Sandy Soils and Crop Residues are Perfect Companions

Crop residues used as mulch are known to benefit crop growth. How this benefit occurs has not been fully examined. A study at the ICRISAT Sahelian Center in Niger attempted to answer that question. The study compared the chemical and physical characteristics of soils planted to pearl millet (Pennisetum glaucum) but subject to different management over a five-year period. The soil management treatments analyzed were millet residue as surface mulch; P and N fertilizer; and fertilizer plus residue.

Many soils in West Africa's semi-arid Sahel are eolian sands over lateritic gravel, sandy to sandy loam in texture, with low clay and organic matter content, low cation exchange capacity, little available phosphorous, and relatively high aluminum saturation. Research in the region has shown that millet responds favorably to nitrogen fertilizer once its phosphorous requirement is met. While manure for use as an organic soil amendment is in great demand by farmers, its supply is limited. Millet crop residue is more readily available, although it is used as fuel and livestock feed as well.

Residue Management

The objectives of a study to compare residue and chemical management of sandy soils were to examine differences in soil chemical traits after five years of surface mulch and/or fertilizer additions; and deduce possible mechanisms that increase millet biomass yield upon addition of crop residues.

After five years of treatment, soils in the test plots were sampled in 10-cm increments to a 120 cm depth. The residue treatment was aboveground stalks and leaves of millet grown in the same plots the previous year. The P+N fertilizer treatment was simple-super phosphate at 13 kg/ha, plus urea at 30 kg/ha applied annually. The third treatment combined all of these inputs. Control plots received none of these treatments.

The biomass yield of pearl millet from fertilizer-only plots (1396 kg/ha) was slightly less than the yield from the residue-only plots (1662 kg/ha), and much lower than fertilizer-plus-residue plots (4,027 kg/ha).

Residue Changes the Soil

Soil pH was significantly higher in the plots where millet residue was applied as mulch, compared to the fertilizer-only and control plots, but only in the upper 10 cm of the soil profile. A complete absence of exchangeable aluminum in the upper 10 cm was observed in treatment plots receiving crop residue. Plots subject to the residue treatments also exhibited significantly higher exchangeable Ca and K contents (Table 1).

As for available phosphorous, plots receiving either residue treatment had significantly more P than the control plots in the upper 10 cm. Available phosphorous levels in the fertilizer-only and the residue-plus-fertilizer plots were greater than the residue-only treatments to a depth of 20 cm (Table 2).

Organic carbon levels in the top 30 cm of the treatment plots ranged between 1.7 and 2.2 g/kg, with no significant differences between treatments. The researchers suggest that these organic carbon levels were at equilibrium which, in the Sahel, is influenced by climate and termite activity. Termites are known to consume surface mulch within one year of its application.

Just the same, available results from research conducted in the Sahel on crop residue management bring an important hypothesis to question: An increase in millet yield after residue additions increases phosphorous availability due to the binding of aluminum by soil organic complexes.

Recognizing micro-topographic differences between plots with and without residue additions, ICRISAT researchers suggested three ways that residues improve soil fertility: As a source for nutrient recycling; as a trap for wind-blown soil materials; and as soil surface stabilizers.

Previous studies at ICRISAT/Niger suggested that enhanced millet growth in plots where crop residues were added could be due, in part, to an increase in nitrogen-fixing soil bacteria. The reasoning is that more bacteria increase root surface area, nitrogen content and uptake of phosphorus by millet, a major factor affecting growth.

Finally, the authors refer to earlier research in Niger by Chase and Boudouresque (1987) which demonstrated that the entrapment of wind-blown materials on denuded, crusted soils by crop residue results in natural revegetation, improved soil fertility, and better water infiltration. Other research in Niger by Drees et al (1990) suggested that air-borne dusts, if entrapped, can add up to six kg/ha of calcium, and two kg/ha of potassium to soils. All of the findings reviewed here have clear implications for leaving or removing crop residues from fields following harvest.

Geiger, S.C., A. Manu, A. Bationo. 1992. Changes in Sandy Sahelian Soil Following Crop Residue and Fertilizer Additions. Soil Science Society of America Journal. Vol. 56, N. 1. Jan-Feb.