Recently, CSBP soil test results highlighted the decline of potassium (K) in many soils across most ag zones in WA. Growers are advised to start or continue to monitor soil K levels and to plant test during the season.

While certain crops like oaten hay remove large amounts of K (10 - 15 kg/tonne of hay) and can, therefore, quickly deplete soil K, other grain crops can also remove a relatively large amount of K if the yields have been high (e.g. a 6 t/ha barley yield can remove 27 kg K/ha).

Over time, the removal from higher yields can add up, and yields have been above average in most parts of the state over the last few years. In addition, K may be lost in leaching events on deep sandy soils in the south and some K can be lost from burning stubble.

The K rundown may not be identified from a 0 - 10 cm soil sample because crops deplete subsoil reserves. The growing crop root system accesses deeper K. This is moved through the plants and a large proportion is then returned to the topsoil via senescent leaves that fall to the ground, or stubble that is left behind at harvest. Both K movement from the subsoil to the surface, and fertiliser K placement onto the topsoil, have contributed to K stratification on many no-till farms.

Figure 1 shows significantly higher K levels in the top 10 cm compared to 10 - 30 cm depth. Unless deeper soil sampling or plant sampling is undertaken, this K deficiency can fly under the radar. By the time K deficiencies start to impact yields, and symptoms such as windrow effects are visible, the soils will need capital K applications to rebuild soil K stocks.

Sub and top soil ag zone maps

Figure 1.

A topsoil map (0–10 cm; left) and subsoil map (10–30 cm; right) of all ag zones in WA, generated from CSBP soil test data taken in 2016–2021.

CSBP has monitored soil nutrient trends over time and some ag zones, like Ag Zone 4 (the Eastern Wheatbelt), show a rapid K decline while phosphorus (P) levels are slightly building up in the soil (Figure 2). The continuous rate of K decline in the topsoil could limit crop production in the next five years when considering that K reserves are much lower in the subsoil. Soil nutrient trends over time provide a good understanding of where deficiencies might occur in the future. We can use this information to catch deficiencies before yield is severely penalised. Monitoring both subsoil and topsoil nutrients is essential for good nutrition recommendations.

Speak with your local CSBP account manager to find out about the trends for pH, organic carbon, P and K in your area. If you haven’t taken any subsoil samples recently, then a plant sample is a good alternative. If there isn’t enough nutrition in the topsoil to compensate for a nutrient-depleted subsoil, then this will show up as a nutrient deficiency in the plant test. This is often the case for K.

Figure 2 Median P and K soil levels

Figure 2.

Median P and K soil levels over the last 25 years for top and subsoils in Ag Zone 4 (Eastern Wheatbelt). 

Ask your local CSBP account manager to help plan your NUlogic Plant Analysis to better understand your K requirements and meet your target yield.

  

 

Andreas Neuhaus
By Andreas Neuhaus
- Agronomist (Data Analysis & Modelling)

Andreas joined CSBP in 2008 and brings 30 years’ experience in technical and scientific agricultural research and development to the role. Working closely with the research and agronomy teams his data analysis and modelling expertise has been an integral part of the development of NuLogic.

Andreas is actively involved in national and regional agronomy research projects, CSBP innovation projects and represents CSBP as a subject matter expert at industry conferences as well as writing technical articles for scientific publications.

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