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By AJ Horrocks, CS Tregurtha, S Maley and ED Meenken. Plant and Food Research.

The results of a five year study of using municipal Compost in pasture, forage cropping, arable cropping and intensive vegetable operations show that compost is a financially viable way to boost production.

The results of the series of trials carried out in South and mid-Canterbury

Indicate that compost can enhance crop production for at least two years after a single application in arable, pastoral and forage cropping systems. The trials examined the benefits of compost on yields, soil health and financial viability.

Small regular applications of compost are the most financially viable. The most financially viable scenario according to the study is to apply small regular amounts of compost at around 8-12 tonne/ha (fresh weight) with fertiliser N offset by one third. The results suggest that applications should be made regularly (i.e. every one to two years). The second most profitable scenario in the arable rotation was a one-off application of 50 t/ha compost with no offset of N fertiliser. Larger one-off applications of 25-50 t/ha may require reapplication of the same rate every three to four years. Over a three-year cropping rotation 50 t/ha was more financially viable than 25 t/ha. In a forage rotation, one-off applications of 25,50 and 100 t/ha of compost where fertiliser N offsets were not tested over two consecutive kale crops resulted in cumulative yield increases of 12 percent, 31 percent and 45 percent respectively, with the 50 t/ha rate being the most profitable. In a forage brassica rotation, compost application rates of 12 t/ha with a 40 percent reduction in fertiliser N increased yields by 18 percent. This one-year scenario was financially viable for freight costs up to $14/tonne. The research included a forage crop trial and a pasture trial at Karina Downs, South Canterbury, an arable crop trial at Lincoln, a kale crop trial on ex-forestry land trial at Bankside, mid-Canterbury, and an intensive vegetable trial at Christchurch. The forage crop, pasture, and kale trials examined simply compared no compost plots with plots treated with 50 tonnes of compost per ha. The Lincoln cropping trials consisted of three replicates of 16 different treatments (48 plots). These included one-off applications of 0, 25, 50 t/ha and application of 8.3 and 16.7 t/ha each year for three years. Following application, the compost was incorporated in the top 10cm of soil. Four crops were grown in Lincoln trials: maize, wheat, oats/ryegrass and peas. In addition to compost rate, four rates of nitrogen fertiliser (0, 33, 67, and 100 percent of standard rate) were applied. The intensive vegetable production trial had five reps with four different treatments: 0, 14 and 28 t/ha of compost and one application of chicken manure at 10 t/ha. The cost benefit analysis calculated the standard cost of production needed to grow each crop, i.e. cultivation, drilling, fertiliser and management of weeds and pests. Other conclusions drawn from the forage cropping trial are that if applying one-off amounts of 25 to 50 t/ha compost, reapplications should be made every three to four years. Over a three-year cropping rotation 50 t/ha is more financially viable than 25 t/ha. The trials revealed some other benefits from using compost. Particulate organic matter, a short-term reservoir for plant nutrients, and available N were still elevated where compost had been applied at the end of both the arable and forage crop trials. This suggests that benefits beyond the scope of these trials are likely.

Soil organic matter and carbon content increased significantly where compost was applied. Even though soil organic matter performs a number of important functions in soil such as minimising compaction pressure, soil physical parameters such as bulk density remained largely unaffected by one-off compost applications over the timescale of these trials.

With high rates of compost, there was a trend towards improved soil structural stability and water holding capacity, suggesting further improvements with time and sustained applications are likely.

Complete substitution of fertiliser with compost is not recommended. Compost increases utilisation of available N so to get the best out of compost it needs to be applied with fertiliser N. A crops ability to respond to available N (from soil, fertiliser and compost reserves) increases where compost has been applied. The mechanisms that underpin this observation are not well understand and require further research to elucidate the key processes and critical factors.

Fertiliser treatment has a strong effect on crop N uptake, which increased with higher fertiliser rates for all crop except peas. There is evidence that the compost led to greater N uptake; particularly in the oat and grass crops.

The 50 t/ha rate of compost had the greatest effect when the full rate of fertiliser was applied. Where no compost was applied there were no relative differences between the total N taken up by all crops between the 67 and 100% fertiliser N rates (739 and 737 kg/ha respectively). However, when compost was applied total N taken up by the crop between the 67 and 100 percent fertiliser N rates went up from 760 to 811 kg/ha. Other important crop nutrients besides N (such as P, K, Mg and Ca) are provided by compost. The application of compost made by the crops better able to uptake these nutrients over time. Soils that have been cropped for a number of years or that are inherently low in nutrients such as P and K may especially benefit from using compost. The amount of nitrate leached did not increase when compost was applied. This may be because available N in the compost is in the non-leachable ammonium form. Higher application of fertiliser N create more nitrate leaching so it may be possible to reduce total N leaching if compost applications offset fertiliser use. Results from the laboratory incubation study indicated that there was more microbial activity in the presence of compost and a high starting point for mineral N, suggesting short term benefits to crops. The rate of net accumulation of N (mineralisation) over the 89 days of the incubation, however, did not differ from the plots without compost as most of the C and N from the compost was in resilient pools that were slow to decompose, suggesting the nutrients were released slowly overtime. Another conclusion of the research is that compost helped improve soil structure and water holding capacity. These were only apparent with high rates of compost, however, and it is possible that soil structural improvements may require sustained applications of compost over time. For more information on using compost in a farming operation, contact Dr. Michael Quintern, Ecocast Ltd.

Ph 027 544 0042.

FIGURE 1: EFFECTS OF COMPOST TREATMENT AND N

(SOIL PROFILE MINERAL N TO 1.5m DEPTH, PLUS FERTILISER

N) ON SILAGE-MAIZE DRY MATTER YIELD (T/HA) AT

THE ARABLE CROP TRIAL (2009-2010).