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Constraints to cropping soils in the northern grains region Rockhampton Emerald A decision tree Qld Bundaberg Taroom Kingaroy Roma Is your paddock or parts of your paddock showing poor crop growth and yield, despite good starting moisture and adequate in-crop rainfall? If so, look for the following: diseases, insect pests, nematodes, herbicide damage, weeds, or frost damage. If these are not the cause, your soil may be limiting plant growth. Use the decision tree below to help identify soil constraints. BRISBANE Toowoomba St George Goondiwindi Brewarrina Narrabri NSW Tamworth Dubbo Newcastle Do the roots show honey brown discoloration, dark lesions, or knotting? Biological constraints Do the leaves or stems show characteristic symptoms of nutrient imbalance? Nutrient constraints Is the soil surface crusting, hard setting, sealing or waterlogged (water ponds for several days after rain)? Surface sodicity Coffs Harbour Is the rooting depth restricted by rock, a dense clay layer, hard pan, or a traffic or plough pan (roots deformed or growing at a right angle)? Physical constraints Are fresh roots absent from the rooting zone (less than 1 metre soil depth)? Is the subsoil wet after a dry finish? Chemical constraints Is subsoil structure coarse or dense? Is subsoil drainage lacking? Subsoil sodicity Biological constraints Nutrient deficiencies Good and bad soil organisms affect plant growth. Problems occur when there is a reduction in the activities of good organisms such as earthworms and vesicular-arbuscular mycorrhizae (VAM), or an increase in pathogens or plantparasitic nematodes. Better crop varieties, improved agronomy and higher yields, along with continuous cropping without adequate fertiliser, have resulted in widespread soil nutrient depletion—especially of nitrogen, phosphorus and zinc. Pathogenic fungi and nematodes often Crown rot build up in monoculture cereal cropping systems in the northern grains region. These include crown rot and common root rot, as well as root-lesion nematodes that can be as deep as 60 centimetres at peak times. Subsoils are generally lower in nutrients than the surface layer. This can reduce yield in dryland regions, where continued root growth and function are essential to enable crops to extract water from the subsoil. Signs of nutrient deficieny include: pale yellow old leaves (lack of N), dark green foliage and purple old leaves (P), and water-soaked strips on young leaves (Zn). What can I do? What can I do? Identify the problem then focus on farm hygiene, crop rotation and growing resistant crops and cultivars. Identify the nutrient disorders. To restore soil nutrients, use a balance of organic and inorganic fertilisers, and incorporate legumes and ley pastures in crop rotations. Encourage the build up of beneficial organisms through moist soil and a good supply of organic substrate (full stubble retention and no-till). Use direct drilling because mechanical tillage is detrimental to VAM and earthworms, and it promotes attack by nematodes and soil-borne diseases. Physical constraints Physical constraints decrease oxygen and water movement in the soil. Compacted soil and soil with a high physical strength impede root growth. Compacted soil layers or layers of high bulk density (>1.5 g/cm3) are widespread in the region. Subsoil compaction is natural and also caused by heavy traffic and tillage on wet soils. Phosphorus (P) deficiency Root problem Man-made compaction occurs in the top 20 centimetres in most cropping soils, but can be deeper especially below wheel tracks. Compacted layers may be visible, measured as high penetration resistance (>2 MPa), or indicated by distorted root growth. What can I do? Deep ripping may benefit soils with hardpans or compacted layers; but it can be costly, has varying levels of success, and could bring up highly sodic subsoil. Deep ripping is rarely worthwhile financially without some type of Controlled Traffic Farming (CTF), as compacted layers quickly re-form under wheel tracks. Opportunity cropping and growing deep, tap-rooted pasture plants such as lucerne help form pathways through a compacted layer. Fibrous-rooted plants can remove more moisture from the soil and help shrink and crack Vertosols, which assists self-repair. Zinc (Zn) deficiency Nitrogen (N) deficiency Chemical constraints solubilities. Only highly soluble salts such as sodium chloride inhibit water extraction or are directly toxic to roots. Chemical constraints can be toxic to plants and also limit the plant’s ability to take up water by decreasing the osmotic potential of the soil water. Soil salinity is commonly measured as electrical conductivity (EC) in a 1:5 soil:water extract and expressed as dS/m. EC measurements do not discriminate between salt types. Acidity In acid soils (pH <5.5), aluminium and manganese become more soluble; they may become toxic as their concentration in the soil water rises. Aluminium inhibits root growth in most plants and induces calcium, phosphorus and molybdenum deficiencies. In the northern grains region, subsoil acidity is common in soils dominated by N2-fixing brigalows and belah. Gypsum: EC values will be high if significant quantities of gypsum are present. If the high EC reading is due only to gypsum there are no concerns for crop growth. The dissolution of soluble gypsum will not restrict the roots from extracting water unless other salts are present. What can I do? Lime is the most economical ameliorant for surface soil acidity, but it leaches very slowly making it unsuitable for rapid amelioration of acid subsoils. Deep placement is effective, but costly and difficult. It is cheaper to prevent subsoil from acidifying in the first place, by adding lime. White gypsum banding Chloride and sodium: Measuring sodium and particularly chloride concentrations in soil provides a better indicator of ion toxicity and why a crop has problems extracting water (see table 1). Chloride salts are highly soluble and can accumulate to toxic Chloride affected wheat concentrations in plant tissue. Chloride accumulation in some species causes toxicity and a drop in grain yield. Research shows that acidity in lower layers is slowly reduced when lime is applied to maintain the surface soil at a pH of 5.5 or more. Alkalinity In alkaline soils, an abundance of anions such as carbonates and bicarbonates contribute to the high pH. At soil pH >8.0, toxicity of carbonate and bicarbonate can reduce crop growth and yield, and/or induce nutrient deficiencies. What can I do? Add elemental sulphur to acidify these soils. Large amounts of elemental sulphur would be needed to reduce the soil pH of clay-dominated soils, which may be costly. Table 1. Concentration of chloride and sodium in soil Low Medium High Surface soil Salinity Salinity is the presence of dissolved salts in soil or water. A high concentration of salts can cause ion toxicity and affect a plant’s ability to absorb water. <300 mg Cl/kg 300-600 mg Cl/kg >600 mg Cl/kg <200 mg Na/kg 200-500 mg Na/kg >500 mg Na/kg Subsoil Several salts are found in northern region soils, including sodium chloride (common salt), calcium sulphate (gypsum) and calcium carbonate (lime). Different salts have different <600 mg Cl/kg 600-1200 mg Cl/kg >1200 mg Cl/kg <500 mg Na/kg 500-1000 mg Na/kg >1000 mg Na/kg Figure 1. Soil classification based on pH (1:5 soil:water) Acidic 0 1 2 3 4 Toxicities of Al, Mn, Fe Deficiencies of Ca, Mo, P 5 6 7 Ideal pH for plant growth 8 9 10 11 12 Alkaline 13 14 Sodicity Toxicity of Na, HCO3 Deficiencies of Ca, K, Zn, Mn What can I do? If the surface and/or subsoil have a high EC reading and a high concentration of sodium and/or chloride, grow crops and their cultivars that are tolerant to chloride. Threshold values of chloride that cause 10 per cent and 50 per cent reduction in grain yield suggest that bread wheat, barley and canola are more tolerant than chickpea and durum wheat (see table 2). Table 2. Thresholds for chloride concentrations in soil (mg/kg) 10% yield reduction 50% yield reduction Bread wheat 700 1500 Barley 800 1500 Durum wheat 600 1200 Canola 1200 1800 Chickpea 600 1000 Field trials suggest that for bread wheat, cvs. Baxter and Sunco may perform better than some other cultivars commonly grown in the northern grains region. For chickpea, Moti has performed slightly better than Jimbour. Work on this is continuing. In paddocks with medium to high levels of chloride, consider growing pasture or forage crops. In paddocks with very high constraints, consider permanent pasture such as grass-lucerne pasture or agro-forestry such as native wisteria (Pongamia pinnata), which may be tolerant to salts. Sodicity Sodicity is an excess of sodium ions relative to other cations (calcium, magnesium and potassium). A high proportion of sodium can lead to soil dispersion. Salinity can mask the dispersive behaviour of sodic soil. Sodicity and salinity both occur in many subsoils of the northern grains region. The effect of these two is nonadditive; plant growth is primarily limited by salinity. However, sodic impermeable subsoil does not allow the drainage that would otherwise help to move salts out of the surface layers. High surface soil sodicity leads to surface crusting, cloddy seedbed, poor water infiltration (more run-off) and increased water-logging. In subsoils, high sodicity often leads to a