You might think that digging over the potato bed has become a long term project.

Still doing a prodigious amount of cropping.  A good harvest this year.

Now you might think that I am taking an interminably long time digging over the potato bed but you are disregarding the opportunity of putting a lot of unwanted stuff in the bottom of a large trench.  If you take out a trench about 2ft wide and 2 ft deep then you can bury an awful lot of stuff in it.  However, I think that I have overdone it a little again.  I have a lot of tourists from the rest of the allotment site coming to look into my trench and ask how much higher I am going to make the allotment beds.

Double digging

I would take a photograph if I hadn't lost my camera.  That just reminded me to look for it again and I have found it in my desk draw, so there may be a picture.  I just need to remember to take some photographs now. The allotment has started to look a little untidy because a lot of vegetables have been harvested and I have not been able to plant green manures - because I have been double digging the potato bed.

It has been taking me so long because I have been looking around the allotment for things to bury while I have a big trench to put them in.  I have buried all of the three year old strawberries and, while I was at it, I planted some of the stolon offsets  in three inch pots using bought peat free compost.  I am going to move the strawberry patch up to where the brassicas are now. They don't seem to survive under a canopy of potato foliage and that is what is going to be planted in the strawberry and roots bed next year.  It helps to sort the strawberries out if I move them.  I can put them into neat rows about 1ft apart, give them a little fertiliser in the form of comfrey inoculated charcoal and maybe a little mychorrhizal fungi. They really  need to be moved now  because I want them settled in before the weather gets too cold.  I'll move them with a large clump of soil so that they will not get too much of a shock and plant them immediately.  I usually get a good crop of strawberries even when treating them like this.  I am hoping to get at least two rows of brand new plants from the plantlets in the pots.

I also cut out the old fruited raspberry canes, cut them up and put them in the trench.  I had several of the canes that grew over seven feet tall.  Is there something about this height of plant and my allotment?  The pink fir apples were this length as well but they were flopping over each other so they did not look 7ft tall.

So I will say again there is absolutely no substance in the internet myth that raspberries and potatoes should not be planted close together.  The only disadvantage that I can see is that the raspberries may shade the potatoes and even this can be alleviated if the raspberries are planted north to south as mine are.

The new raspberry canes were tied onto several wires stretched across the row.  I have three tree stakes, one at each end and one in the middle, holding up the row and two bamboo canes,  got from the inside of rolls of carpets, tied across the top.  Putting the canes across the tops helps to stop the tree stakes from leaning in when the wires are tightened.  Although you don't need to, I reduced the tops to about six inches above the bamboo canes.  I don't need to be greedy  and the unsupported tops will probably break off during the winter anyway.

I don't know how but perennial hedge bind weed, Calystegia sepium  a pernicious weed, reared its ugly head in the raspberries.  There was nothing for it but to take three raspberries out and dig around until all the bindweed rhizomes were discovered and removed.  I doubt that I have found them all so there will be a continuing battle to remove it all over the next few years.  The raspberries are right next to the path so I think that I may have to lift the slabs to see if rhizomes have gone under them.  This is where I put that 3ft x 2ft two inch thick beggar of a concrete slab, which I vowed never to move again, and I am not really inclined to give it another opportunity to fall on my foot.

I replanted the raspberry canes straight away not giving the roots time to dry out and then watered them in with rainwater.  The shed butt has nearly filled up over the last week or so now that it has rained a little.  Several of the canes were ones that I kept from the old row.  These were given to me in 1982 when I took over the allotment and they were a diverse range of raspberry varieties.  I thought that I had only kept the best ones but it seems that one of the varieties I kept produces very small raspberries.  They taste pleasantly sweet but they are very time consuming when picking so I decided not to replant this raspberry and plant another that a fellow allotmenteer  had given me.  I am not sure of the variety.  It was only one plant but I am told that it begins to fruit in July and carries on until the autumn.  Needless to say, it did not fruit at all  this year.

I healed in some Autumn Bliss next to the tap butt last year and they are producing raspberries now but I do not have any room to put them with the other raspberries.  They don't seem to grow as large as the summer fruiting raspberries so I think that I will keep them apart.

I went down the trackway cutting back the hedges and the trees overhanging.  These trimmings went into the bottom of the trench as well.  There is an oak tree that is beginning to shade my allotment with its overhanging branches so I would like to get up a ladder and take them off.  If I do they will go into the bottom of the trenches too.

