Soil health issues

Factors such as a global warming, decline in genetic diversity, pest and disease problems and soil degradation were identified by the Consultative Group on International Agricultural Research) as the most pervasive threats to sustainable agriculture in the new millennium (Lal and Pierce 1991). In the 20 years that has elapsed since this statement was made, soil health has been very firmly under the spotlight in a number of sugarcane industries, as there have been concerns that monocropping of sugarcane over many decades has in a number of industries resulted in nutrient mining, declining levels of soil organic matter and an increase in soil acidity. For more information see

www//soilhealth.cals.cornell.edu/.

2.8.1 The new emerging view of soil health

The terms ‘soil health’ and ‘soil quality’ are becoming increasingly familiar worldwide. Doran and Parkin (1994) defined soil quality as, “the capacity of a soil to function, within ecosystem and land use boundaries, to sustain productivity, maintain environmental quality, and promote plant and animal health.” In general, soil health and soil quality are considered synonymous and can be used interchangeably. The new emerging view is that soil health is a concept that deals with the

integration and optimization of the physical, chemical and biological properties of soil for improved productivity and environmental quality (Haynes 1999).

2.8.2 Yield plateau assessments

Concerns over the possibility that a ‘yield plateau’ has been reached, as demonstrated by the industrial yield data from a number of industries, have led researchers and growers in Australia, South Africa, Swaziland and Zimbabwe to look critically at the way soil is presently being managed.

The major focus of the Sugar Yield Decline Joint Venture (SYDJV) has been to study differences in soil chemical, physical and biological properties and their effect on sugarcane growth in paired old (cane grown for at least 20 years) and new land sites, in conjunction with rotation experiments and a rundown experiment (Garside and Nable 1996). Break crops that have been tested include soybean,

peanut, maize, pasture grass, various legumes and bare fallow for different periods of time. Other studies initiated by the group have centered on sugarcane root systems, soil biology, nematodes, soil carbon, silicon nutrition and strategic tillage.

As in the case of the Australian Industry, the South Africans also acknowledged that yield decline would not be overcome by a single or discreet factor approach. For this reason a multi-disciplinary approach was adopted involving input from soil scientists, nematologists and soil microbiologists.

The program was comprised of the following steps:

 Assessment of industry yield and sucrose productivity trends.

 Survey of industry wide paired old and new cane sites in order to identify differences in soil chemical, physical and biological properties and crop growth on old and new land sites.

 Green manuring trial program comprising field trials employing a range of green manure crops.

 Compaction trials to quantify yield loss and amelioration from inter-row compaction and stool damage.

 Yield potential trials at sites with a history of yield decline.

 Bioremedial amelioration trials with organic amendments such as filter cake, fly ash, chopped trash and chicken litter using vertical mulching as a means of incorporating organic matter to depth.

 Industry wide soil database assessment. Analyses from over 300 000 soil samples, amounting to nearly two million analyses conducted since 1980, covering over 400 000 ha, were assessed to monitor the rate of soil acidification, soil organic matter decline and other fertility related issues.

2.8.3 Paired site outcomes

Selected key research outcomes to restore degraded soils from both countries are given below.

South Africa

South African researchers conducted three surveys in the sugar industry. In the first, soil samples were collected from 29 paired sites in the northern areas of KwaZulu-Natal (van Antwerpen and Meyer 1996). In the second survey, 27 sites on the South Coast were sampled, and in the third survey 38 samples were collected in the Midlands (Qongqo and van Antwerpen 2000). Paired sites consisted of uncultivated (virgin) and adjoining cultivated areas no more than 30 m apart. Virgin areas included natural bush and road reserves with natural grassland. The sites were representative of the main soil forms in the industry. Periods of cultivation ranged from zero years under cultivation (virgin veld) to 30 years in the Midlands, 2-40 years in northern KwaZulu-Natal and more than 50 years on the South Coast. The overall impression from these studies was that chemical properties were more affected by sugarcane cultivation than physical properties (Qongqo and van Antwerpen 2000). In general, the results indicated that the old monocultured land was:

 More acidic. The South Coast pH site declined from an average value of 5.34 to 4.35 after 50 years of cultivation. This decline represented a ten-fold increase in acidification, which was statistically highly significant.

