In general soils become more acid over time. The acidity

Transcription

1 ENCARTE TÉCNICO THE ROLE OF THE NITROGEN AND CARBON CYCLE IN SOIL ACIDIFICATION Greg Fenton 1 Keith Helyar 2 Introduction In general soils become more acid over time. The acidity comes from biological cycling of nitrogen and carbon by plants, usually called the nitrogen (N) and carbon (C) cycles. Effective management of the problems associated with soil acidity is based on an understanding of these chemical processes within the cycles. Soil acidity is part of a whole complex of chemical factors that affect plant nutrition and growth for soils with a ph less than 5.5*. As soils become more acid some plant nutrients become deficient while others increase in availability to toxic levels. These changes in availability of nutrients cause most of the effects on plant growth attributed to acidic soils. In addition, soil acidity may decrease microbial activity affecting nitrogen fixation by some legumes and mineralisation of organic matter (Helyar and Porter, 1989). Acidification of the soil starts when nitric and organic acids are produced in the nitrogen and carbon cycles by lichens and algae that first colonise the rock surface. These acids react with the rock breaking it down to start soil formation and to release the cations that the plant needs to grow. This is called weathering. In time plants replace the lichens and algae. The same C and N cycles occur in plants and continue the weathering process. Thus there is a close relationship between acid production and the accumulation of soil fertility. acidifying N fertiliser (permanent) ammonia type N fertiliser removal of product (permanent) legumes and clover fix N N = Nitrogen NO 3- = nitrate nitrogen NO 4+ = Ammonium nitrogen OM = organic matter erosion of top soil containing OM (permanent) build-up of OM ammonia is oxidised to NO 3- plant takes up NO 3- nitrification produces NO 3- leaching of NO 3- (permanent) mineralisation of OM produces NH 4+ Michel Dignand Figure 1. A diagrammatic representation of the processes and causes of agriculturally induced soil acidity. Those processes marked as permanent means that the acidification by that process is permanent. * In this paper soil ph is determined in a solution of 5:1 0.01M CaCl 2 :soil. This gives almost identical results to the standard method used in Brazil of 2.5:1 0.01M CaCl 2 :soil. 1 Greg Fenton, Project Coordinator, Acid Soil Action, NSW Agriculture, Wagga Wagga NSW 2650, Australia. greg.fenton@agric.nsw.gov.au 2 Dr. Keith Helyar, Director, Wagga Wagga Agricultural Institute, PMB, Wagga Wagga NSW 2650, Australia. Paper presented at the III SYMPOSIUM SOYBEAN/CORN ROTATIONS IN NO-TILL SYSTEM, Piracicaba-SP, July 10-12, ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002 1

2 In an ecosystem that remained closed there would be no change to the acid/alkali balance of the system, only redistribution over time through the system. However loss of organic anions with removal of plant material and through leaching of nitrate nitrogen before the plant can utilise it leaves the eco-system a little more acid. If these losses are small then the small amount of acid added is sufficient to maintain a fertile soil. Such an ecosystem acidifies slowly and this is a major part of weathering of parent material. When the acid produced exceeds the weathering rate then the soil becomes acid. Within the experience of the authors this acidification is not reversed naturally. In time the acidification will mean permanent loss of plant species that are sensitive to acidic soils and reduced the growth of those that remain. In addition the activity and biodiversity of the soil fauna and microbial populations is decreased. Agricultural practices of removal of crops and hay, increased leaching of nitrate nitrogen and erosion of fertile soil (Figure 1) greatly increases net acid production (Fenton and Helyar, 1999). This paper will visit the processes and effects of soil acidity, how acidity varies through out the soil profile and how to manage soil to prevent the formation of acidity. It is written for those who wish to hand their farm on to the next generation in as good, if not better, condition than they received it or if they are not farmers then to assist those who are. In writing this paper the authors recognise that it is difficult for farmers to implement soil conservation practices if they are not making money and suggestions for changes in management are made on the understanding that they will only be adopted if profits are increased. Figure 2. As soil ph falls below 5, aluminium (measured as Al in 5:1 0.01M CaCl 2 :soil) becomes available to plants. Soil acidity and plant growth As soil becomes more acid the availability of aluminium to the plant increases to toxic levels, while the availability molybdenum, phosphorus, magnesium and calcium decreases. In weakly weathered soils manganese toxicity will develop as the soil becomes more acid. These changes in availability of aluminium and plant nutrients cause most of the effects on plant growth attributed to acidic soils. Soil acidity may also reduce nodulation and nitrogen fixation by some legumes, decrease rates of mineralisation of organic matter and decrease microbial activity in general (Robson and Abbott, 1989). Aluminium (Al) Available aluminium increases as ph falls. The available Al at a given ph varies between soils with more aluminium available in well weathered soil than in a weakly weathered soil (Figure 2) (Fenton and Helyar, 1999). The principal effect of aluminium toxicity is to restrict root development by disrupting the structure and function of roots (Figure 3). Symptoms of aluminium toxicity in the leaves are similar to those of phosphorus deficiency and moisture stress as it decreasing the ability of plant roots to grow into acidic soil and extract nutrients and moisture. Crops wilt more quickly during dry periods and an overall reduction in water use efficiency of both