Plant available water

As shown in Figure 25.8, an increase in soil thickness from the design thickness (A) by 50% to (B) may result in only a small increase in plant-available water-holding capacity during a single... 

Base the design on reduced plant-available water-holding capacity (e.g., 10% reduction). 

The next layer of water, held between -30 and —1500 kPa, is available to plants and is therefore called plant available water. The water present between -1500 and -3200 kPa is held in capillaries so tightly that it is not available to plants but can be lost by evaporation. The layer closest to the soil solid is held at more than -3200 kPa and is called hygroscopic water. A soil sample, heated in an oven for 24 hours at 105°C, and then left exposed to the air will adsorb water until a layer of hygroscopic water has been formed, illustrating the strong attraction of soil surfaces for water. 

Water that can easily be removed from soil is called gravitational water. Some of the water retained against the pull of gravity is called plant available water. Which of these two would be expected to have more soluble salts Why ... 

The monitoring of the soil plant available water (PAW) between 0 and 8 m depth in the soil has shown that during the dry season, in active B. brizantha pastures areas, a considerable use of water reserves occurs primarily in the top 2 m of soil. This is similar to observations made in primary and secondary forest areas. However, below 2 m, the depletion of the soil water reserves was greater in the forest ecosystem (Jipp et al. 1998). In general, active pasture ecosystems have a greater proportion of fine roots in soil layers between 0-2 meters, and the water in these layers is depleted more quickly, while a major part of the water reserve in the soil is stored in deeper layers. As the pasture... 

Humus also absorbs large quantities of water. The fully synthesized humus of a mineral soil contains as much as 80 to 90% water by weight. Additionally, micropores within larger soil aggregates hold available water for plants. This increase in plant-available water-holding capacity is a major benefit of organic matter additions to sandy soils. 

To handle a peak cooling load with a reduced size of refrigeration plant, typically to make ice over a period of several hours and then use ice water for the cooling of a batch of warm milk on a dairy farm. This is also used at main creameries, to reduce peak electricity loads. The available water is very close to freezing point, which is the ideal temperature for milk cooling. 

When assessing the potential for RO as an RW treatment option and reviewing standard plant specifications, it is important to compare the rated membrane capacity against the available water source to be treated. Reported RO membrane capacity may be based on a temperature of 77 °F (25 °C) and perhaps only a 1,000 ppm TDS RW. This level of TDS may be much lower than the potential source of RW and the temperature also may vary, making corrections necessary. At lower water temperatures, the viscosity increases and the RO flux decreases (output decreases). This increases the number of membranes required to provide the desired flow. 

Drought is perhaps one of the most complex examples to choose but it illustrates well the possibilities of, and pitfalls to, progress. Drought affects almost every facet of plant function and we are faced with the paradox that yield and evapotranspiration are intimately linked. In general, increases in yield when water supply is limiting are likely to result from characteristics which increase the available water supply, increase water use efficiency or increase biomass allocation to the economically useful plant parts (Pass-ioura, 1986). Additionally, features which maintain cell viability and protect metabolism in water-stressed tissue and allow rapid recovery after dry periods will contribute yield under some circumstances. 

Sornsrivichai P, Syers JK,Tillman RW, Cornforth IS. An evaluation of water extraction as a soil-testing procedure of phosphorus. Glasshouse assessment of plant-available phosphate. Fert. Res. 1988 15 211-223. 

No information could be found in the available literature on the levels of thiocyanate in ground, surface, or drinking water. Thiocyanate is found in concentrations ranging from 100 to 1,500 mg/L in coal plant waste waters (Ganczarczyk 1979 Jensen and Tuan 1993), and from 300 to 450 mg/L in mining (gold extraction) waste waters (Boucabeille et al. 1994b). 

Tap water is undoubtedly convenient it is clean, usually available whenever you want it, and supplied at high pressure. The provision of this service, however, may come at an unnecessary cost—both financial and environmental. The chlorine in tap water may harm your soil s microbe population and damage sensitive plants. Tap water may also have a high pH (see pp.30-31), making it unsuitable for use on lime-hating plants. 

Boron exists in several forms in the soil (USEPA 1975) in soil solntion, it exists largely as the undissociated weak monobasic acid that accepts hydroxyl gronps (Gnpta and Macleod 1982). Most plant-available boron in soils is associated with soil organic matter (Gnpta and Macleod 1982), with the hot-water soluble boron fraction (Hingston 1986), and with soil solntion pH ranges of... 

---