Organic matter in soil have several important functions. It acts as a resevoir of nutrients and water, provides structure, prevents erosion and binds up carbon. Cultivation increases the rate of decomposition of organic matter, inturn releasing CO2 to the atomosphere. Organic matter levels are most likely to decrease under continuous arable cropping where there is little return of plant residues or animal manures. Using a hurling example one hectare of a Tipperary grassland soil contains the equivalent of about 6,000 Galway hurlers in SOM terms (each weighing ~85 Kg). However continuous cultivation over 30 years could half this team to just 3,000 SOM hurlers.
The Dept of Agriculture proposes to set a 3.4 % threshold for soil organic matter (SOM) levels in soil under tillage.
The Department has confirmed that specific farmers involved in continuous crop production and whose holdings are in areas deemed to be “vulnerable” to soil degradation, will be obliged to carry out soil organic matter tests over the next few months.
While such a proposal is welcome in a bid to protect soils from degradation and maintain our Irish soil carbon stocks, more information is needed on how this will be enforced as SOM levels can vary greatly depending on soil type.
A senior Teagasc tillage specialist has questioned a Department of Agriculture proposal to set a 3.4pc threshold for soil organic matter levels in cereals ground.
Professor Jimmy Burke, head of the Teagasc Crops Research Centre in Oak Park, said the cut-off level being proposed was too low and would have serious implications for growers.
To raise SOM levels farmers may implement suggested methods such as the ploughing in of a cover crop, or ploughing in the previous year’s straw or farmyard manure. Other methods like conservation tillage or the application of soil ammendments e.g. biochar could also prove useful.
But Professor Burke contends that the threshold level set by the Department may well result in a lot of tillage ground being put back into grass.
Professor Burke also questioned why the organic matter threshold was set at 3.4pc and asked if there was scientific evidence to support the decision.
The value of protecting exisiting and increasing Irish SOM levels should not be undersestimated. Farmers should be assisted to increase SOM levels and those who maintain high levels should be rewarded.
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