6.1 Freedom from Barriers

Barriers in soil limit root growth and the plant's ability to exploit the available soil volume. As a result, crops may not achieve their potential yield.

If root growth is confined to a soil horizon, two fracture lines, or appears to be stunted or contorted, there is a strong likelihood that the roots are experiencing a barrier to growth. Plants with strong tap roots. such as canola and lupins, or the main root of vines, will clearly highlight a structural problem should it exist, but such impediments to root growth can also be due to chemical factors, such as pH sodicity and toxicity.

Figure 1 - Quality indicators of root growth

Source: A Cass - CRC for Soil & Land Management

It is important to establish the barriers that exist in both the surface and sub-soil and to manage their treatment in the appropriate order.

Physical Barriers

As roots grow through the soil they encounter resistance against which they apply pressure.

Maximum root pressure varies, but is limited to about three mega Pascals (MPa) for vines and 5MPa for cereals. To put this in perspective, your car tyres have about 2MPa pressure.

If a root encounters a soil with strength greater than this, it will be unable to penetrate this soil and unable to extract water and nutrients from it. Soil with high soil strength also requires more power for cultivation.

Soil strength is highly dependent on soil water content and bulk density. Soil is weakest when wet and uncompacted and soil strength should be assessed when the soil is at field capacity, i.e. one or two days after deep soaking rains.

Soil with poor structure, except sand, usually has high soil strength. Poor structure may be naturally occurring due to the nature of soil particle mix, but may also be due to the formation of cultivation pans and traffic compaction or the degradation of organic matter to the point where there are insufficient organic ‘glues’ to hold the structure together.

Measuring Surface Soil Strength

This test should be carried out after good soaking rains when the soil is at field capacity.

Push the sharpened point of a pencil into the soil with the centre of the palm of the hand. If the pencil enters easily without inflicting any pain on the palm of the hand, then the soil strength is less than 1MPa.

If the pencil moves in the soil but causes pain, the soil strength is between 1-3MPa. If the pencil moves into the soil reluctantly, the soil strength is greater than 3MPa.

Interpretation of Soil Strength*:

*Note soil strength may be irrelevant if the soil has many pores and cracks for roots to grow through.

Less than 1MPa - Roots and shoots will grow through the soil without difficulty. Physical quality is good.

1MPa - 3MPa - seedling emergence will be retarded and root growth may be restricted. If resistance is greater than 1MPa at field capacity, resistance is likely to increase to above 3MPa before wilting point is reached. Root growth will be limited to periods when the soil is wet, just after rainfall. Physical quality is moderate.

Above 3MPa - soil strength will limit root growth most of the year and physical quality is poor.

Chemical Barriers

SALINITY

Saline soils are those, which have sufficient levels of soluble salts in the root zone to adversely affect plant growth. Salinity affects root growth in two ways: through direct toxic effects and by increasing the osmotic pressure in the soil solution to a point where plant roots can no longer absorb water. For the plant, the effect is the same as if the soil were much drier. Different species have varying tolerance to salt, but in many situations the concentrations of salt will be too great for plants to survive (see Figure 3)

In arid areas there is insufficient rainwater to leach the salts out of the root zone and salts accumulate as soil water evaporates. Salinity also occurs where drainage is restricted, seepage water gathers as it evaporates and an accumulation of salts is left behind.

The salts present are mainly the cations sodium (Na+), potassium (K+), calcium (Ca++) and magnesium (Mg++), as well as associated anions such as chloride (Cl-), sulphate (SO42-), carbonate (CO32-) and bicarbonate (HCO3-). Sodium chloride normally comprises about two thirds or more of the total salt load, except if gypsum (CaSO4) has been applied to the soil. Therefore, care needs to be taken in interpreting salinity measurement if gypsum has been applied.

The degree of salinity can be assessed either by observing the vegetation or by measuring the soluble salt content of the soil.

Measuring Salinity

The concentration of salt in the soil is easily measured and should be assessed for at least three layers in the top metre. Salinity is measured by electrical conductivity (EC) either in soil saturation extract (SE), or a mixture of one part of soil to five parts of rainwater (1:5). The 1:5 method can be done in the paddock but the SE method requires laboratory facilities.

