The physical properties of any soil are function of climate, vegetation, parent material topography and time. Changes in frequency and intensity of tillage practices alter soil properties, distribution of nutrients and soil organic matter in the soil profile. These changes become stable with time and could affect availability of nutrients for plant growth, crop production and soil productivity (Turan et al., 2009).

Guo et al. (2000) reported that phosphorus (P) was significantly different among sites with time. Results suggested that variation within agriculture practices could influence soil nutrient changes among locations. Variation in texture could result from differences in mineralogy of parent materials; in this case variation is likely to affect the amount and distribution of phosphorus independent of cultivation history. This result suggested that shifting cultivators may have the more fertile sites originally and have only moved into a less fertile one.

Diaz-Zorita and Grove (2002) noted that available phosphorus increased also due to regular application of phosphatic fertilizer. Obviously, continuous use of N alone leads to sever depletion of other nutrient with corresponding decrease in grain yield. Without application of K, soil K eventually drop below the critical level (Zheng, 1999). Similar findings were reported by Elangovan (1984) who reported that K content in 100% NP was lower than that of 100% N alone and unfertilized soils due to the relatively higher crop yields and high removal of K. High K removal in the absence of K addition has resulted in drastic reduction of total K as well as available K under NP alone (Samra and Anand, 2008).

Due to the relatively higher crop yields and high removal of K, the K values were significantly decreased by time. Deficiency of Zn, Fe and Cu are the common feature of these soils. Being arid to semi arid area, about 75% of the cultivated area is irrigated while the rest is rain-fed. The micronutrients availability in soils is a function of the rate of replenishment from soil solids to soil solution. In all soils, replenishment of micronutrients takes place from earth minerals present in the pedosphere or lithosphere. The pedospheric variations due to parent material, topography, climate etc. lead to spatial variation of micronutrients in soils (Katyal and Sharma, 1991).

The distribution of micronutrients may differ among the profiles developed on different parent materials and landforms. With the introduction of high yielding varieties, the removal of nutrients, including micronutrients from the soils, is very large. Continuous use of nitrogenous and phosphatic fertilizers in the intensive cropping system with less use of organic manures results in quick depletion of micronutrients from soils (Dhane and Shukla, 1995).

Previous studies showed depletions in micronutrients (El-Fouly et al., 2010a) and magnesium (El-Fouly et al., 2010b), under different cropping systems and in different soils.

The objective of this study was to examine the long-term effects of intensive cropping system and N or NP applications alone on soil physical and chemical properties after 27 years of continuous cropping. It compared soil properties in 2008 with those of the same soils in 1981 (El-Fouly, 1989).

The soil samples were collected and air-dried, ground, sieved to pass through 2 mm sieve and analyzed. Soil samples were analyzed for texture with a hydrometer (Bouyoucos, 1954), for pH and electric conductivity (EC) using water extract (1 soil : 2.5 water) method (Jackson, 1973). For total calcium carbonate (CaCO₃%), calcimeter method was used as described by Alison and Moodle (1965). Organic matter (O.M%) content was determined according to Walkley and Black (1934) using potassium dichromate (Chapman and Pratt, 1978). Phosphorus was extracted using sodium bicarbonate (Olsen et al., 1954). Potassium (K⁺) was extracted using ammonium acetate. Micronutrients were determined by atomic absorption spectrometer after extraction according tor Lindsay and Norvell (1978). Same procedures were used in both years.

Soil CaCO₃ (%)

Table 4 shows that means of CaCO₃% were different among locations in 1981 and 2008. CaCO₃% ranged between 0.98 and 1.80 in 1981 and between 1.0 and 1.9 in 2008 at surface soil layer. On the other hand, William (2006) found that after 18 years, soils CaCO₃ contents decreased with time.

Phosphorus (mg/100 g soil)

The results presented in Table 5 show that the surface soil P concentration ranged from 0.6 to 1.4 in 1981, and from 2.7 to 4.0 in 2008. The average values of P content increased with time among sites. These results suggest that variation within agriculture practices can influence soil nutrient content. The increase can be attributed to the continuous use of P fertilizers.

Hung et al. (2001) found that long-term P application in the form of single superphosphate caused significant increments. Increases in soil P only in the 0- to 10-cm depth suggest that residual P fertilizer has accumulated mainly in the soil surface. Setia et al. (2009) reported that available P in the control plot decreased whereas in plots with added P, available P increased significantly after 11 and 22 years.

