The question often get asked, “Isn't urea is the best type of fertiliser to use at this time of year?” That’s a bit like saying size nine is the best size of shoe to wear.

It’s not a one-size-fits-all situation. It is also sometimes thought that urea works better than CAN-based fertilisers in colder temperatures and that’s why it’s used in the spring, again not true. Urea is by far the cheapest form of fertiliser per unit of nitrogen, but for ureic nitrogen to be absorbed by plant roots it must first be converted to the ammonium form and then the nitrate form by soil bacteria.

Here comes the science

Water must be added to ureic nitrogen for this conversion to take place.

This can only happen in moist soils and explains why urea is used early in the season and not generally used during the dryer summer months. CAN is 27% nitrogen, half of which is nitrate nitrogen and half is ammoniacal nitrogen.

The nitrate nitrogen is already available for plant uptake, making it quick acting and the other half is slower acting. Because CAN doesn’t have to go through as big a chemical change as urea to make it plant available it is not as vulnerable to being lost to the atmosphere.

However, the response to either form of these types of nitrogen is going to be minimal unless soil PH (lime), P (phosphate) and K (potassium) levels are optimum. The best way to explain it is by saying that growing grass is a bit like running an engine.

Keep the levels right

The nitrogen is the diesel, you fill the tank and when it’s almost gone you top it up again.

The P and K are like the engine oil, you want them to be at the correct level all the time, index three.

We all know that if there is no oil in an engine it doesn’t matter how much diesel you put in, you’re not going anywhere.

The same is true of growing grass – if your Ph, P and K levels are poor it doesn’t really matter how much or what type of nitrogen you apply, you're just not going to grow grass.

Straight nitrogen, be it CAN- or urea-based, is fine on land with optimal soil indexes or on land that has possibly received slurry earlier this spring and has a small cover of grass.

But on bare ground or on index one and two soils something like 18:6:12 will give a much better response.

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