pastures and crops occurs. Plants vary in their tolerance of aluminium. Tolerant plants can survive high levels of aluminium in the soil. These plants are also affected as high levels of aluminium immobilise phosphorus in the soil and can affect the uptake and utilisation of calcium and magnesium. Figure 3. The effect of increasing concentration of Al in solution on the roots of wheat. Plants vary in their tolerance to soluble aluminium. Table 1 gives the tolerance of a number of commonly grown agricultural crops and pastures grown in Australia. The plants are arranged into four tolerance classes (Ring et al., 1993). Table 1. Aluminium sensitivity (tolerance) of some crop and pasture plants. Highly sensitive Sensitive Tolerant Highly tolerant Durum wheat, faba beans, Chickpeas, Alfalfa, medics, strawberry, Balansa, Berseem and Persian clovers, Buffel grass, Tall Wheatgrass Canola, some wheats, barley, albus lupins, phalaris grass, red clover, Caucasian and Kenya white clovers. Some wheats, annual & perennial rye-grass, Tall fescue, Haifa white and subterranean clovers Narrow leaf lupins, oats, triticale, cereal rye, cocksfoot, kikuyu, paspalum, yellow & slender serradella, Maku lotus, common couch, Consul love grass Manganese (Mn) Both toxicities and deficiencies of manganese can occur in plants. Soils most likely to develop manganese toxicity are weakly weathered soils that are high in easily reducible manganese oxides, 2 ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002

3 such as red earths and red brown earths and more highly weathered soils that are high in iron and aluminium oxides that adsorb and accumulate Mn 2+ (Conyers et al., 1997). Not all plants are sensitive to Mn toxicity. In those plants that are sensitive, Mn toxicity causes chlorosis and crinkling of the leaf margins. Deficiencies are only expected where there is an absolute deficiency as in the case of some siliceous sands, or in soils that are very high in calcium carbonate (e.g. > 50 %). High Fe soils do tend to accumulate Mn rather than being depleted in Mn during soil development. This could be a problem in Brazil but the limited literature seen by the authors (Camargo and Falici, 1974) suggest that Mn deficiency may be a bigger problem than Mn toxicity. Increasing the soil ph to above 5.5 with lime can lower high levels of available manganese in the soil. On the other hand Mn deficiency can be accentuated by liming. Molybdenum (Mo) Molybdenum is a trace element required for nitrate conversion to the ammonium form used in plant proteins and in the nitrogen fixation process. Where the ph of a soil is below 5.5 in soils naturally deficient in Mo an application of g Mo per hectare is usually sufficient to eliminate Mo deficiency for 5 to 10 years. In soils high in iron and aluminium oxides more may be required. Calcium (Ca) In the experience of the authors most agricultural soils contain an adequate supply of calcium as a nutrient for the crops and pastures grown. Where there is a moderate calcium deficiency, it is seen first in parts of the plant that are furthest from the main flow of water within the plant such as production of peanuts. Low levels of soil calcium can also adversely affect the nodulation of subterranean clover. Short term severe deficiencies can cause petiole collapse of young expanding leaves. Examples of very severe calcium deficiency are most likely in soils with a ph less than 4 that are sandy and low in organic matter, or where there has been excessive use of highly acidifying fertilisers. Under these circumstances the exchangeable calcium can drop to concentrations less than 40 per cent of the exchangeable cations other than aluminium and this can only occur in soils with a low cation exchange capacity. As these soils usually have very high levels of soluble aluminium, all but the most acid tolerant plants, such as sugar cane, are killed by the aluminium before the symptoms of calcium deficiency become apparent. The symptoms are stubby, weakly branched and discoloured roots and dead shoot growing points. The root symptoms are difficult to distinguish from symptoms of aluminium toxicity. Magnesium (Mg) Losses of production in crops and livestock due to a nutritional deficiency of magnesium are most unusual. In Australia most of soils have adequate supply of Mg in the subsurface soil layers if not in the surface layers. Excess Mg can in some circumstances cause loss of soil structure. Phosphorus (P) Phosphorus is absorbed onto iron and aluminium oxides where it is less available to the plant and this effect increases as soils become more acid. Thus acidic soils high in iron and aluminium oxides need to contain higher concentrations of phosphorus to avoid P deficiencies in plants. The process of acidification The source of the acids in the ecosystem is principally the carbon and nitrogen cycles within the eco-system. The carbon cycle Description of the C cycle starts in the plant leaves with the creation of sugars from carbon dioxide (CO 2 ) and water using the energy of the sun through photosynthesis. These sugars are converted in part to organic acids that dissociate into H + ions and organic anions. The negative charge on the anions is balanced by the base cations, Ca 2+, Mg 2+, K + and Na + to form salts of organic acids. These base cations are taken up through the root. The H + ions are consumed in metabolic reactions or are excreted at the root surface in exchange for excess cation uptake. Note that the plant requires that the charge remain constant across the root membranes when taking up water and nutrients but does not require that a constant acid-base balance be maintained. The result is that the plant material becomes alkaline and the soil becomes more acid. The internal ph balance within the plant is maintained through the Malate-Pyruvate buffer within the plant. When the plant loses its leaves, or dies, the plant tissue becomes part of soil organic matter that in time breaks down to become CO 2 and water again, and releasing the alkalinity stored as organic anions (Figure 4). Fire is the same process but occurs far more quickly. The alkali released in the break down of the organic matter will neutralise the acid in the soil. Sunshine CO 2 + H 2 O The organic matter becomes CO 2 + H 2 O & energy In time the plant dies and becomes organic matter and the acid in the soil neutralises the alkali in the plant In the leaf CO 2 + H 2 O RCOO - + H + In the root cations are absorbed (e.g. C 2+, Mg 2+ ) and H + is excreted into the soils This means the PLANT becomes ALKALINE (RCOO - + M + ) and the SOIL becomes ACID (H + ) Figure 4. The Carbon cycle in a closed system where organic acids are produced from CO 2, water and sunshine as components of plants, then organic matter and return to CO 2, water and energy. ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002 3

