Over the last 20 years, in response to both customer pressure and government policy, the horticulture industry has invested heavily in finding ways to source professional growing media responsibly and more sustainably while reducing peat usage. A huge range of materials considered as 'peat substitute' ingredients has been whittled down to just four: bark, coir, green compost and wood fibre. In the ornamentals sector, many nurseries use combinations of these to cut back the peat content of their growing media by between 10 and 50 per cent, depending on the crop. AHDB, along with Defra and the growing media industry, continues to fund research aimed at giving growers more confidence that specific blends of ingredients can be used commercially with predictable results.
A horticultural fellowship project, CP 095, was established in 2012, in which researcher Gracie Barrett reviewed the chemical, physical and biological properties of growing media materials and undertook trials investigating how nutrient management using organic and inorganic fertilisers was affected by varying the proportions of ingredients in media designed for nursery stock. Her work dovetailed neatly with CP 138, a five-year project funded by Defra, AHDB and the horticulture industry.
The project was commissioned in response to the Sustainable Growing Media Task Force's report in 2012 that highlighted the need for research to demonstrate the technical and commercial viability of new growing media blends and help growers overcome barriers to using them. It takes a different approach from past research by creating and using a model based on the physical characteristics of the raw materials to predict their performance in blends, followed by extensive trials on crops, including vegetable transplants, bedding plants, nursery stock and strawberries.
Results so far
Barrett used her review of ingredients to create 14 peat-reduced and peat-free experimental blends. She analysed their physical, chemical and biological characteristics and, in trials at RI-IS Wisley, assessed how well they performed with viburnum and hebe. While the properties of the blends varied widely, all but one proved capable of producing both crops to a good and uniform quality in the trial, demonstrating that a wide range of media could be used commercially.
In a subsequent trial on nutrient management, using only viburnum, the amount of nutrient leached from the different blends varied significantly, with up to five times more being lost from some compared with others - most of this occurred within the first 14 days after potting. The amount leached was not predictable and could not be related to any one physical property, such as water-holding capacity. Some of the blends proved to be 'over-fertilised' due to the inherent nutrient content of the materials, reinforcing the need to modify fertiliser rates when using them.
A trial looking at a possible alternative source of phosphorus tested biochar infused with phosphate from sewage sludge. Barrett found no significant differences, either beneficial or detrimental, between this and conventional phosphate fertiliser in the five growing media blends it was trialled with.
In the early stages of CP 138, air-filled porosity, available water and bulk density were identified as the three key physical parameters governing growing media performance. Accurate procedures to measure these attributes in growing media materials were then developed. The measurements were used as the basis for a model that could predict the materials' performance in varying proportions in blends.
As part of CP 138, specific trials on soft fruit crops have been running since 2016 to evaluate alternative substrates to the standard coir that is used for both fruit production and soft fruit propagation. Strawberry production was assessed on a commercial farm in Staffordshire where coir was compared to coir-reduced and coir-free products. Root development in all substrates was excellent, with no observable differences in root health. Fruit yields were variable over two years and across different substrates but the evidence suggested that similar yields could be achieved without using 100 per cent coir. Similar trials have been done on a soft fruit propagation nursery in Berkshire on both strawberry and raspberry. In both cases, the growth of plants and their subsequent yield production have shown little difference between 100 per cent coir and coir-reduced substrates.