Helping soil to withstand endless rainfall

URBAN FARMER:Certain types of soil fare better under the type of heavy rainfall we have had this summer – so how can you improve the structure of your soil, asks FIONNUALA FALLON

SHH….DON’T TELL the rain gods but for once it’s not pouring (well at least as I write this, it isn’t). Which makes a more than welcome change from the previous eight months. According to the Met Office, over 590mm of the wet stuff fell on Dublin between January and the end of August of this year (a typical amount for this part of the country should be about 456mm) with July totals alone being over three times the norm.

Cork fared even worse, with a soil-drenching 950.9mm for the same period, while Shannon Airport got 684.5mm. The dubious honour for highest amount of rainfall goes to Valentia, where 1,317.9mm of rain was recorded in the first eight months of 2009, which is 56 per cent above its average.

Surprisingly, up until the end of July, Malin Head came off the best with just above average rainfall, although things took a very soggy turn for the worse in August.

In the OPW’s walled kitchen garden in Phoenix Park, the summer of 2009 also set some new meteorological records, and again for all the wrong reasons.

The garden had its coldest June day ever (9.4C) while the amount of rainfall in July was the highest since 1936. If OPW gardeners Meeda Downey and Brian Quinn were to look for a silver lining to the grey clouds that have been hovering overhead for much of the year (and they’d have to look hard), it’s that they didn’t suffer the fate of Castlerea, in Co Roscommon. which was hit by a tornado on June 12th.

So how has the sodden summer weather affected vegetable and fruit crops, both in the OPW’s walled garden and around the country?

A lot of it boils down to how well-drained your garden is. In the OPW’s walled kitchen garden in Phoenix Park, the soil is free-draining and enriched with plenty of organic matter, which gives it what gardeners call a good ‘crumb structure’. This means that there’s plenty of beneficial microscopic organisms at work, which is key to fertility and productivity.

So despite the heavy rainfall, yields have been surprisingly good. “We’ve had some problems with mildew on the sweet corn, and the heavy rainfall didn’t help when it came to trying to control the spread of white-rot on the onions, which is now affecting the leeks,” says Brian. “But overall, it’s been pretty good. I think the raised beds help a lot.”

Those urban farmers with heavy, poorly drained soils, however, will have found that beneficial bacteria have been far less numerous and less active in their vegetable plots this summer, and that harmful organisms began to thrive instead, encouraging disease. Excessive amounts of rain also lowered soil temperatures, further reducing yields and making the ground unsuitable for early crops.

To make matters worse, the roots of plants growing in poorly drained soils typically suffered from lack of oxygen, inhibiting growth or, in the worst cases, killing the plant (it’s the botanical equivalent of drowning).

And as if all that wasn’t enough to deal with, the final problem for urban farmers this summer was the fact that poorly drained soils are easily made worse by typical methods of cultivation – so if you tried to dig, or rotovate your wet garden, you ran the risk of aggravating the problem by compacting the ground further and destroying any soil structure that there is.

So how do you improve poorly drained soil? Gardeners use a variety of methods to get around the problem of poor drainage, but usually begin by adding lots of organic matter (compost, manure, leaf mould) and horticultural grit.

Avoid digging in really wet conditions and try to stand on a board/plank to spread your weight and lessen compaction. Now that it’s stopped raining for long enough to give your garden the chance to dry out a bit, use the opportunity to fork it over (avoid using a spade which can make drainage problems worse). Don’t try and break down any big clods of soil yourself but leave them exposed to the winter frosts and winds, which will do a much better job of it.

Aside from the measures outllined above, there’s one other thing to check and that’s the soil’s pH. Poorly drained soils or those in areas of high rainfall are often deficient in lime, or what older gardeners call ‘sour’.

Applying garden lime helps by improving the soil’s crumb structure, which then improves drainage – it also frees up other nutrients in the soil, making them more readily available to the plant.

The best time to apply lime is in the autumn (just don’t spread it at the same time as organic matter as this releases damaging amounts of ammonia).

The biggest problem, for most urban farmers, is in knowing exactly how much to apply. Not every soil needs lime and those that do will need it in varying amounts. Too much and you will cause other problems, such as iron, manganese and zinc deficiencies; too little and you’ll have done nothing to address the original problem.

The quickest way to tell whether your soil needs lime is by doing a soil-test, which will measure its pH or acidity/alkalinity: seven is neutral, anything above is alkaline while anything less than seven is acid.

Depending on where you live, you may find that your soil is naturally alkaline, neutral or acid, but other factors can also distort the results of the test, such as imported topsoil or buried builder’s rubble. If your soil pH measures anything above 7.3, you don’t need to add lime – in fact, your soil is already too limey. Anything between 6.5 and 7.3 means that you have a nearly neutral soil while lower readings than this point to an acid soil.

In the OPW’s walled kitchen garden, the soil is acid (somewhere between 5.5 and 6 according to a test that Brian did a couple of years ago). Anything below 5.5 is extremely acid.

While extremely acid soil is fine if you want to grow plants such as camellias, rhododendrons or blue-flowering hydrangeas (in alkaline soils, these will be pink), it’s not much good for fruit and vegetables, which like a pH of somewhere between 6 and 7.3. Fruit-trees and bushes like a soil that’s slightly acid, for example, while brassicas like it slightly alkaline. So if you’ve an extremely acid soil and are planning to grow food rather than flowers, you’ll need to add lime – it’s just a case of working out how much.

To do this, you’ll need to get out your measuring tape and a calculator. Roughly work out the area of your plot (multiply width by length). Once you know your soil type and pH, and you’ve calculated the size of the area that you want to treat, you can then (technically, anyway) calculate how many grammes of lime you’re going to need per square metre.

Remember that the type of lime that you use will also affect your calculations. Garden lime (calcium carbonate) is available at most garden centres and is generally what’s recommended but hydrated lime or builders’ lime (calcium hydroxide) is also often used, but if you are using this you’ll need less of it.

Confused? Then you’re certainly not alone, because most gardeners are. Depending on your soil type, its original pH and the type of lime being used, the amount of lime required per square metre can vary hugely, from as little as 140g to 1,200g. For exact quantities, check out the RHS website (www.rhs.org.uk), which gives precise information on the 24 possible permutations as regards required rates of application.

An easier and more foolproof solution, I think, is to get your soil professionally tested, as Meeda and Brian are going to do this autumn in the Phoenix Park walled garden.

Teagasc has a soil analysis service for €18 (plus VAT), which will tell you the pH of your soil as well as checking for levels of other nutrients. It will also do all of the mathematical calculations as regards your garden’s lime requirements (Teagasc, Johnstown Castle, Wexford, tel: 053 9171200 for further details). Then you’ll only have to spread the stuff, which is another day’s work. You will need weighing scales, safety goggles and a strong pair of gloves to do this. Oh, and a dry day, of course. Which, happily for urban farmers everywhere, suddenly looks a lot more likely than it did a few weeks ago.

The OPW’s Victorian walled kitchen garden is in the grounds of the Phoenix Park Visitor Centre, beside the Phoenix Park Café and Ashtown Castle. The gardens are open daily from 10am to 4.30pm

Next week, Urban Farmer in Property will cover choosing the right apple tree for your garden

Fionnuala Fallon is a garden designer and writer