Why it pays to get the inside story on your soil

URBAN FARMER:The correct balance of nutrients and trace elements is vital for healthy growth

MANY YEARS ago, as a one-time student at the “Bots”, I was made (or should I say encouraged) to learn the essentials of good plant nutrition. Like a schoolchild memorising her ABCs, I learned about N, P & K (the three main plant macronutrients, nitrogen, phosphorous and potassium) as well as Ca, Mg and S (calcium, magnesium and sulphur).

In addition there were the tricky micronutrients, or trace elements to commit to memory, such as Cu, Mo, Mn, Fe, Zn and B (copper, molybdenum, manganese, iron, zinc and boron), all of which I learned are equally necessary to healthy plant growth, just in smaller amounts.

Like every student gardener before me, I then reluctantly performed practical identifications where, on being handed a discoloured leaf, I did my very best to tell the difference between magnesium deficiency, and iron and manganese deficiency.

Or was that zinc deficiency? Hmmm. I was never really all that sure, you see. Because while I quickly discovered that a wide range of plant nutrient deficiencies is surprisingly common, I also found that diagnosing the exact or particular nutrient deficiency is surprisingly difficult. Actually, make that very difficult.

Adding to my confusion was the fact that complex interaction between the different plant nutrients can mean that “too much” of one often results in “too little” of another.

For example, too little calcium (Ca), the symptom of which is yellow, deformed leaves, can be caused by an excess of magnesium (Mg), while conversely, the symptoms of too little magnesium, can be due to an overload of nitrogen (N).

As mentioned earlier, the symptoms of some plant nutrient deficiencies were also bafflingly similar. I discovered, for example, that a deficiency of magnesium (yellowing between the leaf veins, generally on older leaves), could easily be confused with a deficiency of iron (Fe) or manganese (Mn), both of which also cause yellowing between the leaf veins, but generally on younger growth. In its advanced state, however, magnesium deficiency (Mg) can also look an awful lot like potassium (K) deficiency, while a deficiency of zinc (Zn) also results in . . . yellowing between the leaf veins.

Confused? You’re not the only one.

Which is why last summer I had every sympathy for OPW gardeners Brian Quinn and Meeda Downey, as they struggled to identify what exactly was wrong with their pumpkin and squash plants, which were growing rather unhappily in the upper, most recently-restored half of the walled kitchen garden.

Was it a deficiency, they wondered, and if so, of what? Noticing yellowing between the leaf veins (the posh term is “interveinal chlorosis”), Brian sprayed the plants with a dilution of Epsom salts, which has long been a traditional remedy for magnesium deficiency. The result? No improvement.

To complicate things further, the plants were also suffering from powdery mildew, a fungal disease that the gardeners treated by spraying with bluestone, or copper sulphate.

While this seemed to clear the powdery mildew, the plants still struggled. Perhaps, Brian and Meeda thought, the mottled, chlorotic leaves and general poor growth were instead the symptoms of a disease or a virus. Nobody could be certain.

Meanwhile, the gardeners were also worried about the potato crop. Yellow and brown patches were appearing on the leaves, and it looked like the classic signs of blight infection. But Brian had been spraying the crop regularly with a protective fungicide, so blight seemed unlikely. The potato tubers, when dug up, showed no signs of blight infection but were a bit on the small side.

“At that point, we suspected something else,” remembers Meeda. After some research, Brian reckoned he had the answer.

“I came into Meeda the next day and said, ‘I don’t think we’ve got blight, I think we’ve got a manganese deficiency.”

Deciding that it was time for a proper soil analysis to confirm their suspicions, the two gardeners did the sensible thing, and last autumn sent six soil samples (taken in different parts of the garden) to National Agro Chemicals in Lusk, Co Dublin (nad.ie), who in turn sent them to Lancrop Laboratories (lancrop.com )to be analysed.

The results, when they came back, were very interesting. For example, measurements of between 7.5 and 7.7 mean that the pH of the soil in the walled garden (its relative acidity or alkalinity) is higher or more alkaline than is ideal for a fruit and vegetable garden.

Most do best in a pH of about 6.8, but at a pH at or above 7.5 such as that in the walled garden, there is a danger of soil nutrients becoming “locked up” or unavailable to plants.

The only remedy for such overly high alkalinity, according to Joy Larkcom in her book Grow Your Own Vegetables, is to work in plenty of organic manure or composted pine needles and to treat crops with a seaweed-based foliar spray.

As the gardeners had rightly suspected, levels of manganese (Mn) in the walled garden’s soil ranged from “slightly low” to “very low”, as did molybdenum (Mo) and the laboratory marked them both as a priority for treatment.

This, one might assume, was at least one of the reasons for those unhappy pumpkin and potato plants.

Meanwhile, zinc (Zn) amounts were, if anything, too high, to the point where they were possibly interfering with the plants’ uptake of iron (Fe) – a good example of that complex interaction between nutrients that I mentioned earlier. Copper (Cu) amounts were also at least eight times higher than recommended, perhaps as a result of the gardeners occasionally spraying with bluestone or the use of copper-based slug pellets – copper, as mentioned previously in this column, is a heavy metal that slowly accumulates and persists in the soil for many years. Levels of nitrogen (N), phosphorous (P), potassium (K) and iron (Fe) were fine, as were levels of magnesium (Mg), and boron (B), while some other good news for Brian and Meeda was the fact that the walled garden’s C.E.C. (cation exchange capacity) was normal, indicating that its soil is able to both hold and release nutrients as required.

The next step for the gardeners will be deciding, with the expert advice and help of Peter Callaghan of NAD, how best to attempt gently to correct the soil’s existing nutrient imbalances without creating a whole set of new ones. For as Peter Callaghan explained, interpreting and adjusting different soil nutrient levels “is a complete science in itself”. And this is from a man who definitely knows his ABCs when it comes plant nutrition.

WHAT TO: sow, plant and do now

SOW(small amounts, in gentle heat, to move to a cooler but frost-free spot, before planting out in polytunnel from late February onwards, using fleece at night for frost-protection): Sugar peas (Delikett), broad beans, carrots, oriental salad mixes, Mizuna, rocket, Swiss chard, spinach, Ragged Jack kale (for baby leaves), lettuce, white turnips, leeks and onions

PLANT: Very early, chitted potatoes into 2L pots indoors, to move to a cool, frost-free spot before planting in polytunnel mid-February onwards, using fleece at night for frost-protection); Garlic (in individual cloves, direct, outdoors)

DO: Start planning this year's vegetable plot, order seed (catalogues or online), weed polytunnel crops, sort through stored vegetables and discard any rotten/frost-damaged ones.

• 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 4.00pm
• Next week Urban Farmer in Propertywill cover how to prepare a site for a new fruit and vegetable garden
• Fionnuala Fallon is a garden designer and writer