Sand soils are often dry, nutrient deficient and quick draining. They have poor (or no) ability to transport water to plants from deeper layers by capillarity, therefore cultivation of sandy soils in the Spring should be kept to a minimum to preserve the water in the seedbed. The water and nutrient holding capacity of coarse sands can be improved by adding organic matter.
Silty Soils 10% Clay
These soils differ from very fine sands in that they have a greater capacity to form crusts, which are usually very hard. Silty soils can, if overworked, become compacted, giving a lack of infiltration in wet conditions and becoming bricky/hard in dry conditions. They are on the other hand easy to till and can store a good quantity of water. A good reconsolidation is needed but must be avoided in wet conditions.
On these soils crusting can be very severe. The crust often becomes so hard that it has to be broken up. At a low clay and humus content, aggregate formation is often poor.
These soils have a high capillary action but the rate of capillary rise is low, so the water needs of the plants cannot be supplied via capillary water. These soils are darker in colour and the aggregation is more obvious. The aggregation reduces the risk of crusting. Medium clays must be cultivated at the right water content to be easily worked. The risk is to make clods if too dry or cause smearing if too wet. These soils usually have a good self‐structuring ability through climate and time. Soils
Heavy clays have a very high water-holding ability, but most of the water is tightly bound and unavailable to plants. The humus content is usually higher than in other mineral soils. They do not crust on drying. These soils have a very good self‐structuring ability through freezing/thawing and drying/wetting. In cold Winters the clay freezes apart and forms a very favourable aggregate topsoil structure. If the clay dries out without having been frozen, it can become rigid and difficult to work. On the other hand, when it is saturated it becomes sticky and very impermeable to water movement. Because of the high content of colloidal material, the nutrient status of the heavy clays is very high. Heavy clays need high reconsolidation around the seed when dry, but not when wet and plastic. The risk is working them when they are at plastic stage, which will cause compaction.
During any crop establishment, we look to create the most favourable conditions for the seed as possible. The aim is to achieve a strong and uniform emergence, of the maximum proportion of seed, as early as possible in the growing season.
To germinate, a seed needs water, oxygen and a suitable temperature. Germination begins by taking up water (the water content of the seed increases from 13-14% to 45-60%). The swollen grain germinates and if the temperature is high enough the development happens relatively quickly.
The ideal seedbed is to provide the seed with sufficient:
- protection, for a pest free environment
“The seed should be placed on a damp mattress, under a warm, airy cover…”
To make sure the seed will be provided with water, it is important to supply a good contact between seed and soil. This means the aggregates around the seed should not be too coarse. Adequate soil moisture is more difficult to guarantee for the small seeded crop, since it has to be available at shallower depth. In comparison to larger seeds, which tend to have a deeper optimum sowing depth.
The loose soil that covers the seed allows oxygen to be transported to seed and roots, while transporting away carbon dioxide from respiration. It also acts as a barrier to evaporation.
Soil temperature has an important influence on timing of seed germination and rate of seedling growth. A dry, porous soil is easier to warm up than a wet or saturated soil. The greater the amount of water present in the soil, the slower its temperature will rise in the Spring.
To make sure the seedbed is as free of pests as possible, it is important to use appropriate crop rotation and make sure the crop residues from previous years are decomposed.
Preparing The Seedbed
There are different techniques for creating a seedbed. Technique used depends on many factors e.g. available machinery, soil type, cultivation strategy or climate.
Ploughing in of top growth, cultivation of sowing depth with harrow/disc, conventional drilling, applying of fertiliser.
Ploughing & Rapid Drilling
Ploughing in of straw, shallow cultivation, drilling with seed and fertiliser at the same time.
Working in of straw by cultivation, drilling seed and fertiliser at the same time IN the soil/ straw layer.
Shallow working of straw into the surface, drilling with seed and fertiliser at the same time UNDER the soil/straw layer.
Drilling with seed and fertiliser at the same time, without previous straw treatment. Straw remains on the surface. Ploughing warms up the soil and buries the residues so that it will not be an obstacle to the seeding process. However, this can disrupt soil structure and oxidize organic matter. Without ploughing, the organic matter and structure is conserved, but straw may cause problems in seeding process and might transmit plant diseases.
Impact on crop:
- Reduced crop root penetration, resulting in poor short, weak plants
- Soil is made up of 50% soil 50% air, change that ratio and you will have problems
- Poor drainage and slow to warm up
- Considerably reduces fertiliser uptake resulting in wastage
- Causes surface runoff, leaching nutrients.
How to reduce compaction
- Limit heavy machinery, including Land Rovers and other 4x4s!
- Subsoil any maize ground but first check where the compaction is by digging a hole, and setting the subsoiler at the correct depth. Too deep will result in a soil structure collapse.
- Never subsoil when the ground is wet or saturated, as this will result in smearing the soil, squeezing out the air and creating a pan. Only subsoil between the topsoil and subsoil.
The fertiliser recommendations here are for a guide only and are based on a starting level of Soil Index 2. For soils analysing difference from this index, consult a FACTS qualified adviser for more accurate application rates.