erosion-control

Agricultural Practices Which Control Erosion

Clearly a certain amount of erosion has to be expected as a by-product of agricultural land use. But farmers don’t have to put up with a level of erosion that reduces their production, or imposes extra operating costs year by year.

Many farmers already avoid these adverse effects, by simple changes to agricultural management. Often, they are carried out for other reasons before erosion becomes a problem, such as maintaining good soil structure for cropping or reducing loss of soil nutrients from pasture ⁽⁸⁾, and erosion control is a beneficial side-effect. The changes may cost a little extra to implement, but their cost is outweighed by the time and expense that are saved, and the loss of production that is avoided, later on.

Practices which minimise depletion of ground cover

Arable land

The following techniques are used on arable land (cropland, orchards and vineyards), where cultivation exposes soil to windblow, sheetwash or rilling. Affected soils include alluvial soils and peats on plains and valley bottoms nationwide, volcanic ashes on North Island plateaux, and loesses on South Island downs and basins.

Minimum tillage encompasses a wide range of techniques, from direct drilling of seed into stubble or pasture, through spraying followed by direct drilling, or spraying followed by a reduced number of cultivation passes before sowing, to more judicious use of conventional ploughs and harrows.

The techniques do not entail less frequent crops, as they can equally well be an element of either continuous or rotational cropping. While they can entail using specialised equipment, they do not have to. There are some simple ways of minimum tillage with conventional machinery. The critical ones are cultivating soil when the moisture content is neither too high nor too low; and correctly adjusting the plough and harrow to suit the soil and the tractor. If these precautions are observed, two-pass cultivation (once with plough and once with harrow) is often all that is needed to prepare a seed-bed on good soils. Repeated passes with machinery to get a good tilth are avoided and the risk period for surface erosion, between initial vegetation clearance and ground cover by a growing crop, is shortened. Rotational cropping is in itself an extremely useful form of minimum tillage. The intervening years in pasture buildup soil structure, giving it greater resistance to erosion in the year of cultivation, and making cultivation with conventional machinery easier than is the case for continuously cropped soils.

Data from New Zealand crop trials⁽⁹⁾, where these simple forms of minimum tillage have been used in conjunction with appropriate fertilisation, indicate the following findings:

On light arable soils in Canterbury and Otago, minimum tillage maintains grain yields from year to year. This compares with losses ranging from 18 to 62% where soils have been excessively cultivated (combined effects of topsoil loss and fertility depletion; the larger figures apply where soils have been continuously cropped).

On heavy arable soils in the Manawatu and Waikato, it maintains grain yields at original levels, compared to losses of 23 to 45% on excessively cultivated soils (combined effects of topsoil loss and structural breakdown; the larger figures apply where soils have been continuously cropped).

Annual soil loss is reduced by 26 to 52% (only a few trials have measured soil loss as well as crop yield, so the range may be greater).

Similar returns can be expected from minimum tillage of arable soils in other parts of the country. The zero tillage (direct-drilling) technique also reverses yield reductions, but is offset by higher losses due to weed regrowth than are the case for either conventional or minimum tillage, unless it is accompanied by herbicide spraying.

Maize paddock minimum-tilled with conventional plough

Figure 1: Maize paddock minimum-tilled with conventional plough and harrow at optimum soil moisture content. Note good tilth. Manawatu. Photo: G Shepherd

Stubble mulching (Figure 2)

Stubble mulching - to avoid windblow and sheetwash

On continuously cropped soils, stubble can be mulched after harvest and left on the paddock if it is to be direct-drilled the next season. If it is to be cultivated, mulched stubble can be incorporated into topsoil, in the course of cultivation. In both circumstances, a partial ground cover of mulch protects soil from being lifted by wind or runoff until the new crop emerges. If soil particles are detached, most of them are caught in the mulch before they move far. The practice is rarely used in New Zealand, but is becoming widespread in the Australian wheat belt, In the Mid-West USA, it is hard to find cropland that is not stubble-mulched. The following findings have been obtained in Australian and North American studies⁽¹⁰⁾:

Stubble-mulched strips (light) contrast with unmulched strips (dark

Figure 2

Stubble-mulched strips (light) contrast with unmulched strips (dark), in field where autumn-sown grain is emerging in spring. Iowa.

Mulches covering as little as 10% of a field’s surface reduce surface erosion rates by a much larger factor, usually exceeding 50%. Increasing mulch cover to 30% reduces surface erosion by 80% or more.

