Weather – It’s The Variability That’s Hard To Farm With

PART IV - SOLAR RADIATION AND SOIL MOISTURE

Author: Guy Ash, Global Training Manager, Pessl Instruments

Weather variability series is back with PART IV! In Part I of our Weather – It’s The Variability That’s Hard To Farm With series, we explored the impact of precipitation on plant growth, nutrient uptake, and yield variability, emphasizing the importance of in-field IoT devices for precise weather monitoring. Part II focused on temperature, highlighting its role in evapotranspiration, frost risk, and pesticide efficiency. Part III examined humidity, leaf wetness, and wind conditions, demonstrating their effects on disease pressure, spraying accuracy, and crop lodging risks.

Now, in Part IV, we turn to solar radiation and soil moisture, essential factors for crop development, water balance, and maximizing yield potential. This discussion will further reinforce the need for advanced weather monitoring to optimize farm management in an unpredictable climate.

SOLAR RADIATION AND SUNSHINE: impacts growth and development, disease development, drying conditions

Solar radiation can be measured in a variety: W/M², (watts per meter²), J/M² (joules per meter²), KJ/M² (kilojoules per meter²), MJ/M² (millijoules per meter²). Solar radiation sensors come in a variety of types, but the standard sensor provides global measurements of solar radiation in watts per meter squared.

The values vary with cloud cover, time of day and season. This is the energy used for the photosynthesis process of the plant. During warm and sunny days, the absorption of chemical products is generally higher than under cool damp conditions. Solar radiation is also used in the calculation for evapotranspiration, or the water used by the plant/crop each day.

In the picture: Solar Radiation Sensor recording watts per meter2 along with minutes of sunshine each hour.

Depending on how far north or south you are located, the amount of solar radiation can vary tremendously through the seasons. From as low as 250 W/M² in winter months to over 1100 W/M² in the summer months. Along with solar radiation, of course, the photo period shortens and lengthens, and in the summer months the long photo period (day length) can hasten development of crops in short frost-free regions further north.

Solar radiation is essential for plant growth. Plant leaves absorb sunlight and use it as the energy source for photosynthesis. The ability of a crop to collect sunlight is a function of leaf surface area or leaf area index. When a crop is at full canopy, its ability to collect sunlight is maximized.

Agronomic factors such as weed competition, insect feeding or leaf diseases can reduce leaf surface area and interfere with sunlight captured by crops. In theory, as the amount of captured radiation energy increases, crop production will also increase. When plant leaves absorb the energy of the sun for photosynthesis, the temperature of the leaf surface increases. Plants respond by releasing water through the stomata to cool the leaf surface.

SOIL MOISTURE: impacts crop development, plant health, nutrient input & uptake, and yields

Soil moisture can be measured with single or multiple depth probes which provide volumetric or suction estimates at the probe’s sensor location. The soil moisture supply has the most significant impact on yield potential since it’s responsible for carrying nutrients to the plant for photosynthesis. This is why moisture supply is often referred to as the CROPS GAS TANK for yield potential.

In the picture: PI Profile probe measuring soil moisture and temperatures at multiple depths

Each crop has water use efficiency curves related to yield.  Research and field trials over years have identified the number of bushels produced from each inch or 25 mm of soil water used by a crop. For some common crops this equates to 5-6 bu in canola, 7-8 bu for wheat and 10 to 14 bu of corn yield increase for each additional inch or 25mm of soil water added.  The number of bushels produced per inch, or 25 mm of soil water will change over time as new varieties are released with better genetics.

Therefore, the total amount of soil water available (supply) to a crop during the growing season is equal to the amount of soil moisture available at seeding time (determined by soil type) plus the amount of precipitation and/or irrigation (soil moisture) received over the growing season. The use or demand of soil water is determined by temperature and soil type/texture. These two factors (supply and demand) define the yield potential.

The key is to have or maintain soil moisture values at the correct levels to minimize plant stress and maximize yield and quality. This can easily be done by maintaining soil moisture between the Full Point and Refill Point when irrigating. These two levels are determined based on the soil type/texture and probe profile. For dryland production, the farmer is reliant on mother nature based on stored soil moisture and growing season precipitation.

Cost of improper soil moisture levels

Knowing soil moisture levels is very important in dryland and irrigated crops, since the amount of nutrients applied needs to meet the soil moisture levels. If you fertilize for 250 mm or 10 inches, but you have 300 mm or 12 inches of soil moisture, then you are losing yield potential. As an example, for canola this would translate into a 10 to 12 bu/acre (2*5-6 bu/acre) loss or $90 to $120 per acre. The next section will show the economic value of proper soil moisture management for irrigated corn.

In the picture: Soil Probe measuring soil moisture at multiple depth and color-coded chart of Above Full Point (blue), Plant Available Water (green) and Refill Point (red)

Weather variability remains a major challenge for modern farming, affecting everything from seed germination to harvest. Through this four-part series, we have explored the intricate relationships between weather parameters and agricultural success.

Understanding precipitation variability helps farmers predict yield potential and make informed decisions about irrigation and nutrient applications. Temperature monitoring enables precise control over evapotranspiration, frost risk, and spraying efficiency, leading to improved pesticide effectiveness and crop protection. Humidity and wind conditions play crucial roles in disease development, lodging risks, and drying processes, highlighting the need for real-time field data. Finally, solar radiation and soil moisture directly impact crop development and nutrient uptake, demonstrating that the right balance of water and sunlight is essential for maximizing yield potential.

KEY TAKEAWAYS FROM PART IV

Solar radiation powers photosynthesis, but its impact varies with season, time of day, and cloud cover. Soil moisture is just as crucial, ensuring nutrient transport and healthy plant growth. Poor moisture management leads to losses, making real-time monitoring essential. That’s why IoT sensors are important – they help farmers optimize resources, while advanced weather tracking reduces risks and improves farm sustainability.

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