Do We just Add Clover?- Tom Malleson (Sherborne)

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Is nitrogen fixation really as simple as just adding clover to grass swards? The answer to this is of course no – there are many factors to consider in making use of this free resource.  The atmosphere is 78% nitrogen and well-functioning soils should have the ability to capture most if not all their nitrogen needs from the air.  How do we achieve this?

  1. Soil structure.  A well-aggregated soil should consist of crumbs of soil with plenty of pore space in between.  This allows air and water to penetrate the soil fully.  Compacted, platy or blocky soil will restrict N fixation.
  2. Free-living N fixers.  Legume rhizobia are not the only N fixing organisms in the soil.  There are many that function freely within healthy soils.  
  3. Trace elements.  Molybdenum, Cobalt, Sulphur and Iron are necessary for N fixing organisms to function.  Depending on soil type, these elements are generally deficient in UK soils.
  4. Plant diversity.  Trials have shown that swards with greater diversity (of grasses, legumes and herbs) produce more than simple ryegrass/clover mixtures.  Greater diversity of root systems improves soil structure, air and water infiltration.  Competition between species promotes greater growth.
  5. Soil biology.  Nitrogen is not absorbed directly by plant roots.  It relies upon complex interactions between microorganisms within the soil.  Heavily cultivated and/or fertilised soils are bacterially dominated, lacking the organisms (fungi, protozoa etc) that make Nitrogen available to roots.  Composts and liquid soil biology products can rebalance this.
  6. Grazing management.  Legumes and soils need sufficient rest to recover and allow the plant to fix nitrogen.  Set-stocking will severely limit N fixation. Grazing rotations will need to be longer than usual.  Higher residuals may be a consequence but this is more of a problem in ryegrass-dominant swards.

To check if your legumes are working for you, take a spade to the field and look for the nodules attached to the clover roots.  White nodules are inactive, pink nodules are fixing nitrogen for you.  If in doubt, squeeze the nodules between your fingers to see the internal colour.

Contact Tom at tommalleson@fcgagric.com or your local FCG Office if you want an independent assessment of your soil health and gets the soils working for you with less applied Nitrogen.  

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