Nutrient placement critical for sorghum crops

Author: David McRae, Technical Agronomist, Incitec Pivot 

Cropping system across northwest NSW, and southern and central Queensland are generally heavily reliant on stored soil moisture.

The upper soil segment 0-10cm is often dry for a fair proportion of the growing season and this limits nutrient uptake from this soil layer. As a result, crops are forced to take up a greater percentage of their nutrients from deeper layers.

Nutrient availability generally declines with increasing soil depth, as is illustrated in Figures 1, 2 and 3.

Figure 1 shows that 38% of samples taken would likely respond to applied phosphorus (P) if planted to sorghum. If the 0-10cm soil layer dries quickly, and P uptake is reliant on the 10-30cm layer, then sorghum growth and yield would be reduced by low P availability in 82% of samples.

This trend is supported by Phosphorus (BSES) data presented in Figure 2. A large percentage of samples from the 10-30cm layer have a very low reserve of P which will limit the amount available for plant uptake.

Zinc (Zn) data presented in Figure 3 shows a similar story to P. Sorghum would likely respond to Zn applications (depending on soil pH) in 53% of 0-10cm samples or 92% of the 10-30cm samples, if the season is generally dry.

^{Figure 1: Nutrient Advantage Phosphorus (Colwell) soil analysis data from north west NSW, southern and central Queensland 2026.}

^{Figure 2: Nutrient Advantage Phosphorus (BSES) soil analysis data from north west NSW, southern and central Queensland 2026.}

^{Figure 3: Nutrient Advantage Zinc (DTPA) soil analysis data from north west NSW, southern and central Queensland 2026.}

Placement considerations

The placement or location vertically and horizontally of available nutrient is very important. Deep placement research lead by Mike Bell showed that sorghum’s response to applied P fertiliser was greater when placed 20-40cm of depth in the soil profile. Also, plant P uptake of starter P, P applied in the plant/seeding furrow was only between 2-4 kg/ha.

When deciding where to place nutrients horizontally, row spacing and the sorghum hybrid’s root angle/root architecture need to be considered. Sorghum root angles vary between 25-32%, depending on a variety’s tillering ability and stay green level (pers comm T Philp, Advanta Seeds).

Nutrient placed mid-interrow is likely to have low availability for developing sorghum crops as root angle limits the exploration of this area, particularly for wide row spacings. Even nitrogen (N) in lower rainfall season will not move sufficiently from the centre on the interrow space to allow efficient crop uptake.

In situations where deep placement of P, Zn and potassium (K) is required, width between fertiliser application rows will be dependent on the crop spacing. The opportunity to practically band deep nutrients will be limited by soil, weather and crop rotation conditions.

The goal is to enrich a sufficient volume of soil over time to allow crops to access adequate quantities of nutrient, allowing water limited yield to be achieved.

Due to the larger rates of P and K required and the proliferation of sorghum roots systems around a band of P fertiliser, P and K can be evenly distributed over time. Multi band locations will be required as nutrient is only available when soil moisture is adequate.

The low Zn application rates required can lead to poor Zn distribution if blends containing Zn are applied. Compounded fertiliser like Granulock Z® and true liquids such as EASY Liquids – FLOWPHOS 13Z, KEYPRO ZINC and ZN-SUL are more effective at distributing Zn evenly and uniformly.

Table 1 compares distribution of Zn granules per metre of row when Zn is applied with Granulock Z® or a 1% Zn blend. For example, with 100cm row spacing and a fertiliser rate of 50 kg/ha:

• Granulock Z® applies 140 granules containing Zn per metre of row.
• Zn blend 1% only applies 2.5 granules per metre of row.

^{Table 1: Number of granules containing Zn at various row spacing and fertiliser rates.}

Number of granules per metre of row	Row spacing (cm)
	75		100		150
Fertiliser Rate (kg/ha)	Granulock Z	Zn Blend 1%	Granulock Z	Zn Blend 1%	Granulock Z	Zn Blend 1%
25	53	0.9	70	1.2	105	1.9
50	105	0.9	140	2.5	210	3.7
75	158	2.8	210	3.7	315	5.6

A correctly setup liquid applicator will supply a consistent rate of Zn along the application row. Evenly and consistently distributing P and Zn along the seed furrow or in a deep band will help to maximise crop yields.

Compound granular fertilisers such as Granulock Z® and EASY Liquids products – FLOWPHOS 13Z, ZN-SUL and KEYPRO ZINC – provide fertiliser solutions that allow nutrient to be well distributed and correctly positioned (Table 2).

^{Table 2: Analysis of zinc products.}

Product	Nitrogen (N)	Phosphorus (P)	Potassium (K)	Zinc (Zn)
EASY LIQUIDS FLOWPHOS 13Z	9% w/v	13.5% w/v	1% w/v	0.9% w/v
EASY LIQUIDS KEYPRO ZINC			9.2% w/v	9% w/v
EASY LIQUIDS ZN-SUL				16% w/v
GRANULOCK Z	11.6% w/w	21.8% w/w		1% w/w

Summary

In cropping systems that generally rely heavily on stored soil moisture to grow sorghum, simply applying nutrients is not enough.

Nutrients must be strategically placed where roots can access them throughout the season, particularly deeper in the soil profile.

P and Zn deficiencies become much more significant below the surface, making strategic deep placement and uniform nutrient distribution essential for achieving water-limited yield potential.

Compound granular fertilisers such as Granulock Z® and EASY Liquids products – FLOWPHOS 13Z, ZN-SUL and KEYPRO ZINC – provide fertiliser solutions that allow nutrient to be well distributed and correctly positioned.

Further information 

For further information, contact David McRae on 0477 987 321 or by email: david.mcrae@incitecpivot.com.au