Total soluble salt content is estimated from the electrical conductivity (EC) of the soil solution.

Figure 3: A guide to tolerance of plants to salty conditions

Source: Hi-Fert, Plant Nutrition and Soil Fertility, 1997

1. Dry the soil samples on clean plastic under cover. If gravel is present, sieve the air-dried soil with a 2mm sieve.

2. Add 20ml of soil to 100ml of distilled or rainwater in a jar and mix by shaking periodically over one hour.

3. Allow the mixture to stand for half an hour or until the suspension is clear. If the suspension does not clear EC can still be measured but the reading will be overestimated by 0.01 to 0.03dS/m.

4. Calibrate the conductivity meter with a standard solution standard solution, then rinse the electrodes in rainwater and dry by gently touching with clean tissue.

5. Take a reading in the suspension above the soil, ensuring that the electrodes are completely covered.

When the results are more than 0.5dS/m (sands), 0.7dS/m (loams) or 1.0dS/m (clays), the saturation extract method should be used. This method provides the only reliable assessment of soil salinity. Values of more than 8dS/m in surface soils and more than 16dS/m in sub-soils are limiting for most field crop and pasture species. For horticultural plants, the critical value is as low as 2dS/m.

SODICITY

Sodicity is a naturally occurring problem in soils that contain a high level of sodium relative to calcium and magnesium. Although it is chemically induced, the result is the formation of poor physical properties that cause barriers to seedling emergence or root growth.

When in contact with water, a sodic soil will swell and disperse into tiny clay particles that remain in suspension. On drying, these particles block the soil pores resulting in surface crusting or a hard setting layer in the soil horizon. These soils are often susceptible to waterlogging, poor aeration and erosion and can present barriers to root growth and seedling emergence.

Gypsum helps to ameliorate a sodic soil by replacing sodium with calcium. Lime will also supply calcium and will raise soil pH, which may be a beneficial side effect. Neither is very soluble and, for best results, should be mixed into the soil by cultivation or ripping.

Frequently, sodicity is a problem with clay in the B horizon, in this case, best results are obtained by ripping the horizon when nearly dry and ‘blowing’ finely ground gypsum from a tube behind the ripper. Air seeders with tynes spaced at 1m intervals have been used successfully to do this.

Measuring Aggregate Stability and Sodicity

1. From each horizon sampled, select three aggregates or fragments about the size of a pea. Carefully place them in a shallow, clear container filled with enough rain or distilled water to cover the spaced aggregates.

2. DO NOT MOVE THE CONTAINER. Carefully observe the aggregates during the first few minutes and note whether they float on the water or sink immediately. Then record if smaller aggregates detach (slake) from the pea sized aggregate.

3. After two hours record whether slaking is complete, partial or absent.

4. Leave the container undisturbed for 20 hours and then assess dispersion (milky-ness in the water).

5. If NO dispersion occurred, take a handful of soil and remove any gravel or plant fragments. Moisten with rainwater whilst kneading into a ball of about 40mm diameter. Add small amounts of water until the ball just sticks to the hand.

6. Break the ball open, remove three pea sized balls and repeat step 1. Ignore steps 2 and 3, as slaking has no meaning for moist artificial aggregates. Watch and record as for step 4.

Results - Slaking

• Air-dried aggregates float - soil is structurally stable, bulk density of aggregates is very low (less than 1g/cm3). Very good macrostructure quality.

• Air-dried aggregates sink but do not slake - soil is structurally stable, bulk density is greater than 1g/cm3. Good soil macrostructure.

• Air-dried aggregates sink and slake slowly - unstable macrostructure on wetting. Organic matter is probably below optimum. Tilled soil may set hard after rain. Reduced tillage should be adopted and organic matter increased.

• Air-dried aggregates sink and slake rapidly - macrostructure is very unstable, organic matter is below optimum and tilled soil will probably hard set after rain and/or crust badly, will have poor infiltration and will cause runoff and probably erode if on a slope. Reduced tillage should also be adopted and organic matter increased.

Results - Dispersion

• No dispersion after remoulding. Good microstructure stability infiltration and water movement will be good.

