Ken Murphy / 9:01 pm April 12th, 2008

or: Let’s consult the most comprehensive text to date.

Henninger et al’s “Lunar Base Agriculture: Soils for Lunar Plant Growth”, published in 1989 by the American Society of Agronomy, the Crop Science Society of America, and the Soil Science Society of America, weighing in at 255 pages. I’m not qualified enough to determine if there were factual errors hidden in the text, but no typographical errors were noted.

This seems to be the definitive reference with regards to growing stuff in regolith. It starts out by looking at some strategies for getting to the Moon (& Mars), and how to set up shop there. The book then gives an overview of the conditions at the Lunar surface, covering things like the (exceedingly faint) atmosphere, what little magnetic fields there are, the abundant radiation, and micrometeorite flux. Next up is the mineralogical and chemical properties of the Lunar regolith. While this book pre-dates the ‘Lunar Sourcebook’, this chapter is a pretty good summary of what is in the ‘Sourcebook’, with great scanning-electron microscope photos throughout.

Finally, we get to the plants with ‘Pedology [science of soils], Pedogenesis, and the Lunar Surface’. The article’s authors note some of the very good reasons for growing plants at a Lunar base:

“physical support, nutrient reserves, buffering capacity, low maintenance, medium for recycling waste by-products, and nutrient recycling.”

We get an intro to soil concepts, and then they apply the facts of regolith to those concepts. We get such fascinating insights as

“In an Earth-like environment, chemical weathering would be expected to occur because lunar minerals are not currently in equilibrium with an Earth-like environment. Minerals common to the [L]unar surface are among the least stable in terrestrial soils and sediments.”

Finishing up the authors note some items for additional study, like the effect of gravity other than 1g on plant growth. Perhaps a future science fair project would be to create a centrifuge of plant growth chambers to try to simulate a 1/6th gravity while growing plants.

Next is a survey of ‘Nutrient Availability and Element Toxicity in Lunar-Derived Soils’. The authors note the sixteen elements generally considered essential for plant growth, the macronutrients like C, H and O, and micronutrients like Fe, Mn, Zn and B, as well as four supplemental elements found to be beneficial such as Na and Si. We find out that the pH for Lunar dust is 6.32, just a bit under basalt’s 6.6 pH. I like the conclusion, even if it’s not all good:

“The composition of lunar soils and the dissolution properties of the lunar dust studied by Keller and Huang (1971) indicate that a lunar soil has the potential to be an excellent medium for the growth of higher plants. The lunar soil, when exposed to a moist, aerobic, Earth-like environment, can be the source for many of the plant essential nutrients. Additions of N and to a lesser degree, P and K, will be necessary for optimum growth of higher plants. Higher natural concentrations and potential dissolution of certain trace metals, particularly Cr and Ni, may prove toxic to plants and bacteria.”

So there is a note of caution, but overall that seems to be in line with what was shown in the video noted in the first part. The authors do note

“A better indication of the nutrient-supplying ability of lunar soils could be found by measuring the plant-available fraction with extractants common to plant and soil analyses…(list of extractants)…Unfortunately, no such analyses have been performed on the samples of lunar soil collected by the apollo [sic] missions.”

Here we find that part of the reason we still don’t know whether plants are growable in Moon dust is that certain tests we would routinely apply here on Earth haven’t been conducted on the Moon dust. Something to look for in later research.

The next chapter then considers some of the risks identified in the previous chapter, and suggests using ‘Manufactured Soils at a Lunar Base’. Here we get some background on the promising field of zeolites. In this case, the appropriate nutrients are extracted from the Moon dirt and ‘caged’ by the zeolite structure for delivery to plants, while the risky stuff, like excess chromium, is kept away from the plants. It seems rather complicated, if promising, and unlikely something we would be doing right from the start given other priorities for a Moon base.

Next up is a look at ‘Controlled Environment Crop Production: Hydroponic vs. Lunar Regolith’. Here, the authors note that their

“ultimate goal of higher plant research in a CELSS is to remove environmental constraints such that the photosynthetic photon flux (PPF) is the only factor limiting growth and yield.”

So the goal is to produce the greatest amount of edible biomass in the smallest volume possible. We get a thorough overview of hydroponics systems, which the authors note “is widely used to grow high-input specialty crops on the Earth”. They suggest that regolith may help to provide a better root-zone environment for crops, but also note that inorganic salts may be an important consideration, as they are required for plant growth and will need to be shipped up from Earth to be added to the Moon dirt. Criticisms of hydroponics systems are noted, and that their requirements add complexity. The authors, who were researchers at the university that developed the Apogee Wheat noted in the first part, conclude that

“Plant growth in lunar-derived soils is a most interesting topic, but it will be extremely challenging to modify these soils to provide the same root-zone control and optimization that is readily obtained with hydroponic culture…Lunar soil may also provide valuable options for microbial waste recycling.”

So the next chapter looks at ‘Microorganisms and the Growth of Higher Plants in Lunar-Derived Soils’. Here, the thesis is that

“On Earth, microbes are the major biological agents responsible for the conversion of primary minerals into soil. The basic similarities in the composition of terrestrial bedrock and the [L]unar parent bedrock and regolith [refs.] suggest that the latter could be converted into an acceptable soil to support plant growth in an enclosed environment on the Moon if water, energy sources (e.g. from plant and animal wastes), and other factors necessary for the growth and activity of microbes are provided…”

Some relevant microbes and enzymes are identified, though a case is made for GEMS - genetically engineered microorganisms. Ultimately, the author admits that much research needs to be done regarding the role of microbes in converting regolith into functioning soil. Interestingly, he notes a point that I made in the first part:

“The current reluctance to use the limited amount of regolith available (ca. 333 kg ) for such experimentation is understandable. However, the risks are too great to base the designs and predictions of a successful lunar base, that will have to grow its own food and dispose efficaciously of its wastes, solely on studies with simulants.”

