Saturday, 15 May 2010

The art of liming

David and I decided to get lime, stakes and insect netting to prepare for planting out broccoli and the like. But where to get it from? The usual suspects Homebase, B&Q and Dobbies did not appeal at all - to busy, to expensive, to comercial - and then David remembered a previous visit to the harware store on Portobello High Street - a little independent place that stocks everything from paint to plants. The gardening section is brilliant, and includes open sacks and buckets of fertilisers, seeds and bird food, sold by the kilo. And sure enough, 5kg of garden lime, 15m of netting, 26 bamboo stakes and a few pounds later, we were a; stocked up and ready to fend against acidic soil, moths and caterpillars.

Apparently adding manure at the same time kills the neutralising action of lime and releases ammonia. This is one reason for crop rotations, where you lime and manure plots in different years: manure generously and grow potatoes (1), then lime for legumes (2), lime again for brassicas (3) and then grow onions and roots (4) before manuring and growing potatoes again (1). Since we're starting on a clean slate, in very acidic soil, we've manured the potatoes and are liming for brassicas, legumes as well as roots. We'll be putting the beans into isolated rotted manure trenches, which hopefully won't react with the limed soil.

So, how much lime to add? One book says 120g per square metre, per pH point to be raised. Another 50-100g each year. A little internet research  reveals quotes between 100g and 1700g for all sorts of reasons and pH intervals. Clearly an exact science... The gist though is that lime effectiveness is soil dependent - sand requires much less addition than loam, and clay needs lots more. Consensus seems to lie around the values of 100g to 700g per square metre, depending on soil type and starting pH. We borrowed a pH metre gauge, which gave us readings between pH3.5 and pH6, very variable, but really very acidic. We decide on 300g per square metre, which should raise loam soil at pH4.5 to pH6.5.

Hope this works as vegetables, including roots, brassicas and peas/beans don't do well in anything below pH6, as they can't absorb nutrients in acidic soil.

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