• Partial dispersion. The water will be partly cloudy and partly clear. Poor clay stability. Gypsum should be applied as above.

• Complete or partial dispersion after remoulding. Moderate clay stability. In the absence of careful management, soil structure will deteriorate. Compaction, tillage pans and poor infiltration may be problems under these conditions. Tillage in wet conditions should be avoided.

• Partial dispersion (dry aggregate) - the water will be partly cloudy and partly clear. Poor clay stability. If left without surface cover, the soil will ‘seal’ on the surface and cause runoff and erosion on slopes. Gypsum should be applied.

• Complete dispersion (dry aggregate) - Very poor clay stability, the soil will ‘seal’, preventing infiltration and, on slopes, will erode easily. Gypsum needs to be applied. Use a laboratory test to determine how much gypsum should be applied.

SOIL CARBONATES

Calcium and magnesium carbonates are common and important constituents of many South Australian soils. They take several forms:

• fine whitish particles, common carbonate in the soil matrix,

• hard nodules or rubble,

• sheet rock.

The amount and type of carbonate affect

• Plant root depth

• Nutrient availability

• Residual effects of some chemicals

• Drainage.

They can also impact on crop and variety choice. For example, soft carbonates affect the availability of some nutrient elements and may cause lime-induced chlorosis in lupins and stone fruit, for example. If present in large quantities, over 30%, the carbonate will decrease soil permeability and restrict root growth.

Layers of sheet carbonate rock may prevent root penetration and many carbonate nodules will limit the water holding capacity of the soil, as the nodules do not hold water. Soil carbonates are described in the field by noting the reaction to acid.

Measuring Carbonates

Hydrochloric acid of 1N concentration is used. This can be approximately prepared by adding one part commercial Spirits of Salts (HCl) to 10 parts rainwater. Add one or two drops of acid to a soil sample and note the strength of the effervescence (caused by carbon dioxide released by the acid-carbonate reaction). Use Figure 6 to interpret the results.

TOXIC ELEMENTS

Under some conditions, some elements can occur at toxic concentrations.

Boron is associated with certain types of clay, carbonate layers and rock formations. Levels above 15ppm (soil test) are considered toxic and if these levels occur within the potential root zone, damage to the root system can be expected. The only solution is the use of tolerant varieties.

Boron toxicity is associated with low rainfall (<550mm per annum), alkaline and sodic sub-soils. It is a common problem in many soils in South Australia.

Aluminium is present in most soils, but only becomes available at very low pH, acid conditions. It is commonly associated with soils developed on highly weathered rocks.

Manganese, like aluminium, may reach toxic levels in acid soils. This is because its solubility is increased in acid soils

Sulphonylurea Herbicides - In alkaline soils these group B herbicides have been found to break down slowly, often leaving residues that affect susceptible crops beyond the plant back period.

Biological Barriers

The damage caused to root systems by disease and insect attack can be enormous. A stunted root system does not have the capacity to fully explore the soil volume for the water and nutrients that it needs. Recognition of disease and insect problems and their control through appropriate rotations, tillage and spray programs are essential.

Barriers to Seedling Emergence

High and uniform emergence rates of seedlings depend on a disease free surface soil, with adequate nutrition and a good structure to allow adequate rain infiltration and satisfactory seed-soil contact. The topsoil should be formed of stable aggregates to prevent breakdown under rainfall and the formation of a crust.

Poor surface structure can be a significant cause of patchy emergence. Poorly structured surface soils can become hard and blocky after tillage.

Soils with the following features are likely to create barriers to seedling emergence:

• High proportions of fine sand and silt.

• Sodium rich clay (even if the clay content is as low as 10%).

• Organic carbon content in loamy-clay soils of less than 1%.

Water repellence is also a cause of uneven emergence and low germination rates. Water repellence, most commonly found in sandy soils, is caused by the accumulation of waxes on the surface of soil particles. These waxes are produced by the decomposition of specific types of organic matter, often from native vegetation, although some fungi and organic matter from undigested fibrous foods deposited in faeces can also cause a build up of waxes. Cereals do not breakdown to release these waxes and a cropping phase has been found to reduce non-wetting properties over time.

6.1 Freedom from Barriers

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