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Losing Ground - Exposing the Problems

J.E. Pratley

Senior Lecturer in Agronomy, Riverina College of Advanced Education,
Wagga Wagga. NSW 2650

For the past few decades farming in southern New South Wales has involved the use of a pasture phase in the rotation. The advantages of this have been expounded during this time ~ namely the improvement in soil fertility resulting from the nitrogen fixation of the legume-based pastures and the recovery of soil structure which was broken down during the cropping phase. This stable system of farming seemed to be adequate for the long-term future of farming in the Riverina - that is, until things started to go wrong about the middle of the 1970s,

We were, relatively suddenly, confronted with soil structure breakdown, soil acidity and its associated problems, pasture establishment difficulties and poor legume persistence compounded with the occurrence of less favourable seasons. The damage done to the soil in a conventional farming system was further highlighted by the development of minimum tillage techniques which demonstrated, by comparison, the destructive nature of cultivation. Further evidence was provided in the last few months of the 1979-83 drought when much of our valuable topsoil was blown away or subsequently washed away with the drought-breaking rains in autumn 1983. It was indeed fortunate that these rains were relatively gentle in the south, restricting the amount of damage done.

Soil Erosion

Measurements by officers of the Soil Conservation Service in NSW have shown that topsoil losses of the order of 3 tonnes/hectare/year (about 0.25mm) do occur in the cropping areas of southern NSW under conventional cultivation methods (Aveyard et al., 1983) whilst in northern NSW losses of 50t/ha/year (about 4 mm) and greater are not unusual (Marston and Doyle, 1978). The difference in the erosion between north and south is due to the incidence of high intensity summer rains in the north when the soils are in fallow.

This problem is more serious than just the loss of topsoil. It is the topsoil that contains most of the phosphorus which has been applied as fertiliser. Organic matter, nitrogen and other nutrients are also lost. Erosion therefore results in a substantial decline in soil fertility. Further, soil formation takes place so slowly that it is difficult to measure. Estimates in the USA (Seymour, 1980) put the rate of natural soil formation there at between 0.025mm and 0.08mm per year, although some experts consider these figures to be high. It follows therefore, that we are losing our topsoil faster than it forms - that is really not progress.

It has been shown in both southern NSW (Table 1) and in northern cropping areas (Table 2) that the erosion losses can be significantly reduced by a change in method of seedbed preparation.

Table 1. Soil loss from established tillage comparison trials at Wagga and Canberra with rainfall simulation (Packer et al., 1982),

 

Treatments

 

Conventional

Reduced Till

Direct Drill

Sediment loss (kg/ha)

     

- Canberra

330

240

<30

- Wagga

420

291

173

Wagga - rainfall applied at 45mm/hr for 40 minutes, Canberra - rainfall applied at 30mm/hr for 20 minutes.

Table 2. Soil loss from various management practices on a red brown colluvial clay (1 - 3% slope) in south east Queensland. (Cummins et al., 1973 as cited by Marston and Doyle, 1978).

Soil Loss (t/ha/year)

 

Bare Fallow (continuous)

76

Stubble Incorporation

31

Stubble Mulch

12

Not only are we losing the soil, but the portion that remains has, in many cases, deteriorated in both physical and chemical attributes to the extent where productivity has declined significantly. It is worth reviewing some of these aspects.

Cultivation and its Effects

In the beginning was Jethro Tull, sometimes referred to as the “Father of Tillage”. More recently however, he has been referred to as the “Father of Soil Erosion”, Although tillage had been practised in various forms for centuries, Tull was probably the first person to put into print that tilling of the soil would improve crop yields. He would not have foreseen the mechanical revolution that was to take place to convert what was a relatively harmless practice into the destructive force it is today.

Modern cultivation probably began with the development of the horse-drawn mouldboard plough. The “modern” version relates back to a Mr John Deere, in Illinios, USA, who established a factory for its production in 1847 (Smith, 1965). It was a relatively harmless implement in that its action on the soil was reasonably gentle, it was slow to operate, thereby limiting the number of cultivations that could be carried Out, and it gave good weed control by burying weed seeds at depth. It did cause some smearing which, particularly under wet conditions, could produce a hardpan.

