Wheat and barley crop growth and yield is influenced by the supply of nutrients from soil reserves, organic matter and applied
fertilisers, available water and sunlight intensity during the growing season.
The requirement for fertiliser in addition to the soil nutrient reserves is provided reasonably accurately from information based
on soil nutrient tests before the crop is planted. Leaf tissue analysis at late tillering may be used to check the adequacy of the
nutrient supply, particularly nitrogen and trace elements. However soil tests only represent the top 15 cm of soil or the depth of
cultivation. Management of the top 15 - 20 cm of soil has proven adequate for nutrient management but to provide sufficient water
for optimum growth, the effective root zone is actually the top 90 cm of soil. Research conducted in Canterbury (Jamieson, 1994)
on the effect of drought on wheat and barley measured the utilisation of soil moisture down to 130 cm depth. If adequate water
supply is not available to the crop during periods of high evapotranspiration, this is likely to 'cap' the grain yield or the dry
matter grown for forage crops regardless of nutrient supply.
Water utilised from different parts of the root zone has been estimated for the growing season. The top 30 cm provides about 40% of
the water requirement, 30 - 60 cm depth provides about 30% and 60 - 90 cm provides about 20%. Up to 10% may be utilised from soil
below 90 cm if the roots can get down that far.
When crops become water stressed, the yield loss can be as high as 6 to 7.5% per week when evaporation rates are as high as
30mm/week in Canterbury. For a crop with potential yield of 9 t/ha, this is 0.5 to 0.7 t/ha per week.
The amount of water available within the root zone is influenced by soil texture or the relative proportions of sand-silt-clay and
the volume of soil in the root zone. For mineral soils, clay and silt loams have the highest water holding capacity at about 60 mm
per 300 mm soil compared to sandy soils that retain only about half as much. The effective root zone or rooting depth is often
affected by compaction below the cultivated depth and/or low pH or acidity of the subsoil. The amount of water potentially available
to cereal crops is determined by the soil texture throughout the top 90 cm of soil, moisture content and the rooting depth. The
value of increasing the water holding capacity of the soil where crops will experience moisture deficit conditions is 0.22% of the
potential grain yield per mm of deficit that can be averted.
Acidic subsoil often has a layer of compaction or a 'pan' that restricts root growth, the level of exchangeable aluminium may be
damaging to fine roots, restricting root function and the amount of water available to the crop. Increased nitrate leaching to the
groundwater is also associated with increasing soil acidity.
Acidification of soil is an inevitable consequence of growing plants, the process of plant growth has an acidifying effect. Other
sources of acidity in soils are the chemical forms of nitrogen fertiliser used. The amount of nitrate leaching down the soil profile
is related to the efficiency of utilisation of applied nitrogen and whether the crop is grown for grain or silage. Silage crops have
a greater acidifying effect than grain crops due to the removal of basic cations (potassium, magnesium and calcium) as a component
of straw. Removal of the 'whole crop' also has a greater impact on soil physical quality compared to 'grain crop' due to reducing
soil organic matter levels. Lime requirement to compensate for soil acidification under high yielding cereal crops is likely to be
300 to 600 kg lime/ha/year. This is the same as 1.5t/ha every 3 to 5 years.
Growing cereal crops is considered less damaging to soil structure than most other arable crops however soil organic matter levels
are lower than for pasture soils and are prone to compaction. The main causes of compaction are cultivating wet soil, the effect of
heavy machinery wheel pressures on the soil, physical disturbance of the soil with implements. Grazing livestock, especially dairy
cows on winter feed crops may be very damaging to soil structure during wet winter conditions.
Compaction risk is higher where soil calcium levels are low.
Compaction of topsoil is loosened by conventional cultivation however compaction also affects the subsoil which will affect crop
growth, this may have long term effects on soil quality.
The normal practice is to apply lime and cultivate it into the topsoil to achieve appropriate pH and calcium levels from a soil
test prior to planting each crop. This is fine for the topsoil but alleviating compaction in the subsoil is more difficult.
Gypsum is a good option to lift calcium levels below the normal cultivation depth due to different chemical properties in comparison
with lime. This will improve soil structure without raising pH. Use nitrogen fertilisers at appropriate rates and application timing to
minimise leaching of nitrate down the soil profile.
Conduct a soil test (Basic Soil test profile + exchangeable Aluminium) from the soil profile below normal cultivation depth or 30 - 60 cm.
If subsoil acidity is an issue, the pH and calcium levels will be low and exchangeable aluminium will be high which may combine to
restrict root growth. Conduct a visual inspection of the soil profile, looking for any indications of compaction below the normal
cultivation depth. Deep ripping is a good option where the pan can be fractured and this process may facilitate the physical
movement of calcium down the soil profile however if the underlying cause of the compaction problem is not addressed, this is an
expensive short term remedy. Where subsoil acidity is a concern there is reason to increase the target pH and calcium level in the topsoil to minimise
the amount of acidity moving down the soil profile.
Application of gypsum at rates of between 3 and 10 t/ha is known to be more effective than lime in moving calcium down the soil profile below the
cultivation depth but does not increase soil pH. Combining lime + gypsum as a treatment facilitates leaching of calcium into the subsoil. where pH adjustment
is also required, a 60/40 blend of lime and gypsum at about 5t/ha is an option. Incorporation of lime + gypsum by ploughing after application or deep
ripping will improve the effect of this treatment. The objective is to increase the potential yield of cereal crops through increased soil water holding
capacity, water infiltration rate and root growth.
