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Soil Physical Requirements for Germination, Root Development, Plant Production and Survival

Peter Cornish

Agricultural Research Institute, Wagga Wagga. NSW 2650

Most farmers now accept that reducing the amount of cultivation is a good thing, mainly because it can greatly improve the natural structure of surface soils and reduce the risk of erosion. Attention has now turned to deeper soil which, to some extent, has been “out of sight and out of mind”. Can subsoils be improved like surface soils, leading to significant increases in crop or pasture production? There is even the paradox that subsoil tillage may be necessary at times to reap the full benefits of reduced tillage at the surface.

It can be difficult to predict when subsoil loosening will increase productivity. For a start soils can be dense naturally in all or part of the profile and loosening may not produce a lasting effect. Predicting responses can he difficult because a dense soil does not necessarily indicate a problem for root growth or the infiltration of water. Compaction can also occur in conjunction with other problems which need to he rectified as well as loosening the soil. Moreover, some responses to loosening may be obtained more cheaply by other means (for example by applying N fertilizer in cases where loosening reduces N losses from the soil caused by temporary waterlogging). To further complicate matters, we know that some restriction to root growth can be tolerated without affecting the shoots. Although some cases of compaction will be obvious, and responses predictable, it is more generally true that there is only a suspicion that the subsoil is not what it ought to be.

Given this complexity of subsoil loosening, there is a great need for accurate diagnosis of any problem, accurate assessment of its severity and the choice of an appropriate solution. This Conference aims to meet this need. Today’s interest in subsoil loosening is a case of history repeating itself. Subsoil loosening has been in fashion before (e.g. in the 1890’s - Plate 1). Without a sound basis for the practice, the present activity will be just another passing fad.

Plate 1. Subsoiling was a fad in the 1890’s. It will become another fad in the 1980’s unless problems can be correctly diagnosed and appropriate remedies prescribed.

My role in this conference is to introduce the theme by

(i) indicating the crop yield potential where subsoil conditions are not limiting,

(ii) outlining the ways in which subsoil loosening could increase yields and

(iii) by describing the optimal seed bed conditions for plant growth. I shall interpret “seedbed” very loosely, and refer mainly to action below sowing depth, down to the maximum potential rooting depth of 1.0 - 1.5 metres.

A. Crop Yield and Water Use

I want to now examine crop yield potential in light of rainfall, for three reasons:

(i) A knowledge of what our land should produce compared with actual production is the best guide to the need for any corrective measures, including subsoil loosening.

(ii) Water is our most expensive input in dryland crop or pasture production (I will concentrate on crops) and should be used efficiently.

(iii) Tillage practices, especially subsoiling, can have a big effect on crop water use.

(i) Crop yield potential

The best guide to potential yield in dryland crops is the grain produced per unit of water used. Around the world in climates like our own ,crops such as wheat, oats and barley grown with good agronomy hut without irrigation produce about 10 kg of grain per hectare, for each millimetre of water used (10 kg/ha/mm). (Some experiments indicate a yield of 20 kg of grain per hectare above a threshold of about 100 mm.) The approximate yields expected for a given potential water use in south-eastern Australia are shown in Figure 1.

Fig. 1. Potential yield is determined by potential crop water use. You can calculate approximate water use for any previous year (see text) and read from the graph the yield to expect. Actual results from Wagga (1980-83) are shown.

Water is being wasted if yield is substantially below the potential. Late sowing, poor nutrition, diseases etc. can all lead to inefficiency: so can soil physical problems.

You can gauge your expected yield in any season by reading off yield for a given water use. Water-ruse is the sum of water lost from plants and the soil surface between sowing and harvest. It is the sum of the available water at sowing, plus rainfall between sowing and maturity (hard dough), less runoff and deep drainage and any available water left in the soil at maturity. I have used an approximation for water-use in Fig. 1 which will suit most farms in southern N.S.W. : one-third of the January to sowing rainfall (approx. the rain stored in a autumn fallow) plus sowing-November rainfall. I assume that crops can use all of the available water. This should be true of water-use up to about 450-500mm. Nil run-off and drainage beyond the root zone have been assumed. Figure 1 strictly refers to potential water-use, not actual water-use.

