IRRIGATION OF CROPS IN GISBORNE
In Gisborne in the past year irrigation of processing crops was undertaken by a number of growers. Results obtained were not as satisfactory as growers expected, and while in most cases this was due to lateness in starting irrigation, the application of water was not as thorough as it should have been.
Costs of installing and operating sprinkler irrigation systems are high, particularly in Gisborne where sources of water are extremely limited, and in many cases pumping long distances has to be undertaken.
To ensure that returns obtained from the use of irrigation justify the costs involved, efficient and economical use of the water is essential, and it is hoped that the follow- ing points will be helpful to growers in future use of systems.
When water is applied to a soil a fairly rapid downward movement, and to a lesser extent lateral movement, occurs, and water which cannot be held by the soil, passes to lower levels. After 24 to 48 hours, depending on soil types, movement of water will have practically ceased, and the moisture status of the soil becomes stable. The amount of water held by the soil after drainage is the “ﬁeld capacity” of the soil. When the soil moisture is reduced to the stage when plants wilt, this is known as the “wilting point” of the soil. The amount of water between wilting point and ﬁeld capacity is the supply available to plants. Apart from soils that have a water table close to the surface, no upward movement of water occurs, so that the moisture available to plants is contained only on the volume of soil occupied by the roots.
The purpose of irrigation is to supply water to the soil so that available moisture is present in the whole of the root zone of the Crop throughout the growing period.
If available moisture is not maintained, even for a short time under very hot conditions, the check plants receive will not allow them to fully recover and the extent of loss of production and effect on quality will be governed by length of time this condition exists.
As a general average, the available water that can be held per foot depth in different soils is:
Sandy soils ½in to 1 in,
Sandy loams l in. to l½in.
Silt and clay loams l½in. to 2 in.
Clays 2 in. and above
To assure efﬁcient irrigation, soils should be checked by digging 48 hours after application of water, and until this checking has been carried out, a time period of application in each position cannot be arrived at.
APPLICATION OF WATER
Soils should be irrigated well before available moisture has been depleted. Less water per application will be required under these conditions, and penetration of water will be more rapid.
As a general guide, 1in. at each application of water on average soils in Gisborne, before available moisture is depleted, will give satisfactory results, but when soils have been allowed to dry out, as was the case last season, l½in. to 2in. will be necessary. Under such conditions, two applications a few days apart would be most satisfactory.
The penetration rate of water varies considerably, and amounts no greater than ½in. per hour should be applied. These ﬁgures do not apply to other districts.
Under calm conditions, spacing of sprinklers up to 4/5ths of the diameter of the spread apart each way, will give satisfactory results and keep the number of shifts to a minimum. Under windy conditions considerably more overlap is required, and suitable placings of sprinklers can only be arrived at by trial and error. Checking efﬁciency of wetting soil under windy conditions by digging is essential.
Uneven watering of any crop will give poor results for the outlay involved on irrigating, and in the case of crops mechanically harvested at a certain stage, variations in maturity caused by uneven watering can considerably reduce production due to harvesting being governed by the most mature sections of the crop.
Irrigation properly applied will increase development and production of crops, but the extent of increase will be governed by the efﬁciency of its use and the fertility and state of the soil on which it is used. Highly fertile soils of good structure will produce the greatest crops per unit of water.