effects of soil properties on precipitation use efficiency

Irrig Sci (1993) 14:65-73 Irrigation : clence

�9 Springer-Verlag 1993

Effects of soil properties on precipitation use efficiency W. D. Kemper*

Agricultural Research Service, USDA, Bldg. 005, Beltsville, MD 20705, USA

Received: 2 September 1992

Abstract. As other non-agricultural users need and are able to pay higher prices for irrigation waters it will be increasingly essential to make more efficient use of precipitation in crop production. Major problems constrain- ing the efficiency with which rainfall is used are: large portions of this water run off or are evaporated from soil surfaces; root zone water holding capacity is not sufficient to hold water needed to sustain good crop production; and the random nature of the precipitation and associated probabilities of crop failure deter investments needed to achieve improved efficiency of water use on high value crops in rain fed lands. Gravel and organic mulches have shown potential for reducing annual evaporation to as low as 10 cm of water and for facilitating entry of expected intensities of rainfall. Under such mulches, soil water is almost as dependable as "money in the bank", and can be retained for use in seasons when high value crops are most efficiently grown. Measure- ments of such stored water plus past weather records can provide bases for calculating probabilities of adequate water being available for crops whose water requirements are known. Most of these water requirements have been determined when these crops were planted close together and root zones, and in some cases the crop canopy, over- lapped. Research is needed to determine the degree to which wider spacing of high value vegetables and fruit crops under mulched conditions can be used to increase probabilities of economic production when water supplies are marginal. For tree crops, where planting density is not temporally flexible, but small amounts of irrigation water may be available at high cost, management systems are needed to optimize use of rain and ascertain when the economic benefits are sufficiently great to justify use of the irrigation water. Such sporadic use requires that the supply system can provide water on demand. Increases in rooting depths by modifying soils with restricting layers can serve as a cost effective alternative to irrigation under some climatic regimes. Such regimes should be identified and costs of increasing rooting depths should be evaluat-

* Leader of National Program for Soil Management

ed and reduced where possible. Availability of municipal and industrial waste products, whose producers may be willing to pay costs of transport and incorporation, may provide economically feasible means of removing such restrictions. Production levels and environmental conse- quences of incorporating such wastes-cum-amendments in soils need evaluation to provide bases for developing optimized and acceptable management systems.

In many soils there is a need for biochemical means to achieve the large pores in soils needed to facilitate entry of water, retard evaporation at the soil surface and increase rooting depths.

Innovative and sophisticated irrigation systems have improved delivery and application efficiencies. However, since cities and industries can pay more for water, water available for irrigation is diminishing and will continue to diminish. Consequently, survival of agriculture in semiarid regions will become increasingly dependent on efficient use of precipitation.

When irrigation is available on demand, good crops are almost a certainty and high inputs to high value crops are generally a good investment. The investment of the farmer who depends on stored soil water and precipitation to save his crop from water stress and subsequent failure appears to be at much greater risk. Obvious factors affecting the outcome are the amounts, seasonal distribution and variability of the precipitation. There are potential physical feasibilities for changing some of these factors by cloud seeding. Cloud seeding with raindrop nucleating crystals has potential for increasing rainfall immediately downwind from the point of seeding. How- ever, the extent to which such seeding would reduce precipitation in areas further down wind is a legitimate con- cern. Before the economic and physical feasibility of cloud seeding could be ascertained, some States had put others on notice that cloud seeders would be sued, both by those who failed to get rain that they felt was rightfully theirs and by those who felt that they had been damaged by associated storms. Such poised legal action drastically

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reduced the probability that cloud seeding could be com- mercially successful, and also reduced funding available to conduct research that is needed to improve the associated technology.

Weather records which define the amounts and seasonal distribution of precipitation and factors from which the potential evapotranspiration can be calculated are available near most locations. From Stanhill's (1991) map based on such weather records, we can estimate that the average precipitation in Israel north of Beer Sheva approaches 500 mm per year and that the average annual amount of water received in this area is about 7 km 3. This shows that the rainfall received far exceeds the 0.3 km 3 pumped from Lake Kinneret and even the 1.5 km 3 which is the average annual pumping from ground water. A former water commissioner of Israel estimates that more than 2/3 of the precipitation falling on Israel is lost by evaporation from the soil. Such estimates indicate the great potential for improving the efficiency with which precipitation is used in this area.