To look further into these soil workers, we then venture to the ‘Role of Microbes to Condition Lunar Regolith for Plant Cultivation’. The author give a list of 26 trace elements considered essential or growth stimulating for plants, and what the specific function of each one is, and then gives examples of microorganisms that can mobilize these trace elements from minerals. Lunar minerals that could supply these elements are identified, as well as noting two that seem to be in short supply - Molybdenum and Boron, though checking the ‘Lunar Sourcebook’ it seems that there’s just been little data collected on these elements because of their behavior in Lunar rocks, and some of what little data there is may be unreliable. The author notes that while there may be some risk to plants of concentrations of potentially toxic in large amounts elements, such as chromium, Mother Nature seems to have provided a counterbalance in the form of microbial reactions that can reduce that concentration.

In the next chapter, NASA HQ gives us an overview of a ‘Controlled Ecological Life Support System’, which from the photo is the same one I saw down at Cape Canaveral as noted in the first part of the article. This is further explored in ‘CELSS Breadboard Project at the Kennedy Space Center’, which goes into the engineering and operations of the facility. Activities to date (1989) are covered in ‘The CELSS Research Program: A Brief Review of Recent Activities’, largely covering the wheat activities noted earlier, but also expanding to some other higher plants, such as duckweed. We close out the exploration of closing the loop on life support systems with ‘Life Support Systems Research at the Johnson Space Center’. Here we get back to the idea of using Lunar regolith, though the author notes that experiments into the effects of various solvents will be conducted on simulant materials, which calls the results into question, and the author does cover the weaknesses. Looking forward, the author covers a number of the engineering solutions that have been developed to help humans live in a closed environment away from Earth.

Returning more fully to the realm of plant growth in Moon dirt, chapter 15 looks at ‘Physical and Chemical Considerations for the Development of Lunar-Derived Soils’, largely from the context of the functions of rooting media. Particular treatments are prescribed to remove some of the more deleterious elements in the regolith, and what the physical, chemical, and nutrient considerations are. In this chapter we get nice overviews of many of the important nutrients, and comparisons between Earth soils and Moon dirt, making this quite a valuable chapter, and the authors note a number of future areas of research.

One area of these research needs is examined in ‘Geochemistry of Soils for Lunar Base Agriculture: Future Research Needs’. This looks at the mineralogic, lithologic, chemical, physical and textural characterisitics, and notes that much research needs to be done into the dissolution of primary minerals and glass in Lunar minerals, precipitation of colloids, redox change, metal translocation, and other advanced soil research areas. The authors conclude that “the soils formed from [L]unar regolith in the CELSS environment will constitute a large experimental system in which many variables and processes cannot be understood in advance”, but note that carefully constructed experiments with analog simulants can provide usable knowledge that will help us focus future research.

We move from the realm of inorganic rocks to organic plants in the next chapter, on ‘Plant Considerations for Lunar Base Agriculture’. This provides an overview of many of the considerations for choosing which plants to use in a CELSS, and how the unique characteristics of off-Earth environments also need to be considered in the choices. We end with ‘Microbiological Considerations for Lunar-Derived Soils’, which summarized some of the earlier material, but focuses on the Nitrogen Cycle in the plant-growth process. As with many of the other authors, those of this chapter reiterate that using simulants will provide only limited speculative contributions to our knowledge base. Experiments with real Lunar materials will assuredly be exciting.

So there’s a lot of plant-related material to digest, so to speak. This is pretty much the definitive text to date for Moon-based agriculture, but it is pretty frank in stating that research needs to be done with actual Lunar materials. This raises the question of what priorities we have for the use of the Lunar materials we’ve hoarded to date, given that if we are going back to the Moon then we’ll be able to get more. Clearly the priority needs to be given to that research which provides the best results relating to a return to the Moon (which greenhouses is unlikely to be a part of in the nearest term). This is complicated by the fact that if we are going to start out this time around by going to the poles instead of the equator, then we’re likely looking at different sample types, so the results from the equatorial samples might not be ideal for preparing for the polar environment. Still, we’ve got to work with what we have.

As far as the review goes, I have to say that for researchers in the field it might not be quite as useful as hoped (such as if they’d been allowed to use the real stuff for the project), but still the best you’ve got to work with right now. It should definitely be used in conjunction with the ‘Lunar Sourcebook’, which thankfully LPI has made available on CD to alleviate the sticker-shock of the hardbound version. Some of the chapters seem to be used as platforms for the authors to promote particular approaches moreso than report on study data, and overall the amount of info directly relating to Moon dirt is not quite as extensive as could have been hoped for. That being said, the useful chapters are really, really useful, and provide a great introduction to how the Moon dirt compares with Earth soil and why it’s important, making it a great investment for those who want to learn more.

Given that it is really the definitive book to date on the subject, and the quality of the really good chapters, it gets a Full Moon, but I’m going to qualify it as a Full Moon at apogee in anticipation of a more definitive work based on the use of real regolith.

Next time around we’ll try to find some more recent work (post-1989) on the subject, and engage in a little speculation on the future of agriculture on the Moon in:

Of a Garden on the Moon, part III

or: The quest for answers continues

Part I