which is about 50% nitrogen. This is too strong to use on lawns. It's also not suitable for acidic soils because it will make them even more acidic. (My soil is quite alkaline and needs acidifiers.) It has no phosphorus or potassium, but those are not significantly deficient in my soil.
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thanks for that info

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Time for a new nickname.
Janet
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this one is new
contains these words:

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On 10/6/2007 2:33 PM, Academia wrote:

To address this specific question:
The package of fertilizer should have a phone number for contacting the manufacturer. Call. Ask them. They will have the answer.
If there is no phone number on the package, ask the nursery advisor where you bought it. If they can't answer, shop at a real nursery.
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David E. Ross
Climate: California Mediterranean
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thanks

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