If yield is less than the upper limit set by water supply, then some water is being wasted: through run-off, deep drainage, incomplete use of water (caused by low plant vigour or shallow rooting) or a poor pattern of water-use (e.g. too little used early in growth). Some commercial crops reach the limits shown in Fig. 1, but many fall far short, as unpublished work by French and Schultz in South Australia clearly shows (Fig. 2). The physical condition of Australian soil is generally poor by world standards. This deserves careful consideration as a possible reason for yields falling below the potential set by the supply of water.

(ii) The cost of water in a dryland crop

Plants require water, nutrients, light and carbon dioxide for growth.

Fig. 2. Some commercial crops reach their water-limited potential, but many do not. This is shown in a survey of commercial and experimental crops in S.A. (French and Schultz, unpublished). (N.B. The line is slightly different from Fig. 1).

Of these, we normally only cost nutrients as an input. The last two come free, but water has a high price. Land prices are related to potential productivity and this is related to rainfall and its reliability. (The land itself should not be costed as the only resource removed (nutrients) is already costed; unless degraded land requires restoration.) Let us calculate the cost of water : if land is valued at $1000 per hectare, annual rainfall is 500mm ($2 per mm), and interest rate is 10% p.a. (interest = $0.20/mm/ha/yr), then the average wheat crop on that land uses 300mm water worth $60 per hectare (300 x $0.20).

As rainfall is the dominant factor determining yield, and it is also the most expensive input, it makes sense to treat dryland farming as the farming of water. Every mm wasted is indirectly a loss of $l-2 worth of wheat or its equivalent per hectare, per year! The question now is whether better tillage practices, including subsoil loosening, are an economic means of increasing water-use efficiency

(iii) Effects of tillage practices on water-use efficiency

Water-use efficiency is increased by practices which maximise the amount of water available to and used by the plant of our choice, whether crop or pasture (but not weeds). Subsoil loosening can, under the right conditions:

• Increase the total supply of water

• Improve root growth in the subsoil and increase water use

• Alleviate frequent, severe waterlogging.

We can study the range of opportunities for increasing water use efficiency* by looking at the fate of water at Wagga in an average year, between the harvest of successive crops. This is depicted in Fig. 3.

Fig. 3. The fate of water over 1 year of a cropping cycle (Stubble burnt in January). (Estimates of the average for Wagga area, av. rainfall = 547mm)

“Crop Water Use” (Figs1,2).

A.

Transpiration (from leaves)

240mm

(44%)

 

B.

Evaporation from soil

120mm

(22%)

 

C.

Losses from soil, weeds and volunteer pasture between crops(after burning)

95mm

(17%)

 

D.

Losses from evaporation after harvest, before burning and cultivating

95mm

(17%)

 

E.

Runoff

Highly variable, losses at the expense of A to D.

 

F.

Through drainage

Note that ‘C” is as low as 95mm only when weed growth is controlled in summer/autumn.

This conserves on average about 50mm of water at sowing, for the crop. If weeds are not controlled, then C rises to about 130mm, and A 8 drops from 560 to about 325mm.

Each mm fallow water is worth 10-20kg grain per hectare (50mm 0.5 - 1 tonne).

Component A - Transpiration. Only the water passing through the plant (transpired) can contribute to growth. The rest is “wasted” although some of these losses are unavoidable. The “wastage” is high - a crop at Wagga in the “average” year with 545mm rainfall would transpire only about 240mm. As yield is related directly to transpiration, about 300mm of the annual rainfall makes no contribution to grain yield. We will now see where it goes:

Component B - Unproductive losses from the soil surface during the growth of the crop vary greatly, but probably amount to about one-third of total crop water use (i.e. A + B). This loss is reduced by shading the soil, for example with stubble from the previous crop and by practices which ensure rapid early crop growth (adequate fertilizer, early planting etc.).

* Water-use efficiency is defined in various ways, but means here the crop product per unit of potentially available water (PAW). PAW in a continuous crop system equals the annual rainfall.

Rapid infiltration and redistribution are also important. Reduced tillage and subsoil loosening can open up channels for rapid movement of water away from the soil surface where it rapidly evaporates. As well as reducing evaporation, rapid drainage obviously reduces the risk of waterlogging. Waterlogging not only reduces plant water-use through the direct effect on plant vigour, but also by increasing soil evaporation and runoff (component E). Crops which recover from waterlogging may end up using all of the available water, but an increased proportion is used in spring, when more water is used for each kilogram of plant growth.

