The agricultural areas of Western Australia are dominated by sandy soils. They are characterised by low amounts of organic matter and a poor ability to retain water and nutrients. As with wheat and barley crops, oat crops grow poorly without the addition of nutrients. The major nutrients required for healthy growth are nitrogen (N), phosphorus (P), potassium (K) and sulphur (S); and the micro-nutrients copper (Cu), manganese (Mn), molybdenum (Mo) and zinc (Zn).
Oat crops, particularly oaten hay, remove significant qualities of all the major nutrients. It is, therefore, important for growers to use both soil testing and tissue testing to ensure the crop nutrient status is adequate for plant growth. Application of nutrients is required to optimise production either on an annual basis for nutrients like N and P or less frequently for the micro-nutrients like Cu and Zn.
The continued loss of nutrients from paddocks without replacement becomes particularly important when the soils are already marginal or deficient in nutrients. The continued depletion of nutrients, particularly K from soil with adequate amounts will eventually reduce soil K supply and decrease the productivity and quality of produce. Removing nutrients from the soil may also reduce the pH of the soil. As the plant material is removed from the paddock, there is a net export of alkalinity which leaves behind residual hydrogen ions i the soil to maintain electrical balance. Over time, as this process is repeated the soil becomes acidic.
The importance of N management
Nitrogen (N) is largely responsible for setting up the yield potential of the crop. Nitrogen is required for tiller development and required by plants to create protein. The N for plant growth is supplied from both the soil and from N fertiliser application. Nitrogen is taken up by the oat plant when it is in an inorganic form (as either ammonium or nitrate). In the soil over 98% of the N is in an organic form which cannot be taken up by the oat plant until it is mineralised. A large proportion of the oat plants requirement for N is supplied by the soil. Where the available N supply from the soil is inadequate for optimum yield and quality, N fertiliser is required. Soil testing helps estimate the amount of N already available in the soil. Soil type, cropping history, yield potential and the season are important factors to consider in N management decisions.
The amount of N fertiliser required to grow a grain or oat hay can be estimated from your fertiliser decision support programs. As a rule of thumb, N fertiliser at 40-80kg per hectare (N/ha) has been found ideal for most growing condititions in WA. The amount of N required will be modified by seasonal conditions and the oat variety. A dwarf varieties have a higher N requirement, it is suggested that the N application rate used be increased by about 20% above that recommended for non-dwarf varieties. Plant emergence may be reduced if urea at more than 30kg N/ha is drilled too close to the seed.
Oat hay and grain yield increases (response) to applied N depends on the soil moisture available during the season. In a dry season there is usually a poor crop response to applied N due to the reduced rate of mineralisation of granular N fertiliser and possible lack of soil available water. Depending on the crop yield potential, applying a foliar spray of N in drier years may be a better option than granular fertilisers. Poor finishes to the season also reduce crop yield irrespective of how much N is applied.
In wet seasons, leaching of N can occur, particularly in sandy soils. In leaching situations, the N requirement for oats can be delayed and/or split to reduce the N lost by leaching. To maximise hay quality any late N should be applied between tillering (Z25) and stem elongation (Z31). Applying N too late (later than Z33) causes nitrates to accumulate in the plant dry matter reducing hay quality. For grain yield, profitable responses to N application have been found up to 10 weeks after sowing. There is generally little chance of a profitable yield increase to N fertiliser occurring if the N is applied later than 10 weeks after seeding.
Increasing N supply:
- may increase hay yield
- increases hay greenness
- increases stem fibre levels (acid detergent fibre and neutral detergent fibre)
- decreases water soluble carbohydrates (WSC)
- may increase in-vitro digestibility and metabolisable energy slightly
- may sometimes lead to high nitrate N levels - unacceptable in many hay markets
- interacts with variety for fibre and WSC.
Method of N applications is also important. Split applications of N appear important, particularly for hay.
- For hay production, do not apply excessive levels of N as it may decrease hay quality by increasing stem fibre levels and decreasing water soluble carbohydrates. Varieties may differ in their response to amount and of method of applied nitrogen. Increasing N applied to the crop does increase the risk of lodging.
Interaction with seeding rates
- Increasing the seeding rate will increase oat grain and hay yields irrespective of N fertiliser levels. Higher seeding rates will increase grain screenings and reduce leaf greenness in hay. However, higher N fertiliser rates will increase yields.
