Primary symptoms

Crop residues on the surface offer advantages of improved water use and reduced weeds, but can result in problems of land preparation and sowing.

Crop residue has its advantages and disadvantages.

Summary

Many farmers in Asia remove crop residues for livestock fodder and for cooking fuel. Others incorporate the residues during initial tillage operations or later, after they have been passed through animals to become farmyard manure. Burning is becoming more popular as it shortens turn-around time between crops. However, burning leads to reduced soil organic matter and degradation of soil physical properties, in turn increasing the likelihood of waterlogging, crusting and disease.

Residues up to 9 t/ha can come from a preceding rice or maize crop and this amount completely covers the soil with a layer several centimetres in thickness. As shown in the photo, 4 t/ha of residues will just cover the soil surface when spread uniformly. Zero and minimum till equipment is available to sow through deeper layers of residues and still achieve uniform seedling emergence.

The positive attributes of crop residues should not be overlooked. Used as mulch they conserve water, keep the soil cool and free from crusting. They also increase soil microbial activity and maintain or increase populations of earthworms, which in turn increase soil aeration. Retained stubble reduces wind and water erosion and can be grazed by animals, though not when the soil is wet as this can lead to the formation of a soil pan. The negative attribute of crop residues is that if managed incorrectly in wet soils they increase the incidence of disease, which may reduce yield. Root rots such as Rhizoctonia and Pythium species (see diseases) can be a problem. Furthermore, if trash from the previous crop was full of weeds the incidence of weeds in the current crop will also be significant.

Are crop residues a problem?

• Is plant emergence not uniform with sections of seedlings absent from the rows? Is the stubble or residue particularly thick in those areas?
• Is the crop pale with symptoms of nitrogen deficiency? Reports indicate that straw residues may immobilize nitrogen as they decompose, especially when incorporated.
• Are seedling diseases more in evidence than usual? Dig up some seedlings and check their roots. If they are brown or stunted, refer to the section on diseases to identify the problem.

Causes of residue problems

• Too great a thickness of residue remained on parts of the soil surface during seeding which the planting machinery could not penetrate. What method was used for planting and for spreading the residues from the previous crop? How thick were the residues?
• The stubble was diseased. Were there disease problems in the previous crop? Has the soil been wet for a long period, conditions that encourage take-all disease and Pythium and Rhizoctonia root rots? This may have shown as stunting and uneven height of the crop, poor tillering for the leaf number and yellow leaves.
• The planting machinery was not suitable. Did it block frequently?

Doing good things with residues

• Excess residue: Remove any residues that exceed 4 t/ha, equivalent to a thin uniform cover of the soil. Excess residues can be used to stabilise furrows and for general control of erosion as well as for animal fodder and cooking fuel.
• Machinery: Use machinery that has the capacity to sow through thick residues of greater than 4 t/ha.
• Diseases: If diseases have caused problems in the previous crop either incorporate the residues a long time before sowing to allow full breakdown of the straw or remove the residues, or in extreme cases, burn them on site.
• Crop rotation: Include a crop in the rotation that breaks the disease cycle.