4 As the alkali is accumulated in the above ground biomass an equal amount of acid is added to the soil. The return of biomass to the soil neutralises the acid as it breaks down. Some acid will be utilised in the weathering process otherwise the eco-system will retain an alkali/acid balance that, if the system was totally closed, would continue indefinitely. However if plant material is removed from the eco- system as in the harvest of grain or the making of hay then the eco-system is left more acid (Figure 5). If all the nitrate nitrogen produced is taken up by the plant then the acid alkali balance in the eco-system will continue indefinitely. However if any of the nitrate is leached from the root zone before the plant can take it up then the soil is left more acid (Figure 7). Some nitrogen may return to the atmosphere through a further step called denitrification. Atmospheric N 2 N fixation Plant Organic N (RNH 2 ) H + Plant dies In a crop system The grain is removed taking the organic anions (RCOO - + M + ) and leaving the soil a little more acid Soil Nitrate NO 3- N Produces 2H + Nitrification Mineralisation Uses H + Soil Ammonium NH 4+ N Figure 5. If product such as grain is removed from an permanently removed from an ecosystem then that system is left more acid. The nitrogen cycle The description of the N cycle starts with nitrogen being taken from the air by legume plant/rhizobia symbiosis and incorporated into the plant. When the plant dies the nitrogen becomes part of the organic matter that in turn breaks down releasing ammonium nitrogen. The ammonium nitrogen through mineralisation and nitrification becomes nitrate nitrogen with a net release of acid into the soil. As the nitrate nitrogen is taken up by the plant root it excretes an equal amount of anion that will neutralise the acidity as the charge across the root membrane must remain constant (Figure 6). N fixation Atmospheric N 2 Plant Organic N (RNH 2 ) Produces OH - NO 3 - is taken up by the plant Soil Nitrate NO 3- N H + Produces 2H + Nitrification Plant dies Mineralisation Uses H + Soil Ammonium NH 4+ N Of the 2 H + produced by nitrification one is used in the mineralisation and one is neutralised by the OH - Figure 6. The completed nitrogen cycle always returns to the same acid/alkali balance. Note on this diagram is the limited return to the atmosphere of nitrogen gas through denitrification is not shown. If the NO 3 - is leached away before the plant can take it up it leaves one H + behind and soil more acid Figure 7. If NO 3- is leached from the root zone before the plant can take it up the eco-system is left a little more acid. This is an alkaline process that balances the acid produced during the mineralisation and nitrification process.note that contrary to some older texts, the removal of the base cations, Ca 2+, Mg 2+, K + and Na + is not the cause of acidity in the soil but rather the result of acidification of the soil by acids added in the C and N cycles within the eco-system. Creation and destruction of the Stable State of soil acidity In nature most eco-systems have evolved to an almost stable state where the acidity/alkalinity balance changes slowly towards becoming more acid. In this natural stable state the eco-system is producing a small amount of acid that releases sufficient cations from the parent material to maintain fertile soil (Figure 8). For this publication the term stable state ph will refer to the alkali/acid balance in an eco-system that has evolved to the point where changes are very slow. The natural processes that control acid addition in the natural eco-system: The age of the soil. Climate, particularly rainfall, affects the intensity of leaching and of erosion and hence the opportunity for loss of alkaline materials. The vegetation affects acidification through variations in N fixation capacity, in the ability to absorb and recycle nutrients from and in the soil profile, in its ability to minimise the leaching of nutrients through effects on water use and nutrient stripping from the leached water. In addition different plants contain different concentrations of anions and this affects the amount of alkali lost when plant material is removed and alkali returned to the surface from the deeper soil layers. 4 ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002