The disc plough was developed in the late 19th century and was introduced into Australia in the early 1900s. It held greater attraction to farmers because it could be operated faster through the soil than the mouldboard. It had lower maintenance costs and in more recent times has been available in greater working widths - the size of the disc implements in use today is mainly limited by the availability of finance to purchase it and the tractor to pull it. Unfortunately the action of the disc on the soil is not gentle there is a great deal of pulverisation of the soil and there is a smear and zone of compaction at the base of the ploughed layer. The fact that the disc is a relatively quick implement to use means that more soil can be ploughed more often thereby increasing the extent of the damage. The disc, because of its smearing and compacting action, is ideal for producing a hardpan, particularly if ploughing takes place at the same depth year after year. The effect is much greater under wetter conditions,

In the 1960s, we saw the development of soil-incorporated Pre-emergent (R) chemicals for weed control. The first available were diallate (Avadex ), triallate (Avadex BW(R)) mainly for wild oats control and in the early 1970s trifluralin (Trifluralin(R) or Treflan(R)) was released for control of annual ryegrass in particular but also wireweed, fumitory and surface-germinating wild oats, This development was a major breakthrough for the control of these troublesome weeds. To be effective, however, these chemicals required a fine seedbed and double incorporation -, in other words, rather than replace some of the cultivation involved in seedbed preparation, the use of these chemicals actually increased the degree of cultivation needed. As a result, there has been a substantial destruction of soil structure,

Because of the breakdown in soil structure, other problems have been created:

(i) Surface Crusting - In some soils, the breakdown of the aggregate structure of the surface soil has resulted in the formation after drying of an impenetrable crust on the soil surface thereby restricting emergence of seedlings. The importance of this in crop and pasture establishment varies between seasons depending on the timing of post-sowing rains relative to plant emergence. The effect is greater on smaller seeded species such as lucerne and rapeseed. In addition, however, the crusting of the surface must impede rapid infiltration of moisture thereby increasing runoff (and erosion) and reducing the amount of water stored in the soil to finish the crop.

Related to this is the problem of subterranean clover persistence on these soils. As the name 'subterranean' infers, this species tends to bury its seed. Work by Collins et al. (1976) indicates that prevention of burr burial in some varieties drastically reduces the yield of viable seed and also the proportion of hard seed which is so important to the plant’s persistence from year to year. In the soft seeded varieties Woogenellup, Mt. Barker and Seaton Park, only small quantities of hard seed are produced and hence regeneration in the autumn is largely dependent on seed produced the previous spring. It follows therefore, that hard setting surface soils resulting from a quick finish in spring discourage persistence of subclover and these varieties in particular. Where the burr remains on the surface of the soil, it is readily accessible to stock and too high a proportion may be eaten over the summer period to allow rapid regeneration in the following autumn.

It is worth commenting that pasture regeneration following the drought exceeded expectation. Perhaps these pastures should be inspected to identify the varieties of subterranean clover present. It could be that an undesirably high population of hard seeded varieties such as Dwalganup may be present in the pasture, predisposing sheep in particular to fertility decline. Prevention of seedset in this variety in spring may be an important management option which needs to be considered.

(ii) Waterlogging - The breakdown in soil structure, the loss of coarse porosity and the development of a compacted layer or hardpan at the base of the cultivated layer encourages waterlogging and anaerobic conditions to occur. The soils tend to stay wetter for longer, and the oxygen supply to plant roots is restricted. These conditions are conducive in some soils to the development of manganese toxicity (Figure 1) and numerous other problems such as denitrification. Manganese toxicity problems are accentuated year by year as the pH of the soil declines.

Figure 1, Conditions affecting the availability of manganese to plants,

(iii) Soil Workability - The gradual deterioration in soil structure has also resulted in the problem of reduced time during which the soil is in a suitable condition to cultivate (i.e. the period between being too wet and too hard). This had meant

  • either reducing the amount of cultivation undertaken (least likely);
  • cultivating the soil when it was not in appropriate condition (most likely) and therefore accentuating the damage; or
  • delaying sowing (with the resultant penalty in crop yield) to allow the cultivation to be done (quite likely).

