Rootable depth is a very important parameter in determining the quantity of water and nutrients that can be stored in the soil profile and the ability to support plant growth during periods of no rainfall or when irrigation ceases. Soil depth is also important in determining the ability of the profile to support plant rooting, and the ability of the crop to withstand wind damage. Under irrigated conditions soil depth affects drainage, aeration, and water retention properties. Deep soils favour drainage and are therefore optimal for irrigation of dry land crops.
Crops cannot be expected to yield satisfactorily beyond critical ranges of soil depth. Engelstad et al. (1961) found a positive correlation between soil depth and yield, which was also confirmed by Lindstrom et al. (1986). Stewart and Nielsen (1990) indicate that a soil depth of about 30‐40 cm is the minimum depth for maximum yields of all crops. Significant decreases in production are evident for all crops where soil depth is less than 30 cm.
On the other hand, there is an optimum soil depth for crops. According to ILCAO (1989), the optimum soil depth for maize rooting is 50‐90 cm. Landon (1983) indicates that maize roots can reach up to 200 cm but the main uptake is within 80‐100 cm soil depth. Soil depth for high soil fertility should be greater than 100 cm and in conditions of low soil fertility should be greater than 150 cm (Maize Production Manual, 1982). Calvino et al. (2003) conducted a study aimed at investigating the influence of water availability and technological changes over 15 years on the yield of dry land maize crops in the Argentine Pampas. They found that shallow soils presented lower yield than deep soils at a given rainfall.
Mayaki et al., (1976) found that the root development under irrigation and rainfall conditions varied. The author indicates that in irrigated maize, 54% of the root dry matter was in the upper 30 cm with 92% between 0‐90 cm. In non‐ irrigated maize, 30% of the total dry matter was in the upper 30 cm and 70% was in the range of 30‐90cm depth. No maize roots were found below 150 cm. Selkhozpromexport (1980) investigated the relationship between the soil depth and some cereal crops (barley, wheat, maize and sorghum) in north‐east of Libya. The investigations showed that a soil depth of 50‐80 cm is the most favourable for the growth and development of cereal crops (Figure 5.1). Selkhozpromexport observed that productivity of barley and wheat were around 40% in soil depth 0‐30 cm, while the productivity increased to reach more than 90% in soil depth 50‐80 cm.
GMRP (2002) made similar claims in trials set in the North West of Libya in the experimental farm of Fem Melga. The findings of the study confirmed that crops grown at a soil depth of less 30 cm gave the lowest production, whereas the yield markedly increased in soils with depths of more than 50 cm. Mahmoud (1995) states that crop production decreases with the decrease of soil
depth especially in soil where there is a hard pan layer under the top soil (Table 5.6). 0 20 40 60 80 100 0-30 cm 30-50 cm 50-80 cm Soil Depth cm Productivit y % Barley Wheat Maize Sorghum
Figure 5. 1 Productivity response of field crops to varying soil depth Source: Selkhozpromexport (1980)
Table 5. 6 Soil depth and crop production in Libyan Soils
Sub soil layer Soil depth
(cm) The decrease in crop production (%) > 150 0 100-150 0 50-100 20 30-50 20 No hard pan < 30 50 >150 0 100-150 0 50-100 10 30-50 30 Hard pan < 30 60 Source: Mahmoud (1995) It has been suggested that a soil depth of less than 30 cm vastly decreases the crop yield In addition, it is agreed that a soil depth between 50 cm to 80 cm increases the crop yield (Selkhozpromexport, 1980; Stewart and Nielsen, 1990; Mahmoud, 1995; GMRP, 2002; FAO, 2002). Accordingly, the threshold values between land suitability classes for the four land utilisation types can be set.
The basis was the local studies and trials taking into account the studies in similar conditions. Table 5.7 show the suitability classes for the four crops and the critical limits between these classes.