The primary question is whether the inputs needed to improve efficiency will be a good investment. One of the interesting and cost efficient avenues for exploring such possibilities is to use crop production models in conjunction with "climate generators" developed from long-term weather records from specific locations. Sensi- tivity analyses regarding factors such as infiltration rates, soil water-holding capacity available to the crop, etc., can be conducted using these models and climate generators (e.g., Nicks and Lane 1987) to predict effects of changing these factors on crop yields and precipitation use efficiency. Calculations based on Sinclair's (1993) model and long-term weather records indicate that disrupting re- strictive soil layers to allow deeper rooting can increase probabilities of profitable yields of corn and soybeans from about 50% to 95% in some southeastern U.S. locations (i.e., see Fig. 1). This is almost as good as irrigation! Assuming that such predictions are reasonably accurate, achievement of such potentials for improving use of precipitation appears to be dependent on whether it is economically feasible to change soil properties to achieve the desired infiltration rates, rooting depths, evaporation rates, water holding capacities, etc.

The objectives of the following discussion are to identify factors or processes related to soil properties and their management which have cost effective potentials for improving the efficiency of water use, and to identify areas of research which could elucidate these factors and processes, provide bases for soil management to optimize water use efficiency, and be good investments.

Efficiency is generally the ratio of a product to an input necessary to create the product. When defining water use efficiency, one question is: where is the input measured? For the purpose of this discussion, the mea- surement of input is assumed to be where the precipitation or irrigation meets the soil surface because this is the point from which soils and their management significantly affect the efficiency of water use.

Taking this first contact of water with the soil as the starting point, water use efficiency is determined by the following factors:

200

n - O

150 o .

LIJ "-r" t J) D

I00 t / ) Q _J LIJ > -

l,[J N - - 50 <~

100 cm

bU c m

ROOTING DEPTH

YEARS WITH INCREASING PRECIPITATION '~

Fig. 1. Yields as a function of rooting depth as computed by Tom Sinclairs' model for climates during 1969-1988 at Holly Spring, MS

a. Infiltration, i.e., the portion of the precipitation which enters the soil. b. Capacity of the soil to retain water until it can be used by crops. c. Relative rates of evaporation, percolation out of the root zone and crop use. d. Crop production per unit of water used.

The last factor "d", addressed by other contributors to this symposium, will not be discussed in this review.

A. Infiltration and runoff

1. Cultivation, surface sealing, and infiltrate electrolyte concentration

There are few places in the world where some runoffdoes not occur. This is due in part to highly intensive rainfall events. However, the manageable factor is infiltration rates. Low infiltration rates can be a result of organic coatings on the soil minerals which cause the soils to be hydrophobic (e.g., Savage et al. 1972). However, the most common cause of low infiltration rates is small size of pores encountered by water as it enters the soil, and the fact that velocity of water entry is inversely related to the square of the diameter of these pores.

Recognizing this fact in at least a qualitative sense, farmers through many generations have tilled their crust- ed soils into a cloddy, large pored structure in an effort to facilitate the entry of water. However, this achievement has generally been short lived because the open structure disintegrates rapidly as raindrop impacts break segments loose from the clods, fill the large pores and reform the dense surfaces crust. This process is generally aided by the low electrolyte concentration of the rain water. (e.g.,

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Agassi et al. 1985) which causes swelling pressures to de- velop between adjacent particles, breaking bonds that were holding the particles together. This dispersion is moderated in some soils by presence in the surface of slightly soluble minerals such as gypsum, which are sufficiently soluble to provide electrolyte to the water which moderates the dispersion, but are sufficiently insoluble to avoid immediate removal by the first flush of water pass- ing through the surface (e.g., Kemper and Noonan 1970). To some extent the strategic position of such salts at the surface is regained by evaporation of a portion of the water from the soil following the rain. While ability of gypsum treatments to increase infiltration on small plots has been demonstrated, on many soils the effect on fields, including those with some degree of vegetative cover, needs to be determined.

2. Texture and mineral mulches

Sandy soils with less than 20% silt and clay can often bear the beating action of rain drops without sealing as the fine particles migrate down below the zone of physical disruption into the open structure of the underlying soil. However, if such sandy soils are compacted by traffic, destroying the open structure, the relatively small amounts of fine materials are often sufficient to block the remaining smaller pores. Reductions in subsequent infiltration rates to as little as 2% of initial values have been observed (e.g., Akram and Kemper 1979) as a result of one pass of a harvesting truck on such loamy sands when they were moist.