The development of wideline equipment to some extent has alleviated the problem by allowing farmers to cultivate their land in record time, Fortunately the wideline implements are tyned and the soil damage resulting from their use has not been great. Associated with the wideline cultivation is the airseeder attachment which enabled a faster sowing operation. This development has not been without its problems a

  • the extra width of the machine increases the problem of variable seeding depth. In some cases the seed is buried 10 to 12 cm deep and elsewhere deposited on the surface because of unlevel ground. In order to reduce the amount of seed on the surface, there is a tendency to put the sowing boots in too deeply;
  • where the airseeder is not properly adjusted, the bounce of seed and fertiliser also adds to variation in sowing depth and in some cases separation of the seed from the fertiliser, perhaps temporarily slowing down seedling development;
  • particularly where the cultivating points are used at sowing, there is a danger of sowing too deeply. This results in variable emergence of the crop.

DEPTH OF SOWING

More attention needs to be taken with sowing depth. One of the reasons for the poor emergence of wheat in particular is the fact that Mexican semi- dwarf varieties have a shorter coleoptile than the older varieties. The coleoptile is the first structure of the shoot which emerges from the seed at germination, enveloping and therefore protecting the leaves which emerge when the soil surface is reached. The coleoptile is a more rigid structure than the other leaves and therefore has as its main function getting the seedling through the soil. By sowing too deeply however, the coleoptile does not reach the soil surface by the time it has reached its extension limit. The new leaves emerge from the coleoptile but do not have the strength to push through the soil and therefore do not reach the surface. This problem is accentuated where chemicals such as trifluralin and Erex (R) reduce the vigour of the seedling,

Fig 2. The effect of sowing depth on wheat seedling emergence.

In the case of the wideline, the use of narrower sowing points and the removal of the sowing hoses from the boot and attaching them such that the seed falls some distance, say 20 cm, behind the tyne will result in the seed being deposited shallower as indicated in Figure 3 (Rice, 1982).

The effect of the removal of sowing hoses from wideline boots on depth of sowing (Rice, 1982),

(a) With the hose in the boot the seed is planted at point depth.

(b) With the boot removed and the hose hitched back, depth of sowing is determined by soil flow. Trial and error with the length of wire can ensure that seed falls in the 3-5cm zone.

The use of cover crops in pasture establishment

Coinciding with the introduction of the Mexican semi-dwarf wheats has been the difficulty of successful establishment of undersown lucerne. To some extent the poor seasons with late autumn breaks in the 1970s have also contributed to this situation. However compared to the older, taller varieties, the semi- dwarf s have a greater tillering ability, are higher yielding and therefore form a much denser sward. This exerts much greater competitive pressure on the smaller, slower-establishing lucerne seedling, Best establishment occurs where no cover crop is used. However, where a cover crop is used, then the recommendations suggest at most a crop sowing rate of not more than 50% of that normally used. At recommended sowing rates of 35-40kg/ha this means a cover crop rate of less than 20kg/ha. The greater competition from the semi- dwarf varieties increases the need for a reduced sowing rate, The extent of the reduction in lucerne establishment is shown in trials carried out by Simmons and Cregan (unpublished) on the southern slopes (Table 3).

Table_3. The effect of sowing rate of wheat on the establishment of lucerne (Simmons and Cregan, unpublished).

Location

Wheat Variety

Sowing Rate
(kg/ha)

Lucerne
Establishment Cplants/m2)

Junee Reefs

Egret

nil
6
II
17
22
28

22
16
16
II
II
5

Ardlethan

Condor

nil
25
45
70

22
3.5
0.3
nil

Further undersowing with many of the older varieties took place slightly earlier in the season than it now does with the shorter season wheats, Condor, for example, is slightly earlier maturing than Heron and Falcon, the varieties it replaced, and therefore is sown a little later. The mid- season wheats Olympic and Teal are less fashionable now and the late start to the seasons in recent years has also reduced their use. Later sowing works against the establishment of lucerne which is basically a summer- growing species with optimum air and soil temperature for germination and growth of around 25 C (Bula and Massenglae, 1972). Seedling emergence and growth is minimal at temperatures less than 100C which, incidentally, is about the temperature of soils in mid-May at Wagga. The later lucerne is sown in the autumn, the less likely it is to establish due to slower growing conditions and therefore the relatively greater competition it is likely to suffer from the crop which is better adapted to the cooler conditions.

