Growing media for container herbs Susie Holmes, Susie Holmes Consulting Ltd (Earthcare Technical Associate)
Challenges for herb growers Quality must be very consistent, all year round. Shelf-life is an issue. Crop is edible so microbiological safety of product is paramount. Retailers (and government) are pushing for peat-reduced/peat-free production. Soil Association organic herbs must have growing media with at least 51% of the ingredients from organic production systems. Many nurseries grow a range of herbs with slightly differing irrigation and nutritional requirements (e.g. Basil needs good N supply, Parsley & Chives sensitive to high E.C.)
Healthy growing media = healthy herbs Consider factors: PHYSICAL structure, aeration, drainage, capillarity, wettability. CHEMICAL ph, Electrical Conductivity, nutrition, Cation Exchange Capacity. BIOLOGICAL nutrient availability, disease suppression. 21/11/2014 FACTS 2013 3
Growing media ingredients Peat Bark Manufactured woodfibre Forestry residues (eg Sylvafibre) Coir Loam Green compost Perlite (inert so no influence on nutrition)
Physical properties of growing media The size and shape of particles influences the size of pores. Pore size affects air (oxygen) and water holding capacity. Large pores hold air; small pores hold water we need a balance. Particle size and total pore space are related to ingredients in the mix but can change during mixing and potting (e.g. over compaction in pot). 21/11/2014 FACTS 2013 5
Peats Young ( white ) peats Baltic, Finland Medium peats Ireland, Scotland, Older peats ( black ) peats England (e.g. Somerset), Germany
Volume percentage of solids, air and water at 10 cm suction (after Schmilewski) Frozen black peat Green compost Composted bark White peat Perlite < 2 mm Material Coir Vermiculite, coarse Wood fibre Perlite < 6 mm Rockw ool (WR) Rice Husks Polystyrene 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% % % SOLIDS % WATER % AIR
Growing media - physical properties cont. More free draining media such as coir or woodfibre based may need more little and often irrigation than peat media. Implications for shelf life of pot too. Need to maintain good capillary action for ebb and flow irrigation systems. Fine particles in peat/composts/loam can block up pores and inhibit drainage particles of less than 1mm are an issue.
Growing media chemical properties Nutrient availability is related to ph of the substrate, therefore maintaining the ph in the correct range is key. Some substrates hold onto nutrients better than others Cation Exchange Capacity is important. Liquid feeding allows nutrient supply to be tailored to crop demand (which depends on stage of growth) but relies on good reading of the crop and monitoring.
Effect of ph on nutrient availability 21/11/2014 FACTS 2013 10
Cation exchange capacity CEC = the ability of the growing medium to hold and release cations (positively charged ions) eg K+, Mg+ Bark/green compost have higher CEC than peat. Higher CEC is good buffer against high or low nutrient levels esp. for young plants/salt sensitive spp. 11
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Peat Advantages: Low ph and nutrient status Generally stable Disadvantages: Little biological activity Relatively low buffering (CEC) Retailers don t consider to be sustainable
Peat alternatives Barks/woodfibres - need to balance N nutrition to counteract N lock-up. Can be attractive to sciarid fly but drier surface will discourage algae/shore fly problems. Coir naturally high K level initially, higher ph than peat (Ca/Mg supply?), good wettability, must be from reputable source. Green composts tend to have high ph, Electrical Conductivity (salts) and K level but good buffering and slow release of nutrients. Only v high quality compost is suitable (free from herbicide residues/contamination).
Calcium Important in cell wall formation. Demand for calcium is continuous, via the transpiration stream (xylem). Even an interruption for 15-30 minutes will have an impact. Once incorporated in plant tissue it is fixed so deficiency seen in youngest leaves. Deficiency causes stubbing of leaf tip ( tipburn ) and loss of root hairs. Deficiency can be induced by poor ventilation/high humidity issue for herbs grown in winter months in particular.
Ca deficiency leaf tip scorch
Ca deficiency induced by high ammonium level/high EC
Nutrient supply in growing media Calcium and magnesium slow release from lime (but extra may be needed with soft water) Nutrients for first few weeks mostly from base fertiliser. Later nutrients from liquid feed. Too much fertiliser = high conductivity = root damage = root disease. Too little fertiliser = poor growth and colour of crop.
Hard Water Derived from chalk or limestone. Calcium and magnesium salts Temporary hardness is what growers are concerned with = bicarbonates Calcium bicarbonate (= alkalinity on NRM analysis report) Ca(HCO 3 ) 2 > 200 mg/l problems with ph rise in the substrate and deposits in irrigation lines may occur. >300 mg/l problems likely. 19
Soft Water Derived from igneous rocks e.g. granite Rainwater is soft water. Low ph no buffering and ph of substrate may fall in use. Low in nutrients e.g. calcium so feeds containing Ca needed. 20
Monitoring nutrition Observe plants! Growing media analysis. Feed EC check and analysis. Leaf analysis (useful for suspected trace elements problems). Hand held EC meters can be useful if calibrated and checked against lab readings. 21
Growing media analysis ph (target 5.5 6.0 for most spp., ph at point of manufacture of peat based medium will be lower takes time for lime to work). Electrical Conductivity (ideally 100-400µS depending on crop and stage) Ammonium-N no higher than 100 mg/l N,P, K, Mg Index 1-4 Trace elements growing media analysis no use leaf analysis needed.
Leaf analysis Good or diagnosing trace element problems (except Fe). Need standards to compare results against (only available for some spp.) Take youngest fully expanded leaves for sample. 23
Growing media specifications Discuss your requirements, crops, growing systems with the growing media supplier. Have clear written specs for recipe, ph, E.C. and fertiliser rate for each mix. Inspect each load for consistency and keep unused sample in cold store until end of crop. Good communications with supplier important advise them asap if there is a problem. The cheapest mix is rarely the best! The growing media can have a big impact on crop quality. 24