Component C, D - Losses from the soil surface and weeds between successive crops: These losses are reduced by retaining stubbles above the soil surface (not mulched in), by controlling weeds and volunteer pasture growth and possibly by reducing cultivation. The effect of weeds is large, but the effect of stubbles and tillage is less clear. At Condobolin, Fettell (unpublished data) has concluded that stubble retention greatly increases water storage in fallows resulting in a 30% yield increase in 1981. He also concludes that the effect of surface tillage is small and perhaps insignificant, but this may depend on rainfall pattern, soil type and slope. The effect of tillage is significant in northern N.S.W. Taylor and Fischer and others are studying this in southern N.S.W., but the results are inconclusive so far.

Water-use by weeds and volunteer pasture needs special attention. There is now good evidence that some of the yield reduction with direct drilling in the early years of development and adoption were due to the extended weed growth in the graze-spray-sow system. Whilst animal production benefited, crop yields suffered. The average penalty is probably between 0.5-1.0 tonnes per hectare at Wagga. Is this worth more than the extra grazing?

Components E, F - Runoff and deep drainage losses vary enormously depending on soil properties, slope, rainfall intensity and amount etc; but they can be significant.

Fig. 4. Tillage practices can reduce runoff and increase crop water use. In this simulated rain storm (45mm/hr), runoff occurred earlier, and was greater, on conventionally cultivated soil. (Aveyard et al., 1983)

Reduced tillage, leading to better surface structure can lead to increased infiltration and reduced runoff (Fig.4). However, a slowly draining layer of soil such as a compacted plough-pan can act as a throttle, eventually reducing infiltration. In such cases both reduced tillage and subsoil loosening are required for good infiltration and redistribution. The increased water storage that results is one of the big potential benefits of deep tillage. Where the throttle is an impermeable subsoil rather than a compacted layer of soil, deep tillage is likely to have a smaller effect on infiltration and total water storage. In poorly structured, heavy subsoils, loosening improves drainage only in the worked layer (say 30-40cm) whereas removing a hard- pan can influence soil to much greater depths (possibly 1.5 metres).

Subsoiling responses due to increased water storage are most likely in higher rainfall areas, in poorly structured soils. Most wheatbelt soils drain reasonably well and total water storage will not be increased unless a very severe compaction layer is present.

Temporary waterlogging can also result from impeded drainage. This restricts root growth and generally reduces plant vigour so that water is not fully used. This can lead to greater drainage losses if water eventually reaches the subsoil but roots do not.

Residues of Water at Harvest - Although not shown on Fig. 3, it is common for crops in southern Australia to leave available water in the subsoil at harvest. Where this is due to restricted root growth, subsoil loosening could greatly increase yield (up to 50% in relevant overseas studies). However, these big responses could be short-lived, for two reasons. First, the channels created may not be stable. Second, the water may only be recharged in exceptionally wet years. Hence total water use may increase in the first year, but not subsequently. Breaking a restrictive barrier like a hardpan is far more likely to increase total water-use and yield than is loosening only the uppermost part of a deep, impenetrable subsoil.

(iv) Summary of the Main Effects of Deep Tillage on Water-Use Efficiency

By removing hardpans or loosening naturally dense subsoils, deep tillage has the potential to:

- make more water available (i.e. more water stored)

- by increasing the depth of freely draining soil

- by removing a “throttle” to drainage

- promote deeper rooting and increase total water use (i.e. more of the stored water used)

- alleviate some waterlogging, improving crop growth and root development leading to greater total water use and a better distribution of water-use between the winter and spring periods.

B. Optimal Soil Conditions Germination, Emergence, Growth and Survival

I shall introduce this section with a discussion of the conditions at seeding depth and above. Then the bulk of discussion will relate to root growth at greater depths, in keeping with the theme of subsoil conditions.

(i) Conditions at Planting Depth and Above

Soil Conditions at and above planting depth will be related most closely to the germination process, emergence, and nutrient uptake. Below planting depth, soil physical conditions will mainly affect water uptake, but nutrient uptake can also be important. Nutrients like nitrogen, which can leach, will often he taken up in substantial quantities from deep soil, but strong root proliferation is not required for this. As a rule of thumb, root development which is adequate for water uptake will also be adequate for uptake of mobile elements such as nitrogen, but not necessarily for the less mobile ones like phosphorus.

