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Water Management

                                                 By Norman Wilson, Ph.D.

Water is by far the most abundant component of crop plants. Living plant tissue including leaves, stems, roots, and fruit will normally contain 70% to 90% water. Water serves a number of absolutely essential physiological functions in growing plants. Water is the universal component that allows plant roots to take up mineral nutrients, transport those nutrients to the leaves, synthesize food products (carbohydrates, protein, and fat), and store these products in a form that we harvest for yield.

The management and efficient use of water is one of the greatest challenges facing both irrigated and dry land producers. Water represents a major production cost for the irrigated grower and can be the difference between a profitable crop and a total loss for the dry land farmer. In either case, water is the key to efficiently utilizing other expensive inputs. Check just about any local newspaper published in the western half of this country and you will find articles relating to water concerns. Some agricultural areas are depleting ground water reserves, some are in competition with municipalities or environmental groups for available water, and others are struggling with long term drought.

We have established the essential role of water in crop production but you may question exactly what that has to do with soil health. In fact, soil health and water management are absolutely linked. A definition of soil health is probably in order at this point. If we think about those characteristics that make an agricultural soil productive (healthy), we usually list tilth, fertility (CEC), pH, organic matter content, salt content, and maybe such things as flocculation, aeration, and compaction. The point must be made that these things are all directly or indirectly related to soil microbial activity. A productive agricultural soil is defined by its level of biological activity. With this definition in mind, logic tells us that we need to approach soil improvement and/or conservation from a biological rather than a chemical perspective.

An understanding of how soil microbes work to improve and maintain soil productivity is important to developing a water management strategy. In simple terms, soil micro organisms use raw organic matter (crop residue) as a food source to produce energy, air (carbon dioxide or oxygen) and humus. Pore space (air) will occupy 50% or more of the total volume in a well flocculated and biologically healthy soil. This pore space is essential to soil water relationships. Irrigation water or rainfall enters soil pores (infiltration) and displaces air (retention). Continued application allows water to move downward through the soil profile and displace air until the soil reaches its maximum retentive capacity or saturation point. After application ceases, downward movement of water continues in response to gravity. The soil water content at which gravitational movement stops is field capacity. Field capacity is the relatively stable point where water has moved out of large soil pores and is retained in smaller pores by attractive properties of soil particles. Humus can provide these attractive forces and therefore plays a critical role in water efficiency. Capillary water held at field capacity provides the moisture that a growing crop will take up and use to produce yield. The more water stored at field capacity after a rain or irrigation, the more water that is available to produce crop yield.

Water use efficiency is dependent upon soil pore size and number, soil humus levels, and salt concentration. If pore size is too small (inadequate flocculation), infiltration will be slow and water will be lost to runoff and evaporation. If pore size is too large (sandy soil with low humus levels), retentive capacity is low and water will percolate down and out of the soil profile. Sodium salts are critical to water management because they destroy natural flocculation and interfere with water penetration and retention. Where Sodium salts are present, a salt management strategy is essential to water use efficiency. The goal of any water management program should be reduction of waste from runoff, evaporation, percolation out of the root zone, and the negative effects of salt. Water management does not mean denying a crop the water necessary to achieve a profitable yield or excessive irrigation which can result in poor aeration, water logging, and fertilizer leaching.

The relationship between soil biological health and water use efficiency is clear. A strategy to promote and maintain soil biological health should be just as clear. A pre-plant application of ViBasic or ViClout will stimulate aerobic soil microbial activity, enhance flocculation, buffer salts, and help build a soil moisture profile for future crop use. An improved seed bed will promote uniform germination and emergence, faster stand establishment, and better early vigor. ViBasic or ViClout applications during the growing season will maintain a soil environment necessary for good water use efficiency. Now is the time to think about doing your soil, your crops, and yourself a big favor.


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