These levels of mulching do not retard crop emergence or depress crop yields (heavier levels do, while providing little extra protection against soil loss).

Some improvement in soil structure and water-holding capacity is brought about by eventual decay of mulch.

A disadvantage of mulching can be higher incidence of crop disease from infected stubble, leading to an increased spray requirement This has often been observed on continuously cropped land during wet growing seasons. It does not appear to be a problem in dry climates, or on rotationally cropped land where infected stubble decays before the same crop is sown again. On continuously cropped land, the problem can be reduced by tine-chopping stubble and incorporating it into topsoil, where it breaks down faster and does not harbour insects or plant diseases for as long.

Contour cultivation (Figure 3)

Contour plowing - across the slope, not down it

Where sloping ground is cropped, it can be cultivated along the contour as an alternative to the standard practice of cultivating up and down slopes. During rain, soil washed off ridges is trapped in furrows running across-slope, instead of being carried away through downslope furrows by runoff. Sometimes contour-ploughed fields are also strip-cropped, i.e., strips of unploughed grass are left between strips of cropland, as extra sediment traps. The practice is only occasionally used in New Zealand. In the Australian wheat belt, not much land has enough slope to warrant it. In the main grain-growing areas of the USA very little sloping cropland is not contour-ploughed. US data ¹¹ indicates:

Soil loss from contour-cultivated fields is usually reduced by at least 30%. and often 90% or more, compared with fields that are cultivated downslope.

In the short term, this practice does not affect crop yields one way or the other. In the long term yields are higher, relative to sites where soil depth and fertility are depleted by ongoing surface erosion.

A disadvantage is that contour ploughing is unsuited to heavy soils (with high clay content and low infiltration rates), in districts subject to intense rain. Here, runoff can pond in furrows until it breaches the plough ridges, forming downslope rills. Even on light soils, the same problem can occur if slopes steeper than 12 degrees are contour-cultivated. One solution is to direct-drill steeper faces. On soils in wet districts, like Pukekohe, ponding of water in contour furrows causes root-rot in vegetable crops, and is a reason why market gardeners have avoided the technique. The problem does not seem to arise in dry market gardening districts such as Oamaru.

Contour-ploughed wheat and alfalfa

Figure 3: Contour-ploughed wheat and alfalfa (lucerne) in alternating strips. Late summer, New York Photo: G Eyles

Grassed waterways (Figure 4)

Grassed waterways - in hollows, not bare soil

During heavy rainfall on cropland, surface runoff carries soil into depressions which act as temporary waterways. If they are bare, the concentrated flow washes the soil particles out of the field and into watercourses. Retaining grass cover provides resistance to water flow, slowing it to the point where it drops sediment. Grassed waterways have been used on some downland farms in Canterbury and Otago since the 1950s. but there are also many other parts of the country where they could be used on arable land. Again, the technique is widespread in cropping regions of Australia and the USA. Very few New Zealand trials have measured impacts, but overseas data^(12a) indicates :

a reduction of 50% or more in amount of soil leaving fields;

typically, reductions of 5 to 10% in crop yield (proportional to the area of field in grass).

Grassed waterway on sloping field

Figure 4: Grassed waterway on sloping field, cultivated and sown with Swede-grass mix. Otago.

Windbreaks (Figure 5):

Throughout the plains and downlands of New Zealand, windbreaks have been planted for many years. Although their primary role is shelter, they are. also used to protect soil against wind erosion in some parts of the country, notably South Canterbury, Southland, Wairarapa and Hawkes Bay. Little New Zealand data are available about reductions in soil loss on paddocks where windbreaks have been planted. ^(12b)They are known to increase crop and pasture growth by about 10%, with other benefits being better stock condition and reduced deaths during cold weather, but these are consequences of windbreaks’ shelter role, not erosion control.

Thick belts of pine plantation are also used for wind erosion control in sand dune country, along the North Island’s western coastline. Here, the aim is not so much to control wind erosion on farmland, as to prevent pasture from being buried by sand blown from the unstable coastal dunes.

single row windbreak

Figure 5: Permeable single row windbreak of 10 year old eucalypts (nitens) spaced 3 metres apart, protecting soil from westerly gales when pasture is depleted, sheltering stock, and supplying firewood from thinnings, Southland. Photo: SRC

Lowland Pasture

Overgrazing of pasture cannot be completely avoided, given that there are times in any year when feed is short. To minimise windblow, sheetwash or tilling, the critical thing is to maintain just enough ground cover, so that if a gale or a cloudburst strikes, a high proportion of eroded soil particles are trapped in the same paddock before they have a chance to move further.