What is the Ideal Seedbed?

Farmers and scientists for many years have had ideas about what a seedbed should be like and why the soil is cultivated to produce one.

The success of direct-seeding has now challenged all of these ideas. In the past most farmers cultivated the soil until it visually matched their image of a well prepared seedbed, often using a visual “tilth gauge”. Despite good results they did not usually understand what the plant needed, and how the soil best met that need. Because the traditional assessment of seedbeds has been soil-centred rather than plant- centred, it has been difficult to design machines that work well for direct-seeding in a variety of conditions. Hence the multitude and variety of “no-till” drills. Scientists had known little more than farmers, for they understood the requirements for germination in controlled laboratory conditions but had little idea how to meet these requirements under highly variable, fluctuating field conditions.

We now know that cultivation is not essential for the germination and establishment of “cultivated” plants. However, it does appear that the cultivated seedbed was a good “all-rounder” providing some insurance of acceptable establishment in most situations. Deleting cultivation tends to increase the risk of not getting the crop in under some conditions (e.g. soil surface dry and compacted by stock). Better management, and probably better seeding equipment are needed to offset the added risk.

Cultivation is usually thought to control weeds and some pests and diseases, and provide a physical state which is suitable for moisture infiltration, germination, emergence and early plant growth. Since the advent of modern herbicides it has become clear that weed control is the main benefit in seedbed preparation. The following discussion concentrates on soil physical characteristics and plant establishment.

The soil physical requirements for germination and emergence include:

adequate aeration - Poor aeration is not often a problem for germination in non-irrigated crops or pastures. Ripping responses attributed to improved aeration will often have another basis. When aeration is a problem it can rarely be fixed by surface cultivation. Reduced cultivation is more likely to he a long-term solution, perhaps in conjunction with subsoil loosening and drainage. Poor aeration is usually caused by poor drainage (perhaps due to over cultivation). A surface crust can prevent gases from moving between the soil atmosphere and the air above. Biological activity in the soil can then consume the limited supply of oxygen, in competition with seeds or seedling roots.

Occasional recurrence of poor emergence in any paddock due to suspected poor aeration (excess water) warrants some thought. It could be the acute symptom of a chronic, less obvious problem which occurs under more normal conditions. It could herald restricted drainage and poor aeration deeper in the profile where it restricts root growth.

Direct seeding can also encourage poor emergence due to aeration problems in wet soil. This can happen if the seed is pressed firmly into the wall or bottom of a slot which is cut by the machine and simultaneously compacted.

temperature - Most plants happily germinate over a wide range of temperature in the laboratory. Problems arise in the field, however, where the rate of germination is slowed greatly by low temperatures. This prolongs the exposure of seeds and germinating seedlings to risks such as disease and pest attack. Tillage practices have a relatively small effect on temperature, except for stubble retention. Stubbles dampen the daily fluctuations of temperature resulting in lower maxima and higher minima. This is good news for the establishment of summer crops, but bad news for winter crops sown at the usual time. Lower temperature and delayed emergence is most likely to reduce final emergence when other adverse factors also operate, such as excessive moisture.

moisture - This is usually the deciding factor in the success or failure of “seedbed” preparation, whether the seedbed is conventionally cultivated or prepared in the sowing operation.

Most seeds must absorb about their own weight in water before they germinate. It is useful to think that seeds also lose water at times during germination, as well as absorb it. Therefore, when water is likely to be limiting, germination is improved by practices which increase the uptake of water and reduce the loss of water by seeds (Fig. 5).

Fig. 5. Seeds both absorb and lose water during germination. Agronomic practices are designed to increase water uptake and reduce the losses. Germination depends on:

A. Soil water content

B. Soil characteristics which control how tightly water is held

C. Rate of water movement to seed

D. Seed/soil contact areas -increases as soil aggregate size decreases

E. Contact “resistance”. (Some seeds have a “barrier” to uptake e.g. phalaris).

The rate of water uptake depends on the plant species as well as the water content of the soil, how tightly the water is held (clay holds water more tightly than sand with the same content), how freely the water moves through the soil to the seed, and how much contact there is between the seed and the soil. A fine tilth, appropriate sowing depth, harrows and sometimes presswheels have been the traditional means of attempting to meet the requirements for germination. But seeds in good contact with undisturbed soil seem to do just as well given good management and a machine that “works”. Seeding into stubble presents major seed/soil contact problems if much undecomposed straw is incorporated.

