CAN THE NO-TILLAGE SYSTEM AFFECT THE USE OF IRRIGATION IN TROPICAL AND SUBTROPICAL CROPPING AREAS?
The irregular rainfall distribution in tropical
and subtropical regions associated with soil erosion and degradation emphasized
the need to improve new practices of water and soil management. In the
last years, the zero tillage system has had an impressive expansion in
these areas of Latin America and in particular in
Irrigation is an ancient practice that tends to become permanent in order to minimize the variability of agricultural production. According to the climatic conditions, it may be advisable or even indispensable in many countries, depending on the severity of the water deficiency. Lately, water management is improving very fast due to the use of new equipments and technologies.
Rio Grande do Sul (the southernmost Brazilian State) has more than 1 million hectares of irrigated crops mostly devoted to flooded rice production; it represents more than 90 percent of the irrigated crops in the State. The rest of the irrigated areas that are getting the highest increment for grain production, include 30,000 hectares by using sprinklers or conventional equipments and 35,000 hectares with central pivot irrigation. Drip irrigation is used in fruit and vegetable production in about 5,000 hectares. An important part of the horticultural production associates irrigation to the use of protected cultivation. The area of maize production -for grain and seeds- using central pivots covers around 30,000 hectares and is actively increasing. There are more than 400 farm units operating in the State applying high level of irrigation technologies. Many of them are equipped with automatic meteorological stations to monitor water management and for supplementary irrigation (EMATER/RS-ASCAR, 2005).
Irrigated maize production seems to have many advantages. This crop has
a high sensibility to water deficits and a clear potential to respond
to high levels of technology. Under natural rainfall conditions maize
grain yield is very low in all the country, as compared to some traditional
producers like the
In a ten-year series of experiments using different irrigation levels, carried out in the Universidade Federal do Rio Grande do Sul, an average grain yield of slightly more than 10 tons per hectare was obtained in the subtropical region of Rio Grande do Sul. During the same period, the grain yield in non-irrigated areas had an average of less than 6 tons per hectares with a range of 1.5 to 10 ton per hectare (Bergamaschi et al., 2006). This broad maize production range was correlated to the very high variability of the summer season rainfall distribution, in particular when associated to the high sensibility of the crop during the very short period comprised between flowering and the beginning of grain filling (Bergamaschi et al., 2004).
2. The zero tillage system in
Besides irrigation there are several practices aimed to reduce the impact of water deficits or, at least, to improve the water efficiency of an irregular rainfall regime. In the last decades, crop management and in particular soil management contributed with significant improvements. As a consequence, the understanding of water relations in the soil-plant-atmosphere system has changed in a positive way using new management practices that allow crops to better face short dry periods or else to respond to a high level of technology.
The zero tillage is one of the most important improvements introduced in the cropping systems, especially in the tropical and subtropical areas. It represents a very important tool for soil and water conservation, since it may improve several physical, chemical and biological soil properties. Field studies conducted in Southern Brazil showed that soils managed with the zero tillage system may promote a better water availability for maize crops, by delaying the water deficit up to 15 days in comparison to the conventional tillage system (Dalmago et al., 2006).
The adoption of the zero tillage system has had a very large expansion
in the recent decades in many parts of the world. The zero tillage started
to be systematically studied in
A literature review shows a great variability of results and information describing influences of the zero tillage system on the soil-water relations: in general, it alters the soil porosity and density. One of the most important effects seems to be the change of the soil pores size in the range from 50μm to 8.9 μm, responsible for storing the available water for plants in the soil profile. According to Dalmago et al. (2006), the zero tillage system tends to increase the quantity and diameter of pores in that favors the water retention, in particular in the upper layers of the soil close to the surface. As a consequence, soils under zero tillage may retain around 70 percent of the available water for plants from the field capacity to a matric potential of -0.08MPa (the limit for tensiometry). On the other hand, conventionally tilled soils may retain something more than 50 percent of the available water for plants in the same matric potential.
Zero tillage may change the water flow and the soil water availability modifying the components of the soil water balance. It affects either the flow of incoming and outgoing water at the soil surface and mainly the water storage into the soil profile. In addition, since zero tillage modifies the physical conditions of the soil, may affect the plant root development modifying the crop pattern of soil water absorption. This means that the water storage and dynamics in the soil must change in relation to the conventional system, a very important aspect when using the irrigation in zero tillage cropping areas.
