How to properly use compost in the garden

Ripe compost soil
Ripe compost soil

Compost is one of the top fertilizers among gardeners because it is particularly rich in humus and nutrients, and also completely natural. A few shovels of mixed compost provide your garden plants with sufficient amounts of calcium (Ca), magnesium (Mg), phosphorus (P) and potassium (K), and also improves the soil structure in the long term by enriching the soil with humus. If you have created one or two compost heaps in your garden, you can help yourself to the “black gold” at regular intervals. But beware: Just because it is such a valuable fertilizer, it should be used wisely and in the right measure.

Producing good garden compost

To speed up the rotting of your compost and thus the composting process, you should alternate solid (for example, lawn clippings) and loose ingredients (for example, leaves) into it. If the compost is too dry, you can water it with a watering can. If it is too wet and smells musty, you should mix in shrub chaff. The better the waste is mixed, the faster it will mature. If you want to use it in just a few months, compost accelerator can be added. It provides the nitrogen needed for the decomposition of nutrient-poor waste such as wood or autumn leaves.

Finally, when you take mature compost out of the barrel or pile, sift it first before using it so that no coarse debris such as eggshells or pieces of wood end up on the bed. To do this, use a large-area throw-through sifter or a homemade sieve with a minimum 15-millimeter (0.6 in) mesh. Mature, sieved compost is especially important for sowing beds in the vegetable garden, because here you need soil that is as fine-crumbled as possible.

When is compost mature?

Compost develops from the layering of various garden wastes, such as shrub clippings, grass, fruit and vegetable scraps, and leaves. Microorganisms decompose the waste and gradually form valuable humus soil. As a rule, it takes just under six months before so-called “fresh compost” can be taken. This is particularly rich in readily available nutrients, but is very coarse and can only be used as mulch for existing plantings. It is not suitable for sowing beds, because it is much too strong for the tender seedlings. Also, do not work fresh compost into the soil, as there is a risk of rotting.

Mature compost is obtained after about ten to twelve months at the earliest, depending on the composition. Now the components are largely dissolved and produce a fine crumbly humus soil. The nutrient content in mature compost decreases the longer it stands. Therefore, use up the finished mature compost as quickly as possible. A cress test can be used to check the stage of rotting.

How the cress test works

Fill fine-crumb compost samples into shallow trays. Then scatter the cress seeds evenly. If there are several trays, each should get about the same number of seeds. If you have taken samples from different compost piles, be sure to label the trays, so you can tell them apart later. You can simply place the labels in the tray. For a better comparison of the germination results, you can fill another tray with normal garden soil and sow cress in it as well.

After just four to five days, you can compare the germination results. If the compost is too fresh, the cress sown on it will form yellow or brown leaves after a few days. If, on the other hand, the leaves remain green, the compost is ripe. If the cress forms dark green leaves and straight, white rootlets, everything is fine. If the seedlings are yellow, crippled and the roots are puny, you should turn the compost again and let it ripen.

Basic rules for fertilizing with compost

In general, you can use compost as a garden fertilizer all year round. Large-scale initial fertilization with compost is carried out in the spring, when the plants in the garden begin their growth phase. Then regular fertilization is done throughout the year until fall. As a general rule, the more nutrients a plant needs, the more compost may be applied. Magnificent perennials and heavy feeders receive plenty of compost during the growth phase, while wild perennials and woodland edge plants receive considerably less. Bog plants such as rhododendrons and azaleas do not tolerate compost at all, as it is usually too rich in lime. Plants that like to grow in poor soil, such as primroses, horned violets or ox eye, also do well without the natural fertilizer. If you use compost in the garden, make sure you work it in as shallowly as possible with a rake or cultivator.

How much compost do I need?

Of course, the exact amount of compost needed can only be determined after a precise soil analysis, and even then, these are still approximate values, because the nutrient content of compost also varies quite a bit depending on the source material. Nevertheless, there is a rule of thumb for the use of compost in the garden: flowering shrubs, which are very hungry for nutrients, should be supplied with about two liters (4 pints) of garden compost per square meter (10 sq ft) over the course of the year; half that amount is sufficient for ornamental shrubs. For some fast-growing or strong-flowering ornamental plants, compost alone is not sufficient because of its low nitrogen (N) content. Therefore, an addition of about 50 grams (1.8 oz) of horn shavings per square meter is recommended for these plants. Compost can also be used for lawn fertilization. One to two liters (2 pints) per square meter is usually sufficient

Initial fertilization for new planting

To give a good start to hungry ornamental plants, especially woody plants and perennials, you should mix the excavated soil with up to one-third mature compost when planting new beds. If an entire bed needs to be replanted, you can enrich poor sandy soil with up to 40 liters (80 pints) of compost per square meter (10 sq ft). It will provide the plants with the most important nutrients for up to three years, after which you will need to re-fertilize.

Compost in the vegetable garden

You can use compost as fertilizer not only in the ornamental garden, but also in the orchard and vegetable garden. To do this, rake the mature compost flat into the top layer of soil after loosening the soil in the spring. Particularly grateful for a compost fertilization are heavy feeders such as zucchini, pumpkin, potatoes, cabbage and tomatoes. These need up to 6 liters (12 pints) of ripening compost per square meter (10 sq ft). Somewhat less, namely a maximum of three liters per square meter of bed area, is needed for medium feeders such as lettuce, strawberries, onions, spinach, radishes and kohlrabi.

The light feeders among the vegetables should be mulched with a maximum of one liter of compost, but here you can also dispense with the compost application altogether if you have previously grown heavy feeders or medium feeders in the bed. Light feeders mainly include herbs, but also (small) radishes, lamb’s lettuce, peas and beans. Fruit trees or berry bushes are happy to have a mulch layer of compost on the tree disc in the fall.

Compost for pot and container

Mature compost can also be used as fertilizer for flower pots and balcony boxes. To do this, mix one-third garden soil with one-third mature, sifted compost. Depending on the plant, also add one-third sand and/or peat; or peat substitutes. If you are growing your own vegetable or flower seeds in seed trays, you can also use compost to enrich the soil. This soil for growing young plants should not be too rich in nutrients, so a compost/soil mixture in a ratio of 1:4 is recommended here.

Making liquid fertilizer from compost

Compost swill is a fast-acting, natural and inexpensive liquid fertilizer. To make it, add a shovelful of compost to a bucket of water, stir vigorously, and after settling, apply undiluted with a watering can. For plant-strengthening compost tea, let the broth stand for two weeks, stirring thoroughly daily. Then filter the extract through a cloth, dilute (1 part tea to 10 parts water) and spray over plants.

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