DISCUSSION NOTES ABOUT CONTROL OF SOIL EROSION
These notes are based on MAF Policy's Technical Paper 95/4. They have been prepared for educational use, and as an aid for landcare groups. Each topic is summarised, links are given where more detail can be sought, and questions are posed for discussion.
New Zealand is susceptible to high natural rates of erosion because it is a geologically unstable country with a variable climate. Some causes are natural:
Hard rocks uplifted and shattered by earthquakes
Catchment headwaters with steep slopes, high-gradient channels, and severe climates
Others are induced by human activities:
Heavy rainfall on land formed from weak rocks that are not resistant to the force of water runoff
Rain falling on, or wind blowing across, exposed soil where vegetation has been depleted by drought and overgrazing
Is your district subject to any of these?
Erosion can be beneficial. Many of our best soils are eroded material which has been deposited elsewhere in catchments, for instance:
Alluvial soils on river terraces
Loess soils on downlands
Soils formed from volcanic ash
Erosion can also be a problem, if it disrupts people's use of their own land, or when people are affected by the way others use theirs. Some examples are :
Physical destruction of land by erosion
Partial stripping of soil from the land's surface
Loss of crops or pasture, along with the eroded soil
Loss of nutrients, including fertiliser, in eroded soil
Reduced yields from disturbed land afterwards
Damage to farm structures
Downstream damage to channels or river flats
Lowered quality of water available to downstream users
Damaged wildlife habitat
Do any of these things happen in your district?
TYPES OF EROSION AND THEIR EXTENT
The main ones which affect farmland are:
Surface detachment of soil particles by windblow, frost heave, sheetwash and rilling
Gullying by surface and subsurface (tunnel) runoff
Deep-seated mass movement by eathflows and slumps
Shallow mass movement by earthslips and soil slips
Scour and bank collapse by streams and rivers
Collectively these processes affect about 68% of farmland nationwide (tables). However, much of the erosion is slight; in the 1970s it was severe on only 13% of susceptible land (maps). There are no up-to-date nationally comparable data to indicate whether the situation is better or worse in the 1990s.
Which of these processes occur in your district? On your farm? Are they slight, moderate or severe? Is there less or more erosion than 20 years ago?
If there is improvement, why do you think this is occurring?
Surface erosion of topsoil reduces crop yield and pasture growth by at least 20% and in extreme cases more than 60%, where soild structure is disturbed and nutrients are depleted. On susceptible land it typically occurs several times a year, affecting up to half of a paddock's area if vegetation cover is sparse, though only a tenth or less where cover is dense.
Mass movements in subsoil initially reduce pasture growth by 40 to 80% on the fresh scars. Even after they regrass, growth remains depressed by 10 to 40%. Mass movements also damage fences, tracks, drains, water supplies, and occasionally even buildings. Gully erosion has a lesser effect on pasture growth, but it is more disruptive to stock movement, vehicle access and farm assets. Mass movement and gullying happen at irregular intervals, averaging about one year out of every ten in low-rainfall districts (<1000 mm), and more than once a year in high-rainfall districts (>3000mm). Each time, they affect anything from 1 to 10% of farm area.
Streambank erosion eats back into productive farmland, also disrupting farm improvements and access. Minor bank collapse happens several times a year on most streams. It typically affects 10% or less of bank length. Severe scour occurs only in big floods several years apart. It can affect 50% or more of bank length.
All these forms of erosion can have severe impacts downstream, for instance where eroded detritus chokes river channels, or silt is deposited on top of productive river terraces.
Have any of these impacts occurred in your neighbourhood? On your farm? Recently? How often?
WHY PLANNING LEGISLATION REQUIRES FARMERS TO CONTROL EROSION
The Resource Management Act empowers local authorities to control the effects of land use, to the extent needed to ensure sustainable management of natural resources. It is implemented by :
Regional councils through policy statements and regional plans
District councils through district plans
In many regions and districts, Councils write restrictive conditions into their plans, to control the effects of erosion on the environment.