Since such harvesting and associated compaction are often unavoidable, means to alleviate the compaction, such as deep chiseling of the soil, are often effectively used on such soils.

Controlled traffic has avoided such compaction over most of the surface (e.g., Chichester and Richardson 1992). This involves matching the machinery wheels to a given spacing so that all operations can be conducted in the same tracks which cover a small fraction of the total area. High infiltration rates on the uncompacted areas have enabled infiltration of most of the precipitation. However, to infiltrate all the precipitation requires means of diverting water from the compacted furrows with low infiltration to adjacent uncompacted soils.

Sand or gravel mulches on the surface of finer textured soils protect the soils from the beating and sealing actions of raindrops and can be effective in maintaining infiltration rates sufficiently high to allow infiltration of practically all precipitation. While generally expensive to install and maintain, such mulches are also effective in reducing evaporation and will be discussed in detail in the section on evaporation.

3. Organic mulches and biofactors creating macropores

In countries where the values of crop residues are relatively low, leaving them on the surface to protect soils from the beating and sealing action of rain drops is prov-

ing to be a good investment. Leaving these crop residues on the surface prolongs their life compared to plowing them into the soil where moist conditions favor their rapid decomposition by microorganisms. Left on the surface where they persist for months or years, a larger portion of the caloric content of these residues is utilized by mesofauna such as ants and surface feeding earthworms, whose populations build in response to continued availability of this food source. Feeding on the soil surface, these mesofauna create burrows ranging in size from 1 to 10 mm in diameter which are highly effective in draining water accumulating in puddles on the surface into the soil so it does not run off (e.g. Edwards et al. 1988). Looking at the surface of dry soils, these large bio-pores are not always apparent because surface feeding earthworms commonly cast soil over the entrances as they retreat into their burrows. However, surface feeding worms are also attracted to the surface by rainfall and as they emerge they open the entrances to their burrows.

These biopores, providing passageway for water through relatively fine pored soils, are credited with major increases in intake rates observed in soils where tillage has been avoided for several years and a residue mulch has accumulated (e.g. Ehlers 1975). Coupled with the protective action of accumulating crop residues these biopores have increased infiltrability of many soils to a point where there is practically no runoff. Such biopores in soils which are not being tilled are highly persistent due to their basically round configuration which provides arching resistance against potential crushing forces. The strength of this arching resistance is further enhanced by compaction in the soil surrounding earthworms burrows. This compaction results from the earthworm commonly following old decayed roots and ingesting only those portions of the soil that contain large amounts of decaying roots and associated microorganisms. The worm extends its proboscis along this old root channel, rejecting soil endowed with lesser nutrient and forcing it out of the way with muscles which can compact with forces of 0.4 to 0.5 MPa (Kemper et al. 1988). This disrupted and compacted soil surrounding the biopore rapidly gains cohesion (e.g., Kemper and Rosenau 1984) which provides resistance to subsequent disruptive shear and compres- sive forces. Consequently, these biopores often last for years in untilled soils, frequented in sequence by worms and roots and providing for rapid transit of water, oxygen, and mesofauna as they seek protection from preda- tors and extremes of temperature and drought (Wang et al. 1986).

However, some soils do not generate or maintain large populations of such mesofauna. Clay pans, fragipans, compaction pans, and acid subsoils which restrict root growth also restrict mesofauna because the mesofauna extend their burrows primarily to follow organic residues which are their food (e.g., Kemper et al. 1988). Water surpluses or deficits can also limit earthworms population development. They cannot live for extended time below the water table. Their activity ceases and they become dehydrated and die if exposed to soils near or below the wilting point (e.g., Kemper et al. 1987). If a layer or pockets of soil exist where moisture and temperature

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conditions are in the intermediate range where earthworms are viable, some of them may find their way to this safe haven. However, if the water table rises to the surface during winter months when precipitation exceeds evapotranspiration, worms are forced to the soil surface. If this saturated soil condition is accompanied by freezing peri- ods the worms generally die. A few cocoons may survive and prevent complete annihilation of worms in such areas, but their populations will be so low that they will not significantly affect infiltration rates. In soils devoid of these biopore makers, no-till management of the soil can result in infiltration rates that are lower than those of tilled soils. Research is needed to identify the physical, chemical, and biological systems which are hospitable to biopore makers and which can induce them to play their major roles in increasing infiltration rates.

