Plants on the Moon

Why lunar regolith to grow plants on the Moon?

Plants can help solve resupply and food stability issues that accompany missions of progressively longer durations and destinations well beyond low earth orbit. Growing plants also offers a key boost to crew well-being in the isolation and highly engineered environemnts that accompany spaceflight. For a long-term crew presence on the Moon, the resources that need to be shipped from the Earth to grow plants could be dramatically reduced if lunar regolith, the crushed rocks of the lunar surface, could be used as a substrate; i.e., using lunar regolith be used a a basis for a lunar soil.

The challenges of using regolith

Regolith simulants that chemically and physically resemble true lunar regolith can support plant growth. Importantly, in 2022 research from the Ferl/Paul group at the University of Florida has confirmed that plants can also be grown in samples of true lunar regolith returned during the Apollo 11, 12, and 17 missions. However, regolith and its simulants require organic or chemical fertilizer supplements to maintain plant growth and even then, the plants exhibit signs of stress. This need for nutrient augmentation arises because the lunar regolith has limited bioavailable forms of crucial plant nutrients such as nitrogen. Incorporating microbes into the regolith may provide a sustainable approach to reduce the need for these fertilizer additions by capitalizing upon the capabilities of bacteria and fungi to solubilize immobile soil nutrients and enhance plant nutrient uptake. Indeed, the soil microbiome and microbial symbioses with roots are often essential for plant adaptation to nutrient-limiting and stressful conditions on Earth. 

Nitrogen is the most abundant mineral element in plant tissues (after the C, H, and O of the plant organic chemistry), forming up to 5% of total dry matter. Indeed, N is generally recognized as the most important limiting nutrient for plant growth, especially in settings of sustained, high agricultural productivity a scenario likely highly relevant to plant growth facilities forming part of a lunar life support system.

For nitrogen, microbial symbioses can be especially useful as the roots of some plants (principally legumes such as peas and beans) can form a mutually beneficial interaction with soil bacteria. The plant produces a root structure called a nodule to house and feed the bacteria with photsynthetically-made sugar. The bacteria then take atmospheric nitrogen gas and convert it into ammounium, a form usable by the plant. Nitrogen gas is normally unusable by plants and so this bacterial nitrogen-fixation is a method to provide the plants with a biologically accessible nitrogen source without having to add any fertilizer.