Many New Zealand soils do not contain enough phosphorus and sulfur for optimum crop production. These are supplied, along with nitrogen, by applying fertiliser.
Farmers generally test their soils to determine how much fertiliser individual crops will need by taking samples from selected paddocks. A laboratory measures the concentration of elements and makes recommendations about fertiliser.
Superphosphate and nitrogen
Superphosphate, often fortified with additional sulfur, is the standard fertiliser used on arable farms.
Nitrogen is supplied through urea or, if more sulfur is required, ammonium sulfate.
Superphosphate may be spread on the soil surface before cultivation or applied through the drill with the seed. Phosphate and sulfur stay in the soil for a relatively long time, so can be applied two to three months before the plants require them. However, nitrogen is easily lost from the soil, so must be applied close to the time the crop needs it. Nitrogen fertiliser is usually spread onto the growing crop, sometimes several times during the season.
In Canterbury, irrigation is essential for high crop yields. In other arable cropping regions, such as Southland, Otago, Manawatū and Whanganui, the spring and summer rainfall is higher and more reliable. Irrigation costs are not usually more than 10% of total production costs.
In mid-Canterbury, low-cost border-dyke systems were the traditional way of irrigating crops and pasture. Their disadvantage is that the amount of water in the borders varies, which may affect the consistency of the crop yield and the time of ripening within a paddock. These have mostly been gradually replaced by spray irrigation systems.
In the early 21st century farmers paid only for resource consents and the cost of watering the crops, not for the water itself. In the future it is likely there will be increasing restrictions on water available for irrigation in Canterbury, and there may be further costs for acquiring water rights and levies on water used.
Water should be applied at the right rate – too much irrigation wastes water and may cause leaching of nutrients, while insufficient water can dramatically reduce crop yield and quality.
Although early research showed that irrigation at two to three critical growth stages would increase yields, later research has shown that yields were improved by having sufficient water, regardless of the growth stage.
Many farmers determine when the crop needs to be watered by first estimating potential evapotranspiration – the loss of water from the soil due to evaporation and crop transpiration. Another method is to measure soil moisture levels with direct techniques, such as calculating the moisture content of a soil sample from its weight before and after oven drying, or indirect techniques, such as using probes or meters.
Effects of soil moisture
Soil moisture affects not only crop yield, but also quality. Some crops, such as wheat and peas, may be worth less if the plants lack water while the grain is forming. However, protein content of wheat crops may be reduced if grown in soils that are too wet.
If it is too wet before harvest, some grains may begin to sprout, which also reduces the quality of wheat destined for milling.