6.111 The carbon dioxide balance between plants and animals (including humans) should not be overwhelmingly difficult to obtain although the dimensions of this topic have not been scrutinized. Oxygen is a by-product of materials processing in space, but hydrogen must be imported, probably from Earth. The 13 inorganic elements presently considered to be essential for higher plants and therefore called mineral nutrients include nitrogen, phosphorus, potassium, magnesium, sulfur, calcium, iron, copper, zinc, boron, manganese, molybdenum, and chlorine (6.16). These have to be either processed from lunar material or brought from Earth, depending on availability. Table 6.13 gives a sample nutrient solution. Should the lunar soil culture be more like sand than soil, it would be similar to a "slop culture" that must be watered frequently since sand does not hold water well. Also, the pH would have to be closely controlled because sand is not buffered (6.16). VI.11.4: Food Production - Temperature, Light, and Humidity: Plants need a diurnal variation in temperature called thermoperiodicity (6.16). A relatively high night temperature does allow more respiration and therefore higher productivity, but some periods, such as germination, must have specific temperatures for some plants. This implies separate temperature control methods for different fields of crops at different times. The crop area is illuminated with 400 watts/m 2 for 18 hours with 6 hours of night. This daylight is roughly equivalent to 3 or 4 o'clock in the afternoon summer sun. Transpiration, which is essentially evaporation of water from plant cells, is affected by many factors but most strongly by light. Some plants can lose an amount of water equal to their total water content in one day (6 .16). Calculations in Appendix VI.J show that 25% of the incoming radiation distributes itself to evaporate water from plants and ground. To prevent the crops from drying out, water vapor must be actively removed from the air at the rate of 3822 kg/hr of sunlight and recycled to the plants. Typical diurnal variations of plant water uptake, loss, and content are shown in Figure 6.41.
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