5 Cycling of cations, N, S, P & organic acid salts to balance leaching Very limited removal of biomass (acidifying) Leaching of nutrients & H + and OH - Transport of nutrients to the surface made availabe by limited weathering by acid A small amount of NO 3- leached beyond the root zone (acidifying) Figure 8. A diagrammatic depiction of an stable state ph where a small amount of leakage of NO 3 N and loss of biomass creates enough acidity to release sufficient nutrients from the parent material to maintain soil fertility. Soil parent materials affect the rate of acidification due to differences in their acid neutralising capability or buffering capacity. Basalt, and often alluvium, have a high neutralising capability (also called a ph buffering capacity) compared to previously weathered materials such as a sandstone which may contain 98% quartz (SiO 2 ). In Wagga Wagga the stable state ph (0 to 10 cm) is between 5 and 6, increasing with depth to about 8 at 1 metre for soils that have not been disturbed by farming. These soils are generally duplex soil and support open woodland vegetation. This stable state ph has developed over more than maybe 30,000 years and has as part of its genesis the regular burning by the aboriginals who inhabited this land for more than 40,000 years. When the land is turned over to agricultural production acidification rates increase and the natural stable state ph is destroyed. The normal agricultural practices of fertilising and harvesting along with the increase in leaching of nitrate nitrogen accelerate the acidification of agricultural lands. In addition a build up in organic matter takes organic anions out of the eco- system leaving it a little more acid but this is reversed if the organic matter is reduced later (Figure 1). An increase in erosion of the fertile topsoil will remove this organic matter leaving the soil more acid. The challenge to farmers and their advisers is to establish a stable state ph for farming if it is to continue for 100 s and maybe 1,000 s of years. This will mean managing the processes that acidify the soil such as fertilisers, product removal and increased nitrate nitrogen leakage away from the root zone and applying an alkaline material, most likely applying finely ground lime. In Wagga Wagga the stable state ph that cropping farmers strive to achieve is ph between 5.0 and 5.5 by applying finely ground limestone. Most plants and soil biota are not affected by the acidity at this soil ph and the net movement of H + /OH - down the profile is in balance. This stable state ph is economically sustainable as their cropping/pasture rotation is based on plants that are sensitive to acidity and the purchase of lime comes from, and maintains, the farm profits. In an experiment at Wagga Wagga the long term effect of tillage, crop rotations, lime and fertiliser on soils has been running since This experiment indicates dramatic effects on ph (0-10 cm) occurred in the first ten years but this has now stabilised (Conyers et al., 1996). Acid addition is now thought to be equal to the ongoing weathering rate seen as a 7% reduction in fine clay fraction over 20 years (Slattery Pers. Com.). To the east of Wagga Wagga in the non-arable grazing lands lime application does not increase profits sufficiently to cover the cost. The farmers who are establishing a new stable state ph grow perennial, acid tolerant pastures and their agricultural practices are less acidifying. These farmers have a stable state ph that is less than 5.0 and often less than 4.8. While acidification rates are lower than at 5.5 it is not sustainable as the whole profile is acidifying. Distribution of soil acidity over the soil profile (ph profile) To understand how to achieve a fertile stable state ph profile it is necessary to understand the factors that control the distribution of soil acidity down the profile. Planning a cropping or pasture program that will establish a sustainable, fertile ph profile requires management of the acid being added to each layer within the profile. Examples of ways that ph profile influences crops and pastures in an acidic soil: Acid sensitive plants will be affected if the ph is below 4.7 beyond 20 cm. The root of an acid sensitive plant can often break through 10 cm of inhospitable acidic soil, but is less likely to penetrate 20 cm of acidic soil. The effect of lime moves down the profile very slowly and only when the ph in the layer above is greater than 5.5. The response to liming is affected by the ph profile below the limed layers. Where sub-surface acidity occurs it may take a number of years for the economic response to liming to be realised so the ph profile is important in planning a crop and pasture rotations in association with a liming program. Development of a ph profile Different soil profiles develop under different agricultural, forestry and natural eco-systems. For example in many systems the surface 5 cm has higher ph than the sub-surface layers (5-20 cm). In undisturbed soils around Wagga Wagga, for example, the top 5 cm has a ph of 5.5 to 6, a ph of 4.5 to 5.5 in the 5 to 20 cm layer with the ph rising again in lower root zone due to net alkali addition. In other systems the reverse may occur. Where net acid addition occurs on a whole profile basis, the long-term effect is usually a strongly acid profile to depth, with a slightly less acid layer in the top 2-3 cm. Subsequently leaching, diffusion and soil movement or mixing, can redistribute hydrogen and hydroxyl ions and their chemical equivalents (aluminium, carbonate and bicarbonate ions) within the profile. If it is difficult to avoid the development of a highly acid ph profile to depth over laying a less acid deep sub-soil. In the natural eco-system this happens very slowly but where acidification is ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002 5

6 accelerated the profiles will become acid to depth quickly unless finely ground lime is incorporated to at least 10 cm to neutralise any acid addition. The factors that affect the development of the soil ph profile are: Leaching of nutrients affect the location of acid and alkaline reactions in the N and C cycles, and the pattern of excretion of H + and OH - - (or HCO 3 and organic anions) by roots in response to imbalances in cation and anion uptake. Net acid addition can occur close to the top of the profile if nitrate formed from nitrification is leached. Acidity and alkalinity can be redistributed within the soil profile by mass flow in water, by diffusion or by soil movement. In general the movement by water will be acid if the ph is below 5.0 and net alkaline if the ph is above 5.5. The alkaline effect of applied lime moves slowly down the profile (ph > 5.5), unless the surface ph is high (ph 7) and biological activity is high. Plant transport of nutrients and organic anions onto the soil surface. Some worms and ants will bring soil up from depth and will transport lime down the profile both by active movement and faster leaching down the holes they create (Baker et al., 1995). The locations in the soil profile where N and C cycle acids and alkalis are added The primary processes of acid addition to the soil from the N and C cycles are: Nitrification. Dissociation of organic acids in the soil (from organic acids produced in the soil by microflora or organic acids added to the soil in plant residues). Immobilisation or volatilisation of ammonium ions in the soil. Excretion of hydrogen ions by roots in response to uptake of more cations than anions. depth to which ammonium is leached before nitrification occurs. Finally another quantitatively important process, the pattern of excretion of acid or alkali by roots, is affected by the relative availability of the nutrient cations and anions in a given soil layer. The most useful simplification of this balance is that roots usually absorb more non cations than non anions so can be expected to excrete some acid where N availability is low in a soil layer. Where N availability is high, rates of acid excretion can be expected where most of the soil N is in the ammonium form. Conversely moderate to high rates of alkali excretion can be expected where most of the soil N is in the nitrate form. Therefore ecological processes such as the leaching of ammonium after ammonification and the leach of nitrate before absorption by roots, affect the distribution of acid addition to the soil profile. The leaching of NO 3 to depth within the profile before the plant takes it up is one example of how the leaching of nutrient can affect the ph profile. It will leave the point where the nitrification took place more acid and will increase the ph at the point where it is taken up. While this could be a method to correct the acidity in the sub soil it is not always certain that the NO 3 will be taken up and not leached from the root zone. The movement of acid and alkali within the soil profile by leaching and diffusion The net alkalinity or acidity of the soil solution across the common range of soil ph values is shown in Figure 9. Therefore the figure describes the effect of a simultaneous equilibrium between alumino-silicate minerals, Al(OH) 3, H 2 O, CO 2, CaCO 3, on the net alkalinity or acidity of the soil solution (Munns et al., 1992). The primary processes of alkali addition to soils (or hydrogen ion consumption) are: Denitrification, nitrate immobilisation by microflora. Mineralisation of organic N to ammonium. Adsorption of hydrogen ions by organic anions in organic matter added to soils with lower ph than the organic matter. Oxidation of organic anions to carbon dioxide and water. Excretion of OH or HCO 3 by roots in response to greater uptake of anions than cations. Therefore the net distribution of acids and alkali addition to the soil profile depends on the distribution of all the above primary processes of addition of acids and alkalis. Among the quantitatively dominant processes, the oxidation of organic anions (alkaline) and ammonification (alkaline) usually occur near the soil surface where the organic matter is added. The location of the dominant acid process, nitrification, depends on the Figure 9. Net alkalinity or acidity in the soil solution as a function of soil ph, the solubility of Al minerals and calcium carbonate in the soil, and of the carbon dioxide concentration in the soil solution. Key features of this figure are that the solution leaching down the soil profile is net acid below ph 5.2 to 5.6 and alkaline above that ph range. Furthermore, above about ph 5.5 the net alkalinity of the soil solution is very sensitive to the carbon dioxide concentration of the soil air. Thus the mass flow of alkali in leachate is sensitive to the biological activity in the soil and the ph while the mass flow of acid below ph 5.5 is sensitive only to the ph. A further feature is that net movement of acid or alkali by leaching is quite small within the ph range 4.5 to ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002

7 Eucalyptus grass vegetation with: low nutrient and alkali recycling capacity Pastures or trees with: low nutrient and alkali recycling capacity layer can form where plant recycling of alkalinity from the subsoil is low. This results in net acid addition to the surface soil and net alkali addition to the subsoil (Figure 10). The consequences for soil profile development and for soil fertility are profound. Net acid addition Leaching Net alkali addition Leaching Restores acid balance Exaggerates problem of clay dissolution (A horizon) precipitation (B horizon) Result Solodic soils, acid surface, heavy clay subsoil Gradational well structured soil, no strongly acid layer Net alkali addition Leaching Net acid additon Figure 10. Effect on soil ph profiles and soil type of contrasting patterns of acid and alkali distribution in the soil profile that occur under different types of vegetation where net acid addition to the profile is very low or zero. Plant transport of nutrients and alkalinity onto the soil surface Plants with a high organic anion and associated cation content have a greater capacity to recycle soil cations and alkalinity to the surface in plant residues. Thus these species are useful in an ecosystem to achieve an excess of alkali in the upper part of the soil profile by increasing the rate of oxidation of organic anions near the surface. Where this process achieves a ph significantly above 5.5, the leaching of a net alkaline solution downward (Figure 9) helps to neutralise acid addition deeper in the profile. Among the agricultural species clovers and medics are higher in organic anions (75 to 150 cmol/kg) than high quality grasses (50 to 70 cmol/kg). Cereals and low quality grasses that are relatively high in cellulose are