4. Water harvesting

In areas where precipitation is less than 200 mm per year, economic production of crops is generally difficult or impossible even when all the rain water infiltrates. In such areas some desert societies have resorted to using the majority of their area for water harvesting and using the harvested water for household purposes, to water domes- tic animals and to grow gardens and other high value crops on small plots. Under such conditions low infiltration rates are desired in the areas utilized for water harvesting. Historically, removal of organic matter, compaction, and sodification have decreased infiltration rates and increased amounts of water harvested from these areas. The availability of manufactured chemicals which can coat mineral particles and make them hydrophobic has provided a new tool for increasing runoff which, on coarse textured soils, appears to have potential for economic feasibility in some areas where the value of water is high (e.g. Emmrich et al. 1987).

Government agencies and granting institutions, including ARS, ARO, and BARD decided to not continue support to research attempting to optimize runoff on the basis of a conclusion that amounts of water gained would be small and would not benefit a significant portion of the population, compared to the amount of water that might be saved and number of people that might benefit from more conventional irrigation research.

As populations grow and the relative value of water increases it is likely that water harvesting will become economically feasible. Further research on water harvesting and associated desertic use will probably provide alternatives that will be needed in the future. If govern- ments cannot use their limited funding for such research, they should at least provide moral encouragement and access to public lands which would encourage private research.

B. Capacity of soil to hold water for crop use

1. Physical factors

Soil structure, texture and mineralogy of the clay size fraction have long been considered the primary factors

determining the mass of water per unit volume of soils that can be held by a soil against a specific potential (air phase minus water phase pressure). In general, the finer grained materials hold more water because size of pores between the particles are small enough to hold water by capillarity against significant potentials. Recent analyses of Soil Conservation Service data on soils by Hudson (1994) indicate that soil organic matter plays a much greater role in retention of water in soils than we have considered in the past. Available water holding capacity in loam soils is increased by almost 4% of the soil volume when organic matter in the soil is increased one percent of the soil weight. In addition to capillary or surface tension forces, osmotic forces associated with diffuse layer cations also help hold water in soil and make it available through the range of extracting potentials that can be exerted by crops.

The downward force of gravity is highly significant as a factor moving water into and through coarse textured soils. However, the water in the large pores of coarse textured soils is removed at relatively low potentials and the small amount of water retained is primarily in capillary annuli surrounding contact points between adjacent particles. When average size of particles is large there are fewer of these annuli, possibilities of their overlapping or connecting are small and the hydraulic conductivities of coarse textured soils drop to practically negligible values, even at low tensions. Consequently, occurrence of a coarse textured layer in a soil will generally inhibit dewatering and increase the effective water holding capacity of finer soil above that layer. This ability of coarse textured layers to interrupt the hydraulic continuity has provided many shallow soils (i.e., less than 0.6 m deep) overlying such layers with sufficient water holding capacity to satisfy evapotranspiration demands for over a week. When irrigation is available for such soils they can be highly productive.

However, while adequacy of water supply is a primary prerequisite for good production, adequate oxygen supply for root respiration is also a needed for good root growth and function. Consequently, deep chiseling or establishment of large populations of biologic macropore makers (e.g. earthworms) will often improve productivity and water use efficiency of such soils.

The reservoir from which crops can draw water is often limited by layers such as cultivation pans, clay pans, fragipans, acid subsoils, etc., which restrict rooting depth. Removing such restrictions (e.g. Fig. 1) and extending the effective rooting zone from 60 to 100 cm of depth, appears to be highly desirable.

Past studies (e.g., Bradford and Blanchar 1977) have shown that mechanical disruption of impeding layers will allow subsequent crop roots to penetrate to the depth of disruption. Primary factors limiting net benefits this type of improvement of "pan" soils are the high costs of deep soil disruption and the questionable long-term persis- tence of the more open structure.

The costs required have generally prevented such disruption from being economically feasible for field crops. However, as population grows and society recognizes needs to recycle municipal, industrial and animal wastes,

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a new era of economic feasibility of deep soil improvement may be dawning. In the United States, landfill tipping fees in many areas were $10/ton in 1980, rose to about $ 45 in 1992 and are expected to exceed $100 by the end of the decade. About 60% of the material de- posited in these landfills is paper products, lawn clip- pings, leaves, and other biodegradable materials. They can be composted or incorporated directly into soils. Costs of incorporation of these materials to depths of one or two meters in the soil are large, but small in compari- son to the $100/ton tipping fees anticipated at landfills. They take up space while the soil is consolidating and after the soil has consolidated they vacate that space via microbial oxidation and/or ingestion by earthworms. These vacated macropores then play a major role in facilitating invasion of these lower depths by roots and earthworms and sustained habitation and maintenance of this expanded root zone.