Other investigations have also shown that the water infiltration in zero tillage soils is improved, in particular when the straw remains on the surface at the beginning of the crop cycle. The straw also reduces the drop impact of heavy rains (as well as of the irrigation drops) on the soil surface by decreasing the kinetic energy of drops and therefore, reducing the soil degradation on the surface. As a consequence, losses of water and soil caused by superficial runoff tend to decrease, reducing the soil erosion and degradation on the surface. As a result, the efficiency of rains is improved since the tillage system increases water infiltration to the soil, particularly in the case of heavy convective precipitations in the tropical and subtropical regions. In conclusion, there are many evidences in favor of the zero tillage system, in particular for its capacity to increase soil water infiltration and water storage, so improving the soil water availability to plants.
3. Using irrigation in zero tillage areas
Zero tillage tends to change the water relations of the entire soil-plant-atmosphere system. It is a significant fact since the water conditions of any crop tend to respond as an interaction throughout those three elements (or subsystems). In spite of being a new practice of soil management, it may influence plant growth (including the rooting system) as well as the microclimatic conditions at the crop canopy. If considering increments of water infiltration and storage, in particular in the upper layer of the soil, besides of improvements in the pattern of water absorption by roots, the thermodynamics of the crop microclimate must consequently change. The irrigation practices must change in no tilled cropping areas, in relation to those adopted in the conventional tillage system.
With the exception of rice production that prevails in plain and heavy soils, most tropical and subtropical cropping areas are located in irregular areas where the soils have a high clay and low organic matter content. They are very susceptible to erosion and compaction reducing the soil water storage capacity and the penetration of roots. This situation generates a physical degradation of those soils increasing the climatic hazards caused by the rainfall variability. Irrigation has been adopted in some of these areas as a tentative to solve the water deficit. But, obviously this may not the best decision since the sustainability of the cropping system remains in danger.
Another important aspect to consider is that the zero tillage system does not solve the problem of water deficit during long drought periods and/or in the presence of very high evaporative demand, as is common in tropical regions or during the spring-summer seasons of the subtropical areas. As Dalmago et al. (2006) showed that the zero tillage system maintains higher water availability than the conventional tillage for some additional days (4 to 15), depending on the rainfall pattern at the beginning of the crop cycle.
The most significant physical alterations in the soil-water relations occur up to 30 or 40 cm deep in the soil profile. This is a very important aspect at the initial stages of the crop cycle, when the rooting system is concentrated in that part of the soil. The highest content of organic matter and hydraulic conductivity may allow a higher water supply for seedlings in zero tillage soils, permitting the establishment of good crop stands. This becomes a very important fact when the crops are submitted to short drought periods (very frequent in summer seasons), especially during the critical stages when the plants are elaborating the grain yield components. In maize, this aspect is very clear when the plants are subjected to water deficits, even of a few days, from flowering to the beginning of grain filling. According to Bianchi et al. (2005), in those conditions maize plants may profit from a better water condition in zero tilled soils than in conventional tillage.
As a conclusion, evidences show that the zero tillage system is not a definitive solution for the problems of water deficit and hence it should not be against the adoption or expansion of irrigation in tropical and subtropical lands. On the contrary, the zero tillage system combines several new technologies, in particular irrigation. Since zero tillage improves most of the components of the water balance in the soil it must provoke an increment in the efficient use of water by crops. For example, the straw on the soil surface may reduce the evaporation from the soil surface in about 25 percent just after each water application (two or three days), when the water losses to the atmosphere are very high. However, the evaporation process change to an opposite pattern in the following days as the soil drying proceeds, to a difference of about 8 to 35 percent higher in zero tilled soils than in bare soils (Dalmago, 2004). This aspect would have significance to the soil water conservation, mainly at the beginning of the crop cycle.
Another aspect is the reduction of the runoff which tends to reduce losses of water and soil sediments. It would be possible to apply higher amounts of water in each application in the presence of irregular topography, so reducing costs and risks by losses of soil, water, and nutrients. But, this is a critical aspect of the zero tillage system found in Southern Brazil which needs further studies. On the other hand, the wrong principle that the presence of straw on the soil surface is sufficient to solve the problem of erosion has lead farmers to avoid any other practices of soil conservation. Moreover, based on this idea, farmers have adopted a “downhill” sowing system, so exposing the soil to the action of heavy rains and losing a significant amount of water, soil, and nutrients by runoff, in particular when the zero tillage is not still well established in the area. Even so, the zero tillage system has permitted to improve the productive capacity for most of farmers in the Region.
