Soils in extraterrestrial space:Need for studies under microgravity

With the increased interest of plants in space,gravity cannot be ignored when studying soil-plant-water relations.Therefore,it is important to understand the water relations of soils under zero gravity in space or under reduced gravity on the Moon or Mars.This paper outlines the problems of moving water in soils under zero gravity or microgravity,as well as growing plants in these soils.It is suggested that microgravity experiments could be done on Earth using both soils and plants.Results from such experiments could be used to grow plants when mankind establishes bases on the Moon and Mars.

As the need to produce food on space missions may become necessary,understanding soil processes under zero gravity or microgravity is needed.The Moon and Mars are of special interest,with the acceleration due to gravity being 0.17 and 0.38 that of Earth,respectively.These processes include studies of soil physical structure and chemical composition and plant nutrient uptake in an extraterrestrial environment.Water in soils also needs to be studied.The effects of gravity on water are rarely mentioned in textbooks dealing with soil-plant-water relations.Water has unique physical properties that allow it to support life.If life exists on other planets,it probably is based on water rather than any other molecule.Therefore,it is important to know how water is affected by gravity.

Under agricultural conditions on Earth,the depth-wise movement of water,rather than the lateral movement of water,is important.Worms move up and down in soils preferentially(perpendicular to the soil surface)(Darwin,1890),and roots grow down(Barlow,1995).Their channels,in most instances,are vertical.Thus,where worms and roots occur,holes in soils are usually vertical.Morris Muskat was a well-known physicist who was Chief of the Physics Division for an oil company in USA.He wrote a classic book on flow of fluids through porous media(Muskat,1946).Oil engineers are interested in lateral(horizontal flow),not downward flow,because oil deposits are often in lateral layers.Muskat was not much interested in gravity in his work,but soil scientists have long been interested in the effects of gravity on water movement in soils.Both matric (capillary) potential and gravity potential determine the movement of water in soils(Kirkham,2014).Now soil scientists need to know how water moves in soils without gravity or under microgravity.

In spaceships,there is no gravity,and Darcy’s law for water movement under saturated conditions would not work,because it depends upon water being pulled into soils by gravity(Kirkham,2014).To get water to move in space,one could apply a spring to the surface of the water;then the water would be pushed into soils.Or one could put blotting paper at the bottom of the saturated soils,and this would pull water out(Kirkham,2023).In experiments that have been done in space,similar systems(a pumping system or a wick system)have been used to move water to plants(Jones and Or,1999).Plant roots are another way to move water in soils under zero gravity.In addition to springs or wicks,one could use plant roots to pull water out of soils.

Surface materials from the Moon have been brought back to Earth and analyzed (Ming and Henninger,1989),and lunar samples have been sprinkled onto seedlings and growth media.However,plants have never been grown in lunar regoliths as the support matrix.This gap in knowledge was addressed by Paulet al.(2022),who grew on Earth the weed widely studied by plant physiologists,Arabidopsis thaliana(L.) Heynh.,in regoliths brought back from the Moon.While germination occurred in the material,the roots were stunted.Leaves also were stunted and deeply pigmented,a typical indicator of plant stress.These results indicate that under microgravity on the Moon,lunar soils cannot be used to grow plants.

Even though most scientists cannot participate in experiments done in space,studies still can be done on Earth to simulate microgravity and to gain information needed for growth in space.Soil physicists have long used horizontal columns to eliminate or reduce the effects of gravity(Kirkham and Feng,1949;Bruce and Klute,1956).When columns are on their sides,gravity acts only through the diameter depth of the column.This is relatively small compared to when the columns are standing upright.If one puts columns on their sides,one can study infiltration into the columns under gravity-free conditions.The Philip infiltration model gives infiltration as the sum of both the gravity-free absorption into a ponded soil due to capillarity and a second term that represents the infiltration due to the downward force of gravity(Kirkham,2014).With columns on their sides,infiltration is gravity free,and one can study infiltration only due to capillarity.Studies could be done to systematically measure the infiltration rates of different soil types in horizontal columns for infiltration under no-gravity conditions.The horizontal infiltration experiments also could be done with glass beads,which soil physicists use as a model for an ideal soil.Glass beads have actual relevance for the Moon,because lunar soils brought back to Earth have been shown to contain impacted glass beads,which are potential candidates for reservoirs of water (Heet al.,2023).The horizontal infiltration experiments with soils or glass beads could be carried out on Earth.

Horizontal column experiments also could be done with plants on Earth.Using a commercial potting mixture,I grew wheat(Triticum aestivumL.)plants in columns on their sides to determine the stomatal resistance under microgravity(Kirkham,2008).Control plants grew in vertical columns.Figure 1 illustrates the experimental setup.The roots in the horizontal columns did not grow down into the soil,which is observed in space,and the stomatal resistance was high.As noted,on Earth roots grow down,and this response is due to gravity.Out in space with no gravity,roots grow in any direction.Lids must be put on the top of containers to keep the roots in the media(Tibbitts and Bula,1989;Stutteet al.,2006).It is felt that starch (a product of photosynthesis) statoliths,which are amyloplasts,are gravisensors(Morita,2010),and they are capable of swiftly responding when the roots are displaced relative to the gravity vector.Root caps have long been regarded as an organ controlling the georeaction of roots.Information suggests that the amyloplasts contact the cell membrane,which contains gravity receptors.Hormones are then produced,and the hormones produced cause roots to grow down and shoots to grow up (Kirkham,2008;Takahashiet al.,2021).In the horizontal column experiments,different types of crops could be screened to see if some grow better horizontally than others and,consequently,might be better candidates to grow in extraterrestrial media.

Fig.1 Schematic drawing showing the basic principles for microgravity experiments with soil and plants,based on the microgravity experiment reported by Kirkham(2008).Two columns with soil are placed vertically,and two columns with soil are placed horizontally.The horizontal columns simulate microgravity conditions.Seeds are planted in the columns.The columns are clear plastic,so root growth can be observed.A light bank of cool-white fluorescent lights is placed above the columns.Columns are rotated 180° along their long axes every 24 h.Rotation of the columns only affects the orientation of the horizontal columns in the gravitational field;it does not affect the orientation of the vertical columns in the gravitational field.In the experiment by Kirkham(2008),the columns were 40 cm long and 7 cm in diameter,wheat was used,the wheat seeds germinated in the horizontal columns,but the plants emerged poorly and were stunted.

In sum,we need more studies of soils and plants grown under microgravity,so we shall know how to grow plants when mankind establishes permanent stations on the Moon and Mars.

ACKNOWLEDGEMENTS

This is Contribution No.23-316-J from the Kansas Agricultural Experiment Station,Manhattan,Kansas,USA.I thank two anonymous reviewers for their comments.I thank two graduate students in the Department of Agronomy at Kansas State University,USA:Lina Zhang for putting the figure in Word format and Haidong Zhao for putting the references in thePedosphereEndNote style.