Instruction for Soil Sampling
A soil test is only as good as the sample you take Is representative of your land. Hence, It Is very Important that you Collect a representative sample.
Take to the field: Take a notebook, soil sample cartons or polythene bags, two clean buckets, a soil sampling tool such as a spade or soil tube or soil auger (or panga) and ruler.
Make a sketch!) map of the field/plots to be sampled, Indicating the difference in soil, which you recognize. Each soil sample should not represent more than two hectares (5 acres). These may be whole fields or sections’ of fields depending on the following situations: .
1. Let each soil sample represent not more than 2 hectares. For any field or soil area larger than this a correspondingly
larger number of samples must be taken.
2. Irrespective of field size, let a separate soil sample represent parts of the field which differ In:
.:. Soli colour
.:. Soil texture (sand, loam or clay),
‘.’ Slope (if contour formed, sample contour area separately) .
•:. Crop performance. (Crop quality or symptoms indicating varying degrees of nutrient deficiencies) .
•:. Management practices eg Mulched vs unmulched
On the sketch map list your Field or Block designation, sample numbers and indicate/mark approximate distribution of the borings
(spots where a sample was taken). Remember 10 record your address in the soil sampling Information sheet (or notebook).
Instruction for Plant Tissue Sampling
Take 10 The field the Following: Sampling tool (sharp knife), paper bags (not polylhene bags!) and no Ie book. The field should be surveyed and differences in soils and crops recorded.
Factors to consider:
1. Growth size of plants
2. Appearance of the plants
3. Soil type (ie, for different soil types ‘sample differently)
4. Type of crop
Therefore, a separate sample should be taken if the field differs in the above points.
The number of plants to be sampled depends on:
1. Type of crop .
2. Stage of growth
3. Analysis required
The total area sampled Is not critical so long as the field Is uniform In appearance. On the sketch map of the farm, record the Field or Block designation, sample numbers per field, type/variety of crop, stage of growth of crop, name of sampler and date of sampling.
Remember to record your address.
It is recommended that samples be taken (use a sharp knife) from mature leaves just below the growing lip. Put the samples from a uniform field in one paper bag and label. Plants of the same age must be sampled at the same time.
Avoid crops that are:
.:. Covered with soil or dust
.:. Damaged by Insects
.:. Sprayed with foliar feed or pesticides
.:. Moisture stressed over long periods . ..
As a general guide so as to obtain a representative sample, legumes require 10-20 plants to be sampled and vegetables about 40
Samples should be taken to the laboratory as soon as possible for drying.
For details contact KALRO National Agricultural Research Laboratories Tel: 4446989 or 4443376 Nairobi along waiyaki way
Irrigation potential in sub-Saharan Africa
In the dry lands of sub-Saharan Africa, water deficit is the most important environmen tal factor limiting yields in agriculture. When irrigated, these areas can have a high yield potential because of the high solar radiation, favourable day and night tempera ture and low atmospheric humidity, conditions that decrease the incidence of pests and diseases compared to areas in temperate zones. The key to maximizing crop yields per unit of supplied water in dry lands is ensuring that as much as possible of the available moisture is used through plant transpiration and as little as possible is lost through soil evaporation, deep percolation and transpiration from weeds.
In recent years there has been growing concern at the performance of conventional irrigation systems in sub-Saharan Africa. The poor performance of irrigation projects seems to have contributed to stagnation in new irrigation development. Available data suggest that irrigation potential in the region is considerable but largely unexploited
The anticipated long-term yield increases for irrigated land which earlier depended on unpredictable and unreliable rainfall have not always been achieved. This has con tributed to irrigation losing its appeal as an investment strategy. Good performance in irrigation systems is not only a matter of high output but also of efficient use of avail able resources. For example, the inefficient use of irrigation water in arid areas i not only wasteful but often leads to salinization of the soil profile. Irrigation systems that are to be effective and efficient must ensure that drainage, maintenance of soil fertility and salinity-control measures are employed.