The loss of water by seeds in obviously reduced by covering the seed with soil, and further reduced by a mulch of crop stubble. Covering seed with soil also reduces seed loss by predation (ants, mice, birds). Mulches can be a bonus by keeping a crusted soil surface moister and therefore softer.

Turning again to direct-drill seeders, it is my observation that design improvements could markedly increase establishment by improving the water status of seeds. For a seeder to operate well over a wide range of soil types, moisture supply and stubble conditions it will probably have to do a lot of work to at least a small volume of the soil - opening it up, giving some shattering, controlling the flow of broken soil, and firming it into place as required according to soil type and moisture. The Siroseeder developed by CSIRO did a good job on the soil, but at a dreadful cost in terms of both iron (hence $) and draught. It also had almost no stubble handling capacity. A machine developed by Dr. John Baker in New Zealand appears to do a very good job in all departments and is being tested extensively in Australia and New Zealand.

seed placement - Care is needed to ensure that what lies above the seed enhances its water status without providing a physical barrier to emergence. Excessive depth, soil crusts and huge clods all court disaster and are increased risks with direct drilling and stubble retention. Variable seeding depth is the greatest problem.

soil strength - This will be covered in detail later. We need note here only that roots and shoots have to work very hard to penetrate soil. They can exert enormous pressures (up to 200 p.s.1.), but consider for a moment the pressure (per square inch) you need to apply to drive in a steel fence post! Crust strength can easily exceed the maximum emergence force of a seedling. Similarly, any compaction around or under the seed can severely restrict the development of seedling roots. Any prototype seeder should be examined for its propensity to compact the soil under or to the side of the slot it makes.

The most serious effect of reducing seedling root growth is that it increases susceptibility to drought caused by surface drying. The other effect is on nutrient absorption. Surface soil is normally the richest source of nutrients. The plants’ ability to absorb these nutrients depends on root growth in that part of the soil. Deeper soil loosening may inadvertently loosen the surface soil, increasing nutrient uptake and plant growth, and giving the illusion of a response to loosening the deeper soil.

(ii) Root Growth Below Planting Depth

a) Why do plants need roots?

The known work of roots includes the absorption of water and nutrients and the synthesis of growth regulators (plant hormones). Hormone synthesis is beyond the scope of this conference. Nutrient uptake has been covered briefly, so the rest of my paper will deal mainly with root growth and the absorption of water.

b) How extensive is the root system under ideal conditions?

Plants have much more root than most people suppose. The wheat plant can, under ideal conditions, extend roots to a depth of 1.5 metres or more. The final depth in the field in Australia is normally determined by the depth of wetting in winter. This is usually no more than one metre or so (Fig. 6).

The length of root is very striking : possibly as much as one km per plant! Plants do not necessarily need all of this root, however (see section d).

Fig. 6. Wheat, like most plants, can extend roots to great depth. In dryland crops, however, roots only penetrate to the depth of soil wetting during winter. Water is needed for root growth, and moist soils are also “soft”.

A. Dryland wheat

B. Irrigated wheat

(from Weaver, 1926).

c) Which factors control root growth?

One of the most obvious factors is the species of plant. For example, under ideal conditions lucerne is deep-rooted and many vegetables are shallow-rooted. Most plants, however, have remarkably similar root systems when subjected to the field environment, which is less than ideal. Apart from gross differences between species, it is the environment in which the plant grows that determines the extent of its root system, together with some interaction between species and environment

Temperature - Rates of extension depend on temperature, but farmers cannot usually control this, so I shall ignore it.

Nutrition - Plants are able to respond to localised supplies of nutrients such as fertilizer bands. But this does not mean roots grow poorly in zones deficient in nutrients. In fact, root growth is remarkably resistant to nutrient deficiencies. This is especially so in the case of the main roots of plants which are often totally unaffected by quite severe nutrient deficiencies. Branch roots respond strongly to localised supplies of nutrients.

