Soil Aggregation

Soil fertility, one of the important determinants of agricultural productivity, is generally thought to be supplemented through the application of nutrients mainly through inorganic fertilizers. The physical fertility of the soil, which creates suitable environment for the availability and uptake of these nutrients, is generally ignored. The present study aims to characterize the soil physical environment in relation to the long term application of farm yard manure (FYM) and inorganic fertilizers in rice–wheat. The treatments during both rice and wheat crops were (i) farm yard manure @ 20 t ha−1 (FYM); (ii) nitrogen @ 120 kg ha−1 (N120); (iii) nitrogen and phosphorus @ 120 and 30 kg ha−1 (N120P30) and (iv) nitrogen, phosphorus and potassium @ 120, 30 and 30 kg ha−1 (N120P30K30) in addition to (iv) control treatment, i.e. without any fertilizer and/or FYM addition. The treatments were replicated four times in randomized block design in a sandy loam (typic Ustipsament, non-saline, slightly alkaline). Bulk density, structural stability of soil aggregates and water holding capacity of 0–60 cm soil layer were measured.The average mean weight diameter (MWD) was highest in FYM-plots both in rice (0.237 mm) and wheat (0.249 mm) closely followed by that in N120P30K30 plots. The effect of FYM in increasing the MWD decreased with soil depth. The addition of both FYM and N120P30K30 increased the organic carbon by 44 and 37%, respectively in rice. The total porosity of soil increased with the application of both FYM and N120P30K30 from that in control plots. In 0–15 cm soil layer, the total porosity increased by 25% with FYM from that in control plots. This difference decreased to 13% in 15–30 cm soil layer. The average water holding capacity (WHC) was 16 and 11% higher with FYM and N120P30K30 application from that in control plots. The MWD, total porosity and WHC improved with the application of balanced application of fertilizers. The grain yield and uptake of N, P and K by both rice and wheat were higher with the application of FYM and inorganic fertilizers than in control plots. The carbon sequestration rate after 32 years was maximum (0.31 t ha−1 year−1) in FYM-plots, followed by 0.26 t ha−1 year−1 in N120P30K30-plots, 0.19 t ha−1 year−1 in N120P30 and minimum (0.13 t ha−1 year−1) in N120-plots.

We studied the impact of integrated nutrient management practices on the physical properties and structural stability of soil aggregates, and the associated C contents after 18 years of rice–wheat rotation on a sandy loam soil at Project Directorate for Farming Systems Research, Modipuram. Treatments included fertilizer nutrients (NPK), NPK with Zn and/or S; and partial (25%) substitution of N with farmyard manure (FYM), sulphitation press mud (SPM), green gram residue (GR) or rice/wheat residue (CR) in various combinations. Soil aggregate properties and its stability, aggregate associated and particulate fractions of C at 0–7.5, 7.5–15 and 15–30 cm depths were studied to document C sequestration potential of different nutrient management options. The aggregate strength and density were lower with organic substitution (p < 0.05) while water retention by aggregates at field capacity was 2–4% higher with organic inputs. Macroaggregates (>0.25 mm) constituted 58–92% of water stable aggregates and varied significantly among treatments and soil depths. Organic material incorporation improved soil aggregation and structural stability and resulted in higher C content in macroaggregates. The strong linear positive response to C additions indicated C sequestration potential in soils, with preferential location in macroaggregates. However, the kind and source of organic inputs strongly influenced both the soil aggregation and C accumulation in aggregates. A combination of GR in rice and FYM in wheat significantly improved C content in macroaggregates, and residue incorporation was beneficial compared to 100% N application through inorganic fertilizer or GR to rice. Coarse particulate organic matter (cPOM, >0.25 mm) accounted most of the increase in C content within macroaggregates and was substantially higher with CR incorporation. A relatively higher C content in microaggregates-within-macroaggregates (isolated following Six et al., 2002a) in organic-amended soil implies potential in bringing higher C stabilization in intensive rice–wheat system through combination of inorganic and organic fertilizers and crop residues.

Soil aggregation plays a crucial role in soil physicochemical and biological processes, thus influencing soil nutrient retention. It is possible to improve soil aggregation by choosing appropriate agricultural practices. The objective of this study was to evaluate the effects of tillage, crops and fertilizer sources on size distribution of water-stable aggregates (WSA) and nutrient concentrations in aggregate fractions. Soil samples

We investigated C management index (CMI; an indicator of sustainability of a management system and is based on total and labile C) and soil aggregation in medium-textured soils (silt loam and silty clay loam) under different cropping systems as follows: maize-wheat (M-W), rice-wheat (R-W), soybean-wheat (S-W), Guinea grass, and Setaria grass. Field experiments were 6–32 years long and were located in the wet-temperate zone of northwest Himalayas. The plant nutrients were applied through chemical fertilizers (urea, superphosphate, and muriate of potash) with or without organic materials (FYM, wheat straw, and Lantana spp.). The content of total C (CT), labile C (CL), CMI, mean weight diameter (MWD), and aggregate porosity varied significantly under different cropping systems. The range was 1.59 (R-W)–4.29% (Setaria) for CT, 1.23 (R-W)–3.89 mg/kg (Guinea grass) for CL, 52.09 (R-W)–129.77 (Guinea grass) for CMI, 0.90 (R-W)–5.09 (Guinea grass) for MWD, and 41.5 (R-W)–56.8% (S-W) for aggregate porosity. Aggregate porosity was highest (56.8%) under S-W, followed by grasses (50.1–51.2%), and M/R-W (41.5–50.0%). As per these data, (a) continuous use of N alone as urea lowered soil sustainability over control (no fertilizers); (b) use of NPK at recommended rates improved soil productivity over control; (c) the NPK + organic amendments further improved soil sustainability; and (d) the sustainability under different cropping systems followed the order: perennial grasses > soybean-wheat > maize-wheat > rice-wheat.