Sunlight and Water in the Garden: Two Limiting Factors Not to be Overlooked

By Caleb Goossen, MOFGA’s Crop and Conservation Specialist

A foundational principle in agricultural sciences is “the law of the minimum,” which states that plant growth is not limited by the sum total of resources available, but instead is primarily limited by the scarcest resource, also known as the limiting factor. Seemingly all great foundational principles seem obvious in retrospect, and tend to be oversimplifications of the complexities of the world, and this rule of thumb is no exception. However, that does not negate the usefulness of this principle as a lens through which to consider your garden!

Illustration of a barrel full of water with water spilling out of the broken staves; each stave is labeled a different nutrient that plants need
Fig. 1. A plant’s growth is limited by the nutrient in shortest supply, whether that is nitrogen from the soil or carbon supplied by sunlight-driven photosynthesis, just as a barrel’s capacity to hold water is limited by its shortest stave. Illustration by Charlotte Chapman

The law of the minimum is typically used to consider individual plant nutrients obtained from the soil including essential macronutrient elements like nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S) and micronutrient elements like chlorine (Cl), iron (Fe), zinc (Zn), manganese (Mn), boron (B), copper (Cu) and molybdenum (Mo). A good example of a limiting factor in the traditional sense of the law of the minimum would be the micronutrient boron. Plant demands for boron vary by species, and some are actually damaged by too much boron availability, but there are a few common garden plants that can greatly suffer from a boron deficiency — Brassicas and the beet/spinach family are among those that need it the most. You can provide extra nitrogen to your beet crop or your broccoli, but if it doesn’t have enough boron the nitrogen will not provide better growth or a higher quality crop. You’re likely to only get leafier growth and a lower quality crop overall.

This maxim can also be used to consider questions like “what fertilizer do I need?” that are often top of mind for new gardeners. Here’s another example to consider: when a garden has been fertilized for years with animal manures or compost derived from animal manures, there tends to be a buildup of phosphorus in the soil. If you were to continue applying manure to meet nitrogen demands for the crop, you will continue to supply more phosphorus than the crop needs without adding to the crop’s growth potential at all. The limiting nutrient in this situation would likely be nitrogen or potassium, and only supplying more of those nutrients would have a positive impact on growth potential. Supplying additional phosphorus is at best a waste of phosphorus, and at worst a potential source of pollution to waterways or a potential interference to the uptake of other necessary nutrients.

As described in the examples above, plant growth can be limited by the nutrient in the shortest supply in the soil but water and sunlight also provide access to key nutrients for plants. I’m suggesting here that growers first need to address the elements that make up 95% of plant matter — carbon (C), hydrogen (H) and oxygen (O) — all of which are derived from water and air, through the process of photosynthesis.

Sunlight as a Limiting Factor

If a garden isn’t thriving and there are no obvious symptoms of disease or pest pressure, and sometimes even if there are, I first look to rule out some of the most common limiting factors for gardens — lack of sufficient water, excessive water in the soil and/or exposure to light. While these may seem like obvious things to say, and they do tend to be a bigger issue for newer gardeners and newer gardens, any of us can have blind spots and not realize how these basic necessities for plant growth may still be limited or lacking in our gardens. Often an established garden that once had long days of full sun exposure will have had trees grow up around it, limiting the number of hours of direct sun the plants are getting. If you’re not out in the morning to see when the shadow finally leaves the garden, or in the afternoon when the shadow arrives, then you may easily underestimate the hours of sun your garden has lost.

We have likely all learned that plants make their own food, using energy from sunlight. This food is used for each plant’s own metabolism, but also as building blocks for continued growth of all plant parts (like the tomato you want to eat!). While we may know this rationally, it’s hard to have an intrinsic sense of just how much sun is needed to produce the growth that we’re expecting or desiring from our gardens. Most of our common garden vegetables struggle to live up to their potential with less than six to eight hours of direct sun daily during the growing season. Plants described as doing best in partial sun or partial shade typically require four to six hours of direct sun, and shade-loving plants typically do best with less than four hours of sun, or only dappled sun. Many plants that cannot tolerate full sun do best with only morning sun, as afternoon shade protects them at the hottest and driest part of a typical day.

If you think of a leaf as akin to a solar panel, the amount of energy it can create through photosynthesis is directly linked to the amount of sunlight it can intercept. A small seedling needs to grow long enough to make more, larger leaves, until they eventually have enough leaf area to produce the energy needed to yield the crop that you’re trying to grow. This, of course, varies by the crop type and what plant part you’re hoping to harvest. With leafy greens, for instance, we’re actually eating the part of the plant that collects that solar energy — we’re eating the solar panels as it were. Leaves begin producing their own energy shortly after emerging, so they rapidly make up for the energy used to produce them in the first place. That makes leafy greens just about the least energy-intensive crop type for plants to produce. For that reason, I typically recommend that people with limited hours of direct garden light focus their vegetable gardening on leafy greens or leafy herbs like dill and cilantro. 

