Wheat and barley crop growth is influenced by a large number of factors. The main ones are: • soil mineral supply, • soil water supply, • heat unit accumulations, and • sunlight. All are important but at any time the supply of the one most in deficit will tend to limit crop growth and yield. Once you have selected your site, there's not much that can be done about the last of these - heat units and sunshine.
Soil mineral supply
Soil nutrient tests of the top 15 cm of cultivated soil are usually carried out each year before planting. These provide reasonably accurate advance information on the fertilisers required to supplement and replenish the soil's nutrient reserves. Some inaccuracies arise here because of the 'statistical' difficulty of sampling a large area of land in which soil quality can be very variable from place to place. Soil analyses are expensive.
Leaf analyses should be carried out later on in the season at tillering. These provide a second check on the adequacy of the soil nutrients and they are usually also better indicators of the actual availability of soil nutrients to the crop. They are particularly useful for assessing the nitrogen and trace element requirements.
These days, most growers manage soil mineral supplies quite well.
Soil water supply
Regardless of the adequacy of the soil's mineral status, if inadequate water is available during periods of high water use in summer, cereal yield will be severely 'capped'. For example, if a cereal crop is water stressed when water usage is high (30 mm/week) yield losses can be as high as 7.5% per week. For a crop with potential yields of 9000 kg/ha, this represents a loss of nearly 700 kg/ha per week of water stress.
Therefore, while attention to the 15 cm top-soil layer is adequate for managing the crop's mineral nutrient requirements, the deeper soil layers are also very important because the crop extracts water from much deeper down into the soil profile - down to about 100 cm. For example it has been shown that the top layer of soil (0-30 cm) provides only about 40% of the total water usage of a cereal crop, the next layer (30-60 cm) provides about 30%, and the next layer (60-90 cm) provides about 20%. The remaining 10% of water usage is sourced from deeper down still (below 90 cm) - if the roots can get down that far.
The amount of water available to a crop depends 1) on the soil type and 2) on the depth of the root zone.
- Soil type
Soil water-holding capacity depends on soil type (the proportions of sand, silt and clay). Clay and silt loam soils have the highest water capacities (about 20% by volume) and sandy soils much less (about 10%). Managements that raise soil organic matter will also raise the soil's water-holding capacity while those that reduce soil organic matter will tend to reduce it. For example with a silage crop, leaf and stem materials are removed whereas with grain crops more of this remains and can enter the soil to raise its organic component.
- Root-zone depth
The depth of the root zone can be badly affected by soil compaction below the cultivated depth and/or by subsoil acidity. Acid subsoils often also have a layer of compaction (pan) that limits rooting depth and therefore prevents the crop accessing water held deeper down. If the subsoil is too acid, high levels of exchangeable aluminium may also limit root growth and this also limits root-zone depth and access to the deeper soil water.
Soil acidification is an inevitable consequence of intensive cropping with silage crops having greater acidifying effects than grain crops due to the removal of the basic cations (potassium, magnesium and calcium) contained in the straw. Still other causes of soil acidification can be the chemical forms of the nitrogen fertilisers used
Cereal crops are less damaging to soil structure than most other arable crops, however soil organic levels are lower than for pasture soils and such soils are also specially prone to compaction. The main cause of soil compaction is the use of farm machinery (especially on wet soil). The grazing of dairy cows on winter-feed crops can be extremely damaging to soil structure during wet conditions. Compaction risks are usually higher where soil calcium levels and soil organic material levels are low.
Here, the soil management objective is to increase the yield of cereal crops by increasing soil water holding capacity, by improving water infiltration and by deepening the rooting zone.
Normal practice is to apply lime and to cultivate this into the topsoil to achieve appropriate pH and calcium levels based on annual soil tests made prior to planting. The lime requirement to compensate for soil acidification under high-yielding cereal crops is likely to be between 300 and 600 kg lime/ha/year. Application of nitrogen fertilisers of appropriate sorts and at appropriate rates and timings will minimise soil acidification and also minimise nitrate leaching to the groundwater.
While topsoil compaction is improved by cultivation, subsoil compaction is worsened by it. Subsoil compaction will affect crop growth and have long-term effects on soil quality. Gypsum applications raise subsoil calcium levels better than lime due to its greater downward mobility in the soil. Gypsum also improves soil structure without raising its pH.
Soil tests (basic soil test profile + exchangeable aluminium) on subsoil samples taken below 60 cm will identify if subsoil acidity is an issue. If pH and calcium levels are low and exchangeable aluminium is high then these will combine to limit root-zone depth and thus the cropís access to deeper soil water. Where subsoil acidity is of concern, an increase in the topsoil target pH and calcium levels should minimise the amount of acid movement through the soil profile.
A visual inspection of the soil profile will identify if there is compaction below the cultivation depth. Deep ripping is effective to reduce this where the pan can be fractured and it can also facilitate the downward movement of the added calcium through the soil. However, if the underlying causes of subsoil compaction are not addressed, deep ripping will be only a short-term remedy and it is expensive.
Because the calcium in gypsum is water soluble, applications of gypsum at rates between 3000 and 10000 kg/ha are known to be more effective than lime in moving calcium down through the soil profile to below the cultivation depth but they do not increase soil pH.
A combined lime and gypsum treatment will facilitate the movement of calcium to the subsoil and will also increase soil pH. A 60:40 lime:gypsum mix applied at about 5000 kg/ha should be effective. Incorporation of the lime and gypsum into the soil profile after application by ploughing or deep ripping will speed their effects.