The requirements for fertiliser additions to supplement and correct soil nutrient imbalances can be assessed reasonably well from topsoil nutrient tests done before the crop is planted and later on from leaf analyses done at tasseling. Crop nutritional requirements are reasonably well understood and fertilisers are readily available, so maize yield does not usually have to be limited by any mineral insufficiencies or imbalances in the soil.
If insufficient water is available to maize, especially between tassling and maturity, then yield will be reduced. The water requirements of a maize plant increase with development and with summer weather. By the time a crop reaches tassling, average water demand is about 4.5 mm/day (135 mm/month) but in hot, windy conditions this will rise to 6.5 mm/day (195 mm/month). This water usage rate is about double the normal rainfall rate (75 to 100 mm/month over much of New Zealand). Therefore, to avoid water stress and consequent yield reductions, a maize crop must gain access to additional water over and above normal rainfall.
While management of just the 0-20 cm topsoil layer is sufficient to meet the crop's mineral nutrient requirements, to provide it with sufficient water in summer requires either regular irrigation or root zone access to water contained in a much deeper soil profile - roughly from 0-100 cm. Because irrigation is expensive and it is not always an option, it is worthwhile managing the soil so as to maximise root zone access to water 'stored' in the soil.
Research has measured water usage by a maize crop from different levels in the soil down to about 150 cm. It has been found that the top layer of soil (0-30 cm) provides only about 40% of crop water requirements, the next layer (30-60 cm) provides about 30% and the next layer (60-90 cm) provides about 20%. Up to 10% of the water used will come from even deeper down (i.e. from below 90 cm) - if the roots can get down that far that is!
The volume of stored soil water available to a crop depends on two factors: the soil's water-holding capacity and the volume of soil explored by the roots. Clay and silt loams have the highest water-holding capacities (about 20% by volume) whereas sandy soils hold only about half as much. Meanwhile, the volume of soil explored by the roots depends on rooting depth. For a particular piece of land, soil type and so water-holding capacity is a given but the depth of soil that can be explored by the roots (root-zone depth) is affected by soil management.
Under worst-case conditions, the depth of the root zone can be limited to just the top 20-30 cm of soil if negative factors exist in the subsoil layers below. Shallow rooting will severely reduce the volume of soil water available to the crop in a drought. The most common subsoil factors that limit deeper root growth are:
- Soil compaction
- Poor aeration
- High acidity
- High levels of exchangeable aluminium
These factors are usually interconnected.
Compaction: Soil is compacted by the passage of farm machinery. Cultivation remedies compaction in the topsoil layer but increases compaction in the subsoil layers immediately below the cultivation depth. Compacted soil slows root penetration or prevents it altogether. Subsoil compaction can be reversed by deep ripping but this is an expensive and rather temporary remedy if the causes of compaction are not also addressed. An increase in soil calcium (especially with clay soils) and of organic matter (greater earthworm activity) render most soils less prone to compaction. Compaction damage from farm machinery is worst when soils are wet.
Aeration: Heavy and compacted soils suffer poor drainage and so are more likely to become anaerobic in wet weather. This will quickly kill fine roots (especially when the soil is warm) and soil acidity will rise. Raising soil calcium improves drainage and aeration in heavy clay soils.
Acidity: Intensive cropping systems tend to acidify the soil. Maize crops grown for grain have a less acidifying effect than those grown for silage due to the basic cations (potassium, magnesium and calcium) left behind in the leaves and stalks after harvest. Removal of the 'whole crop' for silage also has a greater impact on soil physical quality due to reduced levels of organic matter. Other causes of high soil acidity are the chemical forms of the nitrogen fertilisers used. The efficiency of utilisation of soil nitrogen is also reduced under high-acidity conditions and this tends to increase nitrate leaching.
Lime requirements to compensate for soil acidification under high-yielding maize crops is likely to be between 500 and 800 kg lime/ha/year. Unfortunately lime affects mainly the upper soil layers unless it is incorporated deeper down by cultivation or into the subsoil by ripping.
Aluminium: Under acid-subsoil conditions, exchangeable aluminium levels can increase to a point at which they are toxic to the roots. This means that an acid subsoil can limit root-zone depth and thus the crop's access to subsoil water.
The objective is to increase yield through increased soil water holding capacity, better water infiltration and deeper root growth. Based on soil tests done prior to planting, apply lime and cultivate it into the topsoil to raise topsoil pH and calcium levels. Gypsum is a good way to raise calcium and to improve soil structure without raising soil pH. Use nitrogen fertilisers at appropriate rates and application timings to minimise nitrate leaching.
Conduct a soil test (basic soil test profile + exchangeable aluminium) from the subsoil profile at 30-60 cm (i.e. below the normal cultivation depth). If subsoil acidity is an issue, the pH and calcium levels may be too low and exchangeable aluminium too high both of which will restrict root growth.
Conduct a visual inspection of the soil profile, looking for indications of compaction below the normal cultivation depth. Deep ripping is a good option where the pan can be fractured and this may facilitate the physical movement of surface-applied calcium deeper into the soil profile. Where subsoil acidity is a concern there is good reason to raise the target pH and calcium levels in the topsoil layer with the expectation that this will eventually work its way downwards.
Applications of gypsum are known to be more effective than lime in getting calcium deeper into the soil profile but they do not increase soil pH. Applying lime and gypsum together in a mix combines lime's potential for raising soil pH with gypsum's greater mobility which will facilitate calcium movement into the subsoil layers. Effective treatment will require around 5000 kg/ha of a 60/40 lime:gypsum mix. Incorporation of the lime and gypsum after application by ploughing or deep ripping will further speed the subsoil response to these amendments.