Only soil i have for new raised beds is rotted manure

[mistymoo]

Hi, can anybody help or advise me please? I have 5 very deep new raised beds this year, all we had to fill them is tons of 18 month old manure ( from my own horses). I macerate it so it rots very fast. I have a PH tester and the results are ok for what i grow in them ( onions,brassicas,corn,pak choi at the moment).
My brassicas are still young and now the leaves are turning yellow, i think this may be magnesium shortage am i right? Today i watered with liquid food i got for my toms, now will this suffice or do i need to do something else?
We do keep chickens and have read the poop is good for magnesium levels so i could gather some and put that on the soil ( it would be fresh unless i sift through my compost heap for the last two weeks dirty bedding deposits?) Any tips please?

corn is dead now anyway due to frost, pak choi only been out 3 days so its still early to tell, apart from slug damage no other signs of problems, onions seem ok.

[WendyC]

If the manure is 18 months old I guess it has stopped rotting so should be fine to plant it. I wouldn't add anything else to the beds, pure manure is a rich growing medium already. Yellow brassica leaves suggest nitrogen levels are low, rather than magnesium, probably used up all that was left in the compost they are growing in. The other issue with brassicas is they like a firm soil to grow in and what you might end up with is huge cabbage leaves but no heart. Still edible but perhaps not what you intended! Tomato food is to help with flowers and fruiting which is definitely not what you want your cabbages to do. When you say they are small how small is that. I planted some out yesterday with 3 or 4 true leaves and that's about what I'd usually do. Hope that's of some help.

[mistymoo]

thanks for the reply, so nitrogen low i must have read it wrong! i have not read up on this yet, what do i apply for this please?
the brassicas have around 4 leaves so only young.

[rustylady]

Did you sow your brassicas direct into the bed, or in pots?

[mistymoo]

in pots i did cabbage, early Purple brocoli, cauli, the cabbage has been in for a few weeks and looks ok, its the new stuff only just gone out purple broc and cauli that are yellowing.
my sprouts are in an old bed on other side and they are ok.

[allotmentor]

I would have thought it unlikely that N would be a problem unless you have had a lot of rain through the manure. When you say that the pH was OK, what was it? As well as the firm soil mentioned by @WendyC Brassicas do like a high pH (because of club-root and some mineral deficiencies at lower pHs) - see eg: Growing Brassicas from Allotment Vegetable Growing Advice, General Brassica Advice