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Preparing Soil for Planting Bulbs
Soils for Bulbs
While bulbs can grow in nearly every soil, provide soil ideally suited for bulbs in your landscape. Pay close attention to its texture, nutrients, alkaline-acid balance, and moisture-holding qualities.
Note the three most important qualities of soil: porosity (how dense or loose it is), fertility (what nutrients it contains), and pH (its acid-alkaline balance).
Most bulbs perform best in loose soil with balanced fertility and pH that ranges from slightly acidic to neutral.
Good Soil Depends on Good Practices
Follow these steps to ensure the best soil quality for your bulbs:
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- Eliminate questions quickly by testing samples of your soil yourself, or by using a soil laboratory [see Soil Tests, at left, below]. Follow the soil test’s instructions for improving your soil by adding recommended fertilizers or amendments.
- Most bulbs require loose, well-drained soils with equal parts of sand, silt, and decomposed organic matter.
- Cultivate the beds at least one foot (30 cm) deep. Remove rocks, debris, and deep-rooted weeds. Use a mechanical tiller or double dig—turning the earth two blade-lengths deep with a spade or shovel. If a tiller is used, till once in rows of a single direction, then repeat a second time across the rows in the opposite direction to thoroughly mix the soil.
- Adjust the pH balance of the soil if needed. Lower pH by adding garden sulphur; raise pH by adding garden lime. Apply pH amendments following the package-recommended rates and the soil test recommendation.
- Apply fertilizer, working them into the loosened soil with any pH amendments already applied. Bulbs need three nutrients: nitrogen (N) for foliage growth, phosphorus (P) for vitality and strength, and potassium (K) for intercellular transport of nutrients. Use fertilizer with a 4–10–10 formulation when planting; you’ll need extra nitrogen only if it’s present in insufficient quantities.
- While bonemeal has traditionally been a fertilizer of choice for bulbs, we recommend avoiding it. Several scientific studies suggest that bonemeal may be a adult, child, and animal health hazard. Bone meal also tends to lure animal pests to your plantings by its scent.
- Bulbs also require micronutrients and trace minerals, including iron, magnesium, and zinc. Micronutrients are plentiful in organic compost. Nitrogen is water-soluble and travels down through the soil. Phosphorus and potassium become fixed to soil minerals and stay where they are applied.
- Mix amendments deeply into all the soil where bulbs will be planted, but apply fertilizers only in the root zone area of the bulb species being planted. The depth of fertilizer application depends on how deep each bulb species will be set into the bed. For best results, excavate the bed 1–2 in. (25–50 mm) deeper than the level where bulbs will set, mix in fertilizers, cover the fertilized soil and set the bulbs. Finish by backfilling around them with unfertilized but amended soil.
- For more information on fertilizing bulbs during and after planting, see Fertilizing Bulbs.
Soil Tests
Soil test kits available in garden centers provide reliable results when used as their package instructions direct. One test measures three key nutrients—nitrogen, phosphorus, and potassium—while the other checks the soil’s acid-alkaline balance, or pH. They are easy to use and interpret.
Self-Testing or Using a Soil Laboratory
Another option is to send a soil sample to a soil-testing laboratory. Local university or agricultural extension offices, along with many garden stores offer recommendations.
Taking a proper soil sample is essential for achieving good results.
Dig a hole in the area to be planted that is 2 ft. (60 cm) wide and at least 1 ft. (30 cm) deep. Use a clean hand trowel to scrape soil from the side of the hole at a spot about 6–8 in. (15–20 cm) from the surface.
If the area is large, obtain several samples and mix them to create an average soil for testing. Seal the sample in a clean plastic storage sack until ready to test.
Results of either home-test kits or laboratory tests include recommendations on amending and fertilizing.
Step-by-Step Instructions
Bulbs tolerate many different soil types and compositions.
The best soils are loams—roughly equal mixtures of sand, silt, clay, and organic matter—with a slightly acidic pH.
A soil test will tell you the amount and type of fertilizer to add, and whether an acid or alkaline amendment is needed.
Regardless of fertilizers and soil additives, cultivate your soil to loosen it before planting. Follow easy steps:
Fertilizing for Bulbs
Turn over the soil in the bed with a shovel. Break up any clods. Remove rocks, roots, or debris. Cultivate 12–16 in. (30–40 cm) deep with a cultivating hoe.
Rake equal parts of organic compost and well-rotted manure to cover the bed in layers, with each layer 2–3 in. (50–75 mm) deep.
Work the compost and manure into the bed’s soil with a shovel until it is evenly mixed into the bed’s soil.
Improving Soil Texture and pH
Sandy soils drain too quickly and hold little moisture. Clay soils drain too slowly and hold too much water. Add compost to sandy or clayey soils to improve both soil conditions. A compost layer, 4 in. (10 cm) thick, mixed thoroughly into the bed’s soil will improve drainage.
The ideal pH for most bulbs is 6.0–6.5. If your bed’s soil is too high, or alkaline, lower the soil’s pH by adding garden sulfur. If it is too low, or acidic, raise it by adding garden lime. Follow the recommendations of your soil test for quantities. Repeat treatments may be necessary annually in some soils.