The Waste & Resources Action Programme (WRAP) has been sponsoring scientific trials to measure the commercial and environmental benefits of quality compost for crops as diverse as cereals, top fruit and vegetables since 2005. A total of 24 trials have taken place so far, using quality compost made from garden waste, such as grass cuttings, prunings and leaves, and produced to the nationally recognised BSI PAS 100 specification.
While some of these trials have measured the benefits of short-term compost use, others have been conducted on land where compost has been applied over a period of up to 10 years.
In 2008, WRAP will be trialling the use of both green- and food-waste-derived compost products on a range of crops, including potatoes, organic field vegetables, soft and top fruit, and energy and arable crops. Another trial will also assess the benefits of using digestate (a biofertiliser by-product generated through anaerobic digestion) for barley crops in Scotland.
Some positive findings have been generated from the trials over the past few years. For example, the results have shown how compost can increase the organic matter levels in the soil - thereby improving its structure, water-holding capacity and overall health.
This was borne out in a trial that was started on Park Farm in East Anglia more than six years ago by Enviros Consulting and expanded into six more sites in 2002. Now managed by Westrope Farming, the project assessed the benefits of compost to different crops such as potatoes, sugar beet, barley, onions, wheat and carrots.
The key result from the project was an average increase of seven per cent crop yield - for example, the potato crop, growing at 50 tonnes/ha, increased to 53.5 tonnes/ha over the period of the trial.
This increase was attributed to a long-term change in the soil structure caused by the repeated application of compost. The compost added organic matter to the soil, which improved its structure and made it more workable - allowing for better seedbed conditions and supporting the retention of moisture for longer periods.
The trial also highlighted the commercial benefits that can be derived from compost in terms of the fertiliser replacement value it provides in the form of nitrogen, phosphate and potassium (NPK) but also sulphur, magnesium and trace elements.
When the trial was undertaken, the cost of using standard NPK fertiliser - compared to the cost of applying compost with a reduced nitrogen application over a five-year rotation - produced an average net gain of £116/ha per year. This was based on a farmer paying £2.50/tonne for delivered compost and £1.50/tonne for spreading.
At current fertiliser prices, the typical fertiliser value of the phosphate and potassium (P&K) alone is £7 to £8/tonne for green compost and around £10/tonne for compost containing food waste. At an application rate of 30 tonnes/ha, compost has a nutrient replacement value (for P&K only) of £225/ha for green compost and £300/ha for food-included compost.
Another recent trial has demonstrated the role of compost in protecting soil against adverse weather conditions such as those experienced last summer. The study, which was a continuation of the Park Farm trial, found that the long-term use of quality compost increased onion yields by more than a quarter in the summer of 2007, when many harvests were delayed or wiped out due to torrential rain.
Compost mulch trial
A key trial sponsored by WRAP has involved the long-term application of compost as a mulch on an apple orchard in Kent, where compost increased yields by nearly 50 per cent and significantly boosted profits.
The trial originally started on North Court Farm, near Canterbury, in 2004 when compost was first applied. It was extended in 2006/07 with a second application of compost and the results of this have just been released.
Run by the ReMaDe Kent & Medway programme - which develops markets for recycled materials, such as composts made from garden waste - the 2006/07 trial involved the application of 150 tonnes of quality compost mulch to 13 rows of Cox and Braeburn apple trees at a width of 1m and to a depth of 10cm.
The rest of the orchard, which was the control plot, had been mulched with straw in 2004 and received no further treatment of any kind.
The compost, made from garden waste, was supplied by Shelford Composting near Canterbury and is produced to the BSI PAS 100 specification. This specification, together with the Quality Protocol for Compost, provides reassurance that the product is consistent, reliable, safe and fit for purpose.
Photographs taken of the leaves of the Cox trees in June showed that those that had been mulched with compost were much darker and healthier than the untreated trees' leaves, which were curled at the edges due to lack of moisture. The dark colour of the treated leaves proved that they were getting plenty of moisture and nutrients, and that the process of photosynthesis was working at optimum levels. Improvements in blossoming and fruit set were also noted, which translated into significantly improved yields.
At harvest time, the number of apples per tree increased by 45.6 per cent and 35 per cent respectively for the Braeburn and Cox varieties compared to the control plots.
Fruit weight per tree for the Braeburn apples rose from 20.8kg (51.4 tonnes/ha) to 30.9kg per tree (76.3tonnes/ha), generating an extra profit of £12,972/ha.
For the Cox apples, fruit weight per tree increased by 49.5 per cent, taking the weight of the fruit up from 9.7kg (24 tonnes/ha) to 14.5kg (35.8 tonnes/ha). At 1,000 trees per acre - or 2,470 trees per hectare - this equated to 11,856kg/ha, generating extra fruit worth £6,876/ha.
Although these yields were taken from part of the orchard, they were consistent with typical UK orchard yields at 25.9 tonnes/ha for Cox and 45 tonnes/ha for Braeburn.
Moisture monitoring, which was conducted regularly throughout the trial, had shown that in September, the unmulched soil was already drying out, and the top 30cm of soil continued to do so over the next week. However, under the compost mulch, the soil remained relatively moist. This 0-20cm layer is the important soil horizon where the tree feeder roots are located. It is thought that the increased yields were due to the improved water retention and release of major nutrients, such as potassium, calcium, magnesium, phosphate and nitrogen, provided by the compost mulch.
Robert Balicki, who runs North Court Farm and is also chief executive of marketing organisation Worldwide Fruit, says: "Although these trials were conducted on a small scale, we were very encouraged by the results, which were evident from the start of the growing season with dark, healthy leaves and increased blossom and fruit set.
"These changes translated into impressive yield increases, which provide strong evidence of the potential benefits of compost for our industry."
A further trial involving young apple trees was conducted in 2007 on Home Farm, near Canterbury, Kent. The farm is owned by Paul Mansfield, one of the largest fruit growers in the UK, and the trees were planted in the autumn/winter of 2006. The apples were Braeburn and a patented variety of Jazz, which is not yet cropping commercially in the UK.
A total of 400 tonnes of quality BSI PAS 100 compost mulch was applied to the trial orchards with a further 800 tonnes also being used on the surrounding land. The trial orchards had not been cultivated for fruit before, and the soil varied.
The compost was applied as a mulch in approx 10cm deep, 1m wide rows. There were 4,203 trees in the plot, which covered 1.7ha.
Although 2007 was a relatively wet year, the compost still conserved moisture under the mulch layer, leading to an increase of 31 per cent in apple numbers and a 5.7 per cent increase in average fruit weight. The increase in the weight of fruit per tree was 36 per cent - 3.8kg per tree for mulched Braeburn trees compared to 2.8kg for ones without mulch.
The fruit is not usually collected commercially from a newly planted orchard until the fourth year after planting and the second year's fruit yield is borne on the branches that have grown in year one. Stem-extension measurements showed there was 24 per cent extra growth on the compost-mulched trees - compared to the untreated ones - giving a correspondingly greater yield potential.
These trials show that compost can bring significant commercial benefits to newly planted apple orchards, as well as long-established ones. Its moisture-retention properties may have given young trees a head-start and it significantly increased their yield potential. It had a similar effect on older trees, where tests showed it raised nutrient levels in soil as well.
These scientific trials provide potential users with a clearer understanding of the way in which compost works and the commercial benefits that can be derived. Given increased costs of fertiliser, this may be a good time to consider how compost can help your business's bottom line.
- For more information about compost and previous trials, or to find your nearest quality compost supplier, log on to www.wrap.org.uk/composting or phone 0808 100 2040.
- Solene Le Doze is project officer for WRAP's organics programme.