2. Crop factors and interactions

While disruption of such restricting layers in conjunction with incorporation of wastes is a possibility, it will only be economically feasible in the small portion of our soils which are within reasonable hauling distance of the waste sources. The benefits of such incorporation will only be optimized if plant roots and associated biofactors invade these new depths and continue to find them habitable.

The potential for using years when food production is not needed on all of our lands to grow species of crops with exceptional ability to penetrate deeper in the soils needs to be evaluated. Species of perennials such as alfalfa and switchgrasses tend to push their roots deeper in suc- cessive years. In some cases, they may need concurrent chemical changes in the soil (e.g., gypsum to supply Ca and displace A1 and H in acid subsoils). The cost of the chemical changes may also decrease as our efforts to re- move sulfur from flues of coal-fired power plants with calcium oxide result in deposition of gypsiferous wastes (about 40 million tons per year in the United States). In June of 1992, the United States Environmental Protec- tion Agency (EPA) reversed an earlier decision to ban use of "phosphogypsum" a byproduct of the phosphate manufacturing process which is primarily gypsum. How- ever, EPA placed a limit of 10 pCi/g on the radioactivity allowed in phosphogypsum that is to be applied agricul- turally. This limit apparently will exclude the phosphogypsum from Florida which commonly has 12 to 18 pCi/g of radium-based radioactivity. Research is needed to show EPA the total loading limits of these marginally radioactive materials that can be applied before the crop is significantly contaminated or radon production by the field is significant.

These combined sources will provide enough gypsum to apply over 500 kg per hectare per year to all the culti- vated land in the United States. However, hauling dis- tances and specific soil needs will concentrate them in some areas and exclude them from others.

Gypsum's solubility is sufficient to allow it to move into soil with irrigation or precipitation to substantial

depths. Sumner (1990) found that surface treatments of gypsum resulted in Ca leaching into acid soils to depths of a meter within 5 years, which along with associated displacement of AI, allowed alfalfa roots to penetrate about 50% deeper, access an expanded water supply and increase yields by about 50%.

Data from other crops on other soils are needed to compute the benefits and costs associated with using the gypsiferous compounds in acid soils. Additional data are also needed on uptake of the other constituents of coal fired power plant wastes to determine their benefits and the precautions which must be observed to achieve their safe and beneficial use.

The reservoir of soil water available to crops is determined by the capacity of the soil to hold water against removal by gravity, and the depth to which roots can extend in the soil. The capacity of the soil to hold water is enhanced by small pores (less than 0.01 mm in diameter) and deep rooting which is generally favored by presence of large pores greater than 0.1 mm diameter (e.g. Wang et al. 1986). Consequently, soils which provide plants with large reservoirs of available water should have a broad spectrum of pore sizes from less than 0.001 mm to over 1.0 mm in diameter, or even more ide- ally a bimodal distribution of pores with a few large ones (0.1 to 10 mm) to provide for good root entry and aeration and rapid water entry when needed and many small pores (0.0001 to 0.01 mm diameter) to provide high water holding capacity.

Open, loose and penetrable soil structure in the culti- vated layer commonly provides for easy proliferation of roots in that layer. However, compaction by traffic can be particularly severe if it occurs immediately following tillage (e.g., Bullock et al. 1988) and can cause exclusion of roots from soils in the region under the wheel tracks (e.g. Voorhees et al. 1992). In no-tiU crop production, which is rapidly gaining adherents in the United States, loosening of the soil by tillage is eliminated. While surface traffic immediately following tillage is eliminated by no- till management, there is still some potential for compaction in operations associated with planting, fertiliz- ing, spraying, and harvesting the crop. Harvesting often has major potential for compaction because the grain holding vehicles and combine harvesters are generally heavy and the harvesting operation is often a race to rescue the perishable product from impending bad weather even when the fields are wet and highly com- pactible. Consequently, some compaction does occur which reduces penetrability of soils to roots. Abstinence from subsequent tillage leaves these soils in a compacted state which can result in reduced infiltration and more runoff (Jones 1991).

