A tissue test is a laboratory analysis of plant tissue, usually recently matured leaves, that reports the concentration of each nutrient: macronutrients as percent of dry weight and micronutrients as mg/kg (ppm) of dry weight. Results are compared against sufficiency ranges for the crop.
In practice
Sample the youngest fully expanded leaves (not the newest growth, not old fan leaves), from several plants of the same cultivar and stage, and send them clean and dry. Ranges depend on sampling position and stage, so follow the lab's or extension's protocol. NC State Extension publishes leaf-tissue reference ranges for floral hemp (Suchoff and colleagues), which are a useful starting point for cannabis. A tissue test shows what the plant took up; a solution or runoff test shows what was offered. Use both.
Don't confuse with
- Runoff test: measures EC and pH of drainage, not plant content.
- Water report: analyses the source water.
- Soil test: measures what is in the soil, not what the plant absorbed.
A macronutrient is an essential element plants need in large amounts, measured in tens to hundreds of mg/L: nitrogen, phosphorus and potassium, plus calcium, magnesium and sulfur.The weight of cannabis after drying (and usually trimming), the basis growers use for yield; it is typically about 20-25% of trimmed wet weight.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.A large palmate cannabis leaf with several long leaflets on a long petiole; fan leaves do most of the plant's photosynthesis and carry few trichomes.Hemp is cannabis grown for fibre, seed or other uses and, in many laws, defined as Cannabis sativa with no more than 0.3% THC by dry weight.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Nutrient toxicity is damage from an excess of an element in the plant, either directly or by blocking uptake of others; boron, manganese, copper and chloride are common culprits.Ppm of an element means milligrams of that element per litre of solution (mg/L), the unit nutrient recipes use; it is not the ppm shown on an EC meter.
