Farm Ireland
Independent.ie

Tuesday 28 March 2017

Time to tackle soil compaction troubles

Gerry Bird

The recent fine spell has resulted in the welcome sight of cleared stubble fields ready for the next crop. It's a good time to examine fields for signs of

wear and tear as a result of the past few seasons.

Soil compaction is an important yield-limiting factor in all crops. Compaction mainly occurs where soil is worked in wet conditions, or with inappropriate equipment and traffic. Soil structure is a combination of interlinking large and smaller spaces; the large ones facilitate drainage, the small ones hold moisture and air. Roots require a balance of air and water to grow, compaction squeezes these spaces and reduces water movement and the quantity of air, resulting in poor, constricted root development. Large and deep roots can access more fertiliser, recover water at depth, stabilise the crop and produce a large leaf area -- vital for reaching maximum yield potential.

Vulnerability

Soil type and texture can affect the soil's vulnerability to compaction. Fine clay and silt in clay loams move down through the soil as a result of heavy rain, blocking the small soil spaces. A walk around the fields with a spade can help identify compacted areas prior to the renovation process.

Headlands, due to increased traffic, are more prone to compaction. Tramlines are also obvious areas and a variation in depth may indicate higher soil moisture levels. Check the stubble: weeds like redshank grow in damp areas, and annual meadow grass and scutch grow well where crops are less vigorous. Roots are an obvious sign, with poor, shallow roots a symptom of take-all in all cereals (oats are an exception) in continuous cereal rotations.

Thickened root collar and lateral roots on oilseed rape all indicate restricted root development and penetration. Dig a few holes, note the degree of difficulty in digging and observe the depth of roots. The topsoil should be a good brown colour and loose, with evidence of earthworms and a fresh smell. Last year's straw and stubble at the ploughing depth is evidence of compaction -- it should have been broken down by micro-organisms.

Water movement is the critical factor. To rectify soil compaction, the compacted layer has to be shattered and the soil opened to allow air in and get the good micro-organisms working again. Sub-soiling and deep ploughing are the usual renovation activities. The depth of work depends on where the compaction is found. It is vital to sub-soil only when the soil is dry, with suitable equipment and sufficient horsepower.


Impacts

There are also less obvious impacts from compaction, such as lower soil pH, increased crop uptake of heavy metals and reduced fertiliser uptake. A number of management actions are required in conjunction with sub-soiling to restore the soil's productivity.

Compaction is generally more common on medium- to-heavy loams (more clay/silt than sand in the soil) due to the water retention capacity. The soil pH drops (the soil becomes acid) so wheat, barley and oilseed rape crops fail to reach the yield potential. The application of lime is required to raise the soil pH, neutralising the acidity. Applied nitrogen and phosphorous fertiliser is less available as the pH drops below 6.0-6.5. The availability of nutrients from slurry and other organic manures is also reduced as the microbial activity required to breakdown organic matter is reduced.

Remember that there is a requirement under the SFP scheme to test fields which have been in continuous cereals for six years for organic matter content.

Soil tests on 50pc of the farm have to be carried out in 2010, with the balance tested next year. The threshold is 3.4pc organic matter content and, if the levels are lower, a plan has to be put in place to rectify the situation in conjunction with your FAS advisor.



  • Gerry Bird is an agriculture consultant and member of the ITCA -- gjbird@eircom.net


Irish Independent