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INTRODUCTION
Soils can store different amounts of water depending mainly on their basic physical properties, like texture, structure and the content of organic matter, which influences in a particular way. The total amount of water available in the soil for plants (ADP) is the difference between the stored water at the maximum retention or storage limit known as "Field Capacity" (Cc) and the minimum storage limit, known as "Permanent Wilting Point" (PMP). Both are considered up to the depth of interest for plants or effective root depth (Zr), (Gardner, 1988; Hillel, 1998; Reichardt y Timm, 2004; U.S. Department of Agriculture, 2005; Allen et al., 2006).
A fraction of water depletion from field capacity to a point defined primarily by the characteristics of the crop, among other factors, represents the "water or reserve readily or easily available for plants" (RFU). The adequate selection of this point implies the definition of the ideal operating criterion for irrigation planning (Allen et al., 2006).
The first works carried out in Cuba on water available for plants in Cuban soils date back to the 1960s and were based on the morphological classification of soils valid in Cuba at that time. Only destructive methods of sample processing and different concepts of water retention energy in the soil were used. In this sense, the works of Nakaidze and Simeón (1972), Simeon (1979), Klimes et al. (1980) and others were important.
Later, in the decade of 1980, associated to the works for the elaboration of the map of Cuban soils, at scale 1:25 000 and to the generalized application of the irrigation forecast (Rey et al. ,1982), deeper studies were done about water availability for plants in different Cuban soils by different scientific and technical entities. The former Irrigation and Drainage Research Institute (IIRD), today, Agricultural Engineering Research Institute (IAgric), led them. These studies were made based on the current genetic classification of Cuban soils.
The continuity and updating of these studies have been carried out to date from different works of Master's and Doctorate’s thesis and national and international research projects. They have also been performed from scientific and technical services provided by researchers and specialists of IAgric to different national entities. That has generated a database with information on the physical properties of the main soil types in the country, taking into account, also, correlations with the latest Genetic Classification of Cuban soils (Cid et al., 2011).
The objective of this work is to determine the availability of total water and the reserve easily usable for plants, based on the predominant texture, as a tool that facilitates efficient irrigation programming based on the information available for the most important agricultural soils in Cuba.
METHODS
Database
For the research, the database of Agricultural Engineering Research Institute (IAgric) belonging to the Ministry of Agriculture of Cuba was available. Data were obtained from 131 subtypes of soils corresponding to 11 soil groupings, according to the last classification of soils valid in Cuba (Hernández et al., 2015).
The studied soils belong to the groupings: Alitic, Ferritic, Ferralitic, Fersialitic, Sialitic Brown, Sialic Humic, Vertisol, Hydromorphic, Halomorphic, Fluvisol and Histosol. Only the soils belonging to the Ferralics, Histosols, Poor Evolved and Androsols groupings of this last mentioned classification were missing.
The physical properties of the soils considered and the methods used for their determination appear in Table 1.
TABLE 1 Physical and hydrophysical properties studied and methods used
| Properties | Determination Method |
|---|---|
| Texture | Method of the Pipette |
| Field Capacity | Plazoleta Method |
| Permanent Wilting Point | Method of the Membrane (tension 15 atm.) |
| Bulk Density | Ring method (undisturbed samples) |
| Soil water content | Gravimetric Method |
Data Processing
The layer selected to carry out the studies of each soil profile was from 0 to 0.5 m, taking into account the depth reached by the roots of most of the economic important crops under irrigation in Cuba (Chaterlán et al., 2010; Zamora et al., 2014).
The data of each evaluated soil profile were organized, assigning to it a predominant particle size value and considering the established ranges for the textural classification in the Interpretation Manual of Cuban Soils (MINAG, 1984).
The intermediate values were adjusted according to a logarithmic regression analysis between the values of gravimetric moisture content corresponding to the point of field capacity (Cc) and permanent wilting point (PMP), and the predominant particle size. A similar logarithmic regression analysis was also performed for the corresponding volumetric moisture content values, calculated according to the expression (Cid et al., 2011):
Where:
Wv |
- volumetric moisture content (cm3∙cm-3) |
Wg |
- gravimetric moisture content (g∙g-1) |
Da |
- bulk density (g∙cm-3) |
The quality of the regressions was analyzed from the statistic Coefficient of Determination, R2.
The water depth corresponding to the upper (LSAD) and lower limits (LIAD) of the total available water in the soil for the plant (ADP) were also calculated according to Allen et al. (2006). The mean values of volumetric moisture content at field capacity WvCc and the permanent wilting point WvPMP, expressed in% of volume to the root depth of Zr = 0.5 m, were considered as follows:
(2)
(3)
Finally, the values of the Total Available Water in the Soil for the Plant (ADP) and the Easily Usable Reserve (RFU) were quantified for each profile. For the calculation of the RFU, the results of previous studies were taken into account. Those define that the maximum yields under irrigation are obtained for the treatments where soil moisture is maintained around 85% of the field capacity corresponding to 50% of the total soil water available for plants (Chaterlán et al., 2010; Zamora et al., 2014).
