The potential for using crop stubbles as amendments for subsoil manuring

 

Field crops such as wheat (Triticum aestivum) and canola (Brassica napus) that are grown in the high rainfall zone of southern Australia (500-900 mm mean annual rainfall) generally yield less than their rain-limited potential yields. This is because of environmental constraints such as the widespread occurrence of dense clay subsoils. The dense subsoil particles are closely packed such that there is practically no pore space for air or water to infiltrate into the subsoil. This restricts crop yields, particularly in dry springs, by restricting the roots’ deep penetration into the subsoil to extract moisture from the deep layers.

 

Different approaches have been used to address these subsoil constraints. These include gypsum applications and deep ripping, but results were disappointing. The most promising progress was made when researchers incorporated high rates of organic amendments at the top of the clay subsoil. In 2005, the highest wheat yields, in excess of 12 t/ ha, were achieved by the deep incorporation of 20 t/ha of organic amendments at a depth of 30-40 cm in the clay subsoil. This technology has been given the term ‘subsoil manuring’ by Gill et al., (2008). Not surprisingly the practice is expensive.

 

The main focus of my PhD studies at La Trobe University Bundoora campus will be to investigate ways to make subsoil manuring more cost-effective. Currently the most effective organic amendment for subsoil manuring has been to use 20t/ha (fresh weight) of poultry litter.  However, poultry litter is now in demand, and consequently its price has increased.  I will therefore be conducting a series of field, glasshouse, and incubation experiments to evaluate alternative organic materials for subsoil manuring. These will include crop stubbles, green-chop crops and perhaps animal manures, or a combination of these materials. Such materials need to be readily available on farms. In this paper I will report on my first field experiment at Bundoora in 2010.

 

 

A field experiment was established in June 2010 to test whether chopped wheat straw might be an effective organic amendment for subsoil manuring compared to poultry litter applied at 20 t/ ha fresh weight.  In addition, I investigated whether the addition of fertiliser might improve the effectiveness of the straw amendment.

 

The experiment, consisting of seven treatments with four replications was established on a duplex soil (with dense clay subsoil) on the agricultural reserve at La Trobe’s Bundoora campus. A trencher with a 20 cm wide blade made 3 trenches along the plots and chopped wheat straw at the rate of 16.4 t/ha (oven dried weight) was placed in the trenches at a depth of 20-40 cm subsoil. Other treatments were included as shown in Table 1.   Wheat (Triticum aestivum var. Beaufort^A), pretreated with fungicides, was sown in 16 cm rows in the last week of June at a seeding rate of 100 kg/ ha. Diammonium phosphate (DAP) at 100 kg/ha as a basal dose was also applied. In late September 2010, the field crop was top dressed with urea (90 kg N/ha). Initially, the crop had a very poor establishment due to a number of issues including cold and wet weather, a cloddy seedbed and variable seeding depth and this resulted in low plant density per m² . In addition my field trial experienced duck damage at the seedling stage and cockatoo damage during the grain-filling period. This latter damage affected results at the final harvest.

 

The treatments resulted in significant differences at anthesis in the total biomass and the number of tillers per m² (Table 1).

 

Treatments	Shoot biomass (t/ha)	Fertile tillers (number / m2)
Controls
Control	6.7 b	388 b
Deep ripping	6.7 b	369 b
Straw (chopped 5 cm)
Straw 16.4 t/ha (dry weight)	6.0 b	322  b
Straw 16.4 t/ha + ¼ NPK	5.9 b	278 b
Straw 16.4 t/ha + ½ NPK	7.7  b	403 b
Straw 8.2 t/ha + Poultry 8.2 t/ha	8.0 ab	414 b
Poultry Litter
Poultry litter 16.4 t/ha	10.8 a	562 a
LSD (P=0.05)	2.40	122
Probability	0.007	0.005
Significance	**	**

 

Means within columns followed by a similar letter do not differ significantly (p>0.05).

NPK fertilizer was added to supply the NPK nutrients in 4.1 tonnes and 8.2 tonnes of dry poultry litter respectively. ** indicates differences that were significant at P<0.01

 

 

The 100% poultry litter treatment produced the highest total biomass of 10.8 t/ha at anthesis. The second most productive amendment was the 50:50 treatment comprising 50% poultry litter and 50% chopped straw, as it did not differ from the 100% poultry treatment. The variability at the site meant that the 50:50 treatment also did not differ from the controls, nor did it differ from the 100% straw treatment. Furthermore the addition of NPK fertilizer did not improve the effectiveness of the straw. Similar results occurred for the number of fertile tillers per unit area.

 

The size of the flag leaves at anthesis and during mid grain-filling was also measured. These organs are important in supplying assimilate to the developing grain and so the larger the flag leaf generally the higher the grain yield. The results for the flag leaves mirrored the biomass measurements at anthesis (Table 2). The full poultry litter treatment was again superior and produced the largest flag leaves. The 50:50 poultry/straw treatment and the chopped straw treatments, either with or without fertilizer nutrients, did not produce larger flag leaves than the control treatments.  

Treatments	Flag leaf mass at anthesis (mg/leaf)	Flag leaf mass at mid-grainfill (mg/leaf)
Controls
Control	89  b c	80  b c
Deep ripping	86     c	64   c
Straw (chopped 5 cm)
Straw 16.4 t/ha	92   b c	78   b c
Straw 16.4 t/ha + ¼ NPK	113   b c	81   b c
Straw 16.4 t/ha + ½ NPK	112   b c	99 a b
Straw 8.2 t/ha + Poultry 8.2 t/ha	113 a b	93  b
Poultry Litter
Poultry litter 16.4 t/ha	117 a	121 a
LSD (P=0.05)	36	22
Probability	0.003	0.001
Significance	**	***

Means within columns followed by a similar letter do not differ significantly (p>0.05).

NPK fertilizer was added to supply the NPK nutrients in 4.1 tonnes and 8.2 tonnes of dry poultry litter respectively. ** and *** indicate differences were significant at P£0.01 or P£ 0.001 respectively.

 

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These preliminary results in my first field experiment show the superiority of poultry litter as an organic amendment compared with chopped wheat straw. The performance of the chopped straw was disappointing and did not result in any significant improvement in wheat performance.

 

The results in the wet spring of 2010 most likely reflect the high nutrient concentrations in the poultry litter. However the addition of the NPK fertilizer nutrients, equivalent to ¼ or ½ of NPK nutrients in 4.1 tonnes and 8.2 tonnes of dry poultry litter did not improve the performance of the organic amendment.

 

There will be a need for further investigations into alternative crop materials, or combinations of materials, to try to improve the effectiveness of these organic amendments for subsoil manuring.

 

 

 

I would like to acknowledge La Trobe University for providing a LTUPRS scholarship for my studies, and Dr Peter Sale (Supervisor) and Dr Caixian Tang (co-supervisor) for their kind support and encouragement.

 

 

 

Gill JS, Sale PWG, Tang C (2008) Amelioration of dense sodic subsoil using   organic amendments increases wheat yield more than using gypsum in a

high rainfall zone of southern Australia. Field Crops Research 107, 265-275.

 

 

                  Ph: (03) 9479 3098  

                  Email: sisafi@students.latrobe.edu.au