Preparing the soil for onions

Why Is This So Important?

The root system of onion is fibrous and rather weak, with most roots located in the topsoil layer—only 5–20 cm deep (Autko et al., 2012). Unlike carrots or beets, onions cannot penetrate dense soil layers in search of moisture and nutrients. They need soil that they can easily colonise from the very first days.

Which Soil Is Unsuitable?

• Heavy clay soils. They warm up slowly, allow poor air penetration, and, most importantly, tend to form a hard crust. This crust becomes an insurmountable barrier for tender onion seedlings and hinders normal bulb thickening (Autko et al., 2012).
• Acidic soils. Onions are very sensitive to acidity. In an acidic environment, they absorb nutrients poorly and are more susceptible to diseases, especially fusarium and neck rot (Hochmuth & Sideman, 2023).

A simple practical tip: To find out if your soil is suitable for onions, do a small test. Take a handful of moist soil and squeeze it in your fist.

• If it crumbles into sand—you have light sandy loam, excellent for onions.
• If you can roll a firm ball that does not break apart—the soil is heavy and needs improvement (we’ll discuss this in the next chapter).

Remember: the bulb is a storage organ. For it to grow large and healthy, the roots must easily penetrate deep and wide, finding oxygen and moisture there. Creating the right soil structure is the first step towards that.

Why Is Looseness Critical for Onions?

1. Oxygen access. Onion roots, like all living organisms, respire. In dense, waterlogged soil, there is little oxygen. Under such conditions, roots suffocate, work less efficiently, and the bulb cannot gain mass. Loose soil is saturated with air, which stimulates active root system growth (Autko et al., 2012).

2. Root penetration. The weak root system of onions cannot push through compacted soil. If the soil is hard, the plant wastes precious energy not on forming the bulb but on overcoming soil resistance.

3. Nutrient uptake. A loose structure with many soil aggregates (crumbs) better retains moisture and nutrients, making them available to plants. In addition, beneficial microorganisms are more active in such soil, converting organic matter into plant food.

How to Improve the Structure of Heavy Soil?

1. Adding Organic Matter (more details in Chapter 4).

This is the most effective and natural method. Humus, compost, or well‑rotted manure (not fresh!) act as “looseners.” They bind fine clay particles into larger, water‑stable crumbs. This improves both aeration and drainage. Adding organic matter is a fundamental technique that solves several tasks at once: improves structure, enriches the soil, and increases its water‑holding capacity (Autko et al., 2012).

2. Sand for Clay Soils.

Adding coarse river sand is an old, proven method, but it must be used wisely. Simply adding sand to heavy clay is not enough. Sand grains must “push apart” the clay particles. A combination works effectively: sand + large amounts of organic matter. If you mix clay with sand alone, you can end up with “concrete” (a mixture of cement and sand). Organic matter, on the other hand, creates structure by binding mineral particles into fertile aggregates.

3. Avoid Compaction!

This rule applies to everyone who wants to grow large onions.

• Minimal walking. If possible, make narrow beds so you do not walk on them during care and maintenance. When you step on wet or even moist soil, you compact it, destroying the fragile channels through which air travels.
• Plan pathways. Decide in advance where you will walk, and try not to step into the planting area. This is especially important on heavy soils, which take a long time to recover.
• Do not work with “wet” soil. The golden rule of gardening is not to dig or till soil when it is too wet. In that state, it easily compresses into dense, structureless clods that dry out like stone. Wait until the soil dries out a little. A simple test: squeeze a handful of soil—if it forms a ball but does not stick to your fingers, and when dropped from chest height it breaks into crumbs, it is ready to work with.

Practical conclusion:

Loose soil is the result of proper application of organic materials and careful handling. It is not created in one day, but once you improve the soil structure, you will enjoy the fruits of that labour for several years. In ideal onion soil, there should be no hard clods, and a handful of earth should resemble a crumbly, fragrant “cake” that is pleasant to hold.