On some farms the problem is avoided by undersowing pastures to a barley cover crop before the wheat is sown. Barley has always been considered the cereal of choice as a cover crop because of its more erect habit and earlier maturity. It is important to note that Clipper, the most popular variety, is susceptible to barley leaf scald and other diseases and competition with undersown species is reduced. The release of new varieties of barley in the near future with resistance to such diseases will reduce its effectiveness as a cover crop.

Soil organic matter

One of the effects of cultivation is the breakdown of organic matter and the resultant release of plant nutrients in available forms. Unfortunately most Australian soils are low in organic matter. It is the organic matter which is so essential to the maintenance of good soil structure, to soil stability and to buffering capacity (the ability of the soil to resist change). For example, a loss of 1% organic matter from the soil represents only about one-tenth of the organic matter in European soils (with 9 to 10% organic matter) but it represents perhaps half the organic matter in Australian soils (the organic matter content of Australian soils is only about 2% of the soil). The breakdown of organic matter in Australian soils is therefore of great significance.

In undisturbed soils there is a gradual turnover of organic matter -nutrients are gradually released for plant use through a process called mineralisation while others are returned to the soil in plant and animal remains. However, when the soil is cultivated, the rate of breakdown is accentuated and a flush of nutrients is usually made available over a relatively short period of time, In the case of nitrogen, this has important implications for the soil and for cropping practice. The nitrogen in the organic matter is mainly released as ammonium compounds (NH4+) which are converted to nitrates (N03-), by a process called nitrification. The nitrate form of nitrogen is the most plant-available form but it is also a form which is very soluble in water and therefore very susceptible to being leached out of the root zone. The net effect is an increase in the hydrogen ion (H+) concentration of the root zone - that is, the soil is more acid as a result. (Note, however, that this is only one of the acidifying processes in the soil). The overall reaction as described by Helyar (1976) is:

In cultivated soils there will be an increase in the breakdown of organic matter and release of nitrogen, often in amounts too large for plants to utilise in the short term. The unused portion may be wasted as a result of leaching, thereby hastening the acidification process. In minimally cultivated soils, the organic matter is broken down more slowly, nitrogen is released gradually over time and therefore is more likely to be used by plants and less likely to be leached. The acidification process is slowed down.

Further, the more rapid depletion of the soil's nitrogen reserves in a conventional cultivation situation increases the need for nitrogen fertiliser supplementation in the later stages of the cropping phase. Where the nitrogen is added in the form of ammonia (see Table 3), the potential for soil acidification is greater where plants are unable to utilise a high proportion of the fertiliser,

The efficiency of utilisation by plants of nitrogen from organic matter breakdown must be maximised and the dependence upon fertiliser nitrogen must be minimised. This can be achieved by reducing the amount of cultivation practised during crop establishment.

Table 4. The ammonium content of commonly used commercial fertilisers and their potential acidifying effect (adapted from Glendinning, 1974).

Fertiliser

Total N
(%)

N as Ammonia
(%)

kg lime required to correct acidity per kg N*

Calcium ammonium

     

nitrate

23.0

11.5

0

Easy-N

32.0

7.9

0.5

Anhydrous Ammonia

82.0

82.0

1.8

Nitram

34.0

17.0

1.8

Grower 11(5% S)

11.0

11.0

2.6

Grower 12(9% 5)

11.7

11,7

3.2

Starter 12(3% 5)

12.4

12.4

2.3

Starter 15(10% 5)

15,0

15.0

3.3

Starter 18(17% 5)

17.0

17,0

4.4

Sulphate of

     

Ammonia(24% 5)

21.0

21.0

5.4

*Calculated on the basis of 1.80 plus 0.15 kg for every 1% S in the fertiliser.

