Maximising use of Organic Manures

With fertiliser price once again moving, it is important to remember the value of your organic manures.

The Quality and Quantity, type and timing of application will all have an impact on the nutrient availability of the manure to the crop. It is important to understand these factures to maximise the utilisation of the resource on your farm and ultimately be able to impact on your fertiliser application.

Making the most from what you already have…


The Nitrogen, Phosphate and Potash in an average 5 tonne application of Farm Yard Manure is worth £27.86p at today’s fertiliser prices, and a 2000 gallons application of 6% slurry equates to £23.69p.

This makes the muck heap and lagoon two of the best assets on the farm.

These values apply to ‘average’ samples. The nutrients you have available to you depends on many things and doing a muck and slurry analysis will help to establish just what level of fertiliser is in your organic manures.

Once you have an analysis figure, it must be remembered that current figures suggest only a proportion of these total nutrients are actually available to the crop in the year of application. These amounts can vary with timing, mode of application and the ability of your soil to respond to application.

Nitrogen is the nutrient that is most affected here. As much as 90% of the total nitrogen content of your organic manures may well end up being wasted due to poor application timings, type of application and soil pHs.

Target Applications

Organic manures are a rich source of nutrients, but the ratio of the nutrient content isn’t balanced, especially where sheep and cattle are concerned.

If we look at the table again we can see that for FYM at 5 tonnes an acre we are applying

6:32:79 units which gives us a  N:P:K  ratio of  roughly 1 : 5 : 13  (for a fertiliser like 20.10.10 this would be 2:1:1),  and for slurry 1: 1.3 : 3.6.  If we exclude the nitrogen we can see that in both cases ever unit of P we are apply we are applying roughly 2.5 units of K.  So targeting organic manures to crops and soils which will benefit from potash applications is vital (silage ground and low K index soils).  By doing this, we can start to reduce fertiliser costs and/or improve quantity and quality of grass production.

Consistent over application to the same fields over time will increase soil reserves, but with this comes potential problems such as staggers risks and dry cow problems.

The other thing here to consider is replacement cost. For example, grazing grounds on K index of 2 has no requirement for extra Potash, due to the recycling of nutrients of the animals grazing it. Conversely Silage grounds with a K index of 2 still have a large potash requirement. If we look a the values of potash in the table above every 5t of FYM we apply to grazing ground at a K index 2 we are applying £16 (£12 for 2000 gallon of slurry) of potash that isn’t needed, and could be applied to silage ground.

Improving the value of your slurry.

Just like your soils, your slurry is unique to your farm. When we do calculations like the one above we look at average slurry results which can be very misleading.

Remember that someone who has one hand in the fire and one hand in the freezer is on average comfortable….

Measuring the nutrient content of FYM and Slurry is the way to ensure that your figures are right.  Taking a uniform and representative sample is very important here, as consistency and therefore the nutrient value of the slurry will change as you empty the lagoon or tank.