What Is pH and Why Is It Important for Onions?

pH is a measure of soil acidity or alkalinity, measured on a scale from 0 to 14. A value of 7.0 is neutral; below is acidic, above is alkaline. For bulb onions, the ideal range is 6.0 to 7.0, i.e., neutral to slightly acidic (Hochmuth & Sideman, 2023; Autko et al., 2012).

Why Is This Critical for Onions?

1. Nutrient availability. This is the main reason. In acidic soil (pH below 6.0), many vital elements, especially phosphorus, become unavailable to plants. Onions simply cannot absorb them, even if they are present in the soil. In turn, in too alkaline conditions (above 7.0), plants have difficulty absorbing micronutrients—iron, manganese, boron (Kasynkina & Kudin, 2018). The onion starves at a well‑set table.

2. Diseases. Acidic soil is a paradise for many soil‑borne pathogens. Fusarium wilt and root rots, which are scourges of onions, develop much more strongly at low pH. It is known that fungi causing fusarium thrive best at pH below 6.0, and raising pH to 7.0–7.5 significantly reduces their harmfulness (Hochmuth & Sideman, 2023).

3. Aluminium toxicity. In acidic soils (pH < 5.5), aluminium becomes soluble and is toxic to plant roots, inhibiting their growth and development (Hochmuth & Sideman, 2023).

How to Determine pH?

The most accurate way is to send a soil sample to an agrochemical laboratory. Your local agronomist or gardening centre can advise where to go. This will give you an accurate pH value and fertiliser recommendations.

For the home gardener, a portable pH meter is an excellent tool. It is an inexpensive and accurate device that allows you to monitor acidity on different beds throughout the season. There are also simple indicator tests using chemical reagents, but they are less accurate.

How to Change pH?

The test result will determine your next steps.

If the soil is acidic (pH < 6.0), it needs liming. Use dolomite lime or ground limestone. Dolomite lime is preferable because it additionally enriches the soil with magnesium—an important element for onions (Hochmuth & Sideman, 2023). Application rates depend on the initial acidity and soil type, so follow the instructions on the package or laboratory recommendations.

A very important rule: Liming should be done well in advance, at least six months before planting onions, preferably in autumn under the preceding crop. Do not apply lime immediately before planting! Onions tolerate fresh liming very poorly; it can inhibit plants and cause diseases (Autko et al., 2012). Lime must be thoroughly mixed with the soil and given time to neutralise acidity.

If the soil is alkaline (pH > 7.0), this is less common but possible. To lower pH, you can add elemental sulphur, ammonium sulphate, or acidic peat moss. However, in home gardens, the problem is usually acidification rather than alkalisation.

Brief summary:

Adjusting pH is like setting up the “right kitchen” for your onions. If the soil has all the necessary ingredients (structure, organic matter), but the “stove” doesn’t work (acidic environment), you won’t get a good harvest. Invest time in soil testing and proper liming—it will pay off handsomely.

Now that we know what acidity should be, it is time to talk about what we will “feed” our future bulb. The next step is creating a fertile organic layer. Let me know if you are ready to move on.

Why Do Onions Love Organic Matter So Much?

1. Improvement of all physical properties. Organic matter (humus, compost) acts as a natural structure‑former. It glues fine sand and clay particles into water‑stable crumbs. This, in turn, improves aeration (oxygen supply to roots), water‑holding capacity, and water permeability (prevents waterlogging) (Autko et al., 2012).

2. Nutrition for plants and soil inhabitants. Organic matter is “food” for beneficial soil microorganisms and earthworms. By breaking down organic residues, these creatures convert nutrients that are unavailable to plants into available forms. This is slow but steady and environmentally friendly nutrition for onions.

3. Buffering capacity. Soil rich in organic matter resists sharp changes in acidity better. It smooths out the “peaks,” maintaining an optimal pH for plants.

4. Improved thermal regime. Organic matter is dark in colour and absorbs solar heat better, promoting earlier warming of beds in spring. This is especially important for onions, which love warmth (Autko et al., 2012).

Which Organic Matter Is Best for Onions?