 Lower in organic matter. On the South Coast organic matter levels had declined from 4.7 % to a mean value of 2.4 % at a rate loss equivalent to 0.04 % per year compared to a decline from 6.06 to 5.7 % at a rate of 0.01 % per year in the Midlands.

 Greater compaction. A significant increase in bulk density or compaction was evident after only two years under cane cultivation on the South Coast site. This was probably due to the rapid loss of organic matter, structural breakdown and compaction by infield haulage systems.

 Higher titratable acidity and exchangeable Al.

 Lower cation exchange capacity (ability to hold nutrients).

 Lower K and S reserves.

 Lower air-filled porosity and profile available water capacity (PAWC, 0-10 kPa).

 Lower aggregate stability in cultivated areas compared to virgin soils.

Although changes in soil properties were not consistent across all sites, it was evident from the South African results that there was a strong dependence between history of the cane land (i.e. duration of monocropping) and soil properties such as pH, organic matter, soil aggregate stability (SAS), cation exchange capacity (CEC) and bulk density. Equations were developed to estimate soil degradation from these parameters using the multi linear regression analysis technique. However, the equations still need to be validated for various soil types before they can be used with confidence (Qongqo and van Antwerpen 2000).

Australia

The results from the seven paired sites sampled by SYDJV staff in Tully, Herbert and the Burdekin districts generally paralleled the above outcomes. Additional outcomes associated with the older monocultured land (Garside and Nable 1996) included:

 Less microbial biomass (less soil biological activity)

 Lower infiltration rate

 More root pathogens

 Less trace elements.

Swaziland

Soil degradation due to suspected sugarcane monocropping has been linked to ratoon yield decline on an irrigated estate in Swaziland (Nixon 1992; Henry et al. 1996). A comparative assessment of the chemical, physical and biological properties of a range of soils was conducted to identify soil

properties which might affect the performance of ratoon crops (Henry 1995). Results indicated that cane monocropping often resulted in degradation of soil properties such as surface crusting, low infiltration rate, high bulk density (BD), low total available moisture (TAM), low organic matter (OM), available soil potassium (K) and sodicity, particularly at depth. Haynes and Hamilton (1999) provide a concise synthesis of world literature on the impact of sugarcane cultivation on soil quality. They note that much more is known about impacts on chemical properties of soils than on physical and

biological properties. The main effects identified are:

 Loss of soil organic matter

 Soil acidification

 Changes in soil nutrient levels

 Soil salinization and sodification

 Compaction of topsoil.

2.8.4 Loss of organic matter

The decline in soil organic matter when virgin land is brought under cultivation is a common feature of agriculture in the tropics and sugarcane should not be an exception. Soil disturbance through ploughing causes a sharp initial decline through the decomposition of organic matter, usually followed by establishment of a new equilibrium level after around 20-50 years (Haynes and

Hamilton 1999). Apart from the mineralization of organic matter these authors note that the decline also results from reduced input of organic material. Similar reports have emerged from Australia (Wood 1985), Papua New Guinea (Hartemink 1998), Philippines (Alaban et al. 1990), Cuba (Armas et al. 1991), Fiji (Masilaca et al. 1985), Swaziland (Henry and Ellis 1996) and South Africa (van

Antwerpen and Meyer 1996); Qongqo and van Antwerpen 2000; Dominy et al. 2001). Recent studies

have shown that traces of the original (pre-cultivation) organic material remain detectable in soils under sugarcane cultivation for varying periods: over the first 50 years of cultivation according to studies from Brazil), and between 13 and 50 years depending on soil type (Burke et al. 2003).

Biological indicators of soil quality (such as microbial biomass C, soil respiratory rate, soil enzyme activity and soil earthworm communities) are sensitive to changes in soil organic matter content, and can change markedly before any substantial changes in organic matter content itself are detected. Recent years have seen an increasing interest in the use of biological indicators of soil quality such as microbial biomass C, soil respiratory rate, soil enzyme activity and soil earthworm communities, which are very sensitive to changes in organic matter content (Pankhurst et al.

1997).Both soil microbial biomass C and basal respiration declined under continuous sugarcane cultivation in KwaZulu-Natal, due to a decline in soil organic matter. Studies in Papua New Guinea showed that organic C content of soils under sugarcane cultivation declined over a 17 year period from around 5.5 to 3.2 g/kg (Hartemink 1998).