However, other crops which we grow for their fruit or storage organs (e.g., roots) require a large amount of sunlight energy. Additionally, these energy-intensive portions of plants typically don’t create their own energy the way leaves do. With a potato plant, for instance, you plant a seed potato that has its own energy reserves to get the plant up and going; the first stage of its growth is entirely devoted to creating top growth, stems and leaves, which in turn harness energy from the sun to support the growth of new potato tubers. For the plant, those potatoes represent huge investments of stored energy derived from photosynthesis.

Ways in which Water Can Limit Plant Growth

Insufficient water is likely the most common way in which water is limiting for home gardens, but too much water can also be constraining. Without enough water, a plant’s ability to perform photosynthesis is severely hindered. Plants split water molecules (H2O) to provide the hydrogen and oxygen needed to create sugars, which are in turn used to fuel the plants’ metabolism and growth. Additionally, leaves are cooled by the evaporation of water from within them, which is critical for allowing photosynthetic machinery to operate efficiently. When plants suffer water stress, they close their stomata (small openings that allow them to take air inside their leaves) to conserve water. This reduces the amount of evaporative cooling, and also limits their ability to take in carbon dioxide (CO2) from the air, which provides the carbon used for sugar (C6H12O6) creation. Both elevated leaf temperatures, and limited supply of CO2 can greatly increase a process called photorespiration in which plants accidentally “grab” oxygen (O2) with an enzyme that is supposed to be grabbing carbon dioxide — greatly reducing photosynthetic efficiency. By the time a plant has begun wilting, its capacity to photosynthesize and grow has already been reduced. Wilting, beyond a small amount at the hottest part of the day, is a last ditch effort by a plant to limit water loss by reducing the surface area of leaves receiving direct sunlight, and a sign that you should water as soon as possible.

Figure with a healthy plant and wilting plant with the factors contributed to growth explained
Fig. 2. Plants under water stress. A. When plenty of water is available in the soil, a plant will absorb water through its roots. The water will be used by the plant or released through transpiration by open stomata in the leaves; photosynthesis will also occur normally with CO2 and oxygen being absorbed and released through the open stomata. B. Plants try to prevent water loss through transpiration when limited water is available in the soil by closing the stomata and using a substance called ABA; photosynthesis will decrease because no CO2 can enter through the closed stomata. Less photosynthesis means less energy is produced by the plant and the plant stops growing. Courtesy of Frontiers for Young Minds, from “How Do Plants Deal with Dry Days?” by van der Vyver C and Peters S, 2017. kids.frontiersin.org

Too much water can also act as a limiting factor. Of course, that is typically a situation that is out of our control, like when there’s just days and days of rain. However, poorly draining soil (i.e., heavy soils and/or those with poor soil structure) and your garden location can also play a role. If your garden is situated in a low spot, water will flow toward it; whereas if it’s located in a high spot, the water will flow away from it.

The effects of too much water, if unnoticed, can actually resemble not enough water, which is something that can really trip up new gardeners. Excessive water fills pore spaces in the soil, forcing air out and depriving plants’ roots of the oxygen they need to live. Without oxygen (anaerobic conditions), roots start to die back. If the aboveground water demand is much more than damaged roots can supply, you could have all the sun the plant could want but the plant would still suffer — perhaps even more so as the large photosynthetic demand for water was unable to be met. Excessive soil moisture can also interfere with soil health and nutrient cycling. Saturated, anaerobic conditions can be conducive to some plant diseases, and set up conditions for nitrogen losses through bacterial denitrification; excessive water flow through the soil can also leach nitrogen in the form of nitrate out of the topsoil.

Because water limitations (excess or deficiency) are taking place at the soil-root interface underground, they can be difficult to notice until the plant has been struggling long enough to exhibit symptoms aboveground. For that reason, and because everyone has different soils and weather conditions, the gardening advice that I offer most often is to get your hands dirty: dig a hole or just stick your fingers in the soil around your plants and check the soil moisture status down where the roots are. No matter your soil type, improved soil health and soil structure help to alleviate poor drainage, allow water to infiltrate more readily and also help to hold moisture in periods of dryness, making soil health a critical strategy for increasing your garden’s resilience to water limitations. 

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