Soil water

One other aspect of cultivation that needs some comment is moisture supply. Excessive soil disturbance has made many soils less conducive to long-term storage of water. Apart from the obvious effects of surface crusting and hardpan formation, cultivation destroys the continuous pore structure of the top layers of the soil and results in accumulation of moisture at the surface but often relatively poor percolation into the subsoil portion of the root zone. Infiltration is therefore usually better on untilled soil and there is greater moisture storage at depth to allow the crop to finish - an advantage in dry springs. It is important to note that with each cultivation the dry surface soil is mixed with the subsurface layers and moist soil is brought to the surface, eventually to evaporate - that is, there is potential moisture loss with each cultivation.

Direct drill crops tend to grow more slowly at first which means that such crops do not produce the amount of foliage of conventional crops (Cornish and McNeill, 1982), With less leaf area involved, less water is lost through transpiration. This presumably means better water use efficiency by direct drill crops resulting in more moisture being available in the subsoil in spring to finish the crop,

Stubble

The amount of stubble produced in NSW is not recorded but can be estimated from crop statistics (Table 5),

Table 5. Winter cereal grain production in NSW and the Southern Slopes of NSW in 1979-80 (Australian Bureau of Statistics, 1981).

 

New South Wales

Southern Slopes

 

Area (ha)

Tonnes

Area

Tonnes

Wheat

3.42 million

6.00 million

0.55 million

1.18 million

Oats

0.35 “

0.46 “

0.11 “

0.18 “

Barley

0.45 “

0.69 “

0.12 “

0.20 “

Total

4.22 “

7.15 “

0.78 “

1.56 “

If we assume, conservatively, that stubble comprises about 50% of the dry matter at harvest (it is probably closer to 60% straw), 40% grain), NSW therefore produces in excess of 7 million tonnes of stubble of which the Southern Slopes contributes in excess of 1,5 million tonnes. At an average metabolisable energy value of 5 MJ/kg and a maintenance energy requirement for a dry sheep of 160 MJ/month, there is theoretically enough energy to maintain the NSW flock of 50 million sheep for more than 4 months. This figure is more realistically about 3 months after allowing for the efficiency (about 60%) with which this energy is used. The State’s cattle could be maintained for more than 3 months.

If the calculations are confined to the southern slopes, then the stubbles produced could maintain this region’s 6.7 million sheep for more than 4 months or its 0.56 million cattle for 7 months.

In southern NSW, animals generally have access to the stubble over the summer during which time they consume any weeds and spilt grain and a small proportion of the stubble, A further proportion is trampled by the grazing animals. The remaining stubble, where possible, is burnt mainly because it is easy, effective and gets it out of the way for subsequent seedbed operations. Alternatively the material is ploughed in and, depending on conditions, may or may not create a temporary nitrogen deficiency.

Other options are to harvest the material for straw or to treat it with caustic to make alkalage, neither of which are of significance, or, increasingly being considered is the retention of stubble on the surface as a mulch and erosion preventative.

It seems to me that much greater value has to be placed on the stubble resource. We have to look at improving our methods of stubble utilisation by animals and/or retaining our stubbles in the surface during the initial stages of the next crop,

Improved utilisation by livestock has several advantages, namely;

• a source of energy for livestock during a period of the year when feed is lacking;

• the spelling of pasture paddocks to allow species such as lucerne to produce forage for the autumn;

• the reduction in the stubble load for subsequent cropping operations such as spraying and sowing;

• compensation for the lack of weeds in the stubble as a result of good crop management and the lack of grain through improved harvest efficiency,

It is known that stubble utilisation is improved by:

• stocking immediately following harvest as the stubble feed value is at its highest prior to its deterioration through rain and dew. It is worth noting that almost complete utilisation occurred in 1982-3 when the very dry conditions maintained stubble digestibility and the animals were hungry enough;

• stocking with sheep initially to enable them to select out the more digestible fractions. Cattle can better digest lower quality material than sheep and can maintain their weight for longer. Research has shown that sheep can maintain their weights on stubble for up to 6 weeks (Mulholland et al., 1976) whereas cattle can maintain their weight for up to 12 weeks (Mulholland, unpublished).

More work is needed in the area of supplementation to improve stubble utilisation by livestock. Stubbles are low in protein and in some minerals. Variable responses have been obtained from urea, sulphur and energy supplements and further investigation is needed to achieve these responses economically and more consistently.