The next thing I did was to cut out the fruited canes of the blackberry.  It has done very well this year so I am keen on training next years fruiting stems onto the climbing frame supports.  This will prevent the stems from being blown about in the wind and getting damaged. I might buy another blackberry so that I can say to my daughter I have a brand new blackberry and it only cost me a few quid.  Much better than an iphone.

The cabbages that were left over have gotten rather moth eaten - or rather slug eaten, so I decided to take these out and put them in the trench.  I cut off the roots because I never bury them and took them home to put into the green bin.  There were some with club root which I think has come from the compost from the compost mountain.  This has taught me a lesson - do not take home made compost from alien allotments -just in case. Please note I am not being jingoistic - all allotments except my own are foreign and I should have remembered my rule of not accepting plants, compost or soil from other allotments.

The cabbage tops went into the bottom of the trench with all the other unwanted organic matter.  Some of my part rotted compost went on top and then the bottom spit of soil was replaced with the good topsoil on top of that.
If all that woody material does take nitrogen out of the soil then I don't care at this depth because it is much lower than my top soil.  My own brand of Hugelkultur...

Now I have a problem.  I am running out of things to bury so I am resorting to the shreddings left in the bins by the front gate.  They are mostly laurel shreddings but there are other hedge plants in there as well.  It is all grist to the mill.

You might be wondering why I dig deep trenches two spits down and fill them with woody material.  I could tell you that this is to improve the drainage of the allotment,  or to introduce a large amount of carbon to the soil which can be incorporated into the upper layers  when it has decomposed, or to prevent this material from being burnt and releasing valuable nutrients to the atmosphere or a number of other very valid reasons.

But the truth of it is, I just like digging big holes.

Still digging over the potato bed.

I am still double digging the potato bed.  I am putting partly decomposed compost from the compost heap in the bottom trench.  This will decompose during the winter and hopefully provide a good water sponge for developing plants next year.  There will be some nutrient from the breakdown of the organic material but it will not be a lot.  There will also be better drainage.

The ground is very dry even at the depths that I am digging. I had to water the ground today so that my topsoil did not blow away.  Remarkably, there was a little rain today but the ground is already starting to dry.

When digging out the trenches for the double digging I like to make conical mounds of soil.  I learnt this when I was working in the Glasshouse Crops Research Institute mixing up large amounts of potting and seed composts.  We mixed the composts in large conical heaps because the soil will fall down the sides mixing remarkably well.

I decided to take out the pink fir apple potatoes and put the tops at the bottom of the trench.  There are two reasons why I don't usually do this. Primarily, the tops may be infected with blight and this must not be recycled back into the soil to infect another potato crop. The other reason is that if you miss any little potato tubers the potatoes will grow back between the next crops in the rotation.  They are a bind to get out usually requiring a deep hole to find the offending tuber.

Well, there has been no blight this year so there is little fear that I will be recycling disease.  To prevent the potatoes from regrowing on this bed I carefully inspected the roots to ensure that there were no little ones clinging on within the root mass. I am also hoping that the depth that I am burying the tops will also prevent any I miss from being able to reach the soil surface.

I measured one of the pink fir apple tops and it was about seven feet long.  I knew that the tops had nearly reached the height of the new raspberry canes but I did not realise that they grew quite so big.  As I keep saying to my neighbours it is not how big the tops are but how many big tubers grow beneath them.  I only got one or two pounds of potatoes even from the very biggest tops.  The kestrel was giving me 5-6lb of potatoes from each plant and their tops were not as big.

I am recycling two rows of strawberries by burying them in the digging trench.  This is an if I do this then I can do that kind of scenario.  I am putting the strawberries where the brassicas are now.  It is a fairly large bed and can accommodate the strawberries and the peas next year.  To plant the strawberries I need to clean part of the bed and bury any material that I can.  I do not usually bury any brassicas preferring to take any roots and stems home to put into the recycling bin.  This year I have had a little club root something that I have not seen on the allotment for over twenty years now.  I am not sure but I think that it came from the mega compost heap compost.  I am going to be very wary of having other peoples compost from now on, no matter how good it looks when you have sieved it.

The old cabbage tops that I have not been able to eat and have gone very slug eaten can go into the trench.  I am hoping that the trench is deep enough to prevent any club root from reaching the top soil.  The roots and stems will still go home and be put into the green bin.

