fertilisers

Hi all help is needed, I am have started a course in horticulture with no previous knowledge of gardening of any kind , (only got into garden last year growing veg with my 2year old son) and I am stuck on one question to which I can not fin an answer, I have looked in every book I have and scanned the net ( how I found you).
Q. What are the main differences between slow release fertilisers and controlled fertilisers. I can give examples of each but can not find any more info on them. If any one can shine any light it would be great

 

They do seem to be very similar in meaning, here is some info on slow release which is usually organic

http://www.mggardens.co.uk/fertilisers.htm

and here is info on controlled release
http://www.pottingmix.co.nz/nutricote.htm

 

OK, here's a bit of info on non organic SR/f / CRF. Slow release fertilisers are aimed at slowing down the release of nutrient. so that it isn't all leached out after the first spell of wet weather.

The slowing down process can take several forms, but the non organic ones usually involve the nutruent being coated in a jacket that breaks down slowly through rainfall / irrigation, so that the nutrient is released over a period of time. Obviously this isn't controlled, because a very wet summer would still cause the nutrient to leach out of the soil profile faster than you wanted it to.

Another older way was to use Didin in the fertiliser, which inhibits microbe activity, therefore slowing down the rate at which bacteria make the nutrient available to the plant roots. There's one in the eye for all you Yakult drinkers - we did th egood bacteria / bad bacteria thing years ago :D

An example of controlled release is the same jacket principal, but using a polymer, sort of "semi permeable" jacket. When you apply the fertiliser, the jacket takes in water. As the temperature rises, the water pressure inside the jacket rises as well, and forces nutirent out throught the jacket (or "prill"). When the temperature cools, the water pressure inside lowers, so less nutrient is forced out. In this way, the prill releases nutrent only when the plant needs it - ie when the tempertatures are higher and the plant is actively growing. By some clever calibration, the manufacturers can get this principalto work over a range of temperatures, so you would have both summer and winter products.

Sorry for the spelling - late for work

 

Thanks Stevew, the pagee shave been of great help. the only difference seems to be the controlled release has a resin coating and a control release agent which determines the speed at which the nutrients are released.

thanks again

 

Ahh thanks Will now it is all perfectly clear, hope you wer'nt too late for work.

many thanks to both of you

 

Not too late - it's Friday after all

I forgot to add that most slow release and controlled release fertilisers also have a "quick start portion", which is an amount of readily soluble nutrient which will start working as soon as it gets watered in.

You can have a lot of fun with these things (if you're really sad and boring that is). It's worth spending a bit of time getting familiar with the NPK ratios, and looking at how they differ from soluble feeds to slow / controlled release. All SR / CRF's should show on the bag a percentage figure of slow release nutrient - for the most part just the N, but slow release K is becoming more common as well. So if the slow release portion is 75% , you still have 25% going out as a quick release, if that makes sense.

 

Thats very interesting Will, I hadn't realised that it was all so technical, but I suppose when you think about it, it needs to be.
I presume Osmacote comes under the controlled release?
Pete

 

Yup, Osmocote is controlled release Pete, but I've never used the stuff before, as I only get involved in the turf side of things - so I don't know exactly how it works.