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.