| Information sheets | | | Elsenburg infopaks | | | Irrigation: 3 |
The management of soil moisture
Afrikaans | Xhosa
 Click to enlarge! |
Many fruit and grape producers are ignorant, and even inept with regard to the management of soil moisture. In practice it is found that very few producers actually apply irrigation management. Such management as does occur is seldom based on the conditions and needs of the plants and soil. |
 Click to enlarge! |
The wrong utilisation of water as a resource occurs throughout the growing season - generally too much is used at the beginning and the end of the season, and too little during the peak period. |
 Click to enlarge! |
In the cultivation of fruit and especially grapes quality has a more important role to play than quantity. One of the most important management resources by means of which both quality and quantity can be ensured, is the effective management of soil moisture.
The objective of soil moisture management is not merely the conservation of water, but the effective, economical and responsible utilisation of water. The results of wrong irrigation are to be ascribed either to over-irrigation or to under-irrigation. Over-irrigation usually leads, among other things, to poorer quality fruit, reduced life-span of the orchard or vineyard, development of a weak or diseased root system, fruit with a short shelf-life, soil salination, and the absence of subsoil microbe populations. |
 Click to enlarge! |
Over-irrigation also results in the leaching of nutrients and in weed infestation, and is responsible for high energy expenditure. Moreover, injudicious management of soil moisture can lead directly to excessively rank growth, which results in Botrytis rot, retarded ripening, and infertility. This in turn leads to few and poor fruit, a weak fruit set, heavy bearing with scant sugar, in other words sour fruit, excessive potash absorption, and an increase in the pH of the sap. |
 Click to enlarge! |
If an orchard or vineyard is under-irrigated, the trees or vines will dry
out, under-perform, and eventually die.
The principle of an effective model for the management of soil moisture is couched in the expression: "Irrigate, then wait". It is based upon the fact that with permanent vegetation, root volumes have an important part to play. The weight of the wood below the ground should be the same as that of the wood above the surface. To achieve this, root development, and in particular the development of capillaries, should be stimulated. This is done by ensuring the presence of moisture, oxygen and nutrients at the appropriate root depth. It is important, therefore, for the producer to know how much, and when, to irrigate. |
 Click to enlarge! |
The soil moisture reservoir determines the amount of irrigation that is
required. The reservoir at the disposal of the soil is determined by the percentage of clay, silt, sand, stone and course matter in the soil, the structure of the soil, ventilation and compaction, as well as the capacity of the soil to condense and re-condense.
The potential root depth also plays a part in determining the amount of irrigation required. Root depth can be determined by means of a profile hole which would reveal the potential depth. It is not possible for anyone to determine exactly how much moisture is extracted by any plant at any particular time. To calculate this, evaporation data and plant factors are required. |
 Click to enlarge! |
The rate of irrigation at which an irrigation system applies water to the soil can be calculated by means of a specific formula. However, if a soil moisture management technique is employed whereby the measuring of soil moisture is a continuous process, all uncertainty is eliminated. |
 Click to enlarge! |
The measurement of soil moisture levels can be done by means of a variety of devices, for example the tensiometer and the
Diviner 2000. Graphs are compiled on a weekly basis by making use of daily tensiometer readings. With the help of these, judicious deductions can be made and guidelines laid down with regard to the amount and timing of irrigation.
Whereas the tensiometer is placed at various depths, e.g. 30, 60 and 90 cm to measure the soil moisture potential, the Diviner measures the soil moisture content (%) at every 10 cm depth. Soil moisture readings are captured as data to be downloaded onto a personal computer, where such readings can be graphically represented. This ensures more accurate and correct management of soil moisture. As a general guideline it is recommended that a tensiometer or soil moisture potential model be used on a unit of approximately 10 hectare. The Diviner 2000 can be used very successfully on larger units, since this device can manage up to 99 measuring sites. |
CPJ de Jager
ELSENBURG / WORCESTER extension
