Maize crop growth and yield is influenced by the supply of nutrients from soil reserves and applied fertilisers, available water
during the growing season, sunlight intensity and heat units 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 and leaf tissue analysis at tasseling. 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 maize measured the utilisation of soil moisture down to 150 cm depth.
If adequate water supply is not available to the crop from tassling through to maturity, 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.
Water requirement of maize plants is related to leaf area so by the time the plant reaches tassling the average water demand is
4.5 mm/day (135 mm/month) which can rise to above 6.5 mm/day (195 mm/month) during hot windy conditions. This demand is greater
than the normal rate of rainfall average around 75 to 100 mm/month in regions where maize is generally grown. Maize crops must
utilise soil moisture that is stored within the root zone or receive irrigation to avoid water stress. Yield reduction due to
moisture stress is normally expressed as a reduction in average grain weight.
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 maize is determined by the soil texture throughout the top 90 cm of soil, moisture content and the rooting depth.
Acidic subsoil often has a layer of compaction or a 'pan' that restricts root growth, in volcanic ash type soils the level of
exchangeable aluminium is damaging to fine roots and restricts root function. Increased nitrate pollution of groundwater is
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 stover. 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 maize crops is likely to be 500 to 800 kg lime/ha/year.
Apply lime and cultivate it into the topsoil to achieve appropriate pH and calcium levels from a soil test prior to planting each
crop. Gypsum is a good option to lift calcium levels and 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. Where subsoil acidity is a concern there is reason to increase the target
pH and calcium levels in the topsoil to minimise the amount of acidity moving down the soil profile.
Application of gypsum 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.
Incorporation of lime + gypsum by ploughing after application or deep ripping will improve the effect of this treatment. Effective
treatment will require around 5t/ha of a 60/40 mix of lime + gypsum.
The objective is to increase the potential yield of maize through increased soil water holding capacity, water infiltration rate
and root growth.