Aeration - Roots require oxygen and they compete with soil microorganisms for it. Hence regular exchange of soil air with atmospheric air is required. This takes place through large continuous natural pores, cracks, and channels left by decaying roots etc. These channels are more likely to stay intact in uncultivated soil, a good reason for reducing cultivation. Water also infiltrates through these pores, and roots grow through them. Aeration is normally not a problem in most Australian soils, but there is no doubt that compacted soil layers can introduce aeration problems. The large pores can be lost in compacted soil. The small pores can then fill with water and prevent atmospheric air exchanging with the soil air beneath the compacted zone.

Toxicities - Acid soil layers, rich in Aluminium or Manganese can very severely stunt roots. One effect of soil acidity might be to confine root growth largely to surface layers, even with liming. As this soil dries out in spring such plants would suffer increasing moisture stress despite the presence of subsoil moisture. Evidence of higher grain yields with aluminium - tolerant wheat, even after liming (Scott 1982), points to this possibility. This is why we may need to lime acid soils as well as breed for aluminium and manganese tolerance (Scott, per. comm.).

Pest and diseases - These can limit root growth but are outside the scope of this talk.

Soil water - Roots need water for growth. The roots of establishing plants will penetrate only to the maximum depth of wetting. Deep subsoil water left from previous years cannot be used if a band of dry soil separates the moist subsoil from the surface soil wetted by the current season s rainfall. Both water content and soil properties (such as clay content and structure) determine the critical soil water content for root growth. In many soils, however, it is not water content that limits root growth, but soil strength, which is related to water content.

Soil strength and its relationship with water content and aeration - As a soil dries out, the remaining water becomes less available. At the same time, more air becomes available and the soil becomes stronger. At any water content it is difficult to say what limits growth. The relationship between the three of them varies with soil type, and a knowledge of the relationship can help to predict root growth responses to tillage. These relationships are illustrated in Fig. 7 for a typical tableland soil, the fine silty loam surface (0-30cm) of a soil based on shale. Notice how the strength of this soil (bulk density = l.3g/cm3) increases very rapidly upon drying.

Declining root growth as the soil dries (Fig. 1) could he due to either increasing strength or less available water. This soil in its field condition under pasture makes an interesting comparison with a coarse, sandy loam soil derived from granite (Fig. 8). In Fig. 8 root extension is given for both soil types at. a range of water contents between near-saturation (where there is no evidence of poor aeration) to “dry”, at which the soil holds about 25)~ of its potential available water content.

Fig. 7. The soil physical factors which control root growth interact in a complex way. As the soil dries out (from right to left), the water becomes less available, the soil becomes stronger and there is more air. Root growth declines. The reason for falling root growth can be difficult to find, but needs to be known so that the correct solution can be applied. (NOTE: Soil is normally between field capacity and wilting point.)

Water Potential (-MPa)

Fig. 8. Both soils have a bulk density of 1.3 g/cm but root growth is less in “A” because the soil is stronger. Soil drying also reduces root growth faster in soil “A” than “B” because strength increases at a greater rate.

Notice how root growth in the silty loam soil quickly drops off as the soil dries, but the coarse sandy soil favoured good root extension at all moisture contents. In neither soil did water content directly control root growth. So the rapidly declining growth of roots in the silty loam soil upon drying (Fig. 7) was due to increasing soil strength alone. This soil.. bad not been artificially compacted, and root growth would he very much worse given mild compaction. Root growth in the granite soil, on the other hand, would be much less susceptible to compaction. Notice first how soil strength responds differently to drying and secondly how bulk density is a poor indicator of a soil compaction problem.

The effect of compaction on strength of a red-brown earth (surface soil) is shown in figure 9 (a). The effect of strength on roots in the compacted zone is shown in Fig. 9 (b).

Fig 9 (a) Red-brown earths A Horizons

9 (b) Red-Brown Earths

Fig. 9. Compaction increases bulk density and this increases soil strength (9a). Where a compacted zone of soil is present, fewer roots grow in the compacted zone. The worse the compaction, the fewer the roots (9b) (From Greacen 1981.)

Plant species vary in their ability to penetrate strong soils. This variation seems to he small, compared with the big change in soil strength upon drying most soils. Where soil strength appears to severely restrict root growth and yield, modification of the soil should be the most productive approach to increasing root growth - not a change in the plant.

d) Do plants need all of their roots?