If you and your neighbours could demonstrate that agricultural management on your own properties is sufficiently sound to minimise any adverse effects of erosion, would the Councils need to write so many restrictions into their plans?
If you could do this, would you remove an excuse for other countries to use environmental criteria as barriers to restrict entry of New Zealand produce to their markets?
AGRICULTURAL PRACTICES WHICH INCREASE THE RISK OF EROSION
Farmers can inadvertently create the conditions which accelerate erosion on their land, for instance by depletion of ground cover, which creates opportunities for surface erosion and gullying. Ways this happens include:
Cultivating cropland when conditions are too dry or cold for germinating seeds to form a dense cover
Cultivating steep slopes
Burning stubble after crop harvest
Burning depleted tussock grasslands
Continued grazing of sown pasture during drought, cold or wet conditions
Heavy grazing of sown pasture at any time of year
Allowing sheep grazing at times of high rabbit infestation and weed competition in tussock country, or during drier than normal conditions
In hill country, weakening of soil strength creates conditions for slipping, slumping or flowage of subsoil during heavy rain or protracted wet weather. Ways this can happen on farmland include :
Bush or scrub clearance to create new pasture particularly on steep slopes
Cutting down scattered trees in the course of pasture improvement
Timber-cutting in residual bush or exotic woodlots
Undercutting slopes with farm tracks, drainage channels and spillways.
Once cleared, land no longer has tree roots to reinforce the soil against downslope movement. Where soil is strong, this is not a problem. Where it is weak, land remains permanently susceptible to mass movement erosion.
Along watercourses, damage can happen where farmers graze stock on streambanks for :
Access to water
Extra feed
Suppression of weeds
The resulting loss of soil into channels in turn, causes a loss of water quality through nutrient enrichment of water (by dissolved chemicals in runoff from paddocks), and fouling (by dung, urine and micro-organisms). It also degrades wildlife habitat.
Do any of these things happen in your neighbourhood? On your farm?
All these adverse practices can be avoided, by simple adaptations to farm management. Many farmers have done so for years. Others have not. Here is a summary of the adaptations, how they reduce erosion, and what impact they have on farm production.
AGRICULTURAL PRACTICES WHICH MINIMISE DEPLETION OF GROUND COVER
On arable land:
Minimum tillage
Stubble mulching
Contour cultivation
- reduce soil loss by anything from 25% to 90%, without retarding crop emergence or depressing grain yields.
Grassed waterways in fields reduce soil loss by 50% or more, at the cost of a 5 to 10% reduction in cropped area.
Windbreaks are also thought to reduce soil loss from paddocks (the amount has not yet been measured in New Zealand). They increase crop and pasture growth by about 10% in their lee. They also shelter stock, leading to reduced feed requirement, better condition, and fewer deaths in severe weather.
In lowland pasture:
Rotating stock between paddocks before pasture gets too depleted
Avoidance of mob-stocking or heavy set stocking during drought, cold conditions or wet weather
Temporary de-stocking of depleted paddocks
Supplementary feeding
- maintain sward density. Keeping residual cover at between 20 and 70% of surface area is enough to trap most windblown and water-washed soil before it leaves a depleted paddock. These practices also help reduce pugging and compaction of wet soil, preventing pasture losses of anything between 8 and 91% from these causes.
In hill country, the same practices can be used as on lowland pasture. Additionally, where hill country has been denuded by the combined effects of over-grazing, sheet erosion and fertility depletion :
Pasture improvement by oversowing and topdressing
- reduces surface erosion by 50 to 80%, and reverses yield losses which can be as high as 80%.
In tussock high country :
Keeping the interval between fires, and the spelling period after fires, as long as possible
Topdressing low and mid altitude short tussock, and oversowing with exotic grasses
Retirement fencing of high-altitude tall tussock country
Rabbit control
Weed control
Restocking during drier seasons
- reduce bare ground to 20% or less of surface area, maintain annual dry matter yield anything from 2 to 15 times higher, and enable increased stock numbers on the balance of land that is still grazed.