I have put about 25 strawberry plant runner plants into pots with good bought compost and they have put down adventitious roots.  These will be my new plants to replace the ones that I will bury in the digging trench.

I will start to plant strawberries as soon as I clear the space for them.  If I transplant the strawberries now, they will be able to establish themselves before the winter.  I will plant them with the newly comfrey inoculated charcoal and mychorrhizal fungi.

A myth has grown up about the danger of growing potatoes next to raspberries.  Well if my potatoes grew seven feet of tops while growing right next to raspberries there cannot be much truth to the myth.

The potatoes were great this year and all to the thanks of comfrey inoculated charcoal and mychorrhizal fungi.

Early digging.

I am digging quite seriously at the moment.  I am going down about four spits and burying a mixture of not very decomposed compost; lawn mowings; laurel Prunus laurocerasus shreddings; damson and oak tree branches and weeds.  I am not putting this mixture into the soil to increase the fertility but to improve the drainage while giving the ground a sponge to soak up excess water. In the summer when the ground gets a little dry it may release water into the soil by diffusion allowing plants to access it.    

Since the allotment has now got onion white rot and club root where I put the compost from the giant compost heap, I am more than reluctant to have any weeds or compost except my own.  However, Beryl's allotment had a lot of weeds that needed dealing with so I decided to bury them on my own allotment.  I buried them deep though.

There is a lot of hearsay evidence on the internet that Hugelkultur produces good results when brushwood is added to the soil.  I like to bury high carbon organic matter quite deep in the soil.

About two years ago, I buried big x Cupressocyparis leylandii branches deep in the soil and reported that I could no longer find them.  However, this week I have found some of them when digging in the compost.  They are deeper down than I remember and are being decomposed by fungi growing on them even at this depth.

I put quite a lot of inoculated charcoal into this bed for the potatoes and I had to be very careful to keep the various parts of the soil separate to maintain the charcoal in the top layer.  

Lots of old woody compost was mixed into the second spit soil.  Some quite large branches were being dug up and they looked unsuitable for mixing into the top soil.  However, after examining them closely they could be crumbled into quite fine material that mixed into the soil well.

I have dug over about a quarter of the bed at the moment and I have already started to run out of material to bury.  I think that I will be lopping off some more branches from the hedge soon.


I hope to plant this area with leaves (chard, rocket, American land cress and lamb's lettuce) together with green manures.  The green manures I will be using are crimson clover Trifolium incarnatum, grazing rye Secale cereale and winter tares Vicia sativa leucosperma.  These green manures will be dug in in the spring.  Only grazing rye will not be a nitrogen fixing plant so I am hoping that digging them in will provide the soil with additional nitrogen.  

I am going to sow the green manures in rows rather than broadcast.  I have grown green manures like Caliante mustard Brassicae juncea by broadcast sowing in the past and this has been quite successful.  Until the green manure has developed a canopy over the ground there is a risk of weeds being able to germinate.  They may well be shaded out by mustard but it is best that there is some easy way of weeding.  With this in mind, I am growing in rows to facilitate hoeing.  Also, I get as good a ground cover in rows as I do broadcast sowing.  There are several reasons for wanting a good ground cover.  Green manure will prevent winter rains leaching out nutrients from the top soil.  It provides a good habitat for micro and macro organisms.  It also provides an effective store of nutrients that can be dug into the soil during the spring.  

Is weeding necessary?

We had this discussion today down the allotment.  There are times when you question whether keeping the allotment very clean and free of weeds is the best strategy.

One year I could not keep the allotment weeded due to pressure of work and it got particularly weedy.  I had planted some summer cauliflowers but thought that they had perished under the weight of the overhanging rank mess of weeds.  However, when I came to clear this bed I found that the cauliflowers had formed large heads and were ready to be harvested.  Furthermore, they were as clean as a whistle and had no cabbage white or slugs in them.

Nowadays,  I molly coddle them carefully putting fleece or netting over them to keep the caterpillars and slugs off them and they form infested little heads of no use to man or beast. Actually the beasts do eat them and that's the problem.

So how do the weeds do that?

Weeds can keep carrot root fly at bay as well.   There is an allotmenteer that has not weeded his carrots and they are as big as any on the allotment site.  He just plants the seed in the soil without covering them with a barrier to keep carrot root fly at bay.  How does that happen?  I meticulously cover my carrots with enviromesh every year but if I take the mesh off for a couple of minutes to weed the fly is in like a shot.