low (< 60 cmol/kg) in organic anions and cations (Slattery et al., 1991). Some dicotyledonous weeds such as cape weed and Paterson s curse are high in organic anions. Among the tree species eucalypts and acacias are often low in organic anions (< 100 and often < 60 cmol/kg leaf litter) while other species such as poplars, plane trees and white cedars are particularly high (150 to 250 cmol/kg leaf litter). Limited data on Casuarina spp. indicates they may be in the high end of the eucalypt range and other species have organic anion concentrations between 100 and 150 (Noble et al., 1996). This factor should be taken into account when trying to design more sustainable agricultural and forestry ecosystems. The effect of the major processes affecting the pattern of acidity in a profile have been modelled (Helyar, 2001). This model demonstrates that the equilibrium soil ph profile developed under a given pattern of acid and alkali production is very sensitive to some factors that are subject to management. Examples are the organic anion content of the plant grown, the depth of leaching of ammonium before nitrification and of nitrate before absorption by plants. Furthermore, where the production system results in net acid addition to the soil profile, a deep, highly acid profile will develop over time unless sufficient lime is applied to the surface to neutralise the acid added. Even in natural ecosystems that achieve very low or zero net acid addition, (no nitrate leaching and no export of organic anions from the system), a duplex soil with an acid sub-surface Managing agriculturally induced soil acidity Lime application rates should at least equal the net acid addition To stop a highly acid layer developing in some section of the profile lime application must equal alkali removal as the first step to establishing a fertile ph profile without a strongly acid layer anywhere in the root zone. One exception to this suggestion is where the acid and alkali additions are such that a highly acid layer is created at the bottom of the root zone and the ph is above 5.5 elsewhere. In this situation the acid layer is used to mobilise nutrients from parent material and accumulate them in the main root zone. The soils under Brigalow vegetation may be an example of such a soil in Australia. The surface soil should be limed to ph 5.5 or above This will ensure that the net movement of alkali to the deeper layers within the profile (Figure 9). A soil ph greater than 5.5 will also remove most of the problems associated with soil acidity. Care should be taken not to increase the ph of the surface 10cm significantly above 6.0 because this may induce deficiency in other plant nutrients such as zinc, boron and manganese in well weathered soils. Where the soil is acid to depth it may take many years for the alkaline effect of the lime to move below 20 cm, especially for highly buffered soils. Changes in ph in the subsoil layers of an acidic soil have been measured for 12 years in a trial to the east of Wagga Wagga (Helyar et al., 1997). This has confirmed that if the ph is maintained around 5.5 in the top 10 cm that the ph at 15 to 20 will increase. There is opportunity to correct acidity in sub-surface layers where the exchange capacity of the sub-surface soils is positive (Figure 11). Here gypsum, that is far more soluble than lime, can be used neutralise acidity at depth. > Al-OH + SO 4 2- > Al-SO OH - > Fe-OH + SO 4 2- > Fe-SO OH - Figure 11. The liming effect of gypsum on positively charged soils high in iron and aluminium. Reduce the leaching of nitrate nitrogen In soils with a ph above 4.7 nitrate nitrogen (NO 3 ) is the main form of nitrogen that is available to the plant. Nitrate nitrogen is formed from ammonium nitrogen (NH 4 ) by the process of nitrification. It is easily leached as it is weakly adsorbed by soil materials and is highly soluble. When NO 3 is leached away from the point of nitrification it leaves that point more acid. If the NO 3 is taken up further down the profile this can increase ph at the point of uptake. When the NO 3 is leached below the root zone and into the sub soil aquifers it leaves the soil more acid (Figure 7). ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002 7

8 Table 2 lists the factors that affect NO 3 leaching in the Wagga Wagga district roughly in order of importance, qualifies the effect of each factor and indicates how the effect can be reduced, or increased. The absolute effect of each factor will depend on rainfall and is usually periodic. In wetter climates the effect may occur every year but in drier climates it may occur in some years. Table 2. Factors that affect NO 3 leaching in Wagga Wagga district roughly in order of importance, the nature of the effect of each factor and some indications of how the effect can be reduced, or increased. Factor affecting NO 3 leaching Nature of effect Reducing the effect Poor plant growth Inefficient water use increases leakage of water Efficient water use by healthy well managed crops containing NO 3 into sub soil and pastures dry out the soil profile over summer Annual pastures NO 3 will leach before annual pastures establish Perennial pastures will utilise the NO 3 as it comes available NO 3 will leave the root zone before being utilised Deep rooted perennials will catch the NO 3 before it leaches below the root zone. (Ridley et al., 1990) Pastures Clover/grass ratio High N producing pastures increase NO 3 available Reduce the clover in pastures below 20% but this to be leaching reduces production. Include deep rooted perennials. Annual crops Delay in sowing annual crops will allow the NO 3 Always sow on the rain that breaks the season to move on the first water front ahead of the roots Non acidifying N fertiliser Where urea and anhydrous NH 4 oxidise before the Apply fertiliser after the crop is up in split plant can use it can be leached away application or in bands if pre-sowing. Stubble retention Retaining stubble increase NO 3 