Potassium
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.