The short answer is, “No”. A striking example of this comes from studies with barley grown in a rooting medium of fine glass beads, resembling sand (Russell and Goss 1974). Nutrients and water were supplied in solution. The bed of beads could be compressed to increase its strength and so mechanically restrict root growth. Under severe compression the length of roots was reduced to only one tenth of plants with roots that were not compressed, but there was no effect on the plant shoots (Fig. 10). I have similar results for ryegrass grown in soils of different strength. The mechanical restriction of roots is only detrimental when the supply of water and nutrients in the soil is inadequate.

Fig. 10. A high mechanical resistance can greatly reduce root growth (left), but this need not affect shoot growth.

The shoot growth of both these plants was the same because adequate water and nutrients were provided. (From Russell and Goss 1974.)

Plants have the capacity to produce many more roots than they need under ideal conditions. They do this to minimise the effect of adverse conditions on growth. Therefore we cannot assume that the mere presence of a restrictive hardpan will necessarily reduce plant growth.

Moreover, plants have a remarkable capacity for compensatory growth:

adverse conditions which reduce root growth in one part of the soil can be compensated by increased growth elsewhere. This is one reason why crops can be grown in Australian soils which are very low in phosphorus. They are able to concentrate roots in the surface, around bands of phosphorus fertilizer.

e) How much root does a wheat plant need?

This depends on the job to be done. In the case of nutrient uptake, some nutrients move very slowly in the soil, so a dense network of roots is needed for their uptake, or a concentrated source (i.e. fertilizer). Phosphorus (P) is a good example. Fig. 11 shows how P uptake from soil (but not fertilizer bands) is directly related to root extension. In this case root length was altered by soil compaction.

Fig. 11. Uptake of phosphorus (B) from soil (not current fertilizer) depends on root extension. Note how total P uptake is related directly to the length of root. The soil (silty loam, Fig. B) was compacted to reduce root length. (Cornish, unpublished).

Most available P is in surface soils. Therefore P uptake will be favoured by well structured (easily penetrated) surface soils which promote root extension - whether the structure occurs naturally or is created by cultivation is largely immaterial.

For water uptake, the length of root in a given volume of soil is less important than for phosphorus uptake. One centimetre of root/cubic centimetre of soil (1cm/cm3) is more than adequate for water uptake but complete uptake of soil P (not fertilizer) requires at least 10- 20cm/cm3, depending upon plant species. Lengths of 5-10cm/cm3 are common in surface soils.

At much greater depths (more than 60-70cm) the length of root can be short enough to limit water uptake under some conditions, typically in spring when the need for water is great and the surface layers of soil are dry. Researchers suspect this is important but cannot say so for sure. It could explain why crops sometimes leave available water at depth. Subsoil loosening may assist, but any benefits from increased root density would probably be small.

An alternative reason for subsoil water remaining at harvest lies in the depth of rooting. Water can only move a few centimetres towards roots (except in very wet soils) which means that subsoil water cannot be used unless the roots penetrate to that depth. The main roots of wheat grow downwards at about 1cm per day, and should reach the subsoil to exploit any water there, usually during grain filling. But a hardpan could, theoretically at least, slow roots down so they don’t make it. This idea needs to be tested.

f) Root growth for survival of perennial pastures

Even the most drought resistant perennial (e.g. lucerne, phalaris) needs water to survive drought, even during the normal summer dry period. Only small amounts of water are needed, usually drawn from considerable depth. A newly establishing plant will die unless it penetrates to soil at depth which can supply the small amount of water needed. Once again, hardpans can prevent roots reaching these depths.

References

1. Greacen, E.L. (1981). “Physical properties and water relations”. In Red-Brown Earths. (Eds. Oades, Lewis, Norrish). Waite A.R.1. and CSIRO, Div. Soils : Adelaide) 83-96.

2. Russell, R.S. and Goss, M.J. (1974). Physical aspects of soil fertility - The response of roots to mechanical impedance. Neth. J. agric. Sci. 22 : 305-318.

3. Scott, B.J. (1982). Reponses to lime by cereals. Proc. Aust. Soc. Agron. Conf. Wagga Wagga, July 1982 : 260.

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