There are four main factors aggravating water scarcity:
- Population growth: in the last century, world population has tripled. It is expected to rise from the present 6.5 billion to 8.9 billion by 2050. Water use has been growing at more than twice the rate of population increase in the last century, and, although there is no global water scarcity as such, an increasing number of regions are chronically short of water.
- Increased urbanization will focus on the demand for water among an ever more concentrated population. Asian cities alone are expected to grow by 1 billion people in the next 20 years.
- High level of consumption: as the world becomes more developed, the amount of domestic water that each person uses is expected to rise significantly.
- Climate change will shrink the resources of freshwater.
Water scarcity is expected to become an even more important problem than it is today. There are several reasons for this.
First, the distribution of precipitation in space and time is very uneven, leading to tremendous temporal variability in water resources worldwide (Oki et al., 2006). For example, the Atacama Desert in Chile receives imperceptible annual quantities of rainfall whereas Mawsynram, Assam, India receives over 450 inches annually. If all the freshwater on the planet were divided equally among the global population, there would be 5 000 to 6 000 m3 of water available for everyone, every year.
Second, the rate of evaporation varies a great deal, depending on temperature and relative humidity, which impact the amount of water available to replenish groundwater supplies.
The combination of shorter duration but more intense rainfall (meaning more runoff and less infiltration) combined with increased evapotranspiration (the sum of evaporation and plant transpiration from the earth’s land surface to atmosphere.) and increased irrigation is expected to lead to groundwater depletion.
According to Pr. Andrew Goudie of Oxford University, key changes to the hydrological cycle associated with an increased concentration of greenhouse gases in the atmosphere and the resulting changes in climate include:
- Changes in the seasonal distribution and amount of precipitation
- An increase in precipitation intensity under most situations
- Changes in the balance between snow and rain
- Increased evapotranspiration and a reduction in soil moisture
- Changes in vegetation cover resulting from changes in temperature and precipitation
- Consequent changes in management of land resources
- Accelerated melting glacial ice
- Increases in fire risk in many areas
- Increased coastal inundation and wetland loss from sea level rise
- Effects of CO2 on plant physiology, leading to reduced transpiration and increased water use efficiency
With the population of low latitude regions increasing, water resources are likely to become more stressed in many regions, especially as global warming intensifies.
Increasing intensity of precipitation is likely to increase a region’s susceptibility to a multitude of factors including:
- Rate of soil erosion
- Mass movement of land
- Soil moisture availability
These factors are likely to affect key economic components of the GDP such as agricultural productivity, land values and an area’s habitability. In addition, warming accelerates the rate of land surface drying, leaving less water moving in near-surface layers of soil. Less soil moisture leads to reduced downward movement of water and so less replenishment of groundwater supplies (IPCC). In locations where both precipitation and soil moisture decrease, drying of the land surface is magnified, and areas are left increasingly susceptible to reduced water supplies.
Although projecting how changed precipitation patterns will affect runoff is not yet a precise science, by interpreting historical discharge records, it is likely that for each 1°C rise in temperature, global runoff will increase by 4% (Labat, 2004). Applying this projection to changes in evapotranspiration and precipitation allows us to conclude that global runoff is likely to increase 7.8% globally by the end of the century. This places a region experiencing a higher annual precipitation amount and a larger volume of runoff at an increased likelihood for flooding.
Furthermore, in areas that are already vulnerable due to their limited groundwater storage availability, this cycle intensifies with increased warming and diminishing water supplies. In water stressed regions, variability of precipitation patterns is likely to further reduce groundwater recharge ability. Water availability is likely to be further exacerbated by: poor management, elevated water tables, overuse from increasing populations, and an increase in water demand primarily from increased agricultural production (IPCC).
In a recent global analysis by Dai et al, variations in PDSI indicated that the area of land characterized as very dry has more than doubled since the 1970s while the area of land characterized as very wet has slightly declined () during the same time period. In certain