2.8.5 Acidification

The detrimental effects of toxic levels of exchangeable Al levels on cane growth are well documented for sugarcane (Sumner 1970; Sumner and Meyer 1971; Moberly and Meyer 1975; Turner et al. 1992;

Schroeder et al. 1994). Traditionally, soil acidity problems have been confined mainly to cane growing in the high altitude areas. More recently, an industry wide survey of soil fertility trends indicated that sandy soils on the south and lower south coast of KwaZulu-Natal have become progressively more acidic during the past decade (Meyer et al. 1998).

The results of a more recent investigation based on the use of a soil profile acidification model have shown increased soil acidification on an estate in Zululand and other areas in South Africa (Schroeder et al. 1994). Accelerated acidification of soils under cultivation is most often due to the combined effect of oxidation of ammoniacal fertilizers to nitric acid, mineralization of organic matter and leaching of basic cations from the soil. In virgin soils in tropical areas such as Sierra Leone, where annual rainfall exceeds 3 500 mm per annum, intensive leaching of basic cations over centuries results in highly weathered acid soils with pH values in the range 4 to 5 and very low activity clays (Acrisols).

A decrease in soil pH of virgin land also occurs when the land is cleared for sugarcane cultivation.

This has been specifically recorded in Australia (Wood 1985; Garside et al. 1997), Fiji (Masilaca et al.

1985), Florida (Coale 1995), Papua New Guinea (Hartemink 1998), Puerto Rico (Vicente-Chandler 1967) and South Africa (Schroeder et al. 1994; van Antwerpen and Meyer 1996; Qongqo and van Antwerpen 2000).

Use of the industry wide soil database for the South African sugar industry showed that over the relatively short period of 16 years, average soil pH values in the coastal regions of KwaZulu-Natal declined from 6.2 in 1980-1981 to 5.6 in 1996-1997, and the proportion of soil samples with pH < 5 increased from 18 % in 1980 to 43 % in 1997 (Meyer 1998). A similar outcome was obtained in Papua New Guinea, where the pH of topsoils under sugarcane cultivation decreased from around 6.5 to 5.8 between 1979 and 1996 (Hartemink 1998).

There can be little doubt that the sharp increase in the rate of acidification is mainly caused by the use of acidifying nitrogenous fertilizers such as urea and ammonium sulfate, coupled with nitrate leaching that occurs under the high rainfall conditions that often prevail in cane cultivation areas (Haynes and Hamilton 1999). The drop in pH causes a chain reaction with corresponding declines in exchangeable bases (Ca, Mg, K), cation exchange capacity, increases in exchange acidity, and increases in exchangeable Al (van Antwerpen and Meyer 1996; Qongqo and van Antwerpen 2000).

Sugarcane is more tolerant of acidity than most crops (Hetherington et. al. 1986) and unlike the root system of maize which can be severely affected by subsoil acidity, sugarcane roots survive and grow even under the most severely acid conditions, as illustrated in Fig. 2.10 below. The root distribution of sugarcane on the left is compared with maize on the right, growing in adjoining plots in the same observation trial. The soil tested with a water pH of 4.5, 85 % acid saturation with high levels of exchange acidity, and potentially toxic Al that yielded an Al saturation index (ASI) of 85 %.

Figure 2.10. Root development of sugarcane (left), and poor, shallow root development of maize in a severely acid soil (right) (soil pH (water) 4.5, Al Saturation Index 85%).

Given that a critical ASI of 20 % has been used to determine the lime requirement of sugarcane in the South African sugar industry, the observed response of about 30 % to dolomitic limestone was not unexpected given that the soil test results also showed calcium and magnesium to be highly deficient. More formal liming trials with sugarcane on less acid soils but with an ASI of 40 to 60 % in the subsoil, have always responded to shallow incorporated lime treatments. Deep lime

incorporated treatments using a Nardi plough have to date not shown any additional benefit from applying lime to depth (Moberly and Meyer 1975). Recent advances in research have linked Si deficiency with soil acidification, and in the South African sugar industry liming advice makes provision for using calcium silicate amendments in preference to dolomitic or calcitic limestone in order to supply silicon to the crop as well as raising the pH of the soil (Meyer et al. 2005).