The trouble is I can't stand having an untidy allotment.  So all us allotmenteers with clean allotments might be missing out on getting really good harvests because we weed so carefully.

Got a whole lot of potatoes out today.  Washed them carefully because they did have some slugs in them and have brought them home to bag up.  I haven't weighed them but I reckon on about 6lb or 2kg per plant.  Some big potatoes.

Finishing off the supports for the newly planted peas.

I've finished putting up the chicken wire supports for the Kelvedon Wonder peas.  They haven't grown on as fast as I would like since I planted them but I reckon they will do ok.  The peas have settled in quite well and the dryish weather has not made them wilt.  I did put a watering can of water on each row and this seems to have helped.  I did not use any comfrey liquid so I can see what effect just watering with water has on the plants.  Will they grow as large as the Early Onward?

The sweet peas growing up the climbing French bean supports have really taken over and are shading both the climbing and the dwarf French beans.  As I have not had time to take off the flowers that have gone over there were a lot gone to seed.  I decided to cut them down as far as I could so that light could reach the beans. I gave the whole bed a good hoe to remove any weeds although there weren't that many.

After this, I started to take some more potatoes out.  I filled one of the plastic baskets, which was about half a row, and took them up to be washed.  I like to wash the potatoes at the allotment so that any top soil remains on the allotment and not going down the drain at home.  Several of the potatoes had little round holes in them and when they were cut open the insides were riddled with tunnels.  This is slug damage but I only found one of the beggars in a potato.

The trouble is that if you store potatoes with slugs in them they tend to burrow into others.  This means that the whole bag of potatoes can become useless.

There were only three or four potatoes that had  tunnels within the potato.  They would not store well even with the slug extracted so I put them into the worm bin.

I harvested beetroot - mainly Boltardy, carrots, courgettes, runner beans, climbing French beans and the outdoor tomatoes.  This is the first year ever I have been able to grow outdoor tomatoes.

I talked to Fred for a bit and I remarked on how large his sunflower flowers were.  They were self sown and so were the ones on my allotment.  They grew from the bird seed I had been putting out.  The point is, when ever have sunflowers been able to self seed in this country?  That one crept up on me without me knowing.  Twenty years ago, growing sunflowers involved sowing indoors.  Then carefully planting after the frosts.

There are parts of the allotment that are particularly dry while others where the soil is quite moist.  The moist areas are probably where the springs are. I am going to continue to add lots of organic matter to the soil to increase its water holding capacity.   My next door neighbour allotment holder said that I could have  a large pile of weeds she had taken out.  These will be dug into the potato bed.  When I have packed this organic matter into the soil, I will plant winter green manures to be dug in next spring.  I have grazing rye, winter tares and clover to plant.

I may also plant some winter salad leaves on the old potato bed too.  I was going to plant some carrots but I have far too many with the ones I already have so there is no point in sowing more carrots.  I am going to plant lamb's lettuce, rocket, spinach, chard and American land cress instead.  This will see me through the winter I think.

Tomorrow I will take some photographs of the allotment.  It is a bit untidy this time of year because a lot of things are being harvested leaving large areas of bare soil.  I am endeavouring to make sure that I cover the soil this year.  I will mainly be using green manure to do it.

What to plant after pea plants have been taken out.

As soon as all the peas have been taken, I take the pea plants out and dismantle the supports. This leaves a large area of bare soil.

Bare soil is not really what is wanted in a garden. Although roughly dug bare soil was left in the olden days to be broken down by the frosts in the winter and gave an area that birds could pick over and remove any pests such as slugs and snails, it also meant nutrients would be leached out of the soil and erosion could occur.

The idea nowadays is to make sure that the soil is covered throughout the year. There are several ways of doing this. You can cover the area with black plastic sheets or tarpaulin. This is fine but it does protect slugs and snails that like the damp habitat beneath the plastic. You have to keep this in mind when you are removing the black plastic and collect all the slugs and snails up and remove them from the soil before planting or you will loose all your plants.

