Mulches, stubble

Agricultural management that does not rely on tillage, such as bare-fallow and stubble-mulch practice, can greatly reduce carbon losses (Cambardella and Elliott, 1992). Moreover, converting tilled land to no-till agriculture can lead to rapid... 

Researchers working for the Nebraska Agricultural Experiment Station developed the stubble-mulch or crop residue mulch practice, a highly effective method of soil erosion control (Douley and Russel, 1939). Stubble mulching used subsurface tillage implements that left crop residues on the soil surface and provided some protection against wind and water erosion (Zingg and Whitfield, 1957). 

Zingg, A.W. and C.J. Whitfield (1957). A summary of research experience with stubble-mulch farming in the western states. Washington, DC USDA, Agric. Tech. Bull., 1166, 56 pp. 

Growth inhibition of plants where crop residues are undergoing decay has been observed. For example, McCalla and Army (1961) observed that where wheat straw was left on the soil surface in a stubble mulch system of farming the growth of corn was sometimes retarded. The effect was observed on the very young plants which never recovered during the entire season. 

The scientific basis for most of these benefits resulting from reduced tillage that are not obvious have been discussed in this chapter. The phases of the subject that relate more specifically to stubble-mulch farming are considered in the chapter that follows. 

The term mulch, as used here, refers to any naturally-formed, undisturbed soil covering any material added to serve as a soil cover and to crop residues left in part on the surface as dead or dying materials. This definition is, therefore, broad enough to include essentially all organic and inorganic materials that serve as soil covers. The main emphasis here, however, is on conventional mulches of organic materials, where the soil is usually completely covered to various depths by plant materials added from outside sources. Reference will also be made to stubble- or trash-mulches, where the soil surface is usually only partially covered. Whether the soil is completely or only partly covered, most of the effects produced are very similar, but differ in degree. 

Under Nebraska conditions, Teotia et al. (1950) observed a two- to five-fold increase in numbers of earthworms in stubble-mulched soil over plowed soil. On straw-mulched plots earthworms increased with the rate of straw application up to the maximum application of 8 tons per acre. 

Stubble-mulch farming is commonly defined (McCalla and Army, 1961 Horning and Oveson, 1962) as a year-round way of managing plant residues on cropland in which harvesting, cultivating, seedbed preparation, and planting are performed in such a way as to leave the residues of the previous crop on top of or near the soil surface. 

Residues left on the soil surface decompose at a slower rate than when they are buried, and under some conditions considerable residue may remain from one season to the next (Brown and Dickey, 1970). This is of course very desirable in a stubble-mulch stem of farming. Such resistance to decay is dependent upon the kind of residue, temperature, moisture relations, and extent of soil disturbance during the tillage operations. 

Various types of machinery are used in the stubble-mulch system of farming, depending on the crop grown, soil, climate, amount and type of residue, and local conditions. Carter and McDole (1942), Duley and Russel (1948), Jacks et al. (1955) Horning and Oveson (1962), and many others have described the leading types of equipment used. Of course not all of this equipment is suitable for use in any one farming operation. 

Tillage operations in connection with the stubble-mulch system are ordinarily kept to the minimum necessary to prepare a suitable seedbed, and to keep weeds under control. Usually the sweep or blade is used for lifting the soil initially and killing the weeds. It operates at depths of up to six inches with subsequent tillages with various implements at lesser depths. Draft requirements for the sweep cultivator are usually about 70—80% as great as for the moldboard plow operating at the same depth. 

Comparisons of stubble mulch with complete chemical fallow systems of farming have usually given results that favor the former system. For example. Black and Power (1965) reported that wheat yields on stubble-mulched soils were higher than where spray was used. Furthermore, neither chemical fallow nor combinations of chemical and mechanically-tilled plots resulted in more moisture conservation than with conventional stubble mulch. Wind erodibility was, however, less with complete chemical fallow soils, doubtless because of complete absence of tillage. 

Stubble mulching with adequate amounts of plant materials can markedly reduce... 

The main benefits from leaving crop residues on the surface are in the reduction of wind and water erosion their effects on evaporation are usually minor. When rain falls at comparatively short intervals, mulches of plant residues retard loss by evaporation for a time, but after the surface moisture has disappeared the stubble-mulched soils lose moisture about as fast as a bare soil. This is true chiefly because there is little movement of water by capillarity from the deeper horizons. The extra retention of surface moisture, even if temporary, is often beneficial to seed germination and also to increased infiltration and downward movement of rainfall. 

Studies reported by Army et al. (1961) showed that the soil moisture content below 2 inches was not materially increased by surface residues. Evaporation losses during the summer months from both stubble-mulched and bare fallow soils constitute a high percentage of the total precipitation. A summary of many experiments conducted in the Great Plains showed losses during the fallow period of 76% of the precipitation. In other tests 83% of the rainfall was lost between the harvest of one crop and the seeding of the next (Mathews and Army, 1960). 

Bondy (1968) reported that some farmers in central Kansas are successfully growing continuous wheat, grain sorghum and soybeans according to the stubble-mulch system even without the use of herbicides. The main purpose is to prevent wind and water erosion, but better infiltration also leads to more efficient use of the rainfall. 

The stubble-mulch system is a variation of the complete mulch system (see Chapter 25). Such a mulch seldom covers all of the soil surface, and hence does not absorb all of the raindrop energy. Nevertheless, it is very effective and, furthermore, helps to retard the movement of flowing water. Obviously, the heavier the preceding crop, the better the soil coverage by the mulch is likely to be, and the greater the erosion protection. Any exposed soil may be puddled, but that under the residues is free to absorb water at a rate little affected by the rain. Many experiments have shown that as little as 700 lb. of straw... 

A soil that is cropped and heavily mulched with straw or other suitable plant material is protected from erosion in much the same manner as are forest soils. There is no raindrop splash and little if any runoff. Such soils are nearly always well aggregated and capable of rapidly absorbing any water that reaches them. Where the mulch is thin, as in stubble-mulch farming, there is less protection of the soil but the water is held back long enough for most of it to infiltrate into the soil. 

---