Research has shown the response of oat grain yields to seeding rate is independent of the N application rates. High seeding rates and high N fertiliser rates increase screenings but no other quality parameters. Hay yield response to seeding rate was independent of the level of applied N.
Leaf greenness was the only aspect of hay quality that decreased as seeding rate and N levels increased. At low levels of N there was a larger drop in upper canopy leaf greenness as the number of plants sown increased, compared to greater amount of N applied. Stem thickness of oats decreased as seed rate increased irrespective of N fertiliser.
Interaction with potassium
- Maintaining adequate amount of N and K nutrition are necessary for optimum grain and hay yields. High rates of K resulted in better grain and hay quality
Trials have shown that both N and K are important to optimise yield and quality of oat hay and grain. When soil test K levels are low (Colwell K soil test of less than 80 milligrams per kilogram (mg/kg) the response of oat plants to fertiliser N can be affected by K deficiency. To optimise the response to fertiliser N, adequate K fertliser has to be applied.
Results suggested that both oat hay and grain yields were governed mainly by applied N but required at least 70kg K/ha to achieve their optimum levels.
Whilst N and K interact to influence hay yield, they do not interact to influence hay quality. On K deficient soils, increasing K (regardless of N supply) reduces NDF and crude protein and increases WSC of the hay.
Grain yield increased as combined N and K fertiliser rates increased. The relationship suggests that it would not be economical to add K without an adequate amount of N fertiliser.
As with grain yield, N and K can also interact to influence grain quality. Grain quality is also affected by combined N and K fertlisers. Under low N supply, there is little benefit of K, but with high N supply, a lack of K can affect quality.
N deficiency symptoms
Nitrogen deficiency symptoms of oats appear in the early growth stages and become more severe as the plant grows. When the crop is young, stems are short and thin; leaves and stems are pale green. At flowering, N deficient plants are stunted, have fewer tillers and smaller heads than N adequate plants. At maturity the crop is multi-coloured with upper leaves pale green and middle leaves yellow to pale green with red tips. The oldest leaves my have died, turned brown and fallen to the soils surface. Grain yield is reduced primarily through a reduction in kernels per head and head density.
- Phosphorous (P) is important for oat production. Addition of P fertiliser can increase both hay and grain yield, depending on the soil test P. The optimum P requirement for hay and grain appear to be different. Oat varieties may differ in their P requirements.
Phosphorus is a major nutrient for improved oaten hay and grain production. Phosphorus is a vital component of adenosine triphosphate (ATP), the 'energy unit' of plants. ATP forms during photosynthesis and is used form the beginning of seedling growth through to the formation of grain and maturity. Deficiencies result in slow growth, decreased hay and grain yields, inferior quality and subsequently lost income.
It is suggested that P can be applied at crop establishment since an adequate supply is critical for rapid development. Phosphorus is needed during early growth for plant root development and elongation, so P fertilisers are drilled with the seed during sowing. An economic response is unlikely if the application is delayed for more than 10 days after sowing.
The oat crop response to P will be influenced by the level of Colwell P and the ability of the soil to retain P (phosphorous retention index - PRI). On low P fixing soils (PRI <2 millilitres per gram (mL/g), reactive Fe < 280mg/kg, phosphorous buffering index (PBI) <15), P is held very loosely, making it more available to plant roots and potentially reducing the amount of P required for maximum economic yield. On medium and high P fixing soils (PRI 2-15 and >15mL/g, reactive Fe 280-1000mg/kg and >1000mg/kg, respectively) P is held more tightly with a lesser amount available to plant roots. A better response to applied P is expected where soil Colwell tests are low. Soil testing is, therefore required before deciding what rate of P to apply.
P deficiency symptoms
Phosphorus deficiency results in poor seedling establishment and root development. The deficiency symptoms usually only occur if the deficiency is severe and are more noticeable in young plants as they have a greater relative demand for P than more mature plants. The tips of the old leaves become dark orange-yellow and this colour moves towards the base, usually along the leaf edges. The affected leaves often have green bases, orange-yellow mid-sections and bright red or purple tips and the edges of the leaves are rolled inwards. In severe deficiency, affected areas die and turn red and purple.