Surface soils are generally more dense in no-till production systems. The factor which often appears to com- pensate for the foregone loosening effects of tillage is the development of more macropores by biologic factors. These large continuous pores are commonly formed by mesofauna such as earthworms and ants which find most of their food on the slowly decaying crop residues on the soil surface but live in burrows in the soil. These burrows or macropores, often 1 to 10 mm in diameter can provide

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the large size pores needed to facilitate aeration and root passage through the surface layer and drain puddles of water, accumulating from precipitation on the surface, into the soil.

Observations by several investigators (e.g. Edwards and Amerman 1983; Bruce and Langdale 1990) indicate that no-till systems which leave residues on the surface can increase the infiltration rates sufficiently to absorb practically all of the precipitation. This increase in infiltration rate is due to physical protection of the soil surface from the beating and sealing action of the raindrops, increasing the organic matter and stability of aggregates in the critical surface layer and the biologically induced macropores. All of these factors tend to be enhanced by the no-till systems. However, in hot dry regions there may be insufficient water to allow development of earthworm populations. Under such conditions the loosening achieved by tillage may achieve infiltration rates that are on the average higher than those in no-till systems. It is difficult to discern whether the absence of earthworms is due to climatic factors, or to management factors which can be changed. Earthworms have returned to many fields as a result of no-till where they had been practically absent for decades. Research is needed to define climatic and soil conditions and management practices to facilitate returns of the worms.

The long-term trend is for organic matter to be contin- ually reduced in tilled systems. One of the exciting results of no-till systems is that they halt and in some cases reverse that trend. (i.e. Bruce and Langdale 1990; Ed- wards 1988) and others are finding that organic matter in the top three cm of soil can be increased rapidly in no-fill systems. Organic matter at lower depths in these no-till soils ceases to decrease. When crop residues are left on the surface of no-till soils this increase in organic matter appears to be in the range from 0.01 to 0.03% per year. However, inclusion of a winter cover crop, which is also left on the surface can increase this rate of organic matter increase to 0.05% per year. This long-term increase of soil organic matter generally helps increase infiltration rates and available water holding capacity of no-till systems. In general, no-till management favors high populations of mesofauna which form the large size pores needed for root passage through the top 20 cm of soil, but leaves the 5 to 20 cm layer in a relatively dense condition reducing proliferation of roots therein. In at least some cases the result is deeper root penetration in no-till soils and a larger reservoir of water available to the crop.

C. Evaporation from soils

Diffusive movement of water vapor in the soil air can be calculated with reasonable accuracy and is generally much slower than convective or capillary movement in the liquid phase. Consequently, when continuity of the liquid phase from the body of the soil to the surface is disrupted by a coarse textured mulch at the surface, the rate at which water is lost is greatly reduced. However, it has not been possible to calculate convective movement of water vapor, due to gusting winds and associated

small, but high frequency, changes in atmospheric pressure in coarse textured mulch on the surface of soils. Consequently, Kemper et al. (1994) evaluated movement of water vapor through sand and gravel mulches under field conditions. They found that evaporation from moist soils covered by a few centimeters of gravel or sand mulch was reduced to only about 10 cm per year under their semiarid conditions. Under fallow conditions without the mulches, adjacent soils lost over 30 cm of water. From these data they were able to estimate dispersion flux and associated coefficients for sand and gravel mulches which should be reasonably applicable where wind velocities are in this same range and thereby facilitate estimates of evaporation reduction that can be achieved by such mulches at other locations. Such estimates generally indicate that most of the evaporation from soils can be prevented by gravel and sand mulches.

A primary deterrent to economic agricultural use of such mulches is the cost of putting them in place. Gravel mulches have been used in orchards and landscaping applications, but the tendency for weeds to become estab- lished and transpire the saved water has generally been a problem. Sterling Richards searched diligently for a "one way valve" that would let precipitation into soil, but would block capillary flow to the surface and weed growth. He developed cement blocks with pores through them that were about one millimeter in diameter (Richards 1965). These blocks had few smaller capil- laries, were reasonably effective in accomplishing their objectives and were cost effective for some landscaping purposes. However, they were generally too expensive for agricultural purposes when the cost of water was low. Since irrigation water costs in certain areas are high (e.g. 20 US cents/m 3 in Israel) and are increasing with time the economics of such measures to decrease evaporative water loss should be reevaluated.

