Managing your plot - fertilisers

Prompted by the following question by a member...

Obviously manure of various types can provide nitrogen but I'm not sure which also make the soil more acid. I don't think chicken poo does but it certainly smells of ammonia.

...here are a few thoughts on different fertilisers, and the need for liming.

Firstly, nitrogen fertilisers do have an acidifying effect on the soil. Ammonium containing fertilisers have the largest acidifying effect and ammonium sulphate is the worst. In short, ammonium oxidises to nitrate, effectively giving nitric acid, while the sulphate in sulphate of ammonia results in sulphuric acid. When nitrate and sulphate leach over winter, they remove calcium ions, leaving behind H+ ions in the soil, which if you remember your O-level chemistry means the soil becomes more acidic. The main reason we add lime to soil is to replace the calcium which is lost over winter.

The relative acidifying effects of different fertilisers (measured as the amount of calcium carbonate required to neutralize the effect of the fertiliser) is as follows:

1kg sulphate of ammonia  = 5 - 7 kg CaCO3

1 kg ammonium nitrate, or urea = 2 - 3 kg CaCO3

1kg calcium ammonium nitrate = 1-5 - 2.5 kg CaCO3

1kg calcium nitrate = Nil

So should I be worried ? If the plot you are working on Hill Rise is the original, unadulterated clay soil we have on most of the allotment site (chalky boulder clay) then there is so much calcium in the soil, you won’t need to add lime.

If you have beds and imported top soil, then you may need to check soil acidity (pH) every now and again. Similarly in your garden, if you are down near the centre of town and not on the clay, you could need to check pH. February is a good time to do this, before any fertiliser has been applied.

What about organic manures (cattle muck, horse manure, chicken litter)? Well, exactly the same processes apply. Most muck contains readily available N as ammonum as long as the heap stays anaerobic. In addition, the organic matter the muck contains, slowly breaks down releasing ammonium which is then oxidised to nitrate and so on.

But you are probably applying much less nitrogen as manure, than when applying artificial fertiliser. The exception is chicken manure which is very rich in nitrogen (hence the smell of ammonia noted above!). By the way if you can smell ammonia, it means you are losing the nitrogen. If you are worried about the environment, ammonia will eventually come down somewhere as acid rain. Try and dig chicken manure in within 24 hours.

Compost has very little readily available nitrogen, but you can use it to add organic matter to soils which helps workability as long as you dig plenty in, and the nitrogen will slowly become available over time.

Finally, green waste compost does have some liming (neutralizing) value of its own, depending on source -  you could check with the supplier for a certiicate of analysis.