Soil Moisture Strategies and Monitoring in Citrus in the Central San Joaquin Valley
California Citrus Industry
Citrus crops are a highly profitable commodity in the United States with California and Florida being the two leading States (USDA, 2012). In 2012, U.S. citrus production totaled 11.2 million tons with a value of $3.2 million, of which $2 million was produced in California. The contribution of California citrus crops was 30% of the total U.S. citrus fruit production and 42% of the national value (CDFA. 2014). The California citrus industry is accountable for the production of a number of citrus crops such as navel and valencia oranges (Citrus sinensis L. Osbeck), minneola tangelos (Citrus × tangelo), lemons (Citrus × limon), tangerines (Citrus tangerine), mandarins (Citrus reticulata), clementines (Citrus ×clementina) and grapefruits (Citrus × paradisi), which were grown throughout the State in 267,900 farmable acres.
Crop Physiological Responses to Regulated Deficit Irrigation (RDI)
Crops tend to react in diverse physiological ways to the lack of adequate soil moisture. Insufficient moisture can cause several physiological changes in crops such as reductions in osmotic potential to decrease photosynthetic activity (Turner et al., 1978). The ability to implement RDI during a specific time during the growing season of the crop can avoid a potential reduction in yield and quality as the result of inappropriate RDI management practices (Fereres and Soriano, 2007). Reducing the amount of water applied early in the season can cause the drop of flowers, which reduces potential yield. Yet, reducing the amount of water later in the season will cause a change in physiological activity that can reduce photosynthetic activity, thus minimizing growth of fruit circumference. The reduction of water application can also be the precursor to early fruit maturation in crops and increase Brix levels, causing sugar content along with fruit growth control. As certain physiological characteristics are sought, other important traits might be sacrificed to attain those desired traits (Chaves et al., 2010). If the implementation of a RDI program is not adequately managed, the sudden physiological adaptations crops must undertake when subjected to inadequate soil moisture can possibly increase the potential for petal fall and decrease yield, or in extreme cases, cause tree mortality.
The potential of sacrificing a certain desirable fruit characteristic will be a constant challenge under the current water constraints in California. These challenges create the opportunity for growers to be innovative as they are mandated to use little or no district water, along with poor quality ground water that can be high in salts (Na, Cl, B) or nitrates. Furthermore, the lack of any “additional” water for use as the leaching fraction needed to ensure that salts are moved below the root zone, dictates that irrigators explore alternative irrigation approaches. While a deficit irrigation program may potentially increase the level of stress in the crop, there exists the potential for a reduction in the adverse effect of salt-affected groundwater (Perez-Perez et al., 2008). Crop physiological stress can exacerbate under the presence of other stressors such as a sodic soil condition and weed populations. For instance, soils with high soil salinity (sodium chloride, potassium chloride) levels can create osmotic stress within the root zone and thereby inhibit water uptake by the citrus roots. As for the presence of high weed populations, weed species can potentially vie with citrus crops for water and nutrients, thus further increasing plant stress (Tilman, 2004; Tilman and Wedin, 1991).
It is vitally important to maintain adequate soil moisture throughout the root zone profile. As the crops become subjected to RDI for phenological or cosmetic purposes, crops tend to uptake water from moisture present at lower depths, thus preventing an extreme degree of physiological stress (Geets and Raes. 2009). The lack of adequate deep soil moisture can result in extensive damages to the crop and drop in fruit yield, which can ultimately result in an unproductive crop and an unviable crop stand (Goldhamer et al., 1999). Monitoring and maintaining adequate soil moisture within the root zone is a fundamental part of any management strategy aimed at maximizing crop production with limited water resources.
The need to reduce physiological damage to crop development is the primary need of incorporating a soil moisture sensor to maintain adequate soil water levels. Vice versa, the over-application of irrigation water can produce adverse effects to crop development, creating a need to properly monitor soil moisture levels for the proper development of crop growth. Therefore, having a reliable and dependable soil moisture sensor is vastly important in both the adequate application of water and the strategic limitation of soil moisture, i.e. deficit irrigation.
- Nate Mendez, Agronomist
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