It varies enormously from one soil to the other. In fact the water soil pH found varies from to 4.2 to 9.2, that is to say from acid to a very basic soil (Baize, 1988). The more acidic pHs are found in depth of luvisols and plinthosols, and are associated to the deep clay floor. The minimum values are in the cambisols’ C horizon on schist substratum.
Vertisols and kastanozems present a neutral pH on the surface, which become basic in depth due to the CaCO3 increase. In the solonetz or daya’s soils these pHs are much higher because of the salinity and they reach 9.2 in profile 408.
As described later, these extreme pH values will be at the origin of several nutrient deficiencies.
KCl pH follows the same variation of water pH. The difference between the two varies from 0.5 to 1.5 in the illuvial horizon.
The organic matter, expressed by the organic carbon, is generally low. According to Metson classification (1961; in Landon, 1984), it ranges from very low to medium and varies from 4.55% in the first 10cm of profile 311 to 0.39% in the 404 surface horizon, with a clear difference between cultivated and forest soils.
In fact, in the cultivated soils, a small amount of the organic matter is given back to the soil. The thatch is grazed after the harvest so the roots are the only plant part given back to the soil. Thus, year by year, the organic matter deficit increases. This explains the reason for the practice of fallow rotation by the farmers.
In the vertisols, which are all cultivated, the organic carbon does not exceed 2% but it is uniformly distributed in the profile. The same remark is made for the kastanozems.
For the luvisols this value is clearly higher in forests; it reaches 3.5% in the first 15cm of profile 125 and it does not exceed 2.37% (profile 422) in the bushes. In cultivated soils, it reaches a maximum of 1.9% in the first 20 cm of profile 212. Plinthosols follow the same luvisol repartition but with smaller values.
The C/N ratio is generally satisfactory if 10 as average is chosen. Most of the soils have values between 10 and 14 in the first 10 to 20cm. These values fall quickly to 6 or 7 in the underlying horizons up to 40cm. So, organic matter humification and mineralisation coefficients can be considered as being higher. This is quite normal in a semi-arid climate as the study area one.
Nitrogen values are low to medium, rarely exceeding 0.3%. The majority of soils, in the surface horizon, have values smaller than 0.1%. The cultivated soils are less rich than soils under natural vegetation despite the nitrogen fertilisers input. This effect is due to the sampling date which coincided with the end of cereals’ cycle, when all the nitrogen is concentrated in the plant upper part. Moreover acid soils have lower values than basic soils; this could be partially explained by the low microbiological activity in acid soils.
Phosphorous quantities are very low, generally less than 10ppm. They are higher in cultivated soils than in soils under natural vegetation because of the fertiliser input (still low if compared to the uptake). Moreover in acid soils they are clearly lower than in basic soils. This is due to the iron and aluminium fixation and also to the analysis methodology adopted (Olsen’s methodology) which is more indicated for neutral or/and basic soils.
A particular mention is for profiles 118 and 306 where the phosphorous values are very high, respectively 121 and 127ppm.
Although in general Moroccan soils are known as calcareous soils, those of the study area are an exception. In fact plinthosols, luvisols, cambisols, and leptosols are totally lacking.
Vertisols are divided in the following three groups:
Kastanozems have the same CaCO3 repartition than vertisols. They vary among profiles according to CaCO3 distribution: in some of them CaCO3 is present from the surface (118, 205, 207, 213, and 405), in some less deep ones (208, 222, 417, and 418) it is present only below 30cm, while other ones are decarbonated. Profile 406 is the only one with traces through all the profile.
The CEC values greatly vary from one soil kind to another and also from the surface to depth. The minimum founded is 2.3cmol/kg in the eluvial horizon of profile 215, while the maximum is 66cmol/kg in the vertic horizon of profile 403.
The CEC is a value affected by organic matter and clay content. If we suppose an organic matter’s CEC equal to 200cmol/Kg, the remaining part can be attributed to clay. Therefore we can deduce the CEC value which is derived from the clay contribution, thus obtaining information about clay mineralogy.
By kind of soil, these values are the following.
Plinthosols. The CEC values perfectly follow the texture ones. On the surface they are generally around 10cmol/kg (with an average clay content around 10%) and rarely exceeding 20%. These relatively high values can be attributed to the abdundance of organic matter of the surface horizons. That is confirmed by the decreasing of these values to about half in the underlying horizons which are less rich in organic matter. In the plinthic horizon, poor in organic matter, these values increase a little, but still remain lower than in the surface.
This clay CEC is about 40cmol/Kg in all the profile. Consequently this CEC value give an idea about the clay mineralogy which is in this case dominated by 2:1 lattice minerals, principally illite.
The exchange capacity is almost saturated with exchangeable bases, except for profile 117 which is desaturated in depth, and has a base saturation percent of around 40% in the plinthic horizon. This exchange acidity is dominated by aluminium which show a toxic values at this level (35% of CEC).
On the surface this CEC is almost totally saturated with Ca++ while in the plinthic horizon it is shared at 40% with Mg++. So on the surface horizon the ratio Ca/Mg is very high (>5) and in the plinthic horizon it becomes very low and sometimes decreases to less than 1. In both cases this disruption of the two element values can create some availability problems for one of them.
Luvisols. The CEC values increase with depth with a slight increase in the surface horizon, rich in organic matter. Indeed, on the surface the values are around 10cmol/kg of soil and they increase to 15 in the Bt horizon. Clay CECs are higher on the surface (average of 50 to 60cmol/kg) and lower in depth (average of 36cmol/kg) showing clay mineralogy with illite kind. The complex is generally saturated through all the profile, apart from profile 102 which is saturated at 70% in the Bt horizon, and around 50% in depth.
In the surface horizon Ca++ is largely predominant on Mg++, with ratios between 3 and 9. On the other hand in the Bt horizon this rate greatly decreases and the Mg exceeds Ca, as in profiles 102 and 112.
Vertisols. The CEC values are medium to high (18 to 65cmol/kg). Clay CEC value is 80cmol/kg so showing a montmorillonite clay kind. They are completely saturated by Ca++ and Mg++.
Kastanozems. The CEC values are medium to high with an average of 30cmol/kg. They are almost uniformly spread in the profile. Clay CEC is very variable from one profile to another. The exchange complex is totally saturated with exchangeable bases.
Cambisols. The CEC values are low to high and vary from 7 to 27cmol/kg in the profiles developed on schist substratum. In the other groups (ferralic cambisols) they vary between 2 and 16. For the first group clay CEC is around 40cmol/kg indicating an illite clay kind. For the other group (profiles 103 and 124) this CEC is very low and shows a kaolinitic clay kind. The exchange complex is totally saturated by the bases.
Solonetz. Their CEC is high indicating a montmorillonite clay kind. This exchange capacity is saturated for more than 20% by Na+ (up to 60% in profile 123).
The salinity values are generally low in all the soils (<0.4dS/m), except in the solonetz (5.3dS/m in the natric horizon) and in some other soils close to the depressions.
Naturally texture values greatly vary according to the soil kind. Generally we can consider these points: