Critical nutrient levels for canola in Western Australia

Background

Critical nutrient levels are presented here to provide some interpretation of canola soil and tissue tests for both macro nutrients and micro nutrients.

A ‘traffic light’ format has been used with red indicating greater than 25% yield loss is likely, orange indicating between 15-25% yield loss, yellow between 5-15% yield loss and green less than 5% yield loss. Canola growers can use this information to easily determine the urgency of addressing a particular nutrient issue or soil constraint.

These critical values can be used as a guide to the approximate levels of yield loss expected, for macronutrients and trace elements. They are necessarily simplified and generalised and are not intended or able to replace professional assessment and recommendations. A Quick reference guide (tables only version) is available from the right hand menu under Documents.

To view images of nutrient deficiency in canola, or view information about other constraints in canola, see DPIRD's MyCrop canola diagnostic tool.

Nutrient removal in canola seed and stubble

Harvesting seed in any rotation is exporting nutrients from the farming system so it is important that the nutrient balance in a rotation is maintained for sustainability.

Canola is a valuable crop for the farming systems in Western Australia. It is an ideal break crop for cereals, provides good weed control, lengthens cropping rotations and can be a profitable cash crop. Growers need to be aware of the nutrient concentrations in seed and stubble when planning their cropping and fertiliser program.

Plant available nutrient in the seed and stubble

Canola is considered to be very efficient at extracting nutrients from the soil.

Nutrient concentrations in canola stubble and seed vary greatly depending on season, management practices, available soil nutrient levels and variety. The ranges of the amount of nutrients commonly found in canola seed and stubble in WA crops are shown in Table 1.

Windrowing redistributes nutrients

There is little removal of canola stubble from the cropped paddock. However, important quantities of nutrients can be redistributed into windrows that would not otherwise have occurred in direct harvesting methods. The windrowing and burning of stubble causes little or no direct loss of plant-available nutrients, except for sulfur which may be lost as sulfur dioxide. After burning the nutrients are redistributed and return to the soil in the ash after burning. This is particularly true for potassium with higher soil potassium levels sometimes found under windrowed and burnt canola stubble.

Losses of nutrients, other than in the seed

To maintain the level of plant-available nutrients in the soil in successive canola cropping systems, more nutrients must be applied than is removed in canola seed. This is to compensate for losses, other than in crop products. The nature and extent of other losses vary between nutrients, soil properties and the amount and seasonal distribution or rainfall.

Loss of nitrogen

Nitrogen fertiliser is most prone to losses by biological processes and leaching in WA. Commonly the amount of nitrogen fertiliser needed to maintain soil fertility is about double the amount of nitrogen removed in seed. As there is about 50-70% efficiency in plant uptake and redistribution of nitrogen to seed, the use of nitrogen content in seed usually underestimates the amount of nitrogen fertiliser needed to be applied.

In rotations of canola with legumes, the legumes may add sufficient nitrogen to the soil for that removed in crop products.

Loss of phosphorus

Phosphorus is most prone to losses caused by chemical reactions (known as fixation, sorption, or retention) in the soil. Losses of phosphorus by fixation in WA soils are in the range of 2-10kg of phosphorus per hectare per year, with about 4kg/ha phosphorus being most common.

Growing crops with high analysis fertilisers such as DAP (di-ammonium phosphate), with a low sulfur concentration, may result in a decline of soil sulfur supply for the cropping production system. Periodic application of superphosphate is one strategy to replace the calcium and sulphur removed in crop products.

Loss of sulfur

Sulfur in the sulfate form is easily leached in sandy soils and on sandy soils in WA. As little as 15% of applied sulfur can remain in the top 50cm of soil one year after application. This can result in sulfur deficiency and is more likely to occur in higher rainfall areas or during wet seasons. Loss of sulfate through leaching is less likely on soils with higher clay content at depth (including duplex soils) than on deep sands. This is because sulfate leached from the top soil remains in the clay containing subsoil and becomes available to the crop once its roots systems develop.

Canola crops are generally seeded with a sulfur containing compound fertiliser that supplies enough phosphorus for the entire crop, but only a starter amount of nitrogen and sulfur. Additional nitrogen and sulfur are then applied post emergence.

Loss of potassium

Potassium is prone to leaching on soils with low clay concentrations and low organic matter, especially in high rainfall areas. Only on very sandy soils in the wetter parts of the grain belt are potassium losses by leaching likely to be significant. Many of our sandy soils have low reserves of plant-available potassium. However, soils of the Great Southern and some duplex soils are now found to be deficient in potassium for cropping. Deeper soil testing provides a more complete picture of potassium availability in a paddock and distribution within the soil profile.

Potassium concentration in the stubble of canola is much higher than that found in the seed. There will be a greater proportion of potassium redistributed into the windrows than for other nutrients. Consequently, the need to replace potassium removed in crop products will become increasingly common where the stubble has been windrowed and burnt. Where the stubble has been burnt, the in-between area has often been diagnosed as potassium deficient after swathed and windrowed crops. Potassium problems in wheat following canola have been noted in paddocks where the potassium level in the top 10cm of soil have been less than about 80ppm.

Loss of calcium and magnesium

Most WA soils have large reserves of plant-available magnesium and calcium, however calcium deficiency does occasionally occur in canola; presenting as malformation at flowering and pod formation (known as tipple topple). Magnesium deficiency has not yet been observed in WA canola crops.

Loss of micronutrients

The amounts of micronutrients (copper, zinc, manganese and molybdenum) removed in canola seed have a negligible effect on their supply, in plant-available form, in soils. The need for application or re-application of the micronutrients are almost entirely governed by soil processes other than product removal, such as soil acidity and fixation by soil materials.