Protecting precious soils

A sound understanding of soil characteristics is an essential component of effective farming, especially at a time when there is an increased focus on limiting the impacts of land use on water quality.

Knowing soil conditions and how they vary during the year can help to deliver very practical benefits, including maximising production through efficient utilisation of nutrients in animal effluent and preventing contamination of ground water and waterways.

I find farmers are taking an increasing interest in understanding their soils more to help them manage their business and the environment better.

Soil properties such as texture and structure determine the amount of water and nutrients that can enter and be retained within a particular soil, and the rate at which excess water goes through that soil.

Infiltration rates, water retention, drainage characteristics and consequent leaching losses of nutrients are strongly dependent on these properties, and they vary over time as a result of weather and farming activities.

For example, the Parliamentary Commissioner for Environment recently released a new report on the subject of agricultural greenhouse gases (methane and nitrous oxide), which form about half of New Zealand's greenhouse gas emissions.

The research reports reveal that the wetter soils become the greater will be their rate of nitrous oxide production.

The amount of nitrous oxide emitted (and the amount of nitrate leached into waterways) is greatest in autumn and winter when grass is growing slowly.

It is the time of the year when waterlogged soil is most likely to become compacted by hooves, leading to the microbes in the soil that produce nitrous oxide becoming more active.

Treading damage during grazing, especially in wet conditions, can affect the effluent or irrigation water infiltration rate, limits root growth and nutrient uptake, and reduces pasture production.

So it is best to take extra care with winter grazing in areas where effluent is spread.

Avoid irrigating paddocks with treading damage until the soil recovers.

If in doubt, dig a hole before irrigating and check whether the soil is loose and open or dense and compacted - don't irrigate if the latter condition remains.

A key factor to be cautious of regarding runoff of nutrients to waterways is that suspended solids in farm dairy effluent can accumulate on and just below the soil surface.

This can create an organic layer that temporarily acts as a surface coating reducing infiltration rates to very low levels, generally for a couple of days.

Using low application rate equipment will help to reduce the runoff risk at such times.

Once again, digging a hole to assess soil conditions will help with judging whether it's a suitable time to irrigate.

Another issue is that relatively dry soil contains large pores open at the surface.

When irrigating in such conditions, effluent or water can be rapidly transported below the root zone through a phenomenon known as "bypass flow".

It can particularly occur in soils that undergo shrinkage and fissuring during drying, especially when these soils have been previously compacted by treading. High leaching losses of nutrients can result from this.

That's one reason why Waikato Regional Council has rules limiting effluent application depths.

Ensuring that you have adequate available storage, using low application rate equipment, and keeping a close eye on soil conditions, means you can schedule irrigation to get the maximum benefit, protect your precious soils from damage and care for the environment.