The second way that the ground can be covered is with a green manure. This year I think that I am going to use clovers, tares and grazing rye. In the past I have used ordinary lawn rye grass but this seems to be much slower to germinate and does not put on as much growth as grazing rye. All of these green manures can be planted now and left to their own devices over the winter. I plant the green manure in lines so that I can weed in between them. While the weeds could be used as green manure, if they are allowed to seed they make weeding harder next year. So I weed between the rows of green manure. I am ordering green manure now, although it might have been more opportune to have ordered them in July.

The third way of covering the soil is to plant another crop. The problem with this is that you have to be careful not to plant something that will disrupt your rotation plan. I like to keep all the brassicas together on one bed so that I can control club root. This year I got club root in the turnips and the summer cauliflowers. I'm not sure where this came from because the allotment has been club root free for about 20 years now and I gave the bed a liberal dose of lime. So, I would rather not put winter brassicas on the pea bed.

I could put leeks or winter onions on the bed but I don't really want to. There will be space on the onion bed when the lettuce and cucumbers are taken out and the leeks will be planted there. I am not really in the mood to plant winter onions because of the onion miner fly Phytomyza gymnostoma. It is endemic throughout the allotment site and especially on my allotment. It does affect the leeks too but I will protect them with fleece or enviromesh. Phytomyza gymnostoma has a second generation of egg laying flies around about the end of September and October. This means that all the onions and leeks need to be covered by that time. I would rather plant onion seed in January or February than risk planting sets in October.

The only alternative that would not cause the rotation problems was to plant more peas on the bed. Now the whole philosophy of rotation is to move plants so they are not growing in the same area as they were last year. This is to make sure that they do not deplete nutrients and increase the number of pests in the soil. However, the pea plants that I have just taken out and dug into the soil were very healthy and more or less pest free. Peas can form a symbiotic relationship with rhizobium bacteria and fix nitrogen from the air, therefore lack of this nutrient would not be a problem. So, I thought that I would risk putting in another four rows of peas in. This time of year you need a quick growing variety and I chose Kelvedon Wonder. I have not grown it on my allotment before so this is a bit of an experiment. Also I have never grown peas at this time of the year before.

Will the peas survive healthily and will they produce a crop of peas?


Tried the peas and they did not work. They were very weak and grew poorly. I will not try this again but look for an alternative crop such as rocket, american land cress and lambs lettuce.

Healthy or Fertile soil?

Do we mix up the terms healthy soil and fertile soil? 

My contention is that that a healthy soil is one which has a diversity of species which are living in balance with each other.  An analogy would be when a pond eventually becomes crystal clear because everything is in balance.  The problem with creating a healthy soil is that it cannot be worked on and it cannot be cropped.  To do this would upset the balance by removing recycling nutrients, breaking symbiotic relationships and killing soil fauna and flora. 

Having said that, humans are part of the natural world and taking vegetation to eat in a permaculture kind of way must be fairly sustainable just as long as we return our waste to the environment. 

As we have seen from modern scientific agriculture, we can have a fertile soil which is very poor in biodiversity and is very unlikely to be sustainable. 

The trick is to be able to have a healthy soil which has a diverse population of organisms while also being very fertile providing crops to harvest. 

Although this is more likely to happen if we use organic methods, growing food is not a process that is conducive to producing a healthy soil.  Even in organic gardening we have to alter soil conditions to prevent pest and disease organisms dominating the soil population.  Do we just accept that we have to do this to produce food for ourselves or do we work much harder to find methods of growing that do not involve major disturbance to the soil. 

I would suggest that producing small areas of very intensive fertile soil which is as healthy as possible is the way to garden in the future.  I am not really arguing for no dig methods because they are not really viable in a proper cropping rotation.  This method is fine for handkerchief sized raised beds but not for allotment sized beds, although it does have its place even here.  I just think that a “no dig method” will not achieve a healthy soil when the next rotated crop involves digging to crop it – as potatoes do.   Taking crops out of the soil must    affect mico organisms because we are removing both rhizosphere habitat and food.   

Also, to achieve a high fertility in organic systems large amounts of organic matter have to be added to the soil.  This can be achieved by mulches but I would suggest that it is quicker and more effective to dig the organic matter, such as manures and composts, into the soil. 

If we are going to use the hugelkulture where brushwood is laid on top of the soil and covered with top soil to produce a raised bed or my Montezuma method where logs and branches are buried deep in the soil or even the chinampa method of the native South Americans, we will be breaking the soil and affecting the diversity and balance of organisms within the soil. 