Table 5. 7 Soil depth suitability ratings
Crop Highly Suitable
S1 Moderately Suitable S2 Marginally Suitable S3 Not Suitable NS Barley > 80 80-50 > 50-30 < 30 Wheat >120 120-100 > 100-50 < 30 Maize > 120 120-100 > 100-50 < 30 Sorghum > 80 80-50 > 50-30 < 30
Source: (developed by the author)
5-3-2 Moisture Availability (Available Water-Holding Capacity)
Available water holding capacity (AWHC) is an important agronomic and hydrologic characteristic of soils. It expresses how much water can be stored in soils for plants to use during periods without rain or irrigation. This gives an indication of the drought sensitivity of soils, a key criterion in the selection of suitable crop species and varieties. Available water‐holding capacity is defined as the volume of water retained between field capacity and the permanent wilting point (ILACO, 1989; Landon, 1984).
The available storage capacity of the soil and consequently the water application depth and irrigation frequency are determined by root depth and root distribution as well as by the moisture characteristics of the soil (ILACO, 1989). Landon (1984) states that AWHC of soil profiles can be used as a factor in land suitability classification and in irrigation planning provided the effective rooting depth, the percentage of root distribution and percentage of readily‐ available water is taken into account. He explained that whenever possible
AWHC values for suitability assessment should be derived from the particular crop varieties and cropping pattern on site. However, he introduced a general grouping for irrigation suitability (Table 5.8).
Table 5. 8 Groupings of AWHC values for irrigation planning Rating for Irrigation Suitability AWHC (mm m-1)
Low < 120
Medium 120 – 180
High > 180
Source: (Landon, 1984)
Calvino et al. (2003) state that there is a strong association between yield and available water during the period bracketing flowering in maize crops, adding that grain yield in the study area has steadily increased during a 15 years. Similar results were reported by Leeper et al. (1974) for the U.S corn belt. Hatten and Taimeh (2001) selected AWHC as a land characteristic in their land suitability assessment for different land utilisation types in Jordan. In the
Jordan project, AWHC values of more than 150 mm m‐1 were considered the
upper threshold value and AWHC values less than 75 mm m‐1 the lower limit.
Selkhozpromexport (1980) made similar claims for cereal crops in the north‐east of Libya. Yeha (1982) confirmed that these limits can be used in suitability classification for irrigation in Libya. Table 5.9 shows the threshold values of AWHC for the selected crops.
Table 5. 9 Suitability ratings for Available Water Holding Capacity (AWHC) Available Water Holding Capacity
(AWHC) (mm m-1)
Suitability Classes
Barley Wheat Maize Sorghum
S1 > 150 > 150 > 150 > 150
S2 >110 – 150 > 110 – 150 > 110 – 150 > 110 – 150
S3 110 – 75 110 – 75 110 – 75 110 – 75
NS < 75 < 75 < 75 < 75
5-3-3 Nutrient Availability (Soil Reaction)
Soil reaction (pH) affects a wide variety of chemical and biological phenomena in soils. The effect of soil pH is great on the solubility of minerals or nutrients. Brady and Weil (1999) state that soil pH is the master variable of soil, since it influences many physical, chemical, and biological properties and processes. If the soil pH declines below a critical level, the solubility of aluminium and manganese ions increases, resulting in toxicity and lower yields. Soil acidity affects plant growth in several ways. Toxicity, caused by increased mobility of soil aluminium, is thought to be the most serious of these effects (Black, 1992). Aluminium becomes available when the pH drops below about 5.5. The cation exchange complex of soils becomes largely saturated with aluminium ions at pH 4.0. As a result, plants are deprived of essential cations (Foth and Ellis, 1997). Berglund (1996) states that a soil pH below 5 adversely affects roots and a pH below 4 severely restricts root growth.
If the pH is higher than 8.5 the soils are considered to be alkaline soils. This causes some essential nutrients such as magnesium (Mg) and calcium (Ca) to be unavailable. In addition, there is possible boron toxicity (ILACO, 1989; Landon, 1985).
