Science and the Garden

According to John Bleasdale in the wonderful book "Know and Grow Vegetables" most gardening books tell you to grow things in rows and space them out - in a way that's far too generous.

The point is that plants are capitalists - they compete for nutrients. And they use up the available resources in a circle round them. All gardening is an attempt to contol the conditions under which plants grow.

The first insight is that you can control size by spacing. If you plant them closer together you get smaller produce.

The second insight is that if you plant them closer together you get a higher yield.

And the third insight is that if you plant them on a triangular grid so that they have access to a uniform amount of nutrients on all sides they do better.

The full article goes into the details of what spacings work best for many common vegetables and is well worth a read since these principles make a vast difference to people growing in a restricted space.

We have got used to the idea that to get rid of pests we need chemicals. However we need to be more aware of what we're really doing. Fundamentally when we garden we are manipulating nature to produce food for ourselves or to create a pleasant environment. And we can work with the natural ecology with respect or we can impose our will on it - in the short term at least.

Because we are creating unbalanced environments, this means that in the natural order of things, some specific predators will be attracted, particularly if we have created, lush sappy growth by too liberal fertilisation that's just soft and easy to pierce for the mouthpieces of our pests. If we plant a whole stand of stuff that's all the same (like 4000 acres of wheat or even a hectare or 2 of lettuces it just asks for the pests to pitch up knives and forks in hand.

What can you do? One tried and tested technique is to mix blocks of planting around so there's never too much of one crop together. Sometimes you can plant "companion" plants which help mask the scent or even secrete compounds that deter pests - like our friendly tagetes (marigold) plants we have round the tomatoes this year.

However you can also take a leaf out of nature's book as it were and set pests to catch pests. Mostly these work best in a confined space like a greenhouse but some, like nematodes can be watered onto the ground outside to attack slugs or the chafer bugs and leather jackets that make such a mess of the lawn. In fact nematodes of different types attack a wide range of pests including vine weevil, sciarid fly, thrips and caterpillars.

Other pests that can be dealt with by biological controls are aphids, red spider mite and whitefly. Encarsia, aphidius and the larvae of ladybirds and lacewings are all useful tools to keep pests in balance in the greehouse.

They are working with nature to reduce the pest population to a manageable level rather than eradicate it entirely. And they are a great example of the application of science to maintain a balanced growing environment.

More details about the pest themselves in the next section

Plants are divided into roots and shoots. Even in the seed this division is present. Once the seed germinates the radicle elongates and goes into the ground while the shoot will elongate and push itself out of the ground. The 2 sides of the seed in most vegetable plants become what are called seed leaves and will quickly go green and start photosynthesis to make the sugar that's the basis of much plant chemistry. That gives it a head start.

The goal of the plant is to reproduce itself. Most flowering plants do this sexually with pollen being carried from one to another by insects. Some plants, however reproduce vegetatively by producing fruits directly from axillary buds without flowering - figs and mulberries for instance are like this. Our aim in growing vegetables is to produce the conditions that favour the part of the plant we want to eat. However, plants are quite varied in their approach.

Some plants grow, flower and die in a year. Lettuce, Pak Choy, Rocket are examples. We need to keep these plants vegetative - this can be done by cutting and coming again or by playing tunes with the daylength as they are more likely to bolt when the days are long.

Other annuals like squash, courgette, tomato and beans we want to encourage to flower and fruit.

Other plants grow in one year to produce storage organs which will then sprout the next year. Carrots Turnips and Parsnips are obvious examples. Less obvious ones are Onions, Fennel and from certain points of view Broccoli. Again its important not to let these bolt in year one which some of them may do. These are biennials.

Finally perennials exist from year to year - fruit trees, asparagus, globe artichokes are examples. Permculture is very pro perennials because they reduce the amount of cultivation that the soil needs.

Plants are largely self sufficient in that they use photosynthesis to provide most of their food and structure from, literally, the zephyrs and sunbeams (well CO2 and sunlight at least). However they need some main additional nutrients to be supplied. The big three are nitrogen, phosphate and potassium (NPK) which is what conventional non organic growers use to supplement the plant's growth. They also need various trace minerals - magnesium, calcium and sulphur are the ones needed in largest quantity but they also need some trace elements - iron, manganese, copper, zinc, boron, chlorine, molybdenum and nickel.