coarse or dense structure that restricts soil water and air movement. The subsoil may become oxygen deficient and take on a mottled “rusty” appearance. Crop roots cannot grow in a saturated soil (due to the lack of oxygen). Even though the subsoil is wet, this water is not really available to the crop. Crusting Cloddy seedbed What can I do? Gypsum application improves surface sodicity by flocculating soil, leading to better infiltration and the exchange of calcium for sodium. In soils with moderate surface sodicity (ESP = 12), the application of gypsum at 2.5–5.0 t/ha significantly improves wheat grain yield. For correcting subsoil sodicity, high application rates of gypsum, sufficient rainfall and time are required. Improved subsoil drainage may also help salts to leach from the upper layers. Dispersion is the disintegration of clay aggregates into individual particles when wet, resulting in milky or cloudy water. In the paddock, the soil will be poorly drained and the subsoil may remain saturated for long periods. Sodicity ranging from none to complete (left to right) No Gypsum Gypsum (2.5 t/ha) Chemical constraints decision tree Electrical Conductivity (EC; dS/m) Check soil for electrical conductivity Top 10 cm soil Subsoil Plant growth is not affected by salinity if: Plant growth is affected by salinity if: Low EC <0.3 dS/m Low EC <0.7 dS/m High EC >0.3 dS/m High EC >0.7 dS/m Check soil for exchangeable sodium per cent (ESP) and/or dispersion No dispersion (ESP <6) Check soil for sodium and chloride concentration Dispersion (ESP >6) Cl >300 mg/kg Cl >600 mg/kg Check the soil pH (1:5 soil:water) Cl <300 mg/kg Cl <600 mg/kg and/or Na >200 mg/kg Na >500 mg/kg pH <5.5 Check for gypsum crystals and sulfur concentration pH >8.0 S >100 mg/kg Acidity constraint Alkalinity constraint S <100 mg/kg Sodicity constraint Osmotic effect due to high salt, and Na/Cl toxicity High EC due to gypsum: No constraint to crop growth No gypsum – other salts are causing the problem Table 3. Agronomic practices for varying levels of constraint Low constraints Medium constraints High constraints (<600 mg Cl/kg, <500 mg Na/kg in top 1 m soil depth) (600-1200 mg Cl/kg, 500-1000 mg Na/kg in top 1 m soil depth) (>1200 mg Cl/kg, >1000 mg Na/kg in top 1 m soil depth) • Cereals-legume rotation • Grow tolerant cereals (wheat, barley, canola) • Consider alternative land use (saline forage/pasture production, agroforestry/forestry system) • Consider canola if soil profile is full • Match inputs to realistic yield • Manage crown rot and nematodes • Consider tolerant cultivars • Try opportunity cropping to use available water • Manage crown rot and nematodes • Avoid crops or grow tolerant cereals • Match inputs to realistic yield • Avoid legumes and durum wheat • Try opportunity cropping to use available water Maximising production Good agronomic management helps minimise the water and other physiological stresses imposed by subsoil constraints. In paddocks with subsoil constraints, successful cropping can be achieved by: • • • • • • maximising fallow efficiency with short fallows; effective weed control; suitable rotations for disease minimisation; matching nutrients to realistic yield expectations; appropriate species and cultivar selection; and timely crop sowing. Farmers discuss good agronomic management Further reading 1. Subsoil constraints to crop production in northeastern Australia: A reference manual. 2004. Brisbane: Department of Primary Industries & Fisheries 2. Subsoil constraints to crop production: Impact, diagnosis and management options. 2004. Brisbane: Department of Primary Industries & Fisheries (Crop Note) 3. Cereal diseases: The ute guide. 1999. Canberra: Grains Research & Development Corporation & TOPCROP Australia (GRDC006) 4. Living soils – better soil. 1999. Canberra: Grains Research & Development Corporation. (GRDC070) (for information about beneficial soil organisms) 5. Winter cereal nutrition: The ute guide. 2001. Canberra: Grains Research & Development Corporation & TOPCROP Australia (GRDC004) (for information about nutrient deficiency symptoms) Queensland New South Wales Department of Natural Resources & Water Call Centre Ph. 13 13 04 Email [email protected] Department of Primary Industries – Tamworth Ph. 02 6763 1100 Department of Primary Industries & Fisheries Call Centre Ph. 13 25 23 Email [email protected] © The State of Queensland (Department of Natural Resources and Water) 2007 Acknowledgements: Farmers and advisors from northern grains region Disclaimer: This leaflet is for general information only and does not cover all circumstances. Seek advice and read widely before making decisions or taking action. #scu18876 - 6/2/07 More information