Don’t ignore phosphorus in 2012: base rates may not be the answer!

 

In recent years, significant crop responses to phosphorus (P) application have been rare and now it is all too easy to ignore the value of this element in continuous cropping systems. Put simply, the situation has changed and emphasis needs to be put back on P.   

Wimmera and Mallee farmers have had two reasonably high yielding years, achieved with only base rates of P (5kg/ha) applied at sowing. But it was largely thanks to the wet conditions in 2010 and 2011 which mineralised large pools of available soil nitrogen (N) and P that contributed to the higher than expected yields. Now those reserves are becoming depleted, especially for P.

Most growers are familiar with how important P is for ensuring establishment by improving root growth and early vigour but it is also understood that P it is not very mobile in the soil (this is why generally only the topsoil (0-10cm) is tested for P levels). However, unlike N, the only time to effectively apply P is with the seed at sowing so it is very important to get the rate right initially.

Also, with many farmers taking the opportunity after the wet seasons to grow pulse crops and boost soil nitrogen levels, it is also pertinent to remember that this practice will have little effect on plant available P in the soil. Although there has been some evidence to suggest that break crops can solubilise soil P reserves, the effect is likely to be minimal given that most reserves were broken down in 2010 and 2011. In addition to this, it is common practice not to apply any P at all when growing some pulse crops (particularly vetch).

Soil test results (both Colwell P and DGT[1]) from samples taken between February and April this year suggest most paddocks would now be very responsive to P as levels are extremely low. Conversely, N levels are surprisingly higher than expected, which could easily be attributed to the late February rain and subsequent mineralisation.

According to BCG and University of Adelaide data, the number of paddocks responsive to P has doubled from 30 per cent in 2008 to 60 per cent in 2012 (Table 1).   

Table 1: P-status of paddocks in the Wimmera Mallee (data based on DGT values).

P-status	% of paddocks sampled	Change
2008	2009	2010	2011	2012
Very responsive	9	6	4	12	10	↑
Responsive	21	19	48	39	49	↑
Marginal	7	16	15	17	12	↑
Adequate	33	32	30	27	24	↓
Non-responsive	30	25	4	5	6	↓

What sort of responses can be expected?

In 2011, BCG sowed three replicated trials in paddocks north of Birchip to assess the impact of a range of P fertiliser rates on crop yield and economic returns, and to evaluate the DGT soil test as a measurement for soil available P.

The three paddocks, known as Jil Jil 1, Jil Jil 2 and Jil Jil 3, were chosen because they had different levels of soil P, as indicated by their Colwell P and DGT values (see table below).

Table 2: Soil test results (0-10cm) from each paddock, taken in April 2011.

	Crop type	Colwell P (mg/kg)	PBI	Prediction	DGT (µg/L)	Prediction
1	Correll wheat	23	160	Responsive	18	Responsive
2	Hindmarsh Barley	30	107	Not Responsive	52	Marginal
3	Hindmarsh Barley	30	65	Not Responsive	20	Responsive

The trial was sown with knife points and press wheels and knife points on June 30, 2011. Further details about the methodology used for this trial is available in the BCG 2011Season Research Results handbook (p. 95).

Visual responses were evident in paddocks Jil Jil 1 and Jil Jil 2 throughout the season which carried through to yield (Table 3). The biggest response was seen at Jil Jil 1, where the 75kg/ha of MAP application resulted in a 2t/ha yield increase. The 75kg/ha of MAP treatment saw an additional 16kg/ha of P, as well as an extra 7.5kg/ha of N applied to the crop and as such, the yield response cannot entirely be attributed to the extra P. However, it is thought that an increase of this magnitude is more likely to be a response to the extra N than the extra P.

Table 3: Grain yield results for each paddock.

Fertiliser Application	Grain Yield (t/ha)
MAP rate (kg/ha)	P        (kg/ha)	Jil Jil 1	Jil Jil 2	Jil Jil 3
0	0	2.6	2.1	1.0
25	5.5	3.4	2.9	1.0
50	11	4.2	2.6	1.2
75	16.5	4.5	3.1	1.0
Sig. Difference LSD (P<0.05) CV %	P<0.001 0.5 5.9	NS	NS

Note: 20kg/ha Urea was applied to all treatments to negate the increase in N applied with higher rates of MAP.

In-paddock application

Given that most farmers use MAP (mono-ammonium phosphate) as their main starter fertiliser, this data can be used to determine what rate of P should be employed. 

Importantly, the trial re-confirmed the value of soil sampling. Phosphorous rates should only be brought back to maintenance levels if the soils have been tested and the grower is confident levels are adequate. Decisions about what fertilisers and rates to use at sowing can significantly impact the crop’s outcome and while N can be topped up during the growing season, this is not the case for P. Phosphorous levels cannot corrected in crop. 

However, in paddocks with an unknown soil status which yielded more than 2t/ha of cereal or 1t/ha of canola last season, a minimum of 35-40kg/ha MAP is recommended.