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Manure burn is one of the most common problems in the vegetable garden. We think we’re doing our vegetables a favour, but many vegetables don’t like manure. In this article we’ll share our experience of vegetables that suffer from manure burn, and when to avoid using manure entirely.
I’ve always found soil fascinating; the microorganisms living within it, and the incredible way it can recover from drought and human interventions. So it probably doesn’t come as any surprise that understanding what we add to our soil, manure included, is just as fascinating to me and, I hope by the end of this article, to you.
Why does manure burn plants?
How much ammonia is in manure?
Nitrogen in manure isn’t significantly higher than nitrogen in compost, the difference is the rate of distribution, and method of distribution of those nutrients, with most nitrogen actively carried by ammonia, which is the fastest chemical to leave the manure, meaning it quickly leeches out, and over acidifies, and overfeeds the roots.
Almost all manures contain 15-20% ammonia, apart from rabbit manure. You can tell the ammonia content in manure by the smell. The more pungent the poo, the more ammonia in it. Rabbit poo has virtually no smell at all and carries its high nitrogen levels in other ways.
How much salt is in manure?
When looking for manures, or manure alternatives, the salt content is crucial. Manures from livestock farming tend to contain around 5-10%, which means they can rapidly dehydrate your plants.
I’ll go into more detail about this later but, essentially, even plants that like manure, like lettuce or leafy greens, can quickly dehydrate if fresh manure splashes onto their leaves in the rain (you’ll see white burns on their leaves).
You can get manures with lower salt contents, but the big choice is between ‘hot and cold’ manures. Hot manures, like cow, sheep, or horse manure, need rotting before use. That rotting means they work better and are gentler to plants. Cold manures, like rabbit or chicken manure, are both higher in nutrients, and gentler on plants without any rotting down required.
What is fertilizer burn?
There are two main types of fertilizer burn: foliar, and root. Foliar fertilizer burn is when highly concentrated fertilizer, like manure slurry, or liquid seaweed, accidentally splashes onto the leaves of plants. Root fertilizer burn is when too much fertilizer is used, or manure is used before it is rotted, leading to overly stressed roots.
Fertilizer burn is caused by ammonia and salt in fertilizers which dry out or over acidify your plants.
How to fix fertilizer burn
If your plants have been damaged at the roots, the best thing to do is to flush them out. If they’re in pots, they should be watered through until water drains out of the base of the pot. This helps to reduce nutrients in the soil. Then don’t water them again until the soil is dry to touch on the surface.
If you notice small brown patches or curled leaf edges without any distinct line between brown and green, it’s likely fertilizer burn. Cut off the damaged foliage with clean trimming scissors, and the plant should quickly recover.
What vegetables don’t like manure?
Keep in mind that manure is most often sold as a ‘soil improver’. That means that it can be dug straight through the soil to improve the structure, nutritional value, and water retention of soil, which is the traditional method of manure application.
These days though, we know that no-dig is actually the best way to improve soil structure over time, and by mulching with manure in the right place, we can add more value to our soil than by any amount of digging.
While all vegetables benefit from manure, its nitrogen is often too much for young plants and for fruiting plants to manage.
Not only do they not need manure, but root vegetables actively dislike manure. Root vegetables grow toward nutrients and produce sweeter, truer, roots when they have to work for their food. By planting root vegetables in enriched compost, or soil with manure, you are overfeeding them, which is highly likely to lead to forked carrots or underdeveloped roots.
Fruiting plants (tomatoes, peppers, aubergines, cucumbers)
Manure’s main active nutrient is nitrogen, which is great for developing roots and leaves, but not necessary for the production of strong healthy flowers and fruit. So for plants like tomatoes, peppers, aubergine and cucumbers, manure isn’t necessary at any point in their growing season.
If you grow tomatoes outdoors, a raised bed with manure mixed through it won’t harm them and can improve the soil structure dramatically, but the effects will be the same as mulching with humus-rich compost.
Avoid using manure mulch, or manure feeds in summer on any vegetables. The increased humidity and heavy, irregular, rainfall can lead to leaf burn on any veg or salad crops. Controlled fertilizers are better to support the ripening and development of leaf, fruit, and root crops.
What manures are the strongest?
Which manure burns most?
Horse manure and cow manure have the highest ammonia content and are the most likely to burn your crops or young plants if used fresh. To limit the ammonia in your manure, you’ll have to rot down your manure for as long as possible.
Which manure burns least?
Rabbit manure is one of the best instant use manures you’ll ever use. We have two rabbits here, and they produce mountains of droppings every day. Those droppings are digested twice, and 90% hay. They are slightly acidic, and quick to break down, releasing a good flow of nitrogen into the soil for around one month after laying, which is perfect for an early spring mulch.
Our council bin collection doesn’t take our garden waste over winter, so we collect their droppings and make a bin of them ready for spring, then mulch every flower bed as soon as the frosts are finished.
While you can’t find fresh rabbit manure at garden centres, you can find powdered rabbit manure on Amazon – check this organic option.
Manure is a powerful fertilizer, used for centuries to grow all sorts of crops by commercial and domestic gardeners alike, but as we begin to understand more about our gardens and what we’re putting into them, it’s important to also understand whys some of the old tricks might not be the best way forward.
There are some vegetables that just don’t like manure, like carrots, tomatoes, peppers and parsnips, but in general, it’s worth remembering that no vegetables like manure if it’s splashed all over their leaves!