|
A Technical Guide for Wine Producers
|
|
RECENT ARTICLES | WYNBOER HOME
Organic matter and carbon in vineyard soils
John Wooldridge, ARC Infruitec-Nietvoorbij Soils rich in organic matter (OM) have long been regarded as better for agricultural purposes than those of low organic content. Soil OM and soil health are linked, the management of soil OM being synonymous with that of soil health (Gugino et al., 2007). Recognition of the role of atmospheric carbon in climate change, and the fact that carbon in the form of OM can be sequestered in soils, or released by mismanagement, has given impetus to the need to maximize soil OM content, thereby contributing to carbon neutrality and soil health.
Total soil organic matter (OM) is a complex material composed of living organisms and the remains of once-living organisms. These remains are capable of decay or are products of decay. The term ‘organic’ originally applied to anything produced by living things, although many such compounds are now produced by inorganic processes. Compounds associated with life processes generally contain carbon. Linked carbon atoms form the backbone of the long or branched-chain molecules that are used by plants for structural and energy storage purposes.
The OM contents of soils are easy to determine. It is therefore possible to maintain a record from which changes in OM content can be deduced for carbon audit purposes, or even for the trading of carbon offsets. The method described by Gugino et al. (2007) entails oven drying the bulk soil sample to constant mass at 105ºC, then heating a weighed sample to 500ºC for two hours in a furnace. Mass loss in the furnace is determined by weighing the cooled sample, and is expressed as a percentage of dry sample mass before heating. Percentage mass loss may be converted to OM% by the relationship: OM% = (% mass loss x 0.7) - 0.23. This analysis is usually performed on material that has been passed through a 2.0 mm sieve. Note that OM content is not the same as carbon content. Carbon is only one of the constituents of OM.
Soil OM varies in terms of its availability to micro organisms. Fresh plant residues are better sources of carbon and energy than bacterial cell walls. A measure of availability is the active carbon content. This varies faster in response to changes in crop or cultivation practice than soil OM, and is a leading indicator of soil health (Gugino et al., 2007). Determination of active carbon relies on the fact that soil OM exists in a chemically reduced state and will react with an oxidizing agent such as potassium permanganate. The method of Gugino et al. (2007) involves shaking a soil sample in a dilute potassium permanganate solution. Oxidation of the active carbon by the potassium permanganate causes the deep purple colour of the solution to fade. The extent of this fading, which is measured with a spectrophotometer at 550 nm, varies with the oxidizable carbon content of the sample and is expressed in mg of carbon per kg of soil.
Few vineyard soils in the Western Cape contain more than 1% OM. The rest of the soil consists of mineral grains, stone fragments, and open spaces (pores). The importance of OM is greater than its low abundance may suggest. This is because, relative to soil minerals, OM has a very large surface area on which reactions can take place, and a higher density of chemical reaction sites. Some OM fractions have specific surface areas up to 800 - 900 m2/g, and cation exchange capacities (CEC) of 150 - 300 cmol(+)/kg, which is roughly double that of vermiculite (100 - 150 cmol(+)/kg) and many times that of kaolinite (3 - 15 cmol(+)/kg) (Fitzpatrick1971), which is probably the most abundant clay mineral in Western Cape soils. Soil OM is therefore a powerful exchange medium for macro and micronutrients and contributes a high percentage of the CEC of the soil. Nutrient elements held by OM are usually readily exchangeable by plant roots. Soil OM is able to adsorb metals, which is why copper from sprays is often present in high concentrations in the top few millimetres of soil. Since the stabilities of OM (fulvic acid)/metal complexes follow a sequence (Fe3+ > Al3+ > Cu2+ >…>Ca2+.> Zn2+ Mn2+.> Mg2+; Sparks, 2003), the reactions of OM/metal complexes can, to some extent, be predicted.
Herbicides and pesticides bind to soil OM, which may hold them in inactive form long enough for degradation to take place. Retention is probably due to adsorption into internal voids in the sieve-like humus molecule (Sparks, 2003). The practical significance of retention of herbicides and pesticides is that the required application rate may be affected. Soil OM is characterized by a high water holding capacity, is closely associated with metal oxides and hydroxides, and with clay minerals, on the surfaces of which OM has a range of catalytic functions.
