In part 1 I postulated that with the thousands of different plants that grow in our gardens it seemed unlikely that none had some effect on another. That it was only scientific to investigate how such interactions could take place, and how plants might do so above ground.

Of course whereas plants are reasonably discrete and independent above ground they are very intimately mixed and even joined at the microscopic root level underground. We tend to forget that plants make very extensive root systems; from the big roots we see to the microscopic hair like roots that creep everywhere. A tree's roots extend to more than it's height and even the humble beetroot may have roots stretching metres each way. Thus the foolishness of ripping up weeds, with their roots and all, from amongst crop plants in the midst of the growing season-any losses the weeds might have been causing is nothing as to the disruption done by ripping great chunks out of the crop's root systems! Better to sever the weeds at or just under ground level with a sharp hoe or knife.

Healthy soil is packed full of roots of all sorts and sizes and these bind the soil granules into a mat ensuring a stable and secure footing for all the plants. The roots turn loose soil into a sponge like mass that holds water and breathes air more effectively than the soil on it's own does. And of course roots remorselessly expand cracks in stones and rocks very very slowly breaking them down on their way to becoming more soil.

The most important thing that roots may do for other plant's roots is to die. The decaying roots then make pathways which other roots may follow. Surface and weak rooters may benefit especially by following deeper pathways made by old taproots. Also as each root rots it creates a potentially disease or pest ridden soup for following roots so those of the same plant or genus are unlikely to benefit. However other plants from different families with different requirements and resistance's may find such a soup to be full of useful nutrients.

As some plants root very deeply such as alfalfa these access soil layers other plants can't reach and improve fertility by moving nutrients up to where other plants can eventually reach them once discarded as leaves or other wastes. At the other extreme the most shallow rooted weeds may seem useless at first glance yet they quickly establish and prevent the soil washing or blowing away and as they grow they fix sunlight, water and nutrients creating future fertility when they expire.

Likewise plants, through their wastes improve not only fertility but the soil texture. As organic matter decays it binds with mineral particles to form soil granules. Obviously some plants must produce a different granulation to another. Some plants are well known for leaving a good tilth behind them such as stinging nettles, and flax, and others make saponins, soap like compounds, which aid soil granulation. Spinaches are reckoned especially good as green manures for that reason.

Secondly, worms are good at improving the soil texture and worm casts are not only very rich but very water stable holding their shape and not turning to mud easily. Very little is known about which plants encourage the different sorts of worms, though any plant creating leaf and petal litter, pollen etc. must obviously do so to some extent, and we have no idea as to which plant roots worms prefer to burrow in, through and or eat. As Darwin calculated the worms in a healthy soil contributed over a dozen tons of casts per acre per year then altering the worm population by only a few per cent could give you significant gains in fertility and soil texture.

Of course as I said last month, nature is not altruistic, the plants are only doing what helps themselves. Any advantage another plant gains is inadvertent though we can try and use that effect to benefit our crops. Unfortunately allowing stinging nettles amongst your currants may create a better tilth, it may reduce bird damage, it may even confer some disease resistance and shelf life to the crop BUT the nettles will still be competitive for air, light and water potentially reducing your crop!

Competition is a negative effect but even this can be turned to our advantage -few weeds ever get started where there is dense cover from an existing stand of almost any crop. Some weeds are especially competitive for certain nutrients, for example the thorn apple (Datura) is very pernicious as it robs the soil of phosphates however if it is pulled before it sets seed then all this phosphate is available again once the plant decays.

Green manuring has so far aimed at getting some plant material to rot down for humus and in some cases using legumes to do so and so also gain some nitrogen. We could be more clever and find green manures that accumulated whatever nutrients we are most needy of. Green manures are not normally companions as they are grown either before or after the crop rather than with it to remove the direct competition effects. What would be nice would be if they could be grown at the same time! Seedling crops are one possibility. By germinating a green manure and chopping it down in situ whilst still juvenile it may be possible to increase certain nutrients significantly without much competition with the crop for much other than water.

And indeed this is happening all the time even if you don't notice it. Weed (and all other) seeds germinating and commencing into growth give off vast amounts of nutrients and plant growth factors. If you have ever sprouted Mung beans for bean sprouts to cook you will know what I mean. Or if you germinated beans at school and watched them grow. In either case you have to keep washing the roots with clean water TO REMOVE THE EXUDATIONS which form a scummy festering solution otherwise.