2.8.6 Soil salinity and sodicity

The effects of soil salinity and sodicity in the low rainfall regions of the world have been extensively studied in South Africa (von der Meden 1966; Johnston 1977, 1978; Culverwell and Swinford 1985;

Wood 1991), Swaziland (Workman 1986), Australia (Ham et al. 1997; Nelson and Ham 1998), Egypt (Nour et al. 1989), India (Tiwari et al. 1997), Iraq (Sehgal et al. 1980), USA (Bernstein et al. 1966) and Venezuela (Wagner et al. 1995b).

A primary cause of soil salinization in these regions is the development of high water tables, which allow capillary rise of saline groundwater into the rooting depth of the crop. Poor quality irrigation water may be another source of salts. Chapter 7 deals more fully with the management of saline sodic soils.

2.8.7 Soil compaction

Extracting cane in wet conditions after harvest is unavoidable in many cane growing areas, and uncontrolled infield traffic will cause soil compaction, sealing/capping and physical damage to cane stools. Heavy mechanized traffic has the potential to cause great inter-row compaction, and also stool damage in the cane rows themselves, which is a greater threat to yields than inter-row compaction.

Compaction problems have been reported from many areas where sugarcane is grown, including Australia (Wood 1985; McGarry et al. 1996), Brazil (Centurion et al. 2000), Colombia (Torres and Villegas 1993), Cuba (Armas et al. 1991), Fiji (Masilaca et al. 1985), Hawaii (Trouse and Humbert 1961), India (Srivastava et al. 1993), Papua New Guinea (Hartemink 1998), Thailand (Grange et al.

2002), South Africa (Johnston and Wood 1971; Swinford and Boevey 1984; van Antwerpen and Meyer 1996) and Venezuela (Wagner et al. 1995).

In South Africa, Maud (1960) showed for most sugar belt soils, the tendency to become compacted is greatest when their moisture content is near field capacity. Swinford and Boevey (1984) and Swinford and Meyer (1985) found that moderate and severe compaction on a grey structureless sandy loam caused an increase in bulk density and soil strength and decreased air-filled porosity. Traffic over the row had a greater effect on yield than compaction of the inter-row. Amelioration through ripping was only slightly beneficial. Tines seem to have a detrimental effect due to root pruning, which affects growth of the subsequent crop. It was concluded that yield decline from infield traffic is as much due to physical damage to stools as to a breakdown in structure and sealing/capping from soil compaction, particularly under critical soil moisture conditions.

More recent research outcomes of a compaction trial conducted on a shallow Chromic Luvisol in Mpumalanga, showed increased soil bulk density, reduced water infiltration rates, increased

penetration resistance and reduced root distribution in all compaction treatments (van Antwerpen et al.

2008). The treatments with the higher water content were more susceptible to degradation (Fig.

2.11). It was concluded that even a virgin soil in good physical condition will be degraded over a period of a few years to the physical threshold limits. Permanent traffic lanes should therefore be considered essential in most agricultural systems to protect the productive areas of fields. Proper spacing of inter-rows and the use of low pressure high flotation tires on all axles should be considered as additional measures to achieve this goal.

Figure 2.11. Impact of radial wheel compaction on a virgin soil showing the variable depth of the compacted zone up to a depth of 30 cm (van Antwerpen 2006).

The results of trials conducted on a Mollisol in Colombia have also shown that compaction can have significant effects on cane growth and yield (Torres and Villegas 1993). Highly significant differences in cane yield were found due to the effects of the different infield transporters that were evaluated.

Damage induced by conventional wagons and dumpers running over stools resulted in a yield decline of between 21 and 45 %, compared with only 10 % decline where wheel passes were confined to the cane inter-row. Passage of the grab loader passing over either the stool or inter-row did not cause a

substantial yield decline. Although significant increases in bulk density were generally not associated with any of the treatments, marked treatment effects on infiltration were measured. Changes in soil surface properties leading to surface crust formation, reduced water infiltration, increased runoff and erosion, have also been measured in the Australian sugar industry. Prove et al. (1986) and Davidson (1956), compared cultivated and virgin soils to determine the effect of compaction on bulk density. For subsoils the virgin area was lower in bulk density compared with the cultivated areas for both soil types studied.