Bioirrigation and Its Use in Biointensive Production
by Steve Moore

Bioirrigation, also called hydraulic redistribution or hydraulic lift, is the process where soil water is translocated by plant roots from wet soil, through root xylem vascular pathways, to dry soil areas. This process is driven by a water potential gradient. In short, bioirrigation moves water through roots from high soil water concentrations to low soil water concentrations. This makes water more uniformly available and for longer periods of time, especially in intercropped plants with diverse root architectures.

The root vascular system becomes the irrigation pipes that can move water in a variety of ways. The deep roots can move water from 1) deep wet soil to shallow dry soil or 2) from wet shallow soil to dry deep soil. Roots can move water laterally from wet soil on one side of the plant to dry soil on the other side. Bioirrigation also includes capturing moisture from fog or rain deposited on leaves/needles and transferring it to soils in the root zones for daytime transpiration. In all of the above cases water movement is a night time process in preparation for the day’s demands of plant photosynthesis and transpiration.

In the first two types of water transfer (wet deep to dry shallow and shallow wet to deep dry soils) there are several important criteria for transfer success.

1. There needs to be a diversity of plant root systems (see figure 1).
2. Plant roots need to be in close proximity to each other.
3. A big plus is to create a soil rich in mycorrhizal fungi creating a common mycorrhizal network (CMN). With this CMN tying water donor and water recipient together, water can be directly “piped” from donor plant to recipient plant via roots and fungal hyphae connections. Without the CMN, donor plants move the water to dry soil where it is picked up from the soil by the recipient plant’s roots.

Bioirrigation is an important process in water limiting soil conditions, both in too much and in too little water. It can provide 15% more transpiration capacity and, hence, greater plant growth (and potential yield). Pigeon peas, pearl millet and sorghum are examples of deep rooted crops that can be interplanted with shallow rooted crops such as finger millet and rice.

Bioirrigation increases plant growth and subsequent yields. Part of the story of yield improvement is simply more water. The other part is what the right amount of water in the soil does to increase biological activity (through rhizodeposition), organic matter decomposition (more nutrient availability) and nutrient movement (in water solutions); all of these come together to make more nutrients available to the plant. This movement of nutrients in conjunction with water is nature’s way of fertigation (a combining fertilizing and irrigation). Multiple studies with multiple crops have shown increasing yields and land use efficiency (more production per unit area).

Crops with large tap roots often have shallow roots as well (this is called a dimorphic root system). At night the deep rooted plant moves deep water (high soil moisture) to shallow (low moisture) soil. Daytime plant transpiration needs drives both plants to compete for the same shallow water. Both share in this water movement. There is a production method that allows bioirrigation to be used exclusively by the shallow-rooted plant. This is done by removing the above ground plant material of the deep rooted plant, This leaves the shallow rooted plant to have singular access to the bioirrigated water. A great example where I live in the Southeastern US is Sunnhemp (Crotalaria juncea). It is a summer crop planted right after early spring crops are harvested (bunching onions, spinach, salad mix, poc choi, beet greens, etc.) and then, after two months of growth, the above-ground plant is cut off, in August. Into this Sunnhemp stubble, autumn brassicas (broccoli, kale, cabbage, cauliflower, etc.) are transplanted.

In 50-60+ days, Sunnhemp grows 1-3 m and produces 90-145 kg (wet weight) biomass per metric bed (100 m²) or 83-135 lbs biomass (wet weight) per US bed (100 ft²) . This is a lot of C and N for the compost pile. Sunnhemp, a nitrogen-fixing legume, supplies the soil with lots of N for the next crop (brassicas are heavy N feeders). Sunnhemp can provide 135-155 kgs of N per ha or 120-140 lbs of N per acre. Sunnhemp is also a good fiber crop and has been used in India for fiber for millennia.

Bioirrigation can help mitigate the effects of climate change in many ways. One example is in places that experience both flooding and/ or drought in the same field any given year. Finding ways to reduce food production risk is critical for a constant, steady, supply of food. One clever strategy involving bioirrigation is a closely-spaced, deep-rooted, drought-tolerant crop (i.e., sorghum, pearl millet) interplanted with a shallow-rooted, flood-tolerant crop (i.e., rice). In a drought situation the deep-rooted crop moves water from lower moist areas to dry upper areas to supply water to the rice. In flooding, the rice roots/plant supplies oxygen to the water-saturated soil helping the deep rooted crop to survive. After 50 years of agriculture production and 30 of those years in BioIntensive (BI), I continue to be profoundly respectful of the fundamentally sound principles of BioIntensive. A summary of those BI supportive principles and practices for capitalizing on bioirrigation are listed below.

• Deep soil quality gives the roots the needed depth and soil structure for bioirrigation to work. And, in fully mature BI soils, tillage is minimized allowing for the development and maintenance of strong soil fungal networks (CMN).
• Planting intensification encourages and supports the close plant spacing and intercropping required for bioirrigation to work.
• Functional Biodiversity (also called companion planting) capitalizes on crops with diverse and dimorphic root systems.
• Resource efficiency focuses on water use, nutrient management and land use efficiency.
• Carbon farming for compost
• Use of hand tools and permanent bed/ pathway planting allows close interplanting and above ground top removal of water nurse crops, very difficult to do mechanically.

References Burgess, S. S. O. 2011 Can hydraulic redistribution put bread on our table? Plant and soil Vol 341 25-29 dot 19.1007/s11104-010-0638-1

Ewel, J. J., Schreeg, L. A., Sinclair, T.R. 2019 Resources for crop production: accessing the unavailable. Trends in Plant Science. Vol 24 No. 2 https://doi.org/10.1016/j. tplants.2018.10.008

Hagjioghi, E., Damm, A., Jimenez-Martines, J. 2021. Root hydraulic redistribution underlies the insensitivities of soil respiration to combined heat and drought. Applied Soil Ecology 167 104155. http://doi.org/10.1016/j. apsoil.2021.104155

Izumi, Y., Okaichi, S., Awala, S. K., Kawato, Y., Watanabe, Y., Yamane , K., Iijima M., 2018 Water supply from pearl millet by hydraulic lift can mitigate drought stress and improve productivity of rice by the close mixed planting. Plant Production Science Vol 21 No 1, 8-15 https://doi.or g/10.1080/1343943X.2018.1428494

Mudita, I. I., Chiduza, C., Rishardson-kageler, S., Murunu, F.S. 2008 Evaluation of different strategies of intercropping raise (Zea mays L.) and soya bean (Glycine max (I.) Merrill) under smallholder production in subhumid Zimbabwe Journal of Agronomy 7, 237-243.

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Prieto, I., Armas, C., Pugnaire, F. I. 2012 Water. Release through plant roots: new insights into its consequences at the plant and ecosystem level. Tansley Review New Phytolgist 193: 830-841 doi 10:1111/j.1469- 8137.2011.04039.x

Singh, D., Mathimaran, N., Boller, T. et al. Bioirrigation: a common mycorrhizal network facilitates the water transfer from deep-rooted pigeon pea to shallow-rooted finger millet under drought. Plant Soil 440, 277–292 (2019). https://doi.org/10.1007/s11104-019-04082-1

Sperry, J. S., Love, D.M. 2015 What plant hydraulics can tell us about responses to climate-change droughts, Tansley Review 207: 14-27 https://doi.doi.org/10.1111/ nph.13354

Weaver, J.E. and Bruner, W.E. 1927. Root Development of Vegetable Crops. McGraw-Hill book Company Inc.

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