available to be Increases leaching of NO 3 from top 15 cm by leaching 40%. Most is absorbed down the profile but some Improves structure and therefore drainage is lost (Heenan & Chan, 1992) Reduced cultivation No cultivation and direct drill seeding improved Increased leaching of NO 3 by 18%. Most is soil structure absorbed down the profile but some is lost Soil ph Lower ph inhibits nitrification thus reducing Maintain ph below 4.7 but this will acidify NO 3 available subsurface layers and may well reduce productivity Adsorption onto clay NO 3 is weakly adsorbed by clays slowing leaching Nothing. Where positive point of charge is present the rate of leaching is reduced Erosion of soil containing organic matter or accumulating biomass (eg. forests) A build up of soil organic matter or of live and dead biomass above the soil has the same effect on soil acidity as removing produce from the paddock. Increasing organic matter has many benefits, for example improving soil structure, but it also makes the soil more acid. Lime application should be considered to balance acid added during a phase of increasing soil organic matter if the ph is low enough to affect plant growth and profits from the enterprise. This build up in organic matter will stabilise at a new level after some years. The higher organic matter level is often associated with higher levels of available nitrate nitrogen increasing the potential for further acidification from leaching of nitrate nitrogen. Higher levels of organic matter in the soil (greater than 6%) in the soil will adsorb soluble aluminium minerals resulting in less aluminium being available to the plants. In this situation plants sensitive to aluminium can grow in soils with a low ph. However if the organic mater or biomass is removed in soil erosion event then the soil is left a little more acid. Removal of produce Grain, pasture and animal products are slightly alkaline and their removal from a paddock leaves the soil more acid. If very little produce is removed, such as in beef production, then the system remains almost balanced. If, however, a large quantity of produce is removed, particularly clover or alfalfa hay, products that are relatively high in organic anions, the soil becomes significantly more acid. If the produce is sold off-farm, regular liming is the only way to maintain ph. The rates of lime required to neutralise the acidification caused by removal of produce are given in Table 3. Table 3. The amount of lime needed to neutralise the acidification caused by removal of produce (Slattery et al., 1991). Produce Lime requirement (kg/t of produce) Corn and wheat 9 Soybeans and lupins 20 Meat 17 Grass hay 25 Clover hay 40 Maize silage 40 Lucerne hay 70 8 ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002

9 The acidification caused by removing hay or silage is neutralised if it is fed to livestock in the paddock where it was made and the waste products are distributed evenly over the paddock. Neutralisation of the acid added to forest soils as a result of biomass accumulation and export in logs should be considered as a component of sustainable management of forests. Otherwise long term acidification of the forest soil will occur. The rate of lime required can be calculated from the amount of organic anions removed in the logs plus the amount in the standing biomass that remains after logging. Use of fertilisers The acidification that results from using fertilisers depends on the fertiliser type. Some nitrogenous fertilisers, for example urea, have no effect on ph if all the nitrogen is utilised by the plant. Acidity will only result if the nitrate nitrogen produced from these fertilisers is leached (Table 4). Other nitrogenous fertilisers, for example sulfate of ammonium, have an acid reaction and their use acidifies the soil even if fully utilised by plants. Additional acidification results if nitrate nitrogen is formed and leached. Calcium nitrate, potassium nitrate and sodium nitrate neutralise soil acidity if utilised by plants and have no effect if all the nitrate is leached, but they are expensive and have limited application. Superphosphate has no direct affect on soil ph, but its use stimulates growth of clovers and other legumes. In pasture systems where most of the increased plant growth is returned to the soil, increases of soil N status and a build-up of soil organic matter occur, both of which increase soil acidity. Elemental sulfur is acidifying and requires kg of lime for each kg of sulfur to neutralise its effect. This effect can be avoided by using products that contain sulfur in the sulfate form such as gypsum, potassium sulfate and superphosphate. Nitrogen fertiliser (including urea) that is pre-sown should be drilled into narrow bands to slow nitrification and subsequent leaching. Surface application of nitrogenous fertiliser for crops before sowing, even if harrowed, can result in nitrate leaching and consequent acidification. Nitrate nitrogen formed from post-emergent applications is more likely to be utilised by the crop and will cause less acidification. Lime requirement to maintain a new agricultural stable state ph Based on experience in Australia the following steps may serve as a guide to calculating lime requirements for a nominated paddock. 1. Have the soil tested. Take 20 to 30 samples per paddock to 20 cm deep. Split the samples into 0-10 cm and 10-20cm sub-samples and thoroughly mix the sub-samples for each depth. Have the soil analysed by a reputable laboratory. If the ph is less than 5 then determine the exchangeable aluminium. If the exchangeable aluminium is above 5% it is likely that acidity is already, or will in the future, cost production. The effect will vary with the aluminium tolerance (acid tolerance) of the crop or pasture to be grown. 