Another type of material that might provide a reasonably effective "one way valve" at lower cost is coal-fired power plant wastes containing gypsum. The gypsum content of these materials is a result of using calcium oxide to hold the sulfur oxides from burning coal so that they do not enter the atmosphere. Korcak (1988) applied a layer of this material about 6 cm thick in a strip extending about 1.2 m on each side of the rows in which apple trees were planted in an orchard. The result was improved growth of the apple trees and generally better yields of apples. Weed growth was inhibited. The pores in this material were sufficiently large that all the water from even intense rainstorms percolated through this material into underlying soil. With time the rainfall solubilized more of the gypsum, enlarging the pores and tending to leave a framework of less soluble minerals such as calcium carbonate and silicates. In preliminary studies, it has been observed that such layers of such gypsiferous wastes improve infiltration and reduce evaporative water loss. Even during the first few months after application the gypsum appears to be dissolving away and the pores in these materials appear to be growing in size. It seems probable that with time they will retain less water from precipitation events and will approach coarse sand mulches in their ability to save water. Their bonded na-

ture would prevent them from the movement in response to wind that is a problem with sand mulches.

In Korcak's (1988) study the weeds began to break through the gypsum after a few years. The weakness of gypsiferous barriers appears to be associated with the amount of carbon left in these power plant wastes. Power plant managers believe that they can reduce that carbon content, at little additional cost, and provide a product which would have more strength. Cooperative research is needed between soil scientists and power plant managers to define and obtain optimal byproducts for agricultural u s e .

Korcak's (1988) heavy experimental application of these gypsiferous wastes did not bear contaminants such as boron, selenium, or heavy metals in sufficient amounts to constitute a hazard to the apples or their consumers. The quality of the apples was improved in some respects. However, potential heavy applications of this type should be preceded by analyses to determine what the waste product contains and solubility of components which have potential to benefit or be toxic to plants or animals. For instance, Ransome and Dowdy (1987) found that their power plant waste contained substantial amounts of boron. Recognizing this fact they applied it to boron deficient soils at several rates, measured plant response in the range from boron deficient to toxic, and developed guidelines for optimal use. Additional research of this type is needed to help recycle waste products and obtain potential benefits therefrom, including assessment of increases in infiltration and evaporation reduction. Developing procedures and protocols to avoid negative effects is an essential part of such research.

In the United States, the materials most commonly available for establishing mulches which will reduce evaporative losses from soils have been crop residues. In some countries these residues have such high value for animal feed or fuel that they cannot be spared for this purpose. In countries such as Israel where mechanical power units have replaced animal power, the residues are now available for soil and water conservation.

Cultivation by man, animal, and tractor power was instituted primarily to eliminate competitive natural veg- etation and provide crop plants with the water, nutrients, and light needed for good yields. This cultivation also caused more rapid oxidation of residual organic matter, making its associated nutrients (e.g., N, P, S, etc.) available to the plants. Plowing under the crop residues also placed them under moisture and temperature conditions that tend to optimize their rate of decomposition and make their components more quickly available to the succeeding crops. Availability of relatively low cost fertil- izers to supply needed nutrients for our crops has practically eliminated the need for cultivation as a factor in nutrient supply.

When crop residues are plowed under, subsequent high rates of microbial activity generate fungal hyphae and intermediate products of residue decomposition which hold soil particles together and stabilize the aggregates and structure of the soil (e.g., Miller and Kemper 1962). However, because the decomposition processes involved are rapid, these products are relatively short lived

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and the soil is commonly back to near its preplowdown condition within a couple of months.

When the crop residues are left on the surface, where conditions are too dry for optimum microbial activity, substantial portions of them commonly persist for several months of growing season, and can persist for years in cold and dry climates. These persistent residues provide protection of the surface and generally maintain higher infiltration rates as discussed earlier. They also form an organic mulch which decreases evaporative losses. This decrease in evaporative loss resulted in 2 to 5 cm more water in the rooting depth profile at the end of a fallow season as applications of straw mulch ranged from 2.2 to 6.6 Mg/ha (Greb et al. 1979). Some additional savings due to the mulch can be expected between seeding and time of canopy cover if no-till drills are used which plant the seed by slicing through the mulch and cause minimal burial of the crop residues.