Which of these practices would you use on your farm? Why? What problems would you encounter? How would they affect your day-to-day management?
AGRICULTURAL PRACTICES WHICH COUNTERACT LOSS OF SOIL STRENGTH IN HILL COUNTRY
In hill country grazing land:
Widely spaced tree planting in pasture on unstable ground
Close planting of trees on very steep faces retired from grazing
Reversion of very steep faces to scrub and bush (or just leaving them that way instead of clearing them)
- reduce mass movement erosion by 50 to 90% or more, conserving pasture yield, enabling better feed utilisation, and controlling expenditure on fence and track repair. Retirements occur on the least productive parts of a farm, and so long as they are less than about a fifth of the farm's area, they usually have a insignificant effect on the stock numbers and the performance of the property.
Where erosion has already occurred, slip scars on steep faces can be gradually brought back into production by oversowing, topdressing and initial spelling for 1 to 2 years. Slip debris on footslopes, and earthflow terrain, can be brought back into production by cultivation and drainage. Deep-seated mass movements which threaten farm buildings or roads can be stabilised by engineering measures such as sub-surface drains, horizontal bores and gabions.
Which of these practices would you use on your farm? Why? What problems would you encounter? How would they affect your day-to-day management?
AGRICULTURAL PRACTICES WHICH REDUCE DAMAGE TO WATERCOURSES
In arable and lowland pasture:
Permanent riparian fencing
Tree planting, so long as non-suckering deep-rooted species are used
Controlled grazing, inside temporary electric fences
- all reduce bank erosion and sediment loads by at least 50% and often more than 90%. Permanent riparian fencing also reduces dissolved chemical pollutants in watercourses by similar percentages, and avoids organic fouling of watercourses with dung, urine and associated micro-organisms. Where weed growth is a problem on banks, controlled grazing a few times each year suffices to suppress them, and still maintains water quality for most of the time.
In hill country pasture, the above three techniques are still practical on valley-bottom channels. On steep-gradient hillslope watercourses they are prohibitively expensive, and would disrupt grazing management. The alternatives are :
Spaced plantings of deep-rooted trees to reduce the length of channel damaged during floods
Debris dams to stabilise eroding gully floors while trees establish
Close-planting of trees to stop particularly severe open gullies from eating headward into good grazing land
These techniques do not reduce solute loadings or organic fouling, but they can cause very substantial reductions in sediment input to downstream watercourses.
In tussock high country, on broad valley bottoms the same practices as those recommended for lowland pasture can be used. For steep,high-energy torrents on mountainsides, similar practices to those recommended for pastoral hill country can be attempted. But in both situations, success will be constrained by :
The size and ferocity of high country rivers
The difficulty of revegetation in a high-altitude climate
The sheer size of high country runs
For these reasons, effort usually has to be concentrated on short channel reaches where erosion control is vital to maintaining road access or improving wildlife habitat.
Which of these practices would you use on your farm? Why? What problems would you encounter? How would they affect your day-to-day management?
You don't have to put up with a level of soil erosion that depresses farm production, or imposes extra costs, or affects downstream neighbours. There are simple ways that you can control erosion yourself.
The techniques are tried-and-tested farm management practices. They are all already in use, by farmers somewhere in the country. They cost a little bit of time and money - not a lot.
The benefits are:
Less production loss from farms
Reduced expenditure on damage repair
Extra income from diversified land use
Better flood capacity and water quality in rivers
Less disruption to roads, railways and power transmission lines
Improved wildlife habitat
Keeping erosion at an acceptably low level gives local authorities one less reason to impose planning measures that restrict farmers' use of land. It also gives export markets one less excuse to restrict access for New Zealand produce.
How does your farm shape up, compared with your neighbours?
Do you already use some of the techniques in this booklet? What did they cost? Did you see any benefits?
Can you use any of the others? What returns could you expect if you invest some time and money in soil conservation?
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