On the other hand, since the irrigation tends to increment biomass production it should offer some advantages to the zero tillage system since this practice needs a big amount of straw in the soil surface. This aspect is of particular importance if the rainfall regime alternates a rainy season with a dry season, as is common in most of the Brazilian tropical area (the “Cerrado” Region). In that large agricultural area the zero tillage system may have problems at the beginning of the summer cropping season when the quantity of straw is very low, due to the long dry period of autumn-winter; in addition, the high temperature even during the rainy season stimulate the decomposition of the straw. The adoption of irrigation during the dry season tends to prevent that problem and winter crops reach high yields.
From the technical point of view, the correct implementation of the zero tillage system, as a practice of soil management, demands improvements and/or adaptations in the irrigation management. Firstly, because the zero tillage affects positively the water condition of crops by increasing the water storage and dynamics in the soil profile, mainly into the upper soil layer close to the surface. In practical terms, this means the possibility of delaying the first water application by irrigation in zero tillage areas, in comparison to the conventional system, in particular if the crop stand was already established.
The second aspect is related to the increment of the water infiltration in zero tillage soils, meaning that it should be possible to apply higher irrigation rates in comparison to conventional tilled areas, and also delaying the next water application, as Bayer and Mielniczuk (2001) showed in maize crops.
The third aspect is the influence of the zero tillage in the growth and distribution of the rooting system. For example, if the water supply is regular and sufficient at the beginning of the crop cycle (either by rainfall or irrigation), the growth of plant roots tends to be smaller than in the case of limiting water conditions (Dalmago, 2004). In zero tillage soils there is also a tendency to higher concentrations of nutrients and water content into the upper layer of the profile reducing the root growth into the soil. This condition may have a significant effect for irrigation management by reducing both the amount of water and the interval between each water application. On the other hand, in the case of a limited water supply during the crop vegetative growth (even with a low water deficit), the rooting system tends to have a deeper distribution in the soil, so allowing to increments in the water amounts and interval between each irrigation. Those influences should be more significant in the case of sensitive crops like maize, during the critical stages due to both a high water demand and a high physiological sensitivity to water stresses. That situation have a big chance to occur in the subtropical areas of Brazil and neighboring countries due to the high variability of the rain distribution during the spring-summer seasons, and in particular if considering the influence of the El Niño – La Niña phenomenon.
The relationships between the zero tillage system and irrigation need further scientific studies in order to adjust new parameters and models, considering their interactions in soil properties or in the soil-plant-atmosphere as a whole system. Irrigation management needs an appropriate crop monitoring taking into account the water requirements of plants and soil. Bergamaschi et al. (2004) showed that a precipitation of around 50mm during the critical growing period of maize allowed for a grain yield of about 8 tons per hectare in a dry season. On the other hand, during a rainy summer season a short drought period, with a very high evaporative demand during the same stage of the crop reduced the maize yield to around 1.5 tons per hectare. This example illustrates clearly the importance of using adequate technical criteria in managing crops, soils and water by considering the interactions of all the components influencing the water relations in cropping systems.
There are many aspects to take into consideration if considering the sustainability of the cropping system as well as the entire ecosystem. As the irrigation is introduced in large zero tillage cropping areas both practices may promote reciprocal improvements in terms of technical, economical, and ecological advantages. They support each other interacting for the sustainability of the cropping system and also of the environment as a whole.
[Click on thumbnails to view full pictures]
Figure 2. The straw on the surface of no-tilt soils reduces the impact of rain (and irrigation) drops, decreasing the soil erosion and compaction, and increasing the infiltration of water to the soil profile.
Figure 4. The infiltration of water (from rain or irrigation) is increased in the no-tilt soils, due to the presence of straw on the surface and because of changes in the soil structure.
Figure 6. The number of ear per plant and grain per ear are affected when the water stresses occur during the critical period of maize crops, so reducing strongly the grain yield. This may happen when combining dry spells to low water storage in the soil.
Figure 8. The sprinkler irrigation in conventional tillage has frequent problems of low water infiltration due to the soil compaction and degradation, so increasing losses of water, soil sediments, and nutrients by runoff on the soil surface.