Fertiliser/lawn/horticulture question

2 years ago I re did my garden and made a completely new lawn, I laid turf on about 30cm of compost, sand and manure all dug in.  It didn’t settle very well and I cut it too short so after this winter’s rain it mostly died.  So I removed the few surviving grass plants, added some more sand and compost and planted seed.  It didn’t go that well because we had the warm spell then it got cold again so only half of it germinated, the rest has either died, been eaten or come up slowly. However, it mostly looks like a lawn with a few patches where the cats have been shitting and pissing in it.  I’m not sure if the patches existed which caused the cats to go there, or they made the patches.

However, here’s the question:  around the patches and where poo has laid on the grass has grown much thicker and darker. So clearly this has fertilised the soil.  But I don’t understand how the rest of it can possibly be deficient in nutrients after the amount of organic matter I added.  I’m guessing the urea has delivered nitrogen to the soil, but how much nitrogen can a lawn need? Is there something else in the cat piss that isn’t in compost and manure? I have given it lawn feed now of course.

 I laid turf on about 30cm of compost, sand and manure all dug in

That’s a lot of compost!

But I don’t understand how the rest of it can possibly be deficient in nutrients after the amount of organic matter I added.

Organic matter =/= nitrogen in the short term .  In the short term the bacteria and fungi colonizing the organic matter take up the nitrogen for themselves, it’s only once they die back that it’s released into the soil in an available form.  It’s why you shouldn’t dig things like grass clippings, leaf mulch, chipper wood/bark or manure into the soil until it’s very thoroughly composted.

Mines not looking too bad this year (apart from the 2 tonnes of topsoil I’ve piled up at one end after building a patio round by the summerhouse).  I managed to get in early in a dry spell and scarify it thoroughly, then treated it with iron sulphate,  and timed it just right with the grass seed. It took a little longer than expected to germinate but hasn’t done badly. There’s still dark green patches where the dogs been inspite of all that though!

In the short term the bacteria and fungi colonizing the organic matter take up the nitrogen for themselves, it’s only once they die back that it’s released into the soil in an available form.

Hmm interesting. I dug it all in but most of what I added was new, there wasn’t really any existing topsoil.

You sound like you’re trying to grow grass on a vegetable patch?! Grass doesn’t grow in compost or sand, it grows in soil. If you want a nice lawn from scratch then look at how a farmer grows wheat. Prepare the soil in late summer/early autumn (a fine tilth), then sow, shoot the pigeons, then fertilize it as it grows. Turf is a waste of time unless you absolutely have to be walking on the ground immediately.

Right but soil is made of organic matter, sand and clay isn’t it? It’s a clay area, there is still a lot of clay in the lawn hence topping it up with compost and sand, to even it out.

It’s not that simple I’m afraid. If you’ve got a clay soil then you’ve got a clay soil, no amount of sand is going to change that. If you add compost you’ll make it more loamy, but that’s a process that will take years of repeated applications, and it’s still not what grass needs. Grass will grow fine on clay, in fact clay is ideal for growing grass because it’s nutrient rich and moisture retentive. If your soil is especially poor, then buy in some good quality topsoil and that will get things going. But you don’t need that much soil to grow grass. The main thing is that you start it at the right time of year and make sure you grow varieties (plural for resilience) that suit your heavy soil. Any decent supplied should be able to advise you. Starting now is pretty much the worst time of year for a lawn (seed or turf). Autumn is good if it won’t get much use over winter, or else early spring.

Try TheGrassPeople for seed as they explain what their different seed mixes are best for and have lots of advice for getting grass to grow.

Grass will grow fine on clay

Not in my experience. I’ve lived in this house for 17 years now, the original lawn was rock hard in summer and damp and poorly draining in winter, and that’s with topsoil added by the builders.

If you add compost you’ll make it more loamy, but that’s a process that will take years of repeated applications

I created a brand new surface 50cm above the old one with about 2/3 ton of organic matter and sand, in a 3×5 area. That wasn’t a slow process.  Some of the existing clay is in there and some of the sub “soil” which is glacial till.

Starting now is pretty much the worst time of year for a lawn

That’s why I did it in March.  The question was more about how fertiliser works and the nutrient content of soil.

It’s why you shouldn’t dig things like grass clippings, leaf mulch, chipper wood/bark or manure into the soil until it’s very thoroughly composted

I’ve learned something useful from this thread!

I lazily dig half rotted matter into the soil thinking it will do the rest in there – now I know not to, thanks!

Hmm.. if adding manure is a bad idea, is it the cat poo or the pee that has added nitrogen?

My only experience is from a female dog perspective, cats may be similar. When they wee on grass it all goes in a very concentrated area. The nitrogren content in the urine at that point is too strong and burns/kills the grass in the immediate area. But as it weakens on the periphery then it acts as a good fertiliser. This is why you get a dead patch surrounded by very strong growth around it.
We use the volcanic dog rock things in our dogs water. I don’t know the exact science behind it, but they neutralise or reduce the nitrogen content produced and, much to my sceptical surprise, they actually work. You just need to be persistent for about 10 weeks to see the full effect.

I have nothing to add on how to grow a good lawn, we’re lucky that we have good draining soil and grass seems to be happy to grow when I just chuck some seeds on some lightly raked areas in the spring.

Male dog’s pee makes the grass grow – my parent’s lawn always had patches where the male dog’s had pee’d that grew well – no burn unlike female dogs.   Iron sulphate is good stuff, and cheap, for promoting lawn growth. Dilute crystals in watering can, and apply. My last 2kg just ran out after about 3 years.