Wednesday, 26 July 2017
Wet soils a threat to farm production and environment
To get the best out of soil for pasture production and the environment, air and water need to move freely in and out of the soil - between particles.
With the recent rain events across the country farm soils have become increasingly vulnerable to pugging and soil compaction that prevents plant roots, earthworms and soil micro-organisms to take up nutrients, and create a soil environment that promotes good pasture growth.
Dr Ants Roberts, Chief Scientific Officer for Ravensdown, says pugging and soil compaction occurs when stock are on moist-to-wet soils that do not have the strength to support the animals’ weight. This causes a pugged surface layer and a compacted zone at about 5-10cm depth.
“When soils are pugged, soil structure is lost, its density increases and porosity (the spaces between soil particles), drainage and aeration decreases. The soil stays wetter for longer, and wet soils are more easily pugged. This is called the pugging cycle.
This compaction changes soil properties causing greenhouse gas emissions to rise, and increases surface water run-off, nutrient loss, earthworm death and anaerobic soil conditions,” Dr Roberts says.
It also reduces nutrient uptake by plants and root penetration, leading to pasture ‘pulling’ by grazing animals and increased ‘droughtiness’ of pasture when the summer/autumn dry arrives.
Impacts on production
Many New Zealand farms are on soils considered to be physically resilient, such as volcanic ash and pumice-based soils. But research in the Waikato has shown that just one soil treading/compaction event depressed pasture production by more than 50 percent. It then took two months to recover.
“Treading damage decreases the soil’s macroporosity (the big spaces between soil particles) and ‘integrates’ the pasture, nutrient and biological effects of soil compaction,” says Dr Roberts.
Dr Roberts says practical alternatives to keeping stock off wet soils include short grazing periods, putting stock onto feed pads until soil conditions improve.
“Many people also move stock from pasture into the race or yard when heavy rain occurs.”
“Appropriate artificial drainage of perennially winter-wet and heavy soils will be beneficial and helps decrease the period of soil wetness and the time when damage is likely to occur,” Dr Roberts says.
In either autumn or spring when soil moisture conditions allow, subsoiling, using a soil aerator (no deeper than 20-25cm) is another technique to partly alleviate compaction effects, allowing the natural soil processes to remediate soil structure.
“Soil’s physical structure is just as important as maintaining correct soil fertility levels, to achieve the best results for your pasture production and environmental footprint. It’s not always easy in this weather to prevent your animals from turning a paddock to slush, but just remember your soil is the core of your business and shouldn’t be treated like dirt.”