Perhaps the most important property of OM is its ability to act as a cement, or binding agent. This holds mineral grains together, facilitating the formation of micro aggregates. These aggregates, which remain stable in water, increase soil stability and pore volume. An abundance of interconnecting pore spaces is necessary for the infiltration and drainage of water, and for the exchange of gasses between soil and the overlying atmosphere. Aggregation also reduces soil strength and compaction, thereby increasing ease of root penetration and of soil faunal activity. Note, however, that even in the presence of root exudates from which the grain-binding materials are synthesized, the formation of water stable aggregates requires the mechanical kneading action of root growth. Ideally, aggregates form at a rate that, for a time at least, equals or exceeds that at which they are broken down by micro organisms. Though stable until the cementing material degrades, such aggregates are fragile and are rapidly broken down by cultivation which increases aeration and oxidation rate. Tillage-induced OM loss is a powerful argument in favour of zero tillage in well-aggregated soils, and of the use of cover crops which increase soil OM.
Organic matter is a product of the natural cycle of growth and decay. This process entails the breakdown of complex materials and the formation of materials that are progressively simpler and more resistant to further breakdown. Life processes utilize energy, ultimately solar, to temporarily reverse this process, creating complex tissues and structures from simpler ingredients. These materials, which are rich in nitrogen as well as carbon, are capable of releasing a great deal of energy. Following the death of the producer organism its’ structures and compounds are broken down, initially by the soil fauna, then by hydrolysis and enzymatic oxidation. This breakdown is associated with the release of gases such as nitrogen, carbon dioxide and methane, and facilitated by soil micro organisms. These release the contained energy in a progressive, multi-stage process during which energy and breakdown products are used to generate new compounds in hierarchies of soil micro organisms which themselves break down and are recycled at the ends of their life cycles. Eventually, residues reach the stage where no further microbial attack occurs. Providing that conditions do not change, as through tillage, these stable products of decay may persist in the soil for centuries (Berg & McClaugherty, 2003).
A consequence of the diversity of forms which OM may take, and of the ability of OM to complex with soil mineral material, is that the chemistry of soil OM is not fully understood. As a result the organic constituents of soils are only broadly categorized. As described earlier, total soil organic matter is a general term that includes everything within the soil that is living (biomass), plus the no-longer-living products of life processes. This latter group includes a mixture of plant and animal residues in various stages of decomposition, substances synthesized chemically or biologically from the breakdown products, and tissues and products from the hierarchy of micro organisms that successively derived their energy and materials from earlier forms. Collectively, the heavily decayed, amorphous materials (humus) represent a complex system having non-humic and humic components (Sparks, 2003). Of these, humic compounds are the most abundant.
1. Non-humic materials consist of residues which may retain recognizable tissue structures, and which contain carbohydrates, proteins, fats and waxes in identifiable, little-altered form. Non-humic substances are rapidly attacked by microbes and persist in soils for only a short time.
2. Humic substances resist microbial degradation and are dark coloured and amorphous, with molecular weights between a few hundred and several thousand. They are classified according to their solubility.
All three humic substances have similar structures, but fulvic acid has a lower molecular weight and more oxygen-containing functional groups per unit weight than humic acid and humin. Due to the presence of these functional groups, humic substances are powerful chelating agents able to form stable complexes with metals and hydrous oxides, and to react with clays, particularly where polyvalent metal cations such as Al3+, Fe3+ and Ca2+ are present on the negatively charged clay (Schnitzer & Kodama, 1977; Sparks, 2003). Clays therefore have a stabilizing effect on soil OM. The effect of OM on exchangeable Al3+ is marked, is greatest at low pH’s and increases with OM content up to about 2.5%. Thus, small increases in OM may result in reductions in exchangeable Al3+, and on Al3+ activity in the soil solution. Complexation of Al3+ with soil OM explains why plants may not be affected by Al3+ even in acid soils (Sparks, 2003).
Fulvic acid and the lower molecular weight humic acids attack soil minerals by complexing metals, hastening rock breakdown. This mechanism is used by lichen to obtain nutrients from rock surfaces.
Humic compounds are easily transported in drainage water. This is notably the case in fynbos areas where streams and dams fed by organic-rich soil water have a characteristic reddish brown coloration (Wooldridge & Bothma, 1993). There are three main practical considerations.
The maintenance of reasonably high levels of soil OM is clearly desirable from the carbon sequestration and soil health viewpoints. At any given time the abundance of organic material in the soil will reflect the relative rates of biomass production and breakdown. Within limits, the rates of both these processes increase with temperature, but proceed only where oxygen and moisture are available in adequate quantities. In waterlogged, oxygen-deficient soils breakdown is slow and OM may accumulate. This is why poorly drained valley bottom soils are richer in OM than those on slopes. Although breakdown of OM on valley bottom sites can contribute nitrogen to vines, these sites may be difficult to drain effectively and are therefore unsuitable for viticulture.
Soil OM content and management are linked. Even where management is poor, elevated soil OM contents are reasonably easy to achieve at high latitudes where soil temperatures remain low, and where rainfall is adequate. This is not the case under Western Cape conditions where summers are long, warm and dry. However, even under these conditions soil OM content may be increased by repeatedly adding compost or green manures. These materials stimulate microbial activity and the generation of reasonably stable organic compounds, but do not necessarily lead to improved structure. A disadvantage of incorporating OM into the soil is that unless the soil and OM are deeply and thoroughly mixed, tree and vine roots may become concentrated in the OM-enriched layer and never penetrate into the subsoil. Such root systems are unable to utilize the full potential rooting depth and are, in consequence, intolerant of drought and of long irrigation cycles. Greater benefits can usually be gained by using OM as a mulch. Organic material that has been composted and has lost most of its easily decomposable components in the process lasts longer than fresh OM. Mulches reduce peak summer soil temperatures, a pertinent factor in view of the global warming trend. Mulching also prevents crusting, thereby facilitating infiltration, gas exchange and soil faunal activity, as well as conserving water, which is an increasingly scarce resource. These water savings can be considerable. In a field trial, mulching increased irrigation cycle length from 11.6 to 26.0 days, representing a seasonal water saving of 55% (Wooldridge, 1992). Further, organic residues from surface-applied mulches are rapidly transported down into the soil by the soil fauna, greatly increasing OM content, cation exchange capacity and water retention in the root zone. Disadvantages are that mulches may hold excess water in winter, reduce the rate of soil warming in spring and serve as an above ground growth medium for roots.
A factor that may be critical for vine growth is the carbon to nitrogen (C:N) ratio of the OM. Organic residues that have high C:N ratios, such as wood chips and straw, need extra nitrogen for decomposition. This N demand renders applied nitrogen less available to the crop. Insufficient nitrogen is one reason why high C:N mulches decay slowly. Slow decay rates are desirable where the objective is to keep the soil surface cool and moist, but this advantage may be offset by the higher nitrogen requirement. Conversely, mulches that are high in nitrogen may cause excessive vigour and loss of control over yields and fruit quality. There is also a tendency for mats of fine roots to develop at the base of long lasting or frequently renewed mulches. These roots are prone to mechanical damage and drought.
Increasing soil OM contents has potential growth and environmental advantages, and should be regarded as a prime management objective. Reducing tillage, mulching and the use of cover crops are probably the most effective ways of conserving soil OM.
For further information contact John Wooldridge at wooldridgej.agric.co.za.
REFERENCES Berg, B. & McClaugherty, C., 2003. Plant litter. Springer, Berlin. Fitzpatrick, E.A., 1971. Pedology. A systematic approach to soil science. Oliver & Boyd, Edinburgh. Gugino, B.K., Idowu, O.J., Schindelbeck, R.R., Van Es, H.M., Wolfe, D.W., Moebius, B.N., Thies, J.E. & Abawi, G.S., 2007. Cornell soil health assessment manual, edn 1.2.2. Cornell University Communications Services, Geneva, NY. Schnitzer, M. & Kodama, H., 1977. Reactions of minerals with soil humic substances. p. 741 - 770. In: J.B. Dixon & S.B. Weed (eds). Minerals in soil environments. Soil Science Society of America, Madison, Wisconsin. Sparks, D.L., 2003. Environmental soil chemistry, 2nd edn. Academic Press, Amsterdam. Wooldridge, J., 1992. Effect of certain surface management practices on internal soil environment, irrigation requirement and tree performance in ridges. Decid. Fruit Grow. 42, 289 - 294. Wooldridge, J. & Bothma, K., 1993. Stratification of irrigation dams: a way to improve water quality. Decid. Fruit Grow. 43, 104 - 108.
|
|
Wynboer is incorporated in WineLand, magazine of the SA wine producers.
Visit our sister sites:
|
| UP |
COPYRIGHT (C) 2000 WineLand |