Most plants and soil micro‐organisms thrive best in soils of pH 5 ‐ 7.5. However, it must be recognised that plant species and even varieties may differ
in the degree to which they favour or tolerate pH values beyond their range (Brady, 1974). In the next sections the toleration of the selected crops for soil pH is examined. • Barley Bland (1971) explains that as long as the pH is over 6.0 the barley can be grown successfully. Rayns (1959) suggested that it is safe to grow barley as low as pH 6.2. Mahmoud (1995) pointed out that barley is grown successfully in pH ranges between 6.5 and 7.8.
• Wheat
Landon (1985) states that the optimum pH for wheat is between pH 5‐7. Gardner and Garner (1953) suggested that wheat can be grown on pH 5. They added that actual figures can vary according to the soil texture.
Alsgear (1983) claimed that the optimum pH for wheat in Libya is 6.5 – 7.5. These limits are widely supported by the reports of ARC trials in the east of Libya.
• Maize
According to the Agriculture Compendium, (1989), optimum pH range for maize growth is 5.5 – 7.5. Bland (1971) states that maize can be grown successfully in pH range 5.0 ‐ 8.0. Similar margins are also indicated by the Maize Growing Manual (1982), Landon (1983), Purseglove (1985) and Berger (1962).
Bland (1971) claimed that for certain maize varieties a 100 % yield could be obtained in a pH range 4.0‐ 9.0 provided that there are no serious nutrient deficiencies, especially of micro‐elements. According to Purseglove (1985), maize yields could decrease where pH values are less than 5.5. The optimum pH range of soils for maize growth is between 5.8 and 8.0. Beyond pH 5.0, toxicities of Aluminium (Al), Manganese (Mn), and iron (Fe) may occur,
although maize is relatively more tolerant to levels of Al and Mn toxicity than other crops (Clark, 1982).
At high pH levels, nutritional problems with elements phosphorus (P), zinc (Zn), and iron (Fe) often occur especially in the pH range 7.4 – 8.0 (Clark, 1982). Chen and Barber (1990) find that with the increase of pH there was a decrease in P concentration in maize shoots and roots. The highest phosphorus (P) concentration was at pH 3.8 and the lowest phosphorus (P) at pH 8.3, both in shoots and roots. Also, the root length and surfaces were affected by low and high pH values. The largest root length and surface had developed at a pH value of 6.5.
Mahmoud (1995) confirmed these limits in Libyan soils. He states that the optimum range for maize is pH 6 – 7. These critical values were based on data and trials from the ARC in Libya.
• Sorghum
Landon (1985) states that the optimum pH for sorghum growth is between 5 – 8. Purseglove (1985) states that sorghum is tolerant of a wide range of soil conditions and pH for growth that is between pH 5 ‐ 8.5. In conclusion, the threshold values of soil reaction for the selected crops were selected by comparing published data from around the world, especially semi‐arid and arid regions. The information about pH was analysed together with data from trials by ARC in Libya and the threshold values were selected. Table 5.12 shows the threshold values of pH for the selected crops. These threshold values were based on the available information and ideally, field investigations should be conducted to confirm these limits.
Table 5. 10 Critical soil reaction limits between suitability classes (pH) Crop Highly Suitable S1 Moderately Suitable S2 Marginally Suitable S3 Not suitable Ns Barley 8 – 6.5 < 6.5 – 5.5 < 5.5 - 5.3 < 5.3, > 8.0 Wheat 7.5 – 6.5 < 6.5 – 5.5 < 5.5 – 5 < 5 , > 7.5 Maize 7 – 6 < 6 – 5.5 < 5.5 – 5 < 5 , > 7.0 Sorghum 8 – 6 < 6 – 5.5 < 5.5 – 5 < 5 , > 8.0
Source: (developed by the author)
5-3-4 Nutrient Retention