All seeds and most plant roots both take in soil water and give out all sorts of substances; some of these are purely waste products. Wastes are produced as storage substances such as carbohydrates are turned into living tissues, and plants unload redundant materials so tobacco and cereals lose potassium as the plants mature. Thus germinating weed seeds are enriching the soil around them. (And 'poisoning' it-clover root extract is said to inhibit clover seed from germinating even when diluted to a few parts per million.)

Some seeds even carry the right micro-organisms to let them flourish later as they grow. Unfortunately not all seeds carry the right micro-organisms so there may be trouble establishing alfalfa and other legumes where none have ever grown before. Orchids are notorious and it is to be suspected that many 'difficult' plants need some microscopic partner or the other. A suitable one may often be introduced with leaf mould, well rotted manure or compost explaining part of their exceptional effects on plant health. In nature most plants have a vast range of fungi and bacteria that coexist near or more often on them in healthy soil. These then exchange nutrients with each other and many plant roots are naturally coated with what are termed mycorrhizal associations. In cereal crops these are rare but on a pine tree there may be several hundred different ones.

So plants do not just suck up soil water, they also give off root exudations to help them dissolve the soil particles, to dump wastes and to 'buy' nutrients from their mycorrhizal associations. Then they suck up the soup of fertility that is created. Perhaps more importantly there are all sorts of active chemicals from antibiotics to growth hormones being created and dispersed in the root zone which are sucked up as well. It may surprise you to learn that school simplified the lessons-plants do not just suck up dilute solutions of a few simple chemical salts. Plants actually suck up and spread throughout themselves all sorts of complex substances including antibiotics created by soil organisms. Plants are thus likely to be altering each other's growth and health far more than appears at first glance.

Anti-'companion gardeners' argue that they can see no mechanism whereby such claimed effects as Alliums discouraging mildews can possibly work. They may grant that pongy smells could distract insect pests but they vehemently deny a plant companion could discourage a disease. Well why not? Surely it is so simple they are exposing their own ignorance of plant growth and soil dynamics. Take, say an apple or a rose; growing on it's own in a bare soil; it may well prosper and flourish depending on the balance of soil organisms as well as the usual factors such as air, light, water, soil type and so on. Put Alliums, or any other genus, nearby and the soil micro flora and fauna will immediately and most definitely alter. The soil solution will have more or less of many different substances floating in it which the crop may now use to it's benefit. Much like you or I can resist colds the better if we have certain fresh foods available.

Dandelions are well known for giving off ethylene which then affects the plants around them. How many other plants give off other substances which we have not as yet noticed? Some plants such as chamomile, borage and red dead nettle are said to be good for other plants near them. Indeed as mentioned above even stinging nettles are reputed to be of benefit to our crops.

Exudates may indeed change the flavour and scent of a plant absorbing them in the same way a cow eating garlic gives tainted milk. This would account for how nasturtiums can drive woolly aphids from apple trees. French marigolds are famous for their pong which keeps pests away but their roots (and those of their close relations) also give off substances that control nematode eelworm populations. Others such as the edible chrysanthemum seem to keep pests away from brassicas -but is it their smell or do root exudates make the brassicas tougher to chew or somehow less appealing.

Indeed what might plant exudates be doing at the microscopic level. For it is down at that level that real fertility is created. The surface of soil mineral particles, dead cells and lumps of humus are where most of the countless micro-organisms exist though some float free in the soil solution. However they are not densely packed, in most soils they are scarce, much like plants in a near desert. And for each organism there is a limiting factor; some nutrient that is in too short supply to allow maximum growth, health or reproduction. If we can provide that then there can be a population explosion creating more wastes and dead bodies and thus more fertility.

Of course it is not all in our favour; some plants such as wormwood and fennel seem to be hostile to everything around them. In trials wormwood was found to be effective at keeping cabbages free of caterpillars but at a cost to the yield as the cabbages did not like wormwood any more than the butterflies did and failed to grow well.

Different plants have different exudates some of which are designed to encourage specific allies, discourage enemies or dissolve certain substances. As yet we are uncertain as to what is going on but it seems quite possible that by growing the right plants with the appropriate exudates then some near magic might occur. Indeed if we did but know how we could give our crops companions providing them their food, minerals, vitamins, antibiotics and pep pills. And all for the cost of the seed.