How do you fertilise?

Since it is now summer, when everything is shooting up in the garden, we want to use this GARDENA Newsletter Special to provide information on the right garden fertiliser. You hear the phrase time and again: "It came from my own garden; there's no fertiliser on that!". But there is something deeply illogical about this proud boast about ostensibly successfully grown fruit and veg. If something's big, then it needs a lot of nutrients. A sufficient quantity of the right ingredients is required for plants which need to form valuable content. For this reason, it is important to fertilise each differently-used part of your garden in exactly the right way!

Is the fertiliser used irrelevant?

No, it isn't. Each fertiliser used - whether it be compost, manure or corn fertiliser - has its own effect on the composition of the soil. And each fertiliser works differently because of its own composition. Finally, the nutritional requirements of different plants are also different: The rockery plant sedum has different requirements from the shrub delphinium and tomatoes have different requirements again. 

What all the plants have in common is that, in addition to a range of trace elements such as copper, sulphur, magnesium and others, they all need four main nutrients: Nitrogen - a growth driver and the main component of proteins but also an important element of chlorophyll; magnesium - a key element of this chlorophyll; phosphorous - mainly to promote flowering; and potassium to firm up the tissue and ripen the fruit.

Various plants need different quantities of these nutrients; for information on which plants consume more and which less, see the specific fertilisation tips below.

What about organic fertilisers?

The answer to the following question is interesting: "Is organic fertiliser better than artificial fertiliser?". Again, the answer is: It depends! According to what we currently know about plants, the plant itself doesn't care whether it receives nitrogen, phosphorous, potassium etc. from an organic or artificial fertiliser. It can only take up the fertiliser in a certain chemical formula in which the fertiliser should be available to the roots - otherwise nothing works. Whether it gets organic fertiliser or not, the actual nutrients taken up into the plant are not processed according to their origin but on the basis of what the plant needs. 

The only benefit of organic fertiliser is usually that it is better for the soil and the creatures which live in it. In an extreme scenario, people who only use mineral fertilisers will ruin their garden's soil in the long run because this fertiliser does not provide the humus nutrients and waste required for the soil and creatures which live there. 

If using organic fertiliser, remember that it takes at least three to four weeks - depending on the weather - for organic fertiliser to dissipate into the soil so that the plants can take up the nutrients. So to provide a plant with food in April you have to apply the organic fertiliser (such as hornmeal or compost) well beforehand in March! Mineral fertilisation makes particular sense if you want to give a plant a lot of nutrients relatively quickly. And there is also something in between the two: Mixed organic-mineral fertilisers - with an organic basis and added minerals.

On the subject of adding things: Garden soils usually have too much phosphate whereas nitrogen is almost always in short supply. So it does not always make sense to use a compound fertiliser. If you add a lot of fertiliser, the nutrient level can be too high (and a lot helps but not too much!). It is better to fertilise in accordance with what is needed than to use the same formula regardless. For example, use a nitrogen/potash fertiliser rather than a nitrogen/phosphorous/potash compound if the soil already has enough phosphate. How do you know/how can you find out? Every couple of years you should take a sample of garden soil and check its fertilisation level. This means that you are always aware of the current situation and you might even be able to recoup the costs of the soil test by saving on fertiliser costs. The one costs about the same as the other. In any case, this method provides better care for the soil in your garden. In addition, it often protects valuable groundwater since excess fertiliser is not flushed into it.

But enough of the explanatory background knowledge - let's get practical! Let's take a walk through your garden together:

What needs to be fertilised now? And above all, how much fertiliser needs to be used?

For balcony and pot plants I use a mineral soil such as a potting mix. The stored fertiliser can be easily raked through and is not washed out by frequent plant watering. The fertiliser reserve of a high-quality flower soil, normally added by the manufacturer, usually lasts for around six weeks. At this point at the latest, you need to add more fertiliser. I therefore add long-term fertiliser to the soil during planting - 2-3 grammes per litre of soil. This is how I provide basic food for my plants up until August/September. I also add a small dose of liquid feed each week - 0.1 to 0.2 parts per thousand - when watering (i.e. 10 to 20 ml of liquid feed for a full 10 l watering can). Again: Follow the manufacturer's instructions on the fertiliser packaging!