The calculations were performed as defined by Allen et al. (2006), from the following expressions:
RESULTS AND DISCUSSION
Figure 1 (A and B) shows the result of the regression analysis carried out between the values of the gravimetric and volumetric soil water content at field capacity, Cc, and permanent wilting point, PMP, and the soil predominant particle sizes.
Logarithmic equations with high values of the coefficient of determination, R2, were obtained in all cases. That makes possible to affirm that the equations defined for each case, can explain more than 95% of the variations in the gravimetric soil water content as a function of the variations of soil textures and more than 80% of the variations for the case of volumetric soil water content. The decrease of the accuracy of the regressions with the volumetric soil water content can be associated to the variability of the values of bulk density, which is not only associated to the textural changes, but to structural changes that can occur in the same type of soil (Cid et al., 2011; López et al., 2016).
It is recommended, for the practical use of these equations, to use those obtained for gravimetric soil water content and then convert the values to volumetric soil water content using the specific density data for each site.
Other authors have also pointed to the possible relationships that can be established between the variation of the size of the predominant particles in the soils that characterize their texture and the soil water content corresponding to different values of tension or hydraulic conductivity (Zotarelli et al., 2013). These relationships have been called pedotransference equations. The functions developed for European soils by Wösten et al. (1998), Jarvis et al. (2002) and Zotarelli et al. (2013) can be cited as examples.
In Cuba, Ruiz et al. (2006), define the pedotransference functions among the methods used to determine the hydraulic properties of soils. In particular, Medina et al. (2002) defined pedotransference functions to estimate the values of the water- tension curve from basic soil physical properties, but only for one type of soil (Ferrallitic).
The fundamental contribution of this work to these antecedent studies is that it involves a wide range of soil types. Besides, it allows the direct estimation of the soil water contents from a single basic soil physical property (the predominant particle size). That define the limits of the total soil water reserve available for the plants, which is essential for irrigation scheduling.
FIGURE 1 Regression curves between the soil moisture values at field capacity, Cc, and permanent wilting point, PMP, and the soil predominant particle sizes: A- for the gravimetric soil water content; B- for volumetric soil water content.
The parameters of the logarithmic equations found were used to represent the tendency of the variation in the soil water content and the water depth corresponding to the parameters that define the soil water availability for the plants, depending on the variation of the soil textural classes (Figure 2).
In this case, the different textural classes, that could be obtained from a laboratory textural analysis, have been represented, as defined in the Cuban Soil Interpretation Manual (MINAG, 1984).
These figures are useful to have a quick approximation to the values that can be expected from the soil water availability for the plants according to the soil textural classification.
A comparison of these results with the classical scheme presented by Hillel (1998), for these relationships (see Figure 3) allows us to define that the ranges of variation of soil moisture at Cc and PMP estimated for Cuban soils are higher, fundamentally for the textural classes corresponding to silt and clay. This may be associated with the characteristics of the predominant clay in many soils of Cuba that present considerable amounts of smectite and vermiculite, which causes them to tend to expand and contract according to the level of present soil moisture (Cid et al., 2004).
The results summarized in Figure 2 have an important practical value. They are a simple tool that can be used by technicians and producers to define the irrigation schedule of crops, especially, the definition of when and how much to irrigate. That would be derived from the knowledge of the predominant texture in the soil to the depth of 0.5 m, which can be defined from previous soil studies or with a specific textural analysis for the site under study.
However, for the extension of these results application in productive practice, it is necessary a strengthening of the soil textural analysis, that is carried out in different laboratories of the country, by different entities, so that the predominant particle size can be more accurately specified. In this sense, it is recommended to use a weighted average of the particle size, taking into account the percentage of particles present in each textural fraction.
FIGURE 2 Variation trend of parametres that define soil water availability for plants as a soil texture: A- for gravimetric soil moisture; B- for water depth.
FIGURE 3 General relationship between the soil water available for the plant and present soil texture, by Hillel (1998).
Recommendations for its Practical Implementation
Take advantage of the potential that is available from international projects and national programs to strengthen the textural analysis of soil laboratories and other entities of the agriculture system.
Train technical personnel directly related to soil and irrigation activity in the production base for the practical use of these relationships.
Use the relationships obtained as part of the tools included in the Irrigation Advisory Services and, in particular, for irrigation forecasting.
Perform field validations of these relationships, based on monitoring soil water content in control fields, using current easy-to-use techniques such as electromagnetic probes (TDR).
CONCLUSIONS
The relationships determined for the most important agricultural soils of Cuba, make it possible to predict, with an accuracy of more than 95%, the variation of the limits of the total soil water reserves and easily usable reserves for the plants based on the predominant particle size in their textural composition.
The ranges of variation of soil moisture to field capacity, Cc and permanent wilting point, PMP, estimated for Cuban soils, are higher than the ranges defined by international studies, fundamentally for the textural classes corresponding to silt and clay, which is associated with the characteristics of the predominant clay in many Cuban soils.
The results obtained have a high practical value as a basis for the efficient irrigation programming of crops and allow reducing the hard field and laboratory work involved in these studies, as well as to promote more precise planning of water resources available for irrigation.