Potassium (K) is an important nutrient for oat production. Hay crops remove large amounts of K. Potassium is required for photosynthesis, transport of sugars, enzyme activation and controlling water balance within plant cells. Deficiency of K results in poor root growth, restricted leaf development, few grains per head and smaller grain size which affects yield and quality.
Potassium deficiency is more common on lighter textured soils where there is less clay and organic matter to retain the K in the root zone. The deeper sands on coastal plains are peaty sands of the south coast are the most prevalent K deficient soils of the high rainfall zone of WA. Potassium deficiency is likely to occur if the soil has less than 80mg/kg of K in the topsoil.
Potassium deficiency can reduce the tolerance of plants to environmental stresses such as drought, frost and waterlogging, as well as pests and diseases. Potassium deficiency can reduce straw or stalk length leading to lodging problems.
Crop requirements for K change during the growing season. Potassium uptake is low when plants are small and increases during late vegetative and flowering stages. Research in WA has shown that oat yield response to added K depends on the soil extractable K (Colwell K) and environmental conditions. Adding K has a positive effect on quality for hay and grain where soil K levels are low to deficient.
Potassium deficiency symptoms progresses slowly and can be costly if not detected in time. Regular soil and plant analysis an nutrient budgeting can ensure that K deficiency does not occur. Muriate of potash (KCl) is the cheapest for of K. It is applied by top dressing either before seeding or up to five weeks after seeding. If K deficiency is diagnosed in the soil by Colwell extractable soil tests, applying 40-80kg K/ha as muriate of potash (90-180 kg/ha) may give an economic yield increase. Potassium at low rates can be banded below or with the seed at sowing, with sulphate of potash safer than muriate of potash. Higher amounts of K drilled with seed can decrease seedling germination, mainly due to salt effect.
Hay crops remove greater amounts of K (about 10kg K/tonne) compared to K losses in grain. The removal of nutrients in hay has to be considered when planning fertiliser requirements for following crops. Practices such as swathing of canola and concentrating and burning of windrows can have significant effects on the spatial distribution of K across the paddock. For these reasons growers should use soil test results in conjunction with plant tissue testing and visual symptoms to determine application rates for paddocks. Decision support tools relate soil test values and other soil characteristics to yield potential to five recommended K application rates.
K deficiency symptoms
Potassium is very mobile in plants. In deficient plants, K is redistributed to the new growth and the deficiency symptoms first appear in the older leaves, which turn pale green and bronze with yellow areas developing in the mid-section of the leaf between the edge and mid-vein. These areas quickly extend towards the leaf tip until the top two-thirds of the leaf is bronze-yellow. Grey-brown spots develop within the bronze-yellow ares. Typically, the deficient plant develops a three tone appearance with green younger leaves, green with yellow to bronze colours on the middle leaves and brown older leaves.
Sulphur has an important role in the formation of proteins an dis essential for the production of chlorophyll. Crops that have a high N requirement must have adequate S to optimise N utilisation and protein synthesis.
Sulphur deficiency in oat crops is rare in WA, mainly because of the widespread use of superphosphate (11% S). As with N and P, most of the S in the soil is in organic form. Soils with low amounts of organic matter are prone to S deficiency. Sulphur in organic matter must be mineralised to sulphate before being taken up by roots. Sulphate is mobile in soils and can be leached out of the rooting zone during winter. Deficiencies therefore most often occur in wetter years. On duplex soils, deficiency symptoms may be only temporary as roots grow into the deeper soil layers where more S is available.
Sulphur deficiency is expected to increase in oat crops in the future as more compound fertilisers containing lower S are used in oat production. Hay production, particularly on sandy soils is expected to increase the risk of S deficiency as hay crops remove about 1.5kg S/ha per tonne of hay.
A soil test value of less than 10mg/kg in the soil surface (0-10cm) may indicate likely S deficiency. However, S in the soil frequently increases down the soil profile, so knowledge of the distribution of S in the soil profile is required. This may involve deeper soil sampling to know the Ss uppl in the soil. Applying P as superphosphate and compound fertilisers that applies S at 5-10kg/ha can avoid S deficiency.
S deficiency symptoms
The youngest leaves of S deficient plants are pale green and then pale yellow across the whole leaf (no striping). Under severe deficiency the entire plant becomes a lemon-yellow colour with red stems.