Maintenance of adequate ground cover (Figure 6)

Maintaining adequate ground cover tits in with things which most farmers do anyway in the course of grazing management: rotation of stock between paddocks, and avoidance of mob-stocking or heavy set-stocking during drought, cold conditions or wet weather. Farmers routinely note amount of feed as they walk through paddocks, and it is possible to note residual ground cover at the same time. If one or two paces out of every ten strike bare soil or mud, ground cover is still enough to protect most of the bare patches against wind and rain. If between two and seven, cover is insufficient to prevent wind and rainsplash from detaching soil particles, but residual grass and litter still trap most of them before they move out of a paddock. If more than seven, detached soil is likely to leave in dustclouds or runoff during adverse weather. The remedy is to spell the paddock if at all possible before it reaches this stage, and temporarily destock it if it does. There is an obvious cost in terms of short-term feed utilisation. On the other hand, there is also faster recovery of pasture, and greater long-term production, if paddocks can be lightly grazed when weather conditions are adverse for plant growth. Measurements on farms¹³ show that maintaining sward density :

traps 90% or more of windblown and water-washed soil before it leaves the paddock;

helps reduce pugging and compaction of wet soil;

prevents pasture growth losses of anything from 8 to 91% the following season.

The upper levels of pasture loss may seem high but have been recorded at diverse locations (Hawkes Bay, Manawatu, Canterbury, Otago). The highest losses occur where topsoil is stripped by water or wind; and when grass is trampled into mud on waterlogged, pugged soil. In either case growth cannot recover for a year or longer, even though climatic conditions have improved meanwhile. Light set-stocking or rotational grazing with adequate spells reduce the risk. These practices do not avoid the need to purchase supplementary feed from off-farm, but they at least control pasture loss during adverse events, and enable faster growth recovery afterwards.

ground cover

Figure 6: Protection of soil by adequate ground cover (right), compared with windblow from cultivated soil (left), Southland.
Photo: SRC

Pastoral Hill Country

On hill country also, depletion of ground cover cannot be completely avoided during droughts or cold, wet winters when there is not much fresh growth. But it can be reduced by the same modifications to grazing management as for lowland pasture, i.e., maintaining greater residual feed in paddocks through rotational grazing, avoidance of mob-stocking or heavy set-stocking, and destocking the worst-affected paddocks (if feasible). There is also an additional technique which makes a big difference to density of ground cover on hill country - pasture improvement.

Pasture improvement (Figure 7)

Pasture improvement - to create a dense sward

Since the 194Os, farmers have converted some 4 million hectares of hill country from low-producing to improved pasture, by oversowing and regular fertilisation. While carried out primarily to increase production, this practice has also had the beneficial side-effect of establishing a dense sward. Sparse, sheet-eroded pasture, once widespread in our hill country, is now a rare sight indeed. Field trials ^(14a) have shown that establishment of improved pasture on hill country:

reduces surface erosion by 50 to 80%. relative to levels measured in unimproved pasture;

reverses pasture growth losses of 40% or more, (combined effects of over-grazing, sheet erosion and fertility depletion). In combination with closer subdivision and better grazing management, this reversal has enabled increases in stock carrying capacity from 2-7 stock units a hectare up to 8-12.

Farmers do not need to be convinced of the benefits of hill country pasture improvement; they already know. Perhaps what needs to be stressed is the importance of fertiliser maintenance; something which farmers cut back in the 1980s in order to save costs. Many hill country farms have had thirty or more years of regular fertiliser application. Despite use by plants and losses through runoff, enough residual nutrients have accumulated in the soil to avoid any immediate effect on production from ceasing fertiliser application. However on some properties where fertiliser applications were lighter and nutrient reserves lower, pasture yield has dropped by as much as half over 5 to 10 years, and a reversion to their 1940s condition is already under way. This is a sign of things to come on other farms if their nutrient reserves gradually drop to the same low levels. Recent trials by DSIR, MAF and AgResearch suggest that adequate applications of fertiliser every 2 to 3 years (exact amount and mix depends on soil characteristics and pasture growth required to support target stock numbers) are advisable to maintain pasture composition and yield at an improved level.^(14b)

Soil protected by improved pasture on spurs

Figure 7: Soil protected by improved pasture on spurs, compared with soil exposed to sheetwash by unimproved pasture in hollow. North Auckland.

