There are three types of problems that occur at planting:
- Land may not be cultivated properly, so that there are clods or crusts, which prevent planting at a uniform depth and even germination.
- Preparation is too long before planting or is patchy, so that the weeds have an advantage over the crop.
- Seeds are planted at the wrong depth.
The primary purposes of land preparation prior to planting are to create a soil structure favorable for crop growth, to incorporate residues, and to control weeds and diseases. In many areas, the soil structure is adequate to allow good growth without cultivation, as long as weeds are controlled by other methods. Residues can often be left on the soil surface, if they do not make other field operations difficult.
A good planting method is one that allows the seed to be placed at the correct depth and provides good contact between seed and soil. The correct depth of planting is deep enough to allow the seed to take up water, to protect it from desiccation or birds, and to prevent it from germinating with light rains, but shallow enough to allow the seedling to reach the surface before depleting its food reserves or being attacked by soil insects or diseases. The suitable planting depth for lowland tropical maize is usually about 5-7 cm, but as deep as 10 cm is often acceptable, if the seed is large and healthy. When planting to use residual moisture, especially in highland areas, farmers may place seed at depths of 20 cm. However, specialized varieties are needed for planting at such depths if the seed is covered completely. When seed is planted in dry, bare soil in warm areas, the depth should be around 10 cm to avoid high-temperature damage.
Is seedbed preparation or planting method a problem?
Determine if plant density or distribution is a problem (see plant density and plant distribution)
Note: These observations should be made shortly after emergence, when the maize is at stage V1-V2.
- Look over the field. Is emergence uniform? If not, dig up seeds in the areas where emergence is poor. Use the “Identify the problem” Key to interpret what you find. If the seeds have germinated, measure the length of the mesocotyl in areas where the plants have emerged and in the areas where the plants have not yet appeared. If the lengths differ by more than 3-4 cm, then the planting depth was quite different in the various parts of the field.
- Walk through the field. Have the seedlings been pulled out of the ground and eaten? Look for signs that birds or rodents have been in the area. If the seeds have been pulled up, planting depth may have been too shallow or the soil was not firmed adequately over the seed.
- Look over the field. Are there weed seedlings that are bigger than the crop? If so, the weed control provided by land preparation was not adequate-the crop was probably planted too long after the field had been prepared. Another possibility is that the preparation was not thorough, and weeds were only buried rather than killed.
- Is the spacing uniform? Mechanical planters can block up, leaving gaps or overplanted spots.
- Look over the field and note the size of clods left from the cultivation. A seedbed with many clods larger than 6 cm in diameter can cause problems of variable seeding depth, poor contact between the seed and the soil, and physical impediments to emergence.
- Check for crusting. If rain has fallen recently, it may not be possible to see the crust in the field. Dig up plants in areas of poor germination, and look for knotted, twisted, pale seedlings which could not break the crust. Look for a silt layer on the surface of the soil.
Possible solution: Low-quality seedbed
Note: Often the farmer does not own the equipment used to prepare the land, and cannot specify just when cultivation will be done. You need to consider this limitation when you are designing solutions.
- To reduce clods: develop guidelines for when soil should be worked, based on moisture content (see Table 4 below). Use secondary tillage to reduce the size of the clods.
- To reduce crusting: reduce the number of secondary tillage operations, and leave some crop residues in the upper soil layer. Another solution is light, superficial cultivation to break the crust after planting but before crop emergence.
Use reduced or zero tillage to minimize problems of land preparation.
Possible solution: Poor planting
- Change or adjust planting implements or land preparation to provide the right depth.
- Change land preparation to reduce clods and improve seed-soil contact, or add a compaction practice to firm the soil over the seed. Note that if the soil is clay and if it is wet at planting (> 50% available moisture), the soil should not be firmed after the seed is placed in the hole, since the seedling may not be able to emerge through the hardened crust.
- Perform tillage operations when the soil is in the correct moisture range (Table 4).
Table 4. Approximate soil moisture contents appropriate for different tillage operations, based on soil textural class. (Identify textural class and estimate available soil moisture.)
|Textural class||Primary tillage operations||Secondary tillage operations|
|Clay loam (fine)||25% (near wilting point)||Within a day after primary tillage|
|Loam (medium)||25-40%||Within a day (maximum of two days) after primary tillage|
|Sandy loam (moderately coarse)*||25-60%||30-50%|
|Sandy (coarse)*||Up to 60%||30-50%|
* In sandy soils, hard pans due to equipment traffic at incorrect moisture can develop at depths greater than is the case for clay and loam soils.