Experimenting with green manure

Simon Watkins
Monday, 3rd October 2011

Last time, Simon talked about growing alfalfa alongside his vegetables. He's gone a step further and planted his carrots next to his garlic and his lettuces under his tomatoes, all with a generous glug of nitrogen, fixed into the soil by alfalfa. But what are the results of this polycultural growing experiment?

Alfalfa – the name almost sounds like a shamanic incantation: soft on the tongue, quiet on the ear, repetitive and meditative. It doesn't look much – just like so much overgrown clover – but it has hidden depths, doing as much under the ground for the soil as it does in the daylight. I'm now convinced that it's responsible for the healthiest tomatoes I've grown among other minor successes.

Alfalfa: a nitrogen fixer

I've been conducting the beginnings of an experiment in which I'm transforming a dormant, plastic sheeted and gravel coated garden into a valuable food source. I've activated one half this year by sowing Alfalfa to reinvigorate the existing soil. However, the experiment has segued somewhat into a look at polycultural growing and I believe I'm seeing some interesting results.

Bronze arrow lettuces

Earlier in the spring my lodger planted out some Bronze Arrow lettuces in a small planter – all together, with bought peat-free compost and a little manure. They did quite well, growing through a nasty white fly infestation and managed to supply sandwiches for a few months. But last month most of them shrivelled and died. A few weeks after these were planted, I took some weedy seedlings of the same variety and tucked them in under the tomatoes, between the carrots, basil and garlic cloves along the path by the alfalfa patch but in otherwise unimproved, compacted soil. Now the alfalfa has grown and been cut back twice, having shaded out pretty much everything else. So I would have expected these lettuces to give up almost entirely!

How to grow lettuce at its best

The healthier lettuceNot so – although almost completely shaded out the lettuces growing under the alfalfa and tomatoes are thriving: big, green, juicy leaves without a sign of withering, disease or bolting. I measured the largest leaves on the batch in the small planter, and the equivalent from the lettuces planted in polyculture conditions. The results are revealing: the former are 15cm by 6.5cm at the largest, whilst the latter reach well over 20cm by 8cm. This isn't merely a case of reaching for the light either: the lettuces in the polyculture are developing dense whorls of leaves at ground level.

Ok, so the cabbages haven't done too well and the basil appears a little intimidated, but I'm also seeing some strong growth in carrot tops and the one dandelion which found its way in (but which I've now filched out) was spectacular. Perhaps this is a selective symbiosis. It certainly deserves more research. That's good news for me as I do like a nice lettuce leaf... 

Simon Watkins has a regular blog at permaculture online.  He is a landscape architect based in Coventry, but as a student of permaculture, he is increasingly guided in his work as much by his stomach as his eyes... visit his website to read more about some of the things he gets up to.