All these methods of introducing large quantities of organic carbon into the soil together with the traditional composts and manures  may well alter the dynamics of the soil balance.  I would like to believe that there is an increase in the diversity and interrelationships of organisms within the soil when organic matter is introduced.  Realistically, there must be a cost to breaking the soil to crop it and this is met in the destruction of soil communities when digging and cultivation occur.

The ancient South Americans produced a very fertile black soil now called Terra preta by adding mulches of inoculated charcoal.  These soils are even now sometimes metres deep.  The literature suggests that Terra preta did have a unique population of micro organisms.  Could these soils be called both healthy and fertile?  The charcoal may provide habitats for useful micro organisms that otherwise would be preyed on excessively by the soil predators.  If this is so then charcoal will increase the diversity and possibly the health of the soil.  
Both increasing the health and the fertility of the soil in a balanced way is extremely difficult but if ancient peoples found a way of doing it, it must be possible.  

More thoughts on Terra preta soils.

I am attempting to create a Terra preta type soil using lump barbecue charcoal.  (Have a look at the links on the right to see the Terra preta film.)

This type of very fertile soil has been found in South America and has been made by people over thousands of years using charcoal as a soil amendment.  The evidence that I originally read was that charcoal reduced the fertility of the soil when it was initially added to the soil.  This discouraged me from doing experimenting on my allotment.  However, after reading around the subject and talking to people on various websites such as Allotments.uk I developed the hypothesis that an "inoculated" charcoal may be able to be used as a beneficial soil amendment to produce  a sustainably fertile soil.

Some people are still trying to develop an inoculated charcoal that is very similar to that found the South American Terra preta.  My reasoning was that this seemed to be over complicated because if the charcoal could be amended by Amazonian nutrients then it should be able to be inoculated with nutrients originating in my local area.

I am experimenting with using comfrey, nettle and sweet cicely liquid fortified with worm bin liquid.

Inoculated charcoal is charcoal that has been marinaded in nutrients.  Charcoal has some remarkable properties that allow it to absorb or adsorb chemicals within its labyrinthine structure.  We can use this property to enable us to deliver nutrients to the soil in a form that might not readily leach away.

I would conjecture that the charcoal acts as a kind of buffering mechanism that allows nutrients to diffuse into the soil when there is a low concentration of them while adsorbing them when there is an excess.

The structure of charcoal also gives a relatively safe place for beneficial bacteria and fungi to live adding more nutrients to the soil.

Research has found that Terra preta soils also contain mychorrhizal fungi which form a symbiotic relationship with plants.  The charcoal could give these fungi a source of nutrient that can be transferred to plants and also a  protected environment for their spores.

Now all of this is complete conjecture because research in this area is in its infancy.  We are trying to follow in the footsteps of ancient Amazonian peoples and it is difficult.

I have had impressive results this year after adding charcoal to the soil.  Whether this is due to the charcoal or to some other factor this year, I don't know.  We are attempting to replicate a soil that has developed in the Amazonian rain forests  areas.  There is no evidence that this can be replicated in places like England.

I would like to think that I have not wasted quite a lot of money buying charcoal and that it does have some beneficial effect on the fertility of the soil.  What worries me is that there is not a rush to add charcoal to the soil by farmers and market gardeners.  Is this because the data is inconclusive or for some other reason such as the cost effectiveness of charcoal.

Now as a warning, I need to emphasise that it is the genuine lump wood charcoal that should be used not the briquets.  Charcoal briquets are made with cement and will alter the pH of the soil significantly.  Charcoal itself is alkaline so any further raising of the pH will significantly change the character of the soil.


Is adding charcoal to the soil any better than just burying unburnt brush and logs?  Hugelkultur raised bed systems, which the South Americans also developed, would suggest that there is little difference.  Both are good ways in which to add large quantities of carbon to the soil.  Is the quantity of carbon in the soil significant in raising its fertility?

Although plants need relatively large amounts of carbon dioxide and water to produce their food, the amount of other nutrients they require is very small.

Anything added to the soil that will help to retain moisture will be beneficial to plants.  There is evidence that charcoal helps to retain moisture in the soil.  Together with water retention there is also a benefit in that addition of long lasting carbon will help with drainage of the soil.

I would conjecture that there is an overall benefit of adding charcoal to the soil and this is similar to adding unburnt organic matter to the soil.