Soil defertilisation refers to the defertilisation of soil in order to reduce the amount of plants that can grow on that soil. It is often done on land not intended for growing food on (city parks, ...).
Soil defertilisation Wikipedia
On land not intended for growing food on (city parks, ...), plants that have not been planted there (weeds) become a great nuisance to the city's communal services, costing effort and money to the people.
In some cases, along with soil defertilisation, the changing of the soil's pH and water content of the soil may also be done. This may create a much different environment, allowing more specialised plants/vegetation to grow and take hold.
Soil defertilisation is done by growing specific cover crops (i.e. facelia, Sinapis alba, Lolium multiflorum, ...) on them and then, instead of ploughing them under, removing them from the soil. By doing this, the nutrients that have accumulated in the crops are removed together with the crops. The crops may be used on other land that needs to be fertilsed (instead of defertilised), for example agricultural land.