The retention of stubble for the subsequent crop has advantages in respect of erosion prevention (Table 2) and in improved moisture supply for the crop. This is achieved by reducing runoff, thereby improving infiltration and by reducing evaporation from the soil surface (Taylor and Lill, 1982). The major limitation to its adoption in this area is technique - what machinery to use, what chemicals to use and how to use them.

At previous Conferences blade ploughs and rod-weeders were identified as implements suitable for seedbed preparation, However, blade ploughs are ideally designed to create compaction layers in the soil and are therefore undesirable, The trash-seeder has been a useful development but it tends to be limited to handling stubbles of less than 2 tonnes, a yield which, hopefully, is below our ambition,

There is also a problem in some cases with achieving a good weed kill with herbicides sprayed into thick stubble. Chemical technology may solve this problem by electrostatically charged herbicides or by other means. Instead, it may be necessary to reduce the amount of stubble present.

Consequently it may be that better utilisation of stubbles by animals will reduce the residues to manageable levels for a stubble retention system of cropping, More research is needed to inform us of the minimum amounts of stubble to leave for maximum benefit to the subsequent crop.

Conclusion

It would be fair to say that we have lived through the mechanical revolution with questionable success, We are now in the midst of a chemical revolution which offers prospects for greater success. It is important to understand, however, that chemicals are not a substitute for good management - they are an aid to good management. They will not compensate for late or deep sowings, for poor crop nutrition, soil acidity or poor farm hygiene. The efficiency of utilisation of these chemicals has to be maximised by using them at the right times at the right rates and in conjunction with other management activities, such as grazing, to minimise their use. We don’t want pollution and we don’t want resistance.

The message is loud and clear;

  • reduce tillage and improve soil structure. This will

o slow down acidification

o improve crop and pasture establishment

o reduce soil erosion

o improve utilisation of legume nitrogen and reduce the need for fertiliser nitrogen;

  • improve the utilisation of crop stubble and avoid the destruction of valuable organic matter and energy.

It only remains for us to learn how to reclaim our problem land, how to establish and manage our pastures and master the techniques of direct drilling and stubble retention. Hopefully we will learn much in this regard from subsequent papers at this Conference and enable us to “recover lost

References

1. Aveyard, J.M., Hamilton, G.J., Packer, I.J. and Barker, P.J. (1983). Soil conservation in cropping systems in southern New South Wales. J.Soil Cons. NSW 39 (1) : 113.

2. Collins et al. (1976). The interrelation of burr burial, seed yield and dormancy in strains of subterranean clover. Aust. J.Agric. Res. 27 787.

3. Cornish, P.S. and McNeill, A,A. (1982). Adaptation of wheat to direct drilling. Proceedings of 2nd Australian Agronomy Conference, Wagga Wagga; 203.

4. Glendinning, J.S. (.1974). Fertilizer Handbook (Australian Fertilizers Ltd: Sydney).

5. Helyar, KR. (1976). Nitrogen cycling and soil acidification. J. Aust. Inst. Agric. Sci. 42(4) 217.

6. Marston, D. and Doyle, A~ D. (1978). Stubble retention systems. J. Soil Cons. NSW 34(4) : 210,

7. Mulholland, J.G., Coombe, J. B., Freer, M. and McManus, W.R. (1976). Aust. J. Agric. Res, 27 : 881,

8. Packer, I. J., Hamilton, G. J., White, I. and Jones, D. (1982). Infiltration and soil erodibility benefits of conservation tillage. Proceedings 2nd Australian Agronomy Conference, Wagga Wagga; 192.

9. Rice, D. (1982). Know-how. The Direct Drill Farmer. Summer 1982 29. Seymour, J. (1980). Soil erosion: can we dam the flood? Ecos 25 : 3.

10. Smith, H.P. (1965). “Farm Machinery and Equipment”. 5th Edn. (McGraw-Hill: New York).

11. Taylor, A.C. and Lill, W. J. (1982). Better lupin husbandry in southern New South Wales. Proceedings of 2nd Australian Agronomy Conference, Wagga Wagga; 196.

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