Bran as a peat replacer - a feasibility study

Abstract

Introduction

To investigate the products of composting bran and other feedstocks for use in horticulture as a soil improver or as a peat substitute in growing media.

Results

Composting

Bran is a good material for composting in that it contains both protein, rich in nitrogen, and carbohydrates. The carbon to nitrogen (C:N) ratio is low, approximately 15:1, and therefore other carbon rich, but nitrogen poor, materials could be co-composted with bran. This would raise the initial C:N ratio to, ideally, between 30:1 and 35:1 and lead to a more balanced and stable end product, plus revenues for accepting such wastes (for example, paper or wood wastes), but result in a greater volume of end product to sell.

Composting the bran alone leads to a more concentrated nutrient end product which could be used as an organic fertiliser base.

Water is required to wet up the bran for composting (approximately an equal weight to the bran to bring it to 60% moisture content) . The water could be replaced with wash water from other processing activities, such as starch production, thereby aiding a disposal problem

End products

The end product of composting bran on its own, as was tested at Levington Agriculture, is a dark-coloured, nutrient-rich material with good water retention and fibre content. Dry matter is lost during composting and may lead to a 50% reduction in mass, although this has not been accurately determined.

One sample was leached of much of its mineral nutrients which had become concentrated during the composting process. Excessive nutrients are adverse when the material is to be used directly as a growing medium. However, the leached material, after further stabilisation, could be tested as a material for tomato growing bags. At 3 kg of dry matter per bag, 50% efficiency of use of raw bran and £0.50 per bag for the peat, the value of the bran is £83 per tonne with a maximum use of 60,000 tonnes of raw bran in UK. The value may be higher if the nutrient content is taken into account.

More extensive washing would enable the material to be tested as a potting compost of higher value. The washings could be re-used to wet the bran in batches that were to be used as organic fertiliser.

The composted bran was mixed with commercially available peat, coir and woodfibre in order to lower the pH and dilute the nutrient content. The rate of mixing was 20% composted bran to 80% of the other material in each case. The bran/peat mixture resulted in a medium suitable for growing tomatoes as was verified by pricking out seedlings which grew well. The coir and woodfibre mixtures resulted in the pH and conductivities still being too high and a lower inclusion rate, for example, 10% may be needed.

Market size

The maximum market size for use as a peat replacer at 10% in mixes providing fibre, nutrients and liming effect is 30,000 tonnes of composted bran in UK, or 60,000 tonnes of raw bran at 50% efficiency. The price of good quality peat is £19/m3 containing 111 kg dry matter. At 50% efficiency the raw bran is therefore potentially worth about £85 per tonne. By co-composting bran with other wastes low in N and K but high in carbon, the end product would be more stable biologically. This is to be recommended where the end product is to be used in growing media. A greater inclusion rate could be used in the peat mixture, and less washing would be required where the composted materials were used predominantly on their own.

The composted bran is an organic fertiliser material having an analysis of 4:7:7 (N:P205:K20). Growmore is a standard fertiliser with analysis of 7:7:7 and a retail value of £200 per tonne when sold in small packs. The raw bran may therefore have a value of, for example, £60 per tonne (at 60% of retail price and 50% efficiency) as base fertiliser material requiring some topping up with nitrogen.

Alternatively, pelleted manures are retailing at £5 per 7 kg to the gardening market. The raw bran, at 50% efficiency and at 60% of retail price, may be worth closer to £200 per tonne. This market is relatively undeveloped at present but should be a good prospect with aggressive selling. Prices should be higher than for the product as a fertiliser due to the added properties of organic matter and liming value as a soil improver.

There is a small market for biodegradable plant pots. These are made from, for example, wood fibre with some peat. The manufacturers of such pots could be approached to test the composted bran, with and without leaching, as a replacement for the peat content.