Sustainable farming

FAO defines conservation agriculture by three principles, each with a limit you can measure:

• Minimum soil disturbance: the seed and fertiliser go straight into the ground through a slot or hole, made with a pointed stick, a jab planter or a no-till drill. The disturbed strip must be under 15 cm wide or under 25 % of the cropped area, whichever is less. Strip tillage within those limits counts; ploughing the whole field does not.
• Permanent organic cover: crop residue, mulch or a living cover crop over at least 30 % of the ground, measured just after sowing. FAO grades it 30–60 %, 60–90 % and over 90 %; below 30 % is not conservation agriculture.
• Diversity: a rotation or mixture of at least three different crop species.

Covered soil takes rain in instead of shedding it, stays cooler, and does not blow or wash away. To measure the cover, stretch a cord with 100 evenly spaced knots diagonally across the rows just after sowing, and count the knots that sit over residue: that count is the percentage covered.

The costs are real. Without ploughing, weeds must be pulled or smothered, especially in the first years; residue left for cover is residue not fed to animals; and in cold, wet soils a thick mulch can delay sowing.

Do not follow a crop with itself, or with another of the same family. Tomato, potato, pepper and aubergine share their soil-borne diseases, as do cabbage, turnip and mustard: to the disease they are one crop. A rotation starves the pests and diseases that build up under one crop, and a legume in it feeds the next: clover, vetch, beans, peas and lupins fix nitrogen from the air through their root nodules, about 30–40 kg of nitrogen for every tonne of dry top growth, roots included (Peoples and others, 2009). That puts a good legume ley on the order of 50–200 kg per hectare per year.

Three things cut what the next crop gets: soil already rich in nitrate, which the legume takes up instead of fixing; harvesting the seed, which carries much of the nitrogen away; and carrying off the straw. A ley or green manure left on the field gives the most. The procedure, with the four-course rotation below, is in open ground and hold fertility.

Heat is what makes compost safe. A pile that stays warm kills the pathogens in manure and waste; a cool one only rots them.

• Mix about 25 to 40 parts carbon to one of nitrogen (the USDA NRCS composting standard); Cornell puts the ideal near 30:1. Dry, brown, woody material is carbon; fresh, green, wet material and manure are nitrogen. Too little carbon and nitrogen is lost as ammonia, and it smells; too much and the pile stays cool.
• Keep it damp, like a wrung-out sponge, and turn it to let air in.
• Hold it at 55 °C or more for at least 15 days, turning at least five times, in a windrow; a closed bin or aerated static pile needs 3 days at 55 °C. That is the pathogen-reduction standard for manure compost in the US organic rule (7 CFR 205.203) and for sewage sludge (40 CFR 503). Other countries set their own, and where a local rule exists it governs. Measure it with a thermometer pushed into the middle.

Compost made from human waste follows the sanitation guide and its longer rules.

Test first: texture, pH and the leaf signs of a shortage are in the soils guide. Then work to the 4Rs of the Global 4R Nutrient Stewardship Framework (International Fertilizer Industry Association, 2009), which the USDA's nutrient-management standard also follows. Nutrient the crop does not take up is not banked for next year: nitrogen leaches or goes to the air, and phosphorus leaves with eroded soil.

The FAO/WHO Code of Conduct defines integrated pest management as using every method available so that pesticides are kept to what is economically justified, and the healthy crop comes first. In practice it is a ladder, climbed only as far as needed:

1. Prevent: rotation, clean seed, resistant varieties, sowing at the right time, healthy soil.
2. Watch: scout on a fixed round, at least weekly while the crop grows and more often in the stages when a pest strikes. Count what you find on a fixed number of plants, and act only when the count passes the action threshold, the level at which the damage would cost more than acting. Thresholds are set per crop and pest; a local extension service publishes them.
3. Use nature and your hands: pick off, trap, and protect the insects and birds that eat pests. Push-pull, below, is the best-proven case.
4. Treat last, with the least hazardous product, on the patch that needs it rather than the whole field.

Push-pull, developed by icipe: Desmodium drilled between the maize rows repels stemborer moths and makes the parasitic Striga weed germinate and die; three rows of Napier or Brachiaria grass round the plot attract the moths away. Farmers using it harvested about 4 t/ha of maize against 1.5 t/ha without it, and fall armyworm damage fell by more than 80 %.

Weeds climb the same ladder, and without ploughing they are the main extra work. Prevent: clean seed, and no weed allowed to set seed. Smother: mulch, a cover crop, close spacing. Remove early: what counts is the critical period, the stretch when weeds cost yield. In maize it ended, on average, at the 14-leaf stage, and began as early as the 3-leaf stage (Hall and others, 1992). Hoe or pull through that window; weeds that come up after it cost little yield, but pull any that are about to seed. Herbicide comes last, under the same rules as any pesticide.

A pesticide is a poison, and the person spraying it takes the first dose. Use only a product registered in your country for that crop and that pest, at the rate on its label. WHO classifies every active ingredient by its acute hazard, the dose that kills half of a group of rats. The class says nothing about cancer, birth defects, or harm to water, fish and bees; those are judged separately:

How much of the water you apply reaches the roots depends on the method. FAO gives indicative field application efficiencies. Layout, soil and upkeep move them a long way, so measure your own: water delivered against the crop's need.

Trees on farmland can shelter crops, feed animals, fix nitrogen and hold soil, and they yield timber, fruit and fuel on the same ground.

• Windbreaks: a row of trees slows the wind for up to about 30 times its height downwind, and stops soil blowing for about 10–20 times (Utah State University Extension). For crops it works best at about 40–60 % density; below about 30 % it does little (USDA National Agroforestry Center). Put it across the prevailing wind. These are design rules of thumb: species, gaps and wind direction shift them.
• Alley cropping: crops or pasture grown in the alleys between rows of trees.
• Living fences and fodder trees along boundaries.

Trees compete with crops for light and water close to their rows, so leave the alleys wide enough for the crop and prune roots or branches where they encroach.

A practice is working when these numbers improve, or hold, year on year. Write them down every season, field by field; the oldest field shows a decline first.