Pre-plant fertiliser for brassica crops on loams

Page last updated: Wednesday, 16 November 2022 - 9:42am

Please note: This content may be out of date and is currently under review.

Granular versus liquid

Preliminary trials have been conducted in Manjimup comparing granular and liquid fertilisers as pre-plant fertilisers for cauliflower. In one trial there was no yield or quality difference between liquid and granular fertilisers. A second trial comparing a commercial NPK blend with two liquid products found the granular treatment increased average marketable curd weight and total yield compared to either of the liquid fertilisers.

At the time, the cost of liquid fertilisers and liquid phosphorus in particular, meant there was no economic benefit to using liquid fertilisers when there was no yield advantage. This may change if the relative difference in price between liquid and granular phosphorus changes.

There may be benefits in transport, storage and handling of liquid fertilisers, which are regarded as more convenient by some farmers.

Strip incorporation

For brassicas such as cauliflower and broccoli, fertiliser placement on loam and clay soils was traditionally in two narrow bands. The bands of fertiliser are placed either side of the seedling transplant, slightly below and offset from the seedlings. An alternative method for these crops is an incorporated strip. The fertiliser is applied to the soil surface in a strip about 15 to 20cm wide and incorporated into the soil to a maximum depth of 15cm with a small rotary hoe.

The number of small rotary hoes fitted to the planting machinery depends upon the number of rows of crop to be planted by each pass of the machine. One row of cauliflower or broccoli is planted in the middle of each of the rows of the incorporated fertiliser. Existing vegetable planting machinery can be modified to fit the rotary hoes.

Placing fertiliser in narrow bands concentrates the fertiliser in a small volume of soil, so proportionately less phosphorus dissolved from the fertiliser granules is retained by the soil and the concentration of phosphorus in the soil solution is higher. More phosphorus is available to the plants as their roots grow through that volume of soil.

Incorporating fertiliser exposes more of the phosphorus to the soil components, therefore more phosphorus is retained. However, substantial crop benefits such as early and more even growth can be gained by incorporating the fertiliser, as the plants have access immediately after transplanting, as the fertiliser surrounds the transplanted root ball.

Experimental work using incorporated fertiliser led to an increase in early crop growth, average curd or head weight and marketable yield as well as fewer harvests required to remove the crop. The same results have been observed by commercial cauliflower and broccoli producers. Commercial producers have also reported there is less chance of ‘fertiliser burn’ of the roots of transplanted seedlings when incorporating the fertiliser.

Fertiliser burn of the roots may occur when fertiliser is banded, as the seedlings can be transplanted directly on top of the fertiliser band if there is a slight movement sideways of the transplanting machinery. When fertiliser is strip incorporated it is less concentrated and mixed evenly around the plant roots. Fertiliser burn can hold back early crop growth.

When using the strip incorporation application plant growth occurs soon after transplanting. The plant roots are surrounded by the fertiliser mixed through the soil, giving immediate access to phosphorus and other nutrients. For banded fertiliser, the plant roots first have to grow to the band to intercept the nutrients and this delay in the plant obtaining adequate nutrients can reduce early and subsequent growth. The early growth advantage when using incorporated fertiliser is carried through the crop life.

When incorporating fertiliser it should contain a high level of water-soluble phosphorus. The incorporation method can be successfully used with fertiliser that has low water-soluble phosphorus content, if the background level of phosphorus in the soil is high (that is, the amount of phosphorus already present in the soil is high from application of fertiliser in previous years). This is often the case if vegetables are grown after pasture and the pasture has been topdressed regularly with a fertiliser containing phosphorus.

Trials of liquid pre-plant fertiliser comparing banded and strip incorporated application methods also found the strip incorporation method gave growth and yield advantages.

This method has been used commercially for lettuce crops.


Sections of this webpage are based on a farmnote authored by Rachel Lancaster and Phil Ross.