GRASS DOESN’T GROW OVER A STUMP, by Larry Williams UF/IFAS Extension Agent

It is difficult to grow grass or other plants where a tree was recently removed.

A condition known as “nitrogen robbing” is partially responsible for this phenomenon. It is a biological response to the sudden introduction of large amounts of dead wood parts to the soil. Whether present as sawdust, stump grindings or what is left of the stump itself, this material is high in carbon.

Parts of today’s article are taken from a previous article on the same topic written by Dan Mullins, retired UF/IFAS Extension Agent.

To microbes in the soil the wood is a food source and they quickly begin multiplying in large numbers and begin to decompose it. This is a natural recycling process.

As these microbes increase in numbers, they must have more and more nitrogen which is converted to proteins. Available nitrogen in the soil is quickly used up. It’s not lost, but is temporarily being utilized in the bodies of these soil borne organisms. It will be released later, when the woody material is completely broken down and the microbes begin to die off.
In the mean time, for a period of months or years, depending upon how much woody stuff there is to be decomposed, nitrogen robbing occurs. Grass, flowers, vegetables or other plants established in a spot where all of this activity is underway simply won’t grow because they are starved for nitrogen. A better term might be “nitrogen borrowing,” because the nitrogen is not actually lost.

This condition can be at least partially avoided by the removal of as many stump parts as possible. This includes sawdust, grindings, bark pieces and roots. These parts can be composted for later use as a soil amendment.

The area can also be provided with some extra nitrogen fertilizer on a regular basis. This helps to provide enough of that nutrient for both the microbes and any nearby plants. The extra nitrogen will speed up decomposition and reduce the number of yellow, stunted grass and other plants in adjacent areas.

This condition has to do with an imbalance in the carbon to nitrogen ratio. The best ratio of these two elements is somewhere between 20 to 1 and 30 to 1, in order for rapid decomposition to occur. The woody material, being the carbon containing portion, is extremely high at first, while the native soil is very low in nitrogen. That’s the reason why these areas where trees once grew are barren for so long.

Left alone, things will settle down to normal without doing anything extra. However, it may be that grass won’t grow well for 3 to 4 years in these areas without some help.

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