• Well‑rotted manure or humus (best option). This is the “gold standard” for onions. Humus is fully decomposed, loose, free from weed seeds and pathogens. It feeds plants gently and in a balanced way, without burning roots (Autko et al., 2012).
• High‑quality compost. An excellent alternative to manure. Compost made from plant residues, tops, leaves, is also rich in organic matter and improves soil structure.
• Green manures (cover crops). This is the most environmentally friendly and modern approach. Sowing green manures (mustard, phacelia, winter rye, vetch) in autumn and then incorporating their green mass into the soil in spring is a great way to enrich the soil with organic matter and restore its fertility.

What You Should Absolutely Avoid?

Applying fresh manure just before planting. This is one of the most common mistakes. Fresh manure contains a lot of undecomposed organic matter, which, when incorporated into the soil, begins to decompose vigorously, releasing heat, ammonia, and other substances that can “burn” young onion roots. In addition, fresh manure is a source of weed seeds and pathogens. Applying fresh manure delays maturity, causes thick necks, and poor storage (Autko et al., 2012). Use only humus or well‑matured compost.

How and When to Apply Organic Matter?

1. Timing. The best option is to apply organic matter in autumn during primary digging or ploughing. Over winter and spring, it will have time to mix well with the soil and partially decompose into plant‑available forms. If you miss autumn, you can apply humus in spring, but 2–3 weeks before planting, so it has time to “settle” in the soil.

2. Rates. For bulb onions, the recommended rate of humus is 4–6 kg per 1 m² (Autko et al., 2012). This amount creates an excellent supply of humus without causing plants to “fatten.”

3. Method. Spread organic matter evenly over the surface and then incorporate it to a spade’s depth (20–25 cm) or with a cultivator. It is important that the organic matter is evenly mixed with the soil throughout the plough layer.

Conclusion:

Adding organic matter is perhaps the most important step in creating a “paradise” for onions. Do not skimp on quality humus, and your bed will become fertile and alive. Onions will reward you with large, firm, and sweet bulbs that store for a long time.

Why Is Crop Rotation Critical for Onions?

Onions are a crop that greatly depletes the soil and, more importantly, accumulates specific diseases and pests. If you plant them in the same place for several years in a row, problems are inevitable. The main reasons:

1. Accumulation of diseases. Soil‑borne pathogens are the main enemies of onions. These include fungi of the genera Fusarium (causing fusarium wilt and basal rot), Sclerotium (white rot), Botrytis (neck rot), as well as stem nematode and onion fly (Autko et al., 2012). With continuous cultivation, their spores and larvae accumulate in the soil, and the risk of infection increases each year. Fighting them later becomes very difficult.

2. Depletion of nutrients. Although onions are not the most “hungry” crop, they remove significant amounts of potassium and phosphorus from the soil. Repeated planting depletes the soil, and replenishing reserves with fertilisers alone can be difficult, especially on a small‑scale farm.

3. Phytotoxicity (soil fatigue). Root exudates of onions can accumulate in the soil, inhibiting the growth of their own plants in the following season. This phenomenon is called “soil fatigue.”

The Golden Rule of Crop Rotation for Onions

Ideally, onions should be returned to the same spot no earlier than after 3–4 years (Hochmuth & Sideman, 2023; Autko et al., 2012). During this time, most pathogens specific to onions lose their numbers, and the soil recovers its structure and fertility.

Which Predecessors Are Best for Onions?

Onions like to grow after crops that leave clean, loose soil and share no common diseases. The best predecessors are:

• Cucumber, zucchini, pumpkin. These cucurbits actively consume organic matter but leave the soil structured and free from onion pathogens (Autko et al., 2012; Kasynkina & Kudin, 2018).
• Early potatoes. Also a good predecessor, especially if organic matter was applied under them.
• Tomato, pepper, eggplant. These solanaceous crops do not share common diseases with onions.
• Cabbage and other brassicas (radish, turnip, daikon). They are also good predecessors for onions.
• Green manures (mustard, phacelia, vetch). This is an ideal option! Sow green manures in autumn, and in spring incorporate them into the soil—you will get a clean, healthy, and fertile bed.
• Perennial grasses (especially legumes). A sod of perennial grasses is an excellent preparation for onions (Autko et al., 2012).

