Potassium (K) is one of 17 essential nutrients required for plant growth and reproduction. It is classified as a macronutrient, as are nitrogen (N) and phosphorus (P). Potassium is essential in nearly all processes needed to sustain plant growth and reproduction.

Plants deficient in potassium are less resistant to drought, excess water, and high and low temperatures. They are also less resistant to pests, diseases and nematode attacks. Because potassium improves the overall health of growing plants and helps them fight against disease, it is known as the "quality" nutrient. Potassium affects quality factors such as size, shape, color and vigor of the seed or grain.

Potassium increases crop yields because it:

• Increases root growth and improves drought tolerance
• Builds cellulose and reduces lodging
• Activates at least 60 enzymes involved in growth
• Aids in photosynthesis and food formation
• Helps translocate sugars and starches
• Produces grains rich in starch
• Increases protein content of plants
• Helps retard the spread of crop diseases and nematodes

Potassium Deficiency Symptoms

• Potassium is a highly mobile element in the plant and is translocated from the older to younger tissue. Consequently, potassium deficiency symptoms usually occur first on the lower leaves of the plant, and progress toward the top as the severity of the deficiency increases. One of the most common signs of potassium deficiency is the yellow scorching along the leaf margin.
• Potassium-deficient crops grow slowly and have poorly developed root systems. Stalks are weak, and lodging of cereal crops such as corn and small grain is common.

Scorching appears on outer edge of leaf while midrib remains green. There may be some yellow striping on lower leaves. Poor root development, defective nodal tissues, unfilled, chaffy ears, and stalk lodging are other symptoms in corn.

Peak potash uptake with cereals occurs around late flowering stage. Peak uptake of potash is much greater than offtake at harvest. If this peak requirement is not available, grain number and grain fill will be affected thus prejudicing both yield and quality. This is a key reason for maintaining the adequate soil reserves which can supply these peak amounts more reliably than fresh fertiliser applications. After flowering, potash is redistributed around the plant with a general reduction from leaves to grain and stem bases. Potash is also returned to the soil with the senescence and shedding of older leaves. The rate at which this potash redistribution process occurs varies widely with different seasons and affects potash levels in the straw.

To determine how much P and K to apply you first must test the soil and find out the soil P and K index. This will allow you to calculate the amounts of P and K required. The most reliable way to do this is through a soil test once every 3 to 5 years. There are four soil indexes with Index 1 soils deficit in P & K and Index 4 soils very fertile.

The P and K indices can be interpreted as follows:

• Index 1 (very low) – The soil is deficit. There will be a definite response to applied P and K.
• Index 2 (low) – Extra P and K required, over and above the quantity the crop is likely to remove.
• Index 3 (medium) – This is the optimum soil P and K index. A maintenance dressing of P and K is required to replace the nutrients removed by the crop.
• Index 4 (sufficient) – No additional P and K is required to grow a satisfactory crop.

Timing of P and K applications

The timing of P and K will depend on the soils fertility status. Index 1 soils (very low), the P and K should be combine drilled / board cast before sowing and incorporated at sowing time. This is beneficial as sufficient P and K is required in the seedbed in the early stages of development (rooting & tillering) in cereal crops. On index 2 (low) and 3 (medium) soils P and K can be applied at sowing or as a fertiliser compound (N, P, K) with the 1st N split in early springtime.

On K – fixing soils (soils that don’t release soil K and fix applied K) it is recommended to apply K close to when the crop requires it. Apply K in the springtime along with the N applications. It can be very difficult to build up soil K levels on these soils therefore apply maintenance levels to meet annual crop requirements.

P and K off takes in cereal crops

Table 1 below shows off takes of P and K in cereal crops. These values are off takes per tonne of grain yield but include the nutrient value of straw where applicable. P and K applications should take account of crop yields and nutrient removed in straw.

Table 1: P and K off takes in cereal crops (kg/ha) per tonne of grain yield

1. K rates based on: Spring barley 6.5t/ha, Spring wheat 4.5t/ha, Spring oats 6.5t/ha
2. For Wheat & barely crops; increase or decrease K rate by 10kg/ha per tonne increase or decrease in grain yield
3. For Oat crops; increase or decrease K rate by 15kg/ha per tonne increase or decrease in grain yield.

Summary

For cereal crops the timing of application will depend on crop type for example winter or spring cropping. For winter cereals on very low to low K index soils it is recommended to apply a portion of the crops K requirements and incorporate at sowing time for example ~30% of recommended rate. The remaining crop K requirements can be top dressed in early spring to coincide with N, P, K & S applications. On Index 3 soils K can be applied at time during the growing season ideally with the 1st application of fertilizers in spring time. For spring crops it is recommended to apply all crop K requirements at sowing time and incorporate into the seedbed. On index 4 soils it is recommended to omit K applications for 1 year and revert back to K index 3 requirements until the next soil sample.

For more information, contact your local Glanbia Ireland Representative, branch or glanbiaconnect.com

You can view the 2020 Spring Agronomy Update Brochure here

First Published 20 April 2020