Zinc is a component of many plant enzymes and essential for healthy plant growth and leaf formation. Oats are highly susceptible to deficient levels of Zn in the soil. After the initial recommended application, most micronutrients have a long residual value in the soil. Therefore, both tissue testing and soil testing,in conjunction, can be used to determine the need for re-application.
Plant symptoms may help to diagnose Zn deficiency, however a tissue test may be required. Zinc concentrations in the young leaves of less than 14mg/kg indicate that the plant is Zn deficient.
An initial application of 1-2kg/ha zinc oxide (75% Zn) will correct a deficiency for many years. A foliar spray of 1kg/ha zinc sulphate (23% Zn) in 50-100L of water should be applied as soon as Zn deficiency is detected to prevent grain and hay yield losses.
Zn deficiency symptoms
Zinc deficiency causes patchy growth, with plants in poor areas stunted with pale green leaves and yellow or orange-red tips. Youngest leaves usually remain green, middle and older leaves turn pale green and pale yellow areas develop between the leaf edge and mid-vein at the tip. Brown spots occur in the affected areas, increasing in size until the leaf tip dies, often turning red-brown to black.
With severe deficiency the stem remains very short and youngest leaves have difficulty emerging fully. The symptoms can be mistaken with that of barley yellow dwarf virus and severe P deficiency.
Oats can be found to be highly susceptible to Mn deficiency which can cause significant yield losses. In severe cases, the crop may die entirely.
Tissue tests and visual symptoms can be sued to help diagnose Mn deficiency. Mn concentrations less than 20ppm (mg/kg) in whole shoots indicate Mn deficiency. The concentrations of Mn in tissues vary for different oat varieties.
Applying manganese sulphate (25% Mn) as a foliar spray at a rate of 4kg/ha (1kg Mn/ha in 50-100 L of water) immediately symptoms appear is usually effective in correcting a Mn deficiency,, however, a repeat spray a few weeks later may be necessary.
The application of ammonium sulphate and ammonium nitrate can markedly reduce Mn deficiency symptoms. Drilling fertilisers enriched with Mn can reduce the risk of crop damage from Mn deficiency. However, even where an ammonium enriched fertiliser has been used severely deficient patches may still require a foliar Mn spray.
Mn deficiency symptoms
In oats, Mn deficiency produces a condition called 'grey speck' with occurs in patches. Oats become pale green and young leaves have spots or lesions of grey/brown necrotic tissue with orange margins. These lesions will coalesce under severe Mn deficient conditions. Plants are weak, stunted, floppy and pale green-yellow and appear water-stressed even when adequate soil moisture is available.
Close examination of the leaf may show slight interveinal chlorosis. The distinction between green veins and yellow interveinal areas is poor. Symptoms can be confused with red leather leaf, which is favoured by prevailing high humidity in high rainfall areas. Symptoms can also be mistaken for take-all.
Oats are less susceptible to copper deficiency when compared to wheat and barley, however copper is essential for growth and development. Plants need Cu to produce new cells and for pollen development (sterile pollen), and hence Cu deficiency severely effects grain yield. Deficient plants that apparently look healthy can produce shrivelled grain reducing both grain yield and quality.
Tissue tests, using the youngest emerged leaf can help diagnose Cu deficiency. Tissue tests with Cu concentrations less than 1.3mg/kg indicate the plant is severely Cu deficient. Applying 3-9 kg/ha of copper sulphate (25% Cu) with fertiliser at seeding in areas suspected to be deficient in Cu correct the deficiency. Copper fertiliser has a long residual in the soil, and a single Cu application at recommended rates can last 20-30 years. Intermittent tissue testing of youngest leaves can maintain Cu at adequate levels.
Cu deficiency symptoms
Copper deficient crops have a patchy appearance with plants in poor areas stunted, pale green and looking limp and wilted even with ample soil water. Lat tillers may develop at notes or joints above round. Young leaves turn pale green while old leaves remain green. Under conditions of severe deficiency, plants may have leaves which die back from the tip and twist into curls.
The ears of Cu deficient plants are shrunken with gaps such as 'frosted heads'. The heads of Cu deficient plants have poor seed-set from sterile pollen thus resulting in 'white heads', similar to the ear heads affected by drought, heat stress and frost.
This information is from DAFWA bulletin 4798 'Growing oats in Western Australia for hay and grain', produced by Raj Malik, Blakely Paynter, Cindy Webster and Amelia McLarty, with additional input from Ross Brennan.