Which Predecessors Are Bad for Onions?

• Any type of onion (bulb, spring onion, leek, garlic), as well as other alliums (e.g., gladioli, tulips). They all suffer from the same diseases and pests.
• Corn and sunflower. After them, specific pests (e.g., wireworms) may remain in the soil (Autko et al., 2012).
• Crops under which herbicides with long residual activity were applied. If you do not know exactly which products were used, it is better not to risk it.

What If the Plot Is Small?

Under limited space, following a 3–4‑year rotation is difficult but possible. Here are some practical tips:

• Divide beds and alternate crops by year. For example, one bed has onions, another carrots, a third tomatoes. Next year, swap them.
• Use green manures. Even if you cannot leave a “clean fallow,” sow mustard or phacelia on the bed after harvesting onions. They will improve the soil and provide organic matter.
• Do not plant onions after garlic or vice versa. They are different crops but share common diseases and pests (onion fly, stem nematode).

Brief summary:

Crop rotation is the best free disease prevention. Do not neglect this simple rule. By planting onions in a new place, you give the soil a rest and recovery, and yourself a guarantee of a stable, healthy harvest. Remember: onions love clean, “rested” soil.

Why Is Autumn Preparation So Important for Onions?

Onions are sown or planted very early in spring, as soon as the soil allows. In autumn, we have time and opportunity to do all the labour‑intensive work without haste. Over winter, the applied fertilisers and organic matter have time to “ripen,” transform into plant‑available forms, and the soil settles and compacts naturally, which is important for uniform seed placement.

Step‑by‑step plan for autumn work:

Step 1. Remove the predecessor and primary tillage

As soon as the plot is free from the previous crop (e.g., after early potatoes, cabbage, or greens), do not postpone preparation.

1. Remove all plant residues. Collect tops, roots, and weeds. If they are healthy, they can be put in compost. If the plants showed signs of disease, it is better to burn them or remove them from the site to prevent the infection from overwintering in the soil (Autko et al., 2012).

2. Carry out “provocative” irrigation (if possible). If the autumn is dry, you can water the bed to provoke weed seed germination. After they emerge, they are easy to destroy during subsequent tillage. This will reduce weed infestation in the next season.

Step 2. Apply organic and basic fertilisers

This is the main task of the autumn period. Onions are very responsive to organic matter, but as we said, fresh manure cannot be applied in spring. Autumn is the perfect time for this.

1. Organic fertilisers. Apply well‑rotted manure, humus, or compost at the rate of 4–6 kg per 1 m² (Autko et al., 2012). Spread the organic matter evenly over the surface.

2. Phosphorus and potassium fertilisers. These are the main nutrients for forming a large bulb and good storage quality. They are applied in the main dose in autumn because they dissolve slowly and need to be evenly distributed throughout the plough layer. A rough guide: per 100 m², about 2–3 kg of superphosphate and 1.5–2 kg of potassium sulphate (potassium sulfate) or potassium magnesium sulphate. Important: potassium chloride is best avoided for onions, as it can reduce bulb quality and storage (Kasynkina & Kudin, 2018).

3. Liming (if needed). As we discussed in the pH chapter, onions are very sensitive to acidity. If your soil test shows a need for liming, do it in autumn. Dolomite lime or lime must be well mixed with the soil and allowed to react over winter and spring. Applying them in spring just before planting is not allowed (Hochmuth & Sideman, 2023).

4. Nitrogen fertilisers. Nitrogen fertilisers (ammonium nitrate, urea) are not applied in autumn! Nitrogen is very mobile and is easily leached by meltwater and also volatilises as gas. All nitrogen is left for spring.

Step 3. Deep digging (ploughing)

After fertilisers and lime are spread, the soil needs to be dug deeply.

1. Depth. Dig to a depth of 25–27 cm, i.e., a full spade’s blade (Autko et al., 2012). Onions have a fairly deep root system, and in loose, deeply tilled soil, roots will develop better.

