Ah, The Good Old Days! Gardening always has a whiff of nostalgia about it. I remember the time when all vegetables, save winter cabbage, sprouts and leeks, were carefully lifted from the ground and gardeners would dig away merrily during the autumn days in order to allow winter frosts to break up very sticky clods. No-one worried much about fertility. Allotments were spacious, fertilizer was cheap and we didn’t cross our fingers in fear if anyone mentioned super-phosphates.
But the roof top veg plot has very different requirements. I grow organically; I irrigate (which can leach goodness out of the soil); I intercrop and I plant salads as tightly as possible. I see bare earth as a sin and any planter more than six inches deep as a gift from God. So during the month of October, when I’m taking out spent summer crops and re-planting, is a good time to consider increasing soil fertility.
Soil fertility is a complex issue, far too complex to cover in just one blog article, so I shall concentrate on some of the methods that work well for the rooftop.
Mulching
Last year’s fallen leaves, straw or grass cuttings all make excellent mulching materials. You can buy forest bark in various stages of decay that also makes excellent mulch. But mulching is not feeding. In fact bark and fallen leaves, though they will rot down to improve the soil will leach out nutrients as they do so. So beware of mulching around crops that need all the nitrogen they can get.
Last year I spread leaves over my fallow raised beds, but then discovered that a covering is essential, as they do tend to fly away.
Never mulch seed beds or seedlings, they won’t like it.
Mulching can tend to attract slugs as well, though I guess if they are eating your mulch they will be less attracted to lettuces on bare soil nearby.
The mulch does keep in moisture, so straw laid around the courgettes in mid-summer, or under strawberries as they crop can improve yields as well as protecting the crop. Straw before it has rotted down is too itchy for slugs, so it keep them off, but as soon as the material starts to rot slugs will be attracted to all that putrefying mess.
Feeding
During the summer I buy liquid feeds. The seaweed one is my favourite commercial variety. If you want to get technical there are different mixes to encourage leaf production, strong roots or fruit. I also gather worm tea from my compost bin. Incidentally, the easiest way to make a litre of liquid plant food is to put all your kitchen waste into a plastic bag and tie the tops together. After about a month you will see the brown uggy liquid collecting in the base of the bag. Simply snip off a corner over a milk bottle and let the amber nectar ooze out. The partly decomposing waste can now be placed into the compost heap, taking up far less room that it would have done previously.
I’ve also seen buckets with a permeable centre cylinder. The idea is that you pack the cylinder with nettles or comfrey, fill the whole thing with water, wait a few weeks for the whole lot to rot down, then pour. A good tip is that the nastier it smells, the better it is.
These all work very well for a quick boost, but they don’t have the umph for a more through boost. If I’ve been growing hungry crops like tomatoes or calabrese and I do have some fallow time, I can sprinkle with chicken pellets. These dried dung-bombs are odourless and easy to handle. But beware; chicken pellets can’t be placed directly around root growth, because they will burn it. However they can be sprinkled into the compost heap, perhaps after emptying all that spent potato soil in there?
Composting
Home-made compost is one of the best mulches for my roof top plot because I can plant it up straight away. It improves the soil and, as long as it is the product of green and brown waste (leaves and stems) it will have a good mix of nutrients. I shall be clearing my tomato and carrot beds this month. Before I start I shall clear all the good compost out of the compost box and bag it up. Then I will have room for spent soil from the beds and pots that can go straight into the compost heap, mixed with a nice proportion of chicken pellets. That will be okay to use within about a month, but my winter needs will be rather modest by then anyway. Then I shall mix some compost from the bags into the spent soil. I may also add some vermiculite to increase the water retentive qualities of the soil, or expanded coya to lighten the soil. I can plants seedlings directly into this mix and they will get away well.
My compost always has a lot of egg shells in it. We run a B&B and get through hundreds of organic free range eggs every month. I crush these into the compost, but they often don’t break down straight away. Never mind. The slugs hate finding egg shells in their paths, the compost worms make little houses out of them and egg shells have been shown to be better than lime in long term field tests. As egg shells break down they slowly release lime into the soil. That will help to balance the acidity of my pots and raised beds, keeping them sweet and helping ensure that the plants can take up other nutrients I place in the soil mix.
The books recommend that you buy special compost worms to populate your heap, but my worms have arrived quite naturally. I dont know where they come from, but they seem to survive quite happily. I provide them with straw for nesting in and I do water the heap if it seems dry. I also have a problem with slugs in the heap. I let them live for a while, but my firnd the compost guro, Mr Compost inspected the heap recently and pronounced the slugs a danger.
"They'l lay their eggs in there," he said.
So now, much against my nature I squash any I see. Slugs are predated by nematodes, small wriggly worms, that I've also seen in the heap. So with a bit of luck the system will right itself.
Fixing Nitrogen
Climbing beans, peas and sweet peas are over now. I’ve left a few runners on the vine for seeds and dried beans, but soon I will start to clear them away as well. However I won’t pull the roots. Instead I shall cut the vines off at soil level, and mix in compost with the spent roots of the legumes. If I look closely I will see pale grey nodules about the size of pepper corns clinging to these leguminous roots. This is the nitrogen that this clever little family of plants produce, or fix. And because it is natural, it won’t harm an immediate planting of tender salads or seeds. Nitrogen is just what they need in order to produce green, leafy growth.
Woodash
I now burn wood briquettes and logs in my fire, instead of smokeless fuel. If you have an open fire, or you can burn the woody waste from the garden in a barbeque, this makes another wonderful mulch. Ash from wood fires can be a useful additive to the compost heap or can be applied directly to fallow ground and dug in. It provides a natural source of potassium, associated with flowering and fruiting and it contains useful trace elements. Like egg shells, it also has a liming effect, so wood ash can remedy the excessive acidity that always tends to form in pots and raised beds.
