The GARDENA gardening expert
Dear Ms. A., the example of liquid manure for plant strengthening you have mentioned is not a magic potion, but is generally used to extract silicates from plants. These are released as the plant disintegrates, and are dissolved into the liquid manure.
If we then spray garden plants with this liquid manure, a silicate film is left once the leaves have dried. This in turn makes it more difficult for leaf fungi to penetrate and enter the leaves with their mycelia, where they can spread (see mildew fungi).
Aphids, too, find it more difficult to reach the plant sap with their probosces. Due to this, we like to take silicate-rich plants, such as stinging nettles or horsetail (generally one kilogramme of fresh plants per 10 litres of manure preparation).
Because the preparation of liquid manure stinks accordingly, we add rock flour to (partially) bind the odour. Those who don't wish to prepare liquid manure themselves can simply make use of a horsetail extract, e.g. “Compositum”.
Liquid manure is generally used diluted in a concentration of 2-5 percent to ten litres of stock solution (1 percent of 10 litres is 100 millilitres). The liquid manure is sprayed every two weeks as a preventative method, or in acute cases once per day for three days, repeating the process on three days the following week.
There are also other ingredients which have the same effect as the plant ingredient silicate, for example bulbous plants generally have a high sulphur content. Liquid manure which is high in sulphur is useful in counteracting leaf fungi, as is sulphur spray.
There are various plant protection and preventative recommendations related to specific plants and manure applications.
These extend right up to general liquid manures made from plant mixtures, which are seldom used the treatment or prevention of plant diseases and pests, but rather to the general strengthening of the plants in your flowerbeds. Such liquid fertilisers are poured onto the plants to promote the soil life and to fertilise the plants in the flowerbed (in particular also with mineral substances and trace elements).
Dear Ms. A., you can of course treat your roses with both liquid manure variants.