2. Do not break up clods! In autumn, do not fine‑till or level the soil. Leave large clods and lumps. This offers several advantages:

• Better freezing. A cloddy structure promotes deep freezing of the soil, which helps destroy pests and disease‑causing organisms overwintering in the top layers (Pantielev, 1986).
• Moisture recharge. In spring, meltwater will be better retained in the furrows between clods, providing a good moisture reserve.
• Self‑loosening. Over winter, the clods, under the action of frost, moisture, and wind, will themselves break down into fine particles, and by spring the soil will be fine‑crumbly and loose.

Alternative method for small plots:

If your plot is small and you do not want to dig deeply, you can use deep loosening with a flat cutter or cultivator to a depth of 15–20 cm. This method is less damaging to soil structure and does not turn lower layers to the surface.

Step 4. Sowing green manures (optional but highly desirable)

If you harvest the predecessor early, say in August, you have time to sow green manures. This is an ideal solution for improving and enriching the soil. Sow mustard, phacelia, or winter rye. Green manures will grow quickly; over winter, their green mass will decompose, and by spring you will have a bed rich in organic matter. In spring, they are simply incorporated into the soil during digging or tilling.

Brief summary:

Autumn preparation is not just digging, but a comprehensive effort to create a fertile and healthy “bank” for your onions. Properly applied organic and phosphorus‑potassium fertilisers, deep digging, and leaving the soil under “winter fallow” guarantee that in spring you only need light tillage before planting.

Why Form Beds at All?

1. Soil warming. Onions are a heat‑loving crop, and the root system actively works only in warm soil. Raised beds, especially on light sandy and loamy soils, warm up faster in spring, allowing planting at optimal early dates (Pantielev, 1986).

2. Drainage. Onions absolutely cannot tolerate waterlogging. Excess moisture causes basal rot and fungal diseases, especially in cool weather. Raised beds ensure runoff of excess water, which is particularly important on heavy soils and in regions with excessive rainfall (Hochmuth & Sideman, 2023).

3. Ease of care. Beds with clear boundaries simplify watering, weeding, and loosening. You do not step on the root zone, so you do not compact the soil. This is especially relevant for onions with weak fibrous roots.

Basic Principles of Bed Formation:

1. Bed height: high or low?

• On light, well‑drained soils (sandy loam, light loam). The best option is low, flat beds with a height of 5–10 cm (Autko et al., 2012). On such soils, water does not stagnate, and a slight elevation ensures good warming. High beds are unnecessary here and may even cause the soil to dry out in hot weather.
• On heavy clay soils and in regions with frequent rainfall. The best choice is high beds (15–20 cm), often called “warm” or “raised.” They provide excellent drainage, allowing roots to breathe and avoid wet feet. In such beds, onions feel comfortable even in rainy summers (Kasynkina & Kudin, 2018).

2. Bed width.

Our goal is to create a bed whose middle you can easily reach by hand from the path without stepping on the soil. The ideal width is 70–100 cm. This allows you to arrange 3–4 rows of onions with row spacings of 15–20 cm (Autko et al., 2012). Narrow beds are easier to work and better ventilated, reducing the risk of fungal diseases.

3. Pathways.

Leave pathways 30–40 cm wide between beds. You will walk, water, weed, and harvest along them. Pathways can be mulched with sawdust, straw, or simply compacted. The main thing is that they are clean and not overgrown with weeds.

4. Bed orientation.

If possible, orient beds north‑south. With this orientation, plants receive maximum sunlight throughout the day, which is especially important for onions, which are very light‑loving. Plants will warm evenly and not shade each other (Autko et al., 2012).

5. Formation technique.

The simplest way: use a rake or hoe to form a bed of the desired height. You can use boards or edging tape to secure the edges. It is important that the bed surface is level—this guarantees uniform sowing or planting depth. To achieve this, after forming, run a rake over the bed, breaking any remaining clods and levelling the surface.

Advice for Northern Regions and Cold Soils:

If you live in a region with a cold and long spring, you can use the ridge planting method. Immediately after planting sets or sowing seeds, form small ridges 10–15 cm high. This allows the soil to warm up faster in the sun, and the ridges serve as natural drainage for meltwater. Onions on ridges often produce earlier and more stable yields.

