Too much muck? (manure)

One of the things I love about gardening is that you’re always learning. There are always new things to discover. And sometimes I find out that I’m wrong, which is pretty valuable in itself. This post is one of those occasions. I thought I knew the answer, but I didn’t.

It started with this email from newsletter reader Alan.

… there is a question I hope you’ll be able to answer if you have the time, as I’m getting conflicting advice. I have access to an ongoing supply of horse manure, and I fill my compost bins up with it, also topped up with regular supplies of grass cuttings throughout the summer. In autumn, I merge these bins into one and leave the one pile over winter. In spring I spread the by now well rotted compost onto the garden prior to planting. Is this too much manure for the garden doing this on a yearly basis? Some sites say well rotted manure is only beneficial, others say it can result in too much potassium and insufficient nitrogen.

What are your thoughts?

My reply (edited slightly),

Back before the motor car, the market gardeners of Paris would pay boys to collect the horse droppings from the streets. This would be composted which, for some reason, involved men stomping through it in bare feet with trousers rolled up – definitely a contender for the worst jobs in history award! Although it did keep their feet warm in winter.

Once well rotted, the manure would be laid on the plot and the crops sown. Each year the plots grew thicker and some were metres higher than when taken on due to layer upon of layer of manure. The quality of the produce was renowned, even in London.

The benefits of manure are not just the macro-nutrients, NPK, a lot are due to the organic matter’s effects on the soil. Improved ecosystem, more worms working below the surface and better water retention in dry weather, better water absorption in dry.

The only problem I’ve found using a lot of manure is the soil being so light and fluffy it can be hard to walk across without sinking below shoe level! With brassicas, you need to really tramp it down to firm them in. One other thing to watch for is the pH, just pick up a test kit and then lime if necessary.

As for diminishing levels of potassium, I simply don’t see it. Manure still rotting down can cause a temporary nitrogen depletion, but it is temporary.

There are a couple of articles on the site you might find helpful:

NPK Nutritional Values of Animal Manures & Compost Etc

Farmyard & Animal Manures to Improve Soil Fertility

Further Thoughts

Later a couple of things occurred to me, so I emailed again as I wondered if there might be Aminopyralid in the manure or a persistent selective lawn herbicide like Weed & Feed had been used on the grass.

Some time later, Alan got back to me with this.

… Last year I wrote to you regarding poor germination, spindly growth, etc. and you suggested the rotted manure I was using may be contaminated with Aminopyralid (think I got that right). I sent soil samples off to the RHS for testing, and they basically said too much manure in the soil. All the mineral elements were within safe limits. Quite surprised at this, as I’ve always mixed green and brown, and left it to well rot, but that’s the test results. They’ve advised me not to manure for at least three years. Thought it might interest you

I replied,

That’s interesting and, to me at least, unexpected. I can see how the manure could cause the soil to become too acid but hadn’t considered how an excess of the main 3 nutrients, NPK, can cause ‘lock-up’ of magnesium and presumably some other micro-nutrients. So, in the same way as we sicken for lack of vitamins, plants cannot thrive without their micro-nutrients.

When the pre-war gardeners developed the three-year rotation, manuring once in the cycle, they knew what they were doing!

Thanks for letting me know.

Further research on excess manure

Being as the weather is pretty nasty, I did a little more research on this. I think I understand how too much manure can cause problems now. In fairness to me, it’s a pretty unusual problem that I’ve not suffered. Even when I was lucky enough to get some manure mountains delivered after my neighbour cleaned out his lambing shed.

The real problem with too much manure isn’t excess nitrogen. Whilst too much nitrogen can scorch roots, which is why you shouldn’t use poultry manure with its high nitrogen levels directly on the soil, it doesn’t remain in the soil that long. NO₃ is the most plant-available form, but also the quickest to disappear.

It can leach out with water or get used up by microbes, so it ceases to be a problem in a matter of a couple of weeks.
Ammonium (NH₄), remains longer because it binds to soil particles. This can take a couple of months to reduce to harmless levels.

How long these last depends on a number of factors:

• Soil type; sandy soils leach nitrogen more quickly than heavy clays.
• Water; heavy rain or irrigation will basically wash out nitrogen.
• Weather; warm weather with lots of moisture in the soil will help microbes ‘eat’ the excess nitrogen.
• Plants; a lot of plants growing foliage will utilise the soil nitrogen quickly.

The Main Problem – Phosphorus

The real problem with excess manure is phosphorus. When large amounts of manure are applied to soil, especially poultry, dairy, or pig manure—the phosphorus (P) concentration can exceed the soil’s capacity to utilise or immobilise it.

Unlike nitrogen, which cycles rapidly through volatilisation and microbial processes as mentioned above, phosphorus is relatively immobile and tends to accumulate in the upper soil horizons. The upper horizons are where plants get most of their nutrients and where soil structure has the most effect on plant growth.

1. Chemical Fixation and Saturation

Under normal conditions, phosphorus binds to aluminium, iron, and calcium compounds in the soil, forming moderately stable complexes. However, when manure is applied in excess, these binding sites become saturated. Once saturation occurs, additional phosphorus remains in more readily plant-available forms, increasing the risk of nutrient imbalances.

2. Antagonistic Effects on Micronutrient Uptake

Arguably the most serious problem, high concentrations of plant-available phosphorus can interfere with the absorption of specific micronutrients, particularly:

• Zinc (Zn)
• Iron (Fe)
• Manganese (Mn)

This occurs through competitive inhibition at the root surface and through metabolic interactions within the plant. Elevated phosphorus levels can suppress the expression of transport proteins responsible for micronutrient uptake, leading to functional deficiencies even when the soil contains adequate amounts of those micronutrients.

3. Physiological Consequences for Plants

Micronutrient deficiencies induced by high phosphorus levels commonly manifest as:

• Interveinal chlorosis – yellowing of the leaves between the veins, especially with Fe and Mn deficiency
• Reduced root elongation
• Stunted shoot growth
• Delayed maturity

Because phosphorus is essential for energy transfer, which uses adenosine triphosphate, plants may continue to grow initially, masking early symptoms. Over time, however, the micronutrient imbalances become limiting factors for metabolic processes such as photosynthesis and enzyme activation.

In short: Excess phosphorus doesn’t directly poison plants, but it starves them of other nutrients they need to thrive.

How much is too much manure?

For most growers, too much manure just isn’t a problem. Most just haven’t that much manure available. I’ve used a wheelbarrow (80 litres) of well-rotted manure per square metre when growing cauliflowers, with great results. The amount you can use on your plot will depend on what sort of soil you have and the base nutrient levels in that soil.

I think that applying a wheelbarrow load per square meter every three years should generally not cause any of the problems of excess manure.