What it is
Biotilling is the use of living plants to break up soil compaction, improve aeration and drainage, and increase nutrient cycling. The biotilling plants are usually vigorous pioneer type plants ranging from annuals to trees which are often interplanted with crop plants.
Biotilling plants can be edible/medicinial, nitrogen fixers, nutrient scavengers, dynamic accumulators, and/or nematode suppressing; and should not grow when laid laid on the ground. The plants are chopped & dropped, parially girdled, or killed to stimulate root dieback, release nutrients, and limit competition with crop plants.
Biotilling can be used to develop the appropriate soil microlife for crops. Woody crops do better in a fungal dominated soil--woody biotillers continually provide the food for the fungi which release plant available nutrients. Herbaceous plants do better in a bacterially dominated soil--herbaceous biotillers provide food for the bacteria which release plant available nutrients.
Certain biotillers can increase soil pH since they contain high amounts of the Hydrogen cations Calcium, Sodium, and Maganesium. Hawai'ians traditionally pollarded Kukui trees in gulches to create a fast acting fertilizer. Kukui can be utilized in polycultures.
Biotillers can provide part shade that most young trees and shrubs are adapted to. Woody plants' bark is sensitve to the sun--even in Washington State, orchardists are encouraged to paint their tree trunks with white latex paint to limit sunscald. Look at wild trees and shrubs in an undisturbed location--you'll notice their sensitve lower trunks are always shaded by other plants (or itself if it grew in full sun). Biotillers are the natural solution in agroforestry.
Some biotill tactics are:
"Direct biotill"--biotill plant is planted in the same hole as the crop plant. The pioneer plant is cut back (pollard or coppice) or partially girdled (ring-barked). This may be the best way to unpave the way for woody crop plants since the roots are going in the same direction. "Coconut" Chris Carter has had success with Coconut planted with Inga edulis. He did an experiment by cutting an Inga back in one hole, while letting the other one grow. The coconut with the pollarded Inga was at least twice as tall as the coconut planted with the un-cut Inga.
This can work well in sites that are overun with deep rooted rhizomatic plants. Planting holes can be placed at 3-4' apart within rows to allow easy weed cutting by weed wacker or hand sickle. Desirable trees or shrubs can be in every hole or can alternate with combinations of biotill plants. Some plants are allowed to grow tall to create shade, which slows down most weeds considerably, while other plants are chopped and piled thickly to smother the weeds. The weeds are cut or removed before any "green chop" is laid on the ground--unless there is enough green chop to completely smother them. Cardboard sheet mulch can be added to the mix to reduce maintenance as long as it doesn't encourage the weeds such as horstail, Wainaku grass, or quack grass.
"Circle biotilling": Biotilling plants can be located in a circle around the desired tree or shrub, with a radius of 1-3'. Biotill plants are cut back at the appropiate height and at the right time to limit competition. This system works best in a weed free setting. Concentric rings of diverse biotillers can be used. Daikon radish can be grown and harvested around young trees and shrubs--it is best to cut its vegetation back at least once to add organic matter.
"Hedge biotilling": A tight row of biotill plants that are cut back and added to low growing crops on either side of the row or within the row. This can work well with sunloving and nutrient demanding plants that resent competition such as pineapples. Pigeon pea has been used for with pineapples. Daikon radish could be added to the mix.
"Low competition biotilling": Biotill plants are incorporated into a planting at establishment or at a strategic time to limit competition. Gobo burdock is a relatively slow growing biennial that has a tap root that goes very deep--some say it goes to China. My guess is that it gets 4-8' deep. It barely competes with other plants until it bolts to flower. The flower shoot is an excellent vegetable requires no digging and is similiar to celery.
The most commonly used biotill system is Daikon radish undersown in corn (maize). It is estimated that up to 70% of the US' corn harvest is grown in no-till systems despite using megalith machinery for planting and harvesting. The radish is undersown in mid-late summer and dies back in the winter. It serves as a nutrient scavenger which increases nutrient cycling.
"Extensive rooting biotilling": Plants that send roots far and wide can be used to break up the upper portions of the soil. Mulberry, Gliricidia, banana, and taro all serve this function. Root competition and root disturbance from harvesting are some considerations to be aware of.
Natural Farming, as promoted by Masanobu Fukuoka and Han Kyu Cho, employs bio-tilling in no-soil disturbance (no-till) systems. They focus on cutting existing weeds to break up compaction, shade the soil, and create organic matter. While this strategy can work, it has also undermined many would-be gardeners due to: weed explosions within crops, weeds that take root when laid on the ground, and rhizomatic weeds which must always be pulled (especially in perennials).
Cho asserts that lactic acid bateria and indigenous microorganism (IMO's) break up compaction as well.
While natural farming offers an excellent goal, practioners that follow a cookie cutter strategy or philosophy from another growing region are bound for problems. Every cultivation system must be site and species specific. Experimentation with cutting weedy species can help develop an effective weed eradication plan. Creatively integrating permaculture, agroforestry, and natural farming together can lead to even higher production levels with less work.
Some of the challenges of biotilling are: potentially difficult weed control, root knot nematodes, finding biotilling species compatible with soil conditions (compaction, pH, organic matter levels), competition for scarce resources, for example, water in arid locations, and knowing how and when to chop plants back or when to sow seeds. It takes experimentation and time to develop systems that work well over the long run.
The benefits of biotilling should be of interest to all cultivators since they will get the most rewards. Reduced costs on fuel, machinery, fertilizers, and weed control in addition to healthier plants can improve the economics of farming, gardening, or subsistence cultivation. Our home, the earth, also benefits since carbon is sequestered by biotillers and less is carbon dioxide is released as mechanical tillage descreases(exhaust from machinery and carbon volization from soil disturbance).
Incorporating biotilling plants is a powerful way for cultivators change their ecological footprint for the better.
One final note, cultivators interested in bioilling should heed Masanobu Fukuoka's advice, "Start small, learn from your mistakes, and build on your successes".