Improving soil structure and sward performance

Dairy farmers are currently trying to manage swards that have been badly poached and compacted by both machinery and livestock as a result of two very wet years.  

How and when to rectify this problem are two questions that are commonly asked.

Water lying on the soil surface for a prolonged period is an indication that soil structure may have been damaged. Soil compaction can only be properly diagnosed by digging an inspection pit in a field and examining the soil layers.  The top soil should break apart easily into small rounded blocks.  When the blocks are angular, they fit together too tightly and water cannot pass through to the subsoil below.  

There should also be plenty of grass roots extending to 15cm in the soil.  A good earthworm population is also an indicator of a healthy soil and a well aerated structure.  It is also important to look below the topsoil to identify if the subsoil is compacted.  Large solid blocks in the subsoil layer will also disrupt root growth and drainage.  Rusty colours in the subsoil are an indicator of water logging and such wet soils would benefit from sub-soiling to improve land drainage.

There are two types of machine that can be used on compacted soils.  Both machines are more effective on heavy soils.  Dry free draining soils which have a lower risk of compaction are unlikely to show a benefit from these machines.

Soil aerator – this machine works by cutting divets in the ground with a spiked roller.  These divets allow air down into the soil and surface water is allowed to drain away more freely.  A soil aerator is ideal for surface compaction which has been properly diagnosed following soil inspection.  The tines or spikes need go into the soil between 10 and 15 cm to be of value.

A soil aerator with water tanks to increase the weight of the machine in dry weather.

Subsoiler - this machine has tines which create a ripple effect 25 – 30 cm below the soil surface across the width of the machine.  It lifts and shatters the soil and in the process opens up the soil structure introducing air and improving drainage.  Discs at the front of each tine open up the sward and prevent excessive damage to the top of the sward.  The roller at the back of the machine leaves the sward level.  This machine is effective on soils where compaction is deeper and where a pan has been created from heavy machinery or excessive poaching from out wintering livestock.

One model of subsoiler on the market that has a variable depth setting.

Soil conditions need to be dry to carry out subsoiling or soil aeration work.  If carried out during wet conditions, the problem may be made worse.  A dry autumn period is the ideal time to carryout this work as it improves drainage and reduces the risk of water logging during the winter months.  This will help to increase spring grass growth.  It is also important to do this work before reseeding or “stitching in” grass seed as there is no point putting new seeds into capped or compacted soils which hinder growth rates.

The benefits of repairing compacted soils are:

It should be noted that soil compaction is not the only reason for poor production in grass swards.  Soil pH and nutrient levels are also very important.  Regular soil analysis will help to identify these problems.  Reseeding older swards will also help to increase grass production and quality where ryegrass content has deteriorated.

Over 100 dairy farmers recently discussed how to rectify compaction problems at two CAFRE reseeding events held in Banbridge and Cookstown.  The feedback was very positive with a number of farmers intending to undertake someone of the two techniques discussed in this article if and when the weather improves.