In shrub gardens, the extent of fertilising measures mainly depends on the size of the mass formed by the leaves and flowers of the plants. I only fertilise rockery shrubs here and there with a handful of hornmeal or a little compost (barely a litre per square metre). Strongly growing bed shrubs can be given around 50 grammes of compound fertiliser per square metre in spring (around March/April). Alternatively, you can give them around three litres of compost per square metre. Weaker growing shrubs only need 30 grammes or 1.5 litres. In mid-June you can feed them again with a second, slightly reduced amount.

In vegetable gardens, the extent of fertilising measures mainly depends on the type of plant, since because vegetable plants can be divided into strong consumers, including cabbage, medium consumers such as tomatoes and weak consumers such as lettuce. Here are some individual fertilising recommendations:

Weak consumers: Add 1 to 2 litres of compost per square metre and, if necessary, additional nitrogen (approx. 70 grammes of hornmeal per square metre) and potassium (approx. 35 grammes of potassium magnesium fertiliser per square metre).

Medium consumers: Add 2 to 4 litres of compost per square metre and, if necessary, additional nitrogen (approx. 120 grammes of hornmeal per square metre) and potassium (approx. 70 grammes of potassium magnesium fertiliser per square metre).

Strong consumers: Add 4 to 6 litres of compost per square metre and, if necessary, additional nitrogen (approx. 150 grammes of hornmeal per square metre) and potassium (approx. 100 grammes of potassium magnesium fertiliser per square metre).

Important: Cabbages, onions and carrots should not be placed on surfaces freshly fertilised with manure. Only add these plants to such areas the following year. There is a risk of intolerance here, since some manure attracts flies whose larvae can mine into the plant.
 

When fertilising grass areas, please note the following (as already stated in the Lawn Special): When you cut the grass, you also remove nutrients from the lawn: For each kilo of trimmings you remove around 30 grammes of nitrogen, 20 grammes of potassium and 10 grammes of phosphorous. You have to replace these nutrients. Decorative lawns need somewhat less fertiliser than lawns which are walked on or are under stress. You normally fertilise lawns every four to five weeks from March/April, ending with a final autumn fertilisation to prepare for the winter at the start/middle of September. However, differences do exist now that long-term lawn fertilisers and special autumn fertilisers have come to the market. As we keep reiterating, you must pay close attention to the manufacturer's information on lawn fertiliser packets. This can also help to answer the question of whether you can use composite fertiliser rather than lawn fertiliser. Take a look at the mix ratio of composite fertiliser - nitrogen: phosphorous: potassium. For lawns, this is ideally 10:3:3-5.

To fertilise trees, you first have to differentiate between deciduous trees and evergreens. Their annual growth cycles are rather different, which is why they need fertilisers at slightly different times.

Deciduous trees mainly need their nutrients at the start/middle of June and then around mid-July to mid-August for a second growth spurt. After that, you should not fertilise them further. Thus, you actually apply fertiliser to deciduous trees for the first time in April (around 55% of their annual requirement) and in June (45%). The figures vary slightly for fruit trees and roses, which require a 50/50 or 60/40 split.

Evergreens need their first nutrients in May and a second, higher dose between the end of July and the end of August, when they are forming cones. The feed distribution for evergreens is therefore 35/65. 

The fertiliser quantity also depends on the plant's requirements, i.e. the mass each plant forms. For small conifers, around 35 grammes of composite fertiliser per square metre and per year is sufficient; for the maintenance fertilisation of trees and bushes around 50-70 grammes of composite fertiliser per square metre and per year should do the trick, and for fruit trees, roses and rhododendrons you can go to the upper limit and add 100 grammes in both doses. 

Please note that the tree roots must be able to access the nutrients at the stated times. If you want to use organic fertiliser, you have to add the waiting period for decomposition to the times stated above.

By the way:

Conservatory and indoor plants are now also in their growth phase - so do not forget to fertilise them regularly up to October. Use a green plant feed for green plants and a flowering plant feed for flowering plants - the latter contains more phosphorous to promote flowering.