FORMULA SHELF
25 of 370
The arithmetic behind the work: what to measure, what to type in, and what the answer comes out as. Every formula says what the symbols mean and gives a worked example, so you can check it against a job you have already done.
SHELVES
Agronomy
25 formulas
The proper way to buy seed. A bag says 80% germination, so you must buy 20% more seed than the target population needs, and more again for field losses.
Target 300,000 plants/ha, peas at 0.25 g a seed, 95% germination, 90% establishment: (300,000 x 0.25) / (0.95 x 0.90) / 1,000,000 = 87.7 kg/ha.
Turn a spacing into a population, then into a seed rate. Work out what you want first, and the drill setting second.
Rows 30 cm apart with plants 10 cm apart in the row = 333,333 plants a hectare.
Every sprayer book starts with this. Get the volume right and the dose per hectare is right. Measure a nozzle with a jug, and time the pass over 100 m.
A 2 L/min nozzle at 8 km/h on a 6 m boom: (2 x 600) / (8 x 6) = 250 L/ha.
The calculation behind a fertiliser plan. Subtract what the soil and the manure already give you, then convert to the product you are actually buying.
Target 120 kg N/ha, soil and manure give 35, using urea: (120 - 35) / 0.46 = 185 kg/ha of urea.
The language of every irrigation schedule. 25 mm is a proper soaking for a root crop, 5 mm is just keeping the surface damp.
Applying 40 m3/ha is a depth of 4 mm, which is too little to reach the roots in a loam.
To know how much grain is in the barn before you sell it, and to check a silo is big enough. Measure the heap or the bin and multiply.
A 6 m diameter, 4 m deep bin holds about 113 m3, which is 85 tonnes of wheat.
For checking whether there is enough hay for the winter, and for working out how many bales to book in.
5 t/ha of dry matter at 220 kg a bale with 10% loss = 20.5 bales a hectare.
Catch the output from one nozzle for one minute to get flow. Measure your forward speed and the nozzle spacing on the boom. This tells you how many litres per hectare you are putting down, so you can mix the right amount of chemical in the tank.
1.5 L/min x 600 / (8 km/h x 0.5 m) = 225 L/ha.
Convenience only. Double-check the result against the worked example or a second method before you use it.
For converting the old imperial yields into today's tonnes a hectare, and for reading the old books on the shelf. A bushel is a volume, so the weight depends on the crop.
Wheat at 2,400 lb/acre: 2,400 / 60 = 40 bushels.
The rule that makes a heap work. Too much carbon and it sits there for a year; too little nitrogen and it goes sour and smells. See Compost Heap.
A heap of 100 kg straw at 80:1 needs about 2 kg of nitrogen, which is 4.5 kg of urea.
Grain dried too fast or too hot goes cracked, and the cracks cost you in the mill. See Storing Grain.
Wheat at 22% harvested to 15% stored, 8 t/ha: (22 - 15) x 8,000 / 100 = 560 kg of water a hectare.
Comparing a field of grass with a field of hay, and working out how much feed a field really gives. The green weight is mostly water.
A field giving 25 t/ha fresh at 20% DM = 5 t/ha of dry matter, which is about 11 round bales.
The two numbers that tell you whether the drill or the seed is at fault. A poor emergence with a good germination test means a drilling problem, not a seed problem.
Target 350 plants/m2 and you count 245: 70% emergence.
The way to work out what a crop is worth before you have even got the harvester in the field. Combine it with a per hectare count of plants and ears.
250,000 plants/ha, 1.6 ears, 60 grains, 45 g: (250,000 x 1.6 x 60 x 45) / 1,000,000 = 108 t/ha.
Whether the cover crop is a real feed or just a cover. It depends on the dry matter, not the green weight, and on how much you leave behind.
4,000 kg DM/ha at 60% grazeable, 20 cows at 10 kg DM a day: 2,400 / 200 = 12 days.
The basic scheduling method. Multiply the daily water use by the crop stage, take off the rain, and divide by how well your system actually delivers.
Full maize at 6 mm/day and Kc 1.0 over 10 days, no rain, 80% efficiency: (60 - 0) / 0.8 = 75 mm to apply.
Lime is the one thing you cannot take back once it is spread, so use a buffer test rather than a pH test. See Soil Ph And Lime.
Buffer pH 6.5, target 6.5, 20 cm deep: no lime needed. Buffer 6.0 means about 0.4 t/ha on that soil.
Oilseed rape and grass are hungry for sulphur. Too little sulphur and the crop cannot use the nitrogen you have paid for, so the response to extra N is poor.
For 90 kg N/ha, apply 15 kg S/ha as ammonium sulphate.
The part of the manure spreadsheet that matters. A load of manure looks like a big pile of nitrogen on the yard, and only a fraction of it is available to the crop this year.
10 t/ha of manure at 0.5% N with 40% availability: (10 x 0.5 x 100 x 0.4) / 100 = 20 kg N/ha.
To check that the pump and the pipes can deliver. A 1 hectare block needing 25 mm in 4 hours needs 17 L/s.
2 ha, 20 mm, in 6 hours: (2 x 20 x 10,000) / (6 x 3,600) = 18.5 L/s.
To work out how long it takes to drill a field, how much fuel you will burn, or how much fertiliser to put on through the drill.
Rows 25 cm apart = 40,000 m of row a hectare.
Convert your target plants per square metre into kg per hectare (check the seed bag or the seed-rate table), then multiply by the area in hectares. This gives total kilograms of seed to buy. See also the Seed Saving depot for saving your own seed.
2 ha at 35 plants per m2 (about 200 kg/ha): 2 x 200 = 400 kg of seed.
Convenience only. Double-check the result against the worked example or a second method before you use it.
The setting that goes on the drill. Work it out from the seed rate, then check it over a measured 10 m and count.
60 kg/ha of maize, 7,700 seeds/kg, rows 0.75 m apart = 104 seeds a metre.
Work out the fill for the area you are about to spray, then measure the product with a proper jug. Over-application is expensive, illegal and dangerous.
2 ha at 250 L/ha is a 500 L tank. At 2 L/ha of product that is 10 L in the tank.
Before you harvest, work out where the crop is going to go, and whether the bin will hold it all.
60 tonnes of barley at 0.70 t/m3 = 86 m3, plus 15% = 99 m3 of bin space needed.
SOURCES & PROVENANCE
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This page is an AI-organised index. The label means the grouping and cross-links were compiled with AI; the information itself comes from the public-domain books, reports and manuals listed below. Open any source to read the local copy or follow the link to the original publication.