Tussock High Country

A different suite of techniques is needed to maintain ground cover in tussock high country, given its peculiar problems. Controversy still rages about whether tussock depletion is due to burning, rabbits, suppression by weeds, overgrazing by sheep, or a combination of the lot. The cause varies from district to district and even from run to run, depending on altitude, climate, original composition of the tussock, and past grazing management. What can be safely said, is that since the 1950s a number of high country runs have successfully rehabilitated tussock country by means which are still applicable today.

Burning control, tussock improvement, retirement fencing (Figure 8)

Burning control, pest eradication, oversowing and top dressing - to resore depleted tussock cover

At low to mid altitudes, tussock is a tire-induced community and in the absence of occasional fires, it reverts to scrub on a time-scale of 100 years or more. There can be no doubt that if runholders are to maintain tussock grasslands in areas prone to scrub reversion, occasional burning is necessary. However, it can take up to 20 years with no grazing for burnt tussock (whether tall or short) to recover to the stage where density and biomass are close to pre-burning levels. This suggests that a burning cycle which covers a run in less than 20 years leads to cumulative decline in dry matter yield. High country surveys⁽¹⁵⁾ indicate that keeping the interval between fires, and the spelling period after fires, as long as possible:

maintains annual dry matter yield anything from 2 to 15 times higher than from excessively burnt tussock country (depending on severity of fires, and time allowed for recovery in between);

maintains soil fertility, through avoiding ignition of soil carbon and nitrogen volatilisation during fires.

Grazing management after burning is a critical factor which affects how fast tussocks recover. Heavy grazing of regrowth greatly slows regeneration, while spelling followed by light grazing aids it. The obvious downside to infrequent burning is a higher percentage of woody shrubs in the tussock, particularly in higher-rainfall districts. This leads to some reduction in area grazeable by sheep, obstacles to mustering, and possible damage to fleeces.

Topdressing low and mid altitude short tussock and oversowing with exotic species (mainly fescue, vernal, cocksfoot and clovers) is a fast way to restore ground cover, where runholders cannot afford to spell a depleted block for 20 years until it naturally recovers. Where short tussock remains dominant in the sward, this technique^(16):

reduces bare ground, often from 50% or more of the surface down to 20% or less

increases annual pasture production from 500-2000 kilograms of dry matter a hectare up to 1000-6300 kg dm/ ha (higher figures, from some MAF or DSIR trials, generally apply to tussock that has been almost entirely converted to sown pasture).

Grazed low-altitude tussock

Figure 8: Grazed low-altitude tussock, improved by oversowing and topdressing, contrasts with mid-altitude tussock, not grazed or burnt in recent years, above retirement fence. High-altitude tussock cover remains depleted long after burning and grazing have ceased. Otago.

Retirement fencing, i.e., fencing off and ceasing grazing, can restore depleted high-altitude tall tussock country where climatic conditions prevent establishment of exotic grasses or legumes. This technique ⁽¹⁷⁾ achieves the following:

It leads to a very slow recovery in ground cover. The high-altitude climate is such that even tussock growth is impaired by cold temperatures, frost and wind.

It has a negligible impact on grazing. In earlier years, grazing high-altitude tall tussock land was thought necessary for summer feed, and to save feed reserves on lower country for winter grazing. However, the retired land generally has a carrying capacity of 1 stock unit a hectare or less. Increases in stock numbers of 20% or higher (sometimes over 100%) were recorded on most high country runs where retirement fencing was routinely implemented as part of farm conservation plans in the 1950s-1960s. typically reducing grazeable area by anything from a tenth to a half. These figures imply that the amount of high-altitude grazing lost was either negligible, or outweighed by what runholders gained from cessation of burning followed by pasture improvement on low- to mid-altitude short tussock country.

All three management techniques have been used less in the high country in the 1980s-1990s, partly because topdressing and fencing costs have risen steeply and are no longer government-subsidised; also because lower wool returns have restricted runholders’ expenditure. On some properties, past benefits have been counteracted by proliferation of rabbits (eating the increased pasture growth) or spread of hawkweeds (suppressing it). In such cases, the techniques may remain valid ways to counteract depletion of tussock cover only if accompanied by measures to control these pests.