Brief summary:

Proper bed formation is the art of creating an optimal microclimate for the roots. Raised beds give onions warmth and drainage, while proper width and orientation provide ease of care and maximum light. Spend 15–20 minutes creating perfect beds, and onions will reward you with healthy growth and large, sweet bulbs.

When Is Disinfection Really Needed?

1. Presence of diseases in the previous season. If in previous years there were outbreaks of fusarium wilt, white rot, neck rot, or other onion diseases, disinfection becomes a necessary measure. The causal agents of these diseases (fungi Fusarium, Sclerotium, Botrytis) can survive in the soil for several years (Hochmuth & Sideman, 2023; Autko et al., 2012).

2. Violation of crop rotation. If you cannot maintain a 3–4‑year rotation and onions are forced back to the old spot too soon, the risk of pathogen accumulation increases. In this case, disinfection becomes insurance.

3. Heavy, poorly drained soils. Under such conditions, onion roots are more vulnerable to root rots, especially in rainy years.

4. Doubtful planting material. If you are not sure about the health of onion sets, their preventive treatment is mandatory, but this relates to treating the planting material itself, not the soil.

It is important to understand: Soil disinfection is not a panacea. It does not replace proper agronomic practices. It is only an additional tool to reduce the initial infectious load. Without crop rotation and proper care, even treated soil will eventually become a source of disease again.

Main Methods of Soil Disinfection:

There are several approaches, from simple and eco‑friendly to chemical. For home gardens and small farms, the first two are optimal.

1. Agronomic method (preventive).

This method includes a set of measures we have already discussed in previous chapters:

• Deep turnover of the plough layer in autumn. Bringing lower layers to the surface helps freeze out overwintering stages of pests and pathogens (Pantielev, 1986).
• Sowing green manures. Some green manures, especially mustard, oilseed radish, and other brassicas, have phytosanitary properties—they suppress soil‑borne pathogens (biofumigation).
• Crop rotation. As we said, this is the most reliable and environmentally friendly way to prevent disease build‑up.

2. Biological method (eco‑friendly).

In recent years, biological preparations based on beneficial microorganisms have become increasingly popular. These are safe and effective products:

• Trichoderma (e.g., products based on Trichoderma harzianum). This antagonistic fungus suppresses many pathogens, including fungi of the genera Fusarium, Rhizoctonia, Pythium. It is applied to the soil in spring, 1–2 weeks before planting (Hochmuth & Sideman, 2023).
• Bacterial preparations (e.g., based on Bacillus subtilis). These bacteria also act as pathogen antagonists, improve root nutrition, and stimulate plant growth. They can be applied to soil or used for treating planting material.

Biopreparations are safe for humans, animals, and beneficial insects. They work “gently,” but require application at recommended doses and under favourable weather (warm, moist soil).

3. Chemical method (last resort).

Chemical disinfection (fumigation) is generally used only in commercial vegetable growing and in cases of serious disease problems. For home gardens, this method is rarely used because of complexity, environmental hazards, and the need for a long interval between treatment and planting (Autko et al., 2012). Using chemical fungicides on small plots is a complex issue requiring strict adherence to instructions and safety measures.

Practical Recommendations for the Gardener:

1. Start with prevention. Proper crop rotation and organic matter application are the best “disinfection.” They create a healthy, balanced soil where pathogens cannot thrive.

2. If problems occurred in the previous season, use biological products. They are safe, effective, and eco‑friendly. Apply Trichoderma or bacteria to the soil in spring, 1–2 weeks before planting.

3. Treat planting material. Dressing sets (e.g., in a fungicide or biological solution) is a much more targeted and effective protection method than treating the entire soil.

4. Use chemicals only as a last resort and strictly according to the label, if you are sure other methods are not working.

Brief summary:

Soil disinfection is a tool that should be applied selectively when there is a real threat of disease. In most cases, for the home gardener, preventive measures (crop rotation, green manures, organic matter) and, if necessary, biological products are sufficient. Healthy, structured soil with a rich microbial community is itself the best protection against diseases. Remember: your goal is not sterile earth, but living, fertile soil in which onions can grow healthily.