In the northern plains of the United States, each extra cubic meter of water stored in this manner increases wheat yields by about 1.6 kg. In the southern plains of the United States, this extra water stored by these mulches increases winter wheat yields by only about 0.6 kg/m 3 (Stewart 1988). The relatively low efficiencies of this stored water use in the southern plains is improved to some extent when sorghum is grown and the increased grain yields are about 1.2 kg/m 3 (Unger 1978). Unger found water storage efficiency could be increased from 22.6% in bare fallow to 43.4% when 8 Mg per hectare of sorghum residue was placed on the surface. When an additional 4 Mg per hectare of residue was applied, the storage efficiency increased by 0nly 2.5%.

The evaporative losses of water involved when there are thick organic mulches on the surface may occur primarily as a result of the mulch absorbing significant quantities of water during the wetting period and subsequent evaporation of this water absorbed by the mulch to the atmosphere. If this is true, water loss after it is in the soil may be as low through crop residue mulches as through gravel and sand mulches. Kemper et al. (1994) provide a simplified equation from which evaporation through sand and gravel mulches can be estimated based on thickness of the mulch, size, and color of particles in the mulch, and annual climatic data (temperature, wind speed, and relative humidity). They provide estimates of the dispersion coefficient through such mulches for water vapor as a function of pore size and wind speed. Actual long-term storage efficiencies under organic mulches should be evaluated because if they approach those under gravel mulches and if the root zones have large storage capacities, this would allow delaying of crop growing seasons to the times when the water will be most efficiently used in crop production, rather than forcing the growing season into the specific time when the rain is received.

Results of Unger (1978), Greb (1979), and others indicate that normal annual production of crop residues is generally not sufficient to provide sufficient mulch to optimize water use efficiency in most semiarid regions. With some crops (e.g., cotton and soybeans) in some warm and windy regions, little of the residue persists longer than a few months, even under no-tiU manage-

72

60

50

45

~"E 401 TOTAL RAINFALL E ~ (Cumulative Monthl ~

= = 35

E o 30 o ~ ~ 1 7 6

. c

o ~, 2 5 �9 .~ a::

~ ) �9 o m 2C Grovel ~ .~ 5cm D e e ~

1 5 [ - �9 �9

I 0

Zcm

AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG 1966 - - 1967

Fig. 2. Water accumulation in soils in Colorado under gravel mulches 1 -5 cm thick

means of getting adequate water for high value crops. Informat ion will also be needed on yield response of the crops to the open canopy configurations to complete analyses of economic feasibility.

Tree fruits are often high value crops which have potential for benefitting f rom surface mulching and spacing adaptat ion to fit precipitation. The high value of produc- ing trees argues strongly against changes in tree spacing by removing trees to adapt to year to year variations. However, recent results obtained by horticultural research in Israel indicate that pruning the trees could change their water requirements sufficiently to make such adaptions.

Costs and benefits of covering bare soil with mulches (assuming that sources of low-cost mulches can be developed) and planting trees at spacings to achieve adequate water in normal years should be determined. Back up irrigation systems to supplement small amounts of irrigation in low precipitation years might also be essential for long-term sustainability.

The general objective of these types of research is to find ways to use precipitation more effectively in crop product ion so that agriculture will survive and thrive, even if it loses access to all or par t of current supplies of irrigation water.

References

ment. Under such conditions applying complementary organic materials such as animal manures and municipal wastes including leaves, grass clipping, and processed paper may be able to help fill this deficit. In the past the standard recommendat ions have been to compost these organic materials or bury them as soon as possible after they were distributed in the field to prevent them or their vapors from moving with the wind and becoming off site nuisances. Such burial is contrary to the objective of keeping degradable organics on the soil surface as mulch to improve infiltration and reduce evaporative water loss. I f means to keep these organic wastes on the surface and avoid their becoming a nuisance can be developed, these materials which are becoming a major disposal problem could become a resource. Optimizing their benefits and avoiding their problems may be achieved most cost effectively by mixing them with other waste stream materials such as coal-fired power plant residues, sewage sludges, etc.

One potential for adapting crops to existing precipitation is spacing plants further apart in areas of low precipitation. I f the rooting extent is great enough to access the water under the bare areas there are possibilities for keeping the plants adequately supplied with water to achieve good yields.

A major deterrent to this alternative has been the high portions of the rain fall in arid areas that are lost by evaporat ion f rom the bare soil. The potential of porous mulches on the surface to reduce the evaporative loss to a small fraction of the rainfall at reasonable cost needs evaluation to determine the feasibility of spacing as a

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