11 | 12 | 2017

Abstract Seedbed and sowing

Abstract Seedbed and sowing.

The seedbed is the soil environment in which crops establish and develop. It provides anchorage, water and nutrients and contains weeds, pests and the source of some diseases. During seedbed preparation the tarmer tries to produce conditions favourable to the cereal and disadvantageous to its competitors.

Before the discovery of herbicides, cultivations were aimed at reducing weeds. With the use of herbicides, the main role of cultivation is to provide soil physical conditions that lead to successful establishment and root growth. Timing is important; Witney and Elbanna (1985) have produced simulations of yield loss due to untimely sowing; within a fortnight of the optimum time, losses were less than 1 per cent. Six weeks after the optimum time, yield loss averaged 10 per cent for winter crops, 20 per cent in spring barley and 30 per cent in spring oats. The farmer has to balance the area he intends to sow against his sowing capacity and the likelihood of satisfactory sowing conditions. If his whole area is to be sown efficiently grouped about the optimum time, some fields must be sown early and some will be cultivated in less than ideal conditions.

When nutrients or water are in short supply, plant performance is related to the proportion of the soil profile that roots exploit. In wet conditions, compacted areas are anaerobic and may produce damaging gases such as ethylene and carbon dioxide. When the soil dries, roots have difficulty penetrating compacted areas. By loosening soil, cultivation has a beneficial effect on root growth (Lockhart 1971). Germination is dependent on seeds absorbing water from the soil. Close seed to soil contact is necessary; a cereal seedbed should be firm but it does not need to be as fine as that required by smaller seeds.

Different types of implements are used in the production of seedbeds. Ploughs invert soil, bury trash and leave a clean cloddy surface. Subsoilers break up pans and compacted areas. Discs cut up clods and turf. Cultivators break up clods; forward-raked tines loosen and may bring up unweathered soil, backward-raked tines crush and consolidate. Rollers, crumblers and flexi-coil flatten, consolidate and break up clods but their effect is limited to the top few centimetres.

Cultivations also have an effect on the biological balance. The incidence of volunteer cereals can be reduced if shed grain is given time to germinate and then ploughed in. Deep ploughing reduces weed pressure, it can also put infected take-all residues out of early reach of the next crop's young roots, thereby delaying and limiting infection. Slugs are damaged by cultivation and are less mobile in firm seedbeds.

Most sowing is done with conventional disc or Suffolk coulter drills. Sometimes fertiliser is combine-drilled with the seed or placed in a band nearby. If soil conditions are too weak to allow for normal cultivation and drilling, broadcasting can be adopted. Seed can also be drilled directly into the stubble using a special direct drill; its use has been limited by surface trash, poor soil structure, inadequate drainage, perennial weeds and slugs.

Ideally seed should be evenly distributed, but in practice most fields are drilled in rows. There is a slight yield advantage (up to 5 per cent) in favour of narrow rows (12 cm v 20 cm) but the narrow drills are more expensive and have difficulty with large stones and other obstructions. The optimum depth of sowing is 2.5 - 4.0 cm of settled soil.

Nutrition: Fertiliser provides the difference between soil supply and crop requirements. Soil analyses and their interpretation are provided by the advisory services. Assuming an accurate sample is obtained, the analysis can be used for several years; but fields should not be sampled if recently limed or fertilised as free nutrient can cause distortions. Routine analysis covers phosphate, potash, magnesium and acidity (calcium). Deficiency in trace elements such as copper is usually related to soil type but confirmed by analysis. Manganese deficiency should be confirmed by foliar not soil analysis. Low sulphur soils can be identified by soil analysis but deficiency should be confirmed by foliar analysis. Nitrogen analysis is used in some countries, but it tends to be unsatisfactory on soils that are wet or lack homogeneity. In Britain, previous cropping is generally used to provide a guide to soil supply. The ADAS Nitrogen Index (ADAS 1982a) has three levels, 0, 1 and 2, in Scotland these are called low, moderate and high (SAC 1985).

Lime: The optimum pH for wheat and barley is 6.0 - 6.5, oats is more tolerant of low pH. Above 6.5 trace element unavailability may lead to deficiency (Fig. 2). On light soils or those with a high organic matter content, the upper limit should be 6.3. Below 6.0, barley and to a lesser extent wheat are susceptible to acidity; below 5.5 crop failure is a significant risk.

On sandy soils or where heavy applications of ammonium fertiliser are used, pH tends to fall more rapidly hence it should be checked more often than once every five years. Small applications of lime every two or three years maintain pH in the optimum range and reduce the variation due to uneven spreading. As lime takes time to become effective it should be applied the year before the most sensitive crop is grown. It can be applied to the stubble, plough furrow or braided crop. If the pH is low the plough furrow is the best place as it is then worked into the seedbed. During hard frost it is often spread onto braided wheat to avoid tracking damage. Application to the stubble should be avoided on low pH soils, or if a single heavy dressing is applied, as the lime is ploughed under rather than being mixed into the seedbed.