At one extreme, plants are grown in hydroponic systems which supply all the nutrients in solution and the plants grow in an artificial, totally controlled environment. At the other they are grown under an organic regime with a lively soil flora with lots of earthworms and soil fungi and a regular mulching with properly prepared compost.

What does science have to say? - well that depends on whether you buy into the reductionist approach or the soil ecology approach. If you take the view that the soil is just there to hold the plants up and we can just chuck in the nutrients that we know are deficient (a kind of outdoor hydroponics) then various things happen. You get run off of nitrates and phosphates which cause algal blooms and some health side effects - which is why we now have EU legislation to limit how much you fertiliser can be used. Typically agriculturalists are a bit trigger happy with the nitrogen bottle which means the plants grow too fast and sappy and are therefore prey to plant pests. The nutrients in compost tend to be released slowly over time so the plants grow in a more balanced way.

Another complication is how many of the trace minerals there are in the soil. Its reasonably straightforward to measure NPK but trace minerals need to be done professionally. You also have the complication of pH - ie how acid the soil is. At a pH of 6-7 most minerals are readily available. Most get held more tightly and are less available as the soil gets more acid - except for Iron, Manganese, Zinc, Copper and Cobalt which are most available in acid soil and are tightly bound above pH 7.

Over time soils tend to become more acid so its necessary to redress the balance with lime from time to time. Traditionally this goes in with the brassica (cabbage, broccoli etc) part of the rotation.

Plants get at nutrients through their roots which enter the soil particles. The nature of the soil can make a real difference too. Sandy soils drain freely but don't have so many nutrients, clay soils have lots of nutrients but a very fine pore structure. This makes them prone to waterlogging and losing soil structure. Loams and silts are in between. The soil character is improved with the presence of organic matter - humus - which traditionally gets applied as compost. This allows for the presence of earthworms who improve fertility by improving the soil structure and moving the pH to the alkaline side.

So for a plant to grow well it needs ready access to nutrients in a slow release kind of way so it doesn't get to much too soon and in soil which stays at a good level of moisture. If a plant dries out (water stress) this will push it toward fruiting (good for tomatoes, terrible for lettuce) and if it dries out enough the plant will be permanently stunted.

Two more points in this little article.

1) contrary to what the hydroponics and agribusiness boys believe, it's actually true that no plant is an Island. Most plants (though not Brassicas) are associated with Mycorrhiza. These are beneficial soil fungi. Some live between the cells of the plant root and then extend into the soil as additional root hairs. others live in the soil but from a sheath round the plant root. The fungi take sugars from the plant that they can't photosynthesise themselves. In return they are particularly good at phosphate uptake. Phosphate isnt very soluble so that in a deficient soil the plant on its own would have to provide a much bigger root system (at the expense of other things the plant could be doing with materials that build the root) than is needed with the Mycorrhiza. Currently there is a lot of interest in commercially available Mycorrhiza - see separate article.

2) Having said all that, the ghastly truth is that plants are capitalists - they compete for nutrients. Thats why its often better to grow them on a triangular grid rather than in rows. It also means that its important to keep the weeds under control. You can do this mechanically by hoeing, by weed suppression using mipex as we do commercially or by spraying with herbicides. In traditional agribusiness you're always trying to do away with labour which is why the first efforts into GM were to produced herbicide resistant spray so that Monsanto could

a) own the world's food supply
b) sell lots of crop spray.

Its also worth noting that excess use of the nitrogen bottle causes plants to be prone to pest attack - and hence allow the agrichemistry industry to sell more pesticides.

When I was a young man this didn't seem such a wonderful idea which is why I gave up Biochemistry as a way of earning my living. ;)

So I've told you that a plant left to its own devices will do what it has to do as long as it has access to enough water and nutrients to go through its life cycle.

When we garden though, we are seeking to control the plant's life cycle for our own purposes. That's covered in a separate article