Guo et al. (2000). and Diaz and Grove (2008) found that the increase in P which accumulated at all locations in 2008 may be attributed to the repeated application of the N+P. McCollum (1991) documented that annual addition of 45 kg P/ha for 20 or 40 years to continuous smooth brome (Bromis inermis) on a silt loam soil resulted in increasing soil P concentration (150 to 250 mg P/kg soil) as compared to unfertilized. The concentration of phosphorus in soil is influenced by some other factors. A study conducted by Chen et al. (2003) concluded that the recycling of P was mainly driven by plant P demand and sustained by root and leaf litter inputs. Besides, seasonal changes in environmental conditions such as rainfall, soil moisture and temperature are also involved in the P availability. Microbial biomass also plays an important role in P availability.

Potassium (mg/100 g soil)

Data in Table 5 show that mean values of K varied between 47.7 to 81.1 in 1981 and from 23.3 to 55.5 in 2008 in surface soil layer. The decrease in soil K after 27 years of intensive cropping may be due to the continuous use of N or N+P fertilizer without any K addition in the soil at any cycle. Obviously, continuous use of N or P alone leads to severe depletion of other nutrients. The decreases in total K content were due to crop removal in the absence of external source of K supply through fertilizers and manure.

Ishaq et al. (2002) reported that the decrease was due to the relatively higher crop yields and high removal of K. High K removal in the absence of K addition has resulted in drastic reduction of total K as well as available K under only NP addition (Harry et al., 2000).

Available Fe

Mean values of Fe content varied from 7.8 to 22.1 among sites in 1981 and from 4.3 to 16.9 at depth of 0 to 30 cm in 2008 (Table 6). In general, mean values of Fe content decreased in 2008 in comparison with 1981. The decline in Fe status with continuous cropping occurred because of Fe removal by the successive crops from the native soil reserve. Khan et al. (2002) reported that continuous application of increasing level of N significantly depleted the available Fe content in the soil .These results agreed with the findings of Nayyar et al. (2001), Li et al. (2007), Machado et al. (2007) and Santiago et al. (2008).

Available Mn                                                           

Table 6 indicates that there were drastic differences among locations in Mn content in 1981 and 2008. In general, Mn contents at all locations showed decrement in 2008 compared with 1981. Mn content ranged between 12.2 to 44.4 in 1981 and between 1.92 to 7.8 in 2008 (Table 6). Khan et al. (2002) reported that intensive fertilization and cropping for a long time have brought remarkable decreases in available Mn in soil. The decrease in Mn content due to continuous cropping can be ascribed to Mn removal by successive crops without any addition from the external source. Cropping sequence year after year have created a favorable environment for strains of Mn oxidizing bacteria or fungi leading to faster depletion. Malhi et al. (1998), Cox et al. (2003) and Carter (2005) found that Mn was negatively correlated with pH and sand, whereas a positive correlation existed with EC, OM, CaCO₃ and clay. From this association, it can be inferred that addition of organic matter encourages the availability of Mn and as the soil become coarser, Mn deficiency become a problem (Setia and Sharma, 2004).

Available Zn

Table 7 indicates that all locations showed decreases in soil Zn content in 2008 compared with1981. Mean values of Zn ranged between 0.87 and 2.6 and from 0.4 and 2.2 in surface soil of 0 to 30 cm depth through 1981 and 2008, respectively. With the introduction of high yielding varieties, the removal of nutrients, including micronutrients from the soils are very large. Continuous use of nitrogenous and phosphatic fertilizers in the intensive cropping system, with low use of organic manures, resulted in quick depletion of micronutrients from soils (Santiago et al., 2008). The accumulation of Zn in surface soil layers might be due to: (i) the addition through plant residues left over by the soils which have also been reported by Katyal and Sharma (1991), Setia and Sharma (2004) and Verma et al. (2005). Similar results were obtained by Khan et al. (2002), Wright et al. (2007) and Ewa et al. (2009), where available micronutrient contents decreased with time.

Available Cu

Mean values of Cu content (ppm) in soil were differed among locations in 1981 and 2008 (Table 7). All locations showed decreases in Cu content in 2008 compared with 1981. The Cu content in 1981 varied from 2.73 to 6.34 and from 0.3 to 2.0 in 2008. Fertilizer addition caused a considerable decline in available Cu status irrespective of the level of application. Different treatments, continuous use of N alone resulted in a significant decline in the Cu content. Nayyar et al. (2001) also reported a decline in Cu content with increasing level of N, P and K in a long-term experiment after 22 cycles.

Long term cultivation of alluvial silty clay loamy soils of small holding under continuous tillage surface irrigation and unbalanced fertilizers use led to negative changes in nutrient contents in the soil. Balanced use of fertilizers, based on yield estimation, soil testing and plant analysis is recommended. Otherwise, continuous depletion of nutrients from the soil will continue, leading to yield decreases and low soil fertility.

The authors have not declared any conflict of interest.