2. Estimate of the acidification of your enterprise based on measurement and experiments from the local district. This will account for the acidification due to nitrate leaching, removal of product and organic matter build-up. Add to this your above average use of fertilisers and adjust for above average production. The figures in Table 5 are estimates of acidification based on known rates that can be used to calculate how much lime is required. Conclusion Nearly every agricultural practice is more acidifying than the natural eco-systems. To establish an agricultural eco-system that will continue indefinitely will mean planning. This plan will be based on an understanding the causes of the acidity. It will mean changing some practices and most likely applying finely ground limestone as the source of alkalinity to maintain this balance. It is difficult to plan without having the soil chemically analysed. In the Acid Soil Action program in Australia, of which one author is the coordinator, the first step in developing a culture sustainable agriculture practice within the farming community is to sample and analyse the soil to depth. Those making this plan must remain mindful that the person who ultimately decides on what happens to the land is the owner of the land. He needs to be well informed and well advised. Table 4. Acidifying effect of nitrogenous fertilisers and legume-fixed nitrogen. Source of nitrogen Lime required to balance acidification (kg lime/kg N) 0% nitrate 50% nitrate 100% nitrate leached leached leached High acidification Sulfate of ammonium, Mono-ammonium phosphate (MAP) Medium acidification Di-ammonium phosphate (DAP) Low acidification Urea, Ammonium nitrate, Aqua ammonia, Anhydrous ammonia, Legume fixed N Alkalisation Sodium and calcium nitrate Equivalent to applying 3.6 kg lime/ kg N. ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002 9

10 Table 5. Typical rates of acidification to used as a guide to calculating lime requirement. Cause of acidification Rate of lime to neutralise acid added Your lime requirement NO 3 leaching 1 - District average Average rate 2 High rainfall - annual crops or grasses 250 kg/year.kg High rainfall - perennial crops and pastures 100 kg/year.kg Medium rainfall - annual crops or grasses 150 kg/year.kg Medium rainfall - perennial crops and pastures 50 kg/year.kg Above average Crop removal (See Table 3) e.g. Soybeans 20 kg/tonne.kg e.g. Corn silage 40 kg/tonne.kg Above average Fertiliser type (See Table 4) e.g. DAP 36 kg/100 kg DAP.kg Total annual lime requirement.kg/year Time to next liming if ph is above /Total ann.lime req Years 1. This rate will be based on local experimental data or if not available will need to be built up with farmer experience over time. 2. This rate is increased by 10 % if stubble is retained in 10 years or more rotation. 5% if there is no cultivation in 10 years or more rotation. This rate is decreased by 20% if the ph is between 4.6 and % if the ph is between 4.2 and 4.5 (below 4.2 there is virtually no acidification) 3 If the ph < 5 it may be time to lime now. Experience in Wagga Wagga is that it is most economical to keep farming until the ph is 4.6 to 4.8 then apply 2.0 to 2.5tonnes per hectare of limestone to neutralise accumulated acidity, usually 10 to 20 years between applications. References BAKER, G.H.; BARRETT, V.J.; CARTER, P.J.; BUCKERFIELD, J.C.; WILLIAMS, P.M.L. and KILPIN, G.P. (1995). Abundance of earth worms in soil used for cereal production in south eastern Australia and their role in reducing acidity. In Plant soil interactions at low ph. Ed R. Date. Kluwer Academic Publishers. CARMARGO, M.N. and FALESI, I.C. (1974). Soils of central plateau and Transamazonic Highway of Brazil. In Soil management of tropical America Ed. Bornemisza E. and Alvarado A. Pub. Nth Carolina State University N.C. USA. CONYERS, M.K.; HEENAN, D.P.; POILE, B.R.; CULLIS, B.R. and HELYAR, K.R. (1996). Influence of dryland agricultural practice on the acidification of a soil profile. Soil and Tillage 37, CONYERS, M.K.; UREN, N.C.; HELYAR, K.R.; POILE, G.J. and CULLIS, B.R. (1997). Temporal variation in soil acidity. Australian Journal Soil Research FENTON, G. and HELYAR, K.R. (1999). The causes and management of acid soils. Invited paper for the Soils Workshop Describing, analysing and managing our soil (25pp.), th November, 1999 (University of Sydney and NSW Branch Australian Soil Science Society). HEENAN, D.P. and CHAN, K.Y. (1992). The long-term effects of rotation, tillage and stubble management on soil mineral nitrogen supply to wheat. Aust J. Soil Res HELYAR, K.R. and PORTER, W.M. (1989). Soil acidification, its measurement and the processes involved. pp In Soil Acidity and Plant Growth. (Ed. A.D. Robson), Academic Press, Australia. HELYAR, K.R.; CONYERS, M.K. and LI, G. (1997). MASTER, Managing acid soils through efficient rotations. Final Report DAN 207 International Wool Secretariat, AWRAP Project NSW Agriculture, Wagga Wagga. HELYAR, K.R. (2002). Soil acidification and nutrient cycling in relation to the sustainable management of agricultural and forestry ecosystems. In Keynote papers, 5 th International Symposium on Plant-Soil Interactions at Low ph, Jagersrust, KwaZuluatal, th March 2001 (in Press). MUNNS, D.N.; HELYAR, K.R. and CONYERS, M. (1992). Determination of aluminium activity from measurements of fluoride in acid soil solutions. J. Soil Sci., 43, RIDLEY, A.M.; SLATTERY, K.R. and HELYAR, K.R. (1990). Acidification under grazed annual and perennial grass based pastures. Australian Journal Experimental Agriculture RING, S.M.; FISHER, R.P.; POILE, G.J.; POILE, G.J.; CONYERS, M.K. and MORRIS, S.G. Screening species and cultivars for their tolerance to acidic soil conditions. Plant and Soil 155/156: ROBSON, A.D. and ABBOTT L.K. (1989). The effect of soil acidity on microbial activity in soils. In Soil acidity and plant growth. A.D. Robson, Academic Press. SLATTERY, W.S.; RIDLEY, A.M. and WINDSOR, S.M. (1991). Ash alkalinity of animal and plant products. Australian Journal Experimental Agriculture ENCARTE DO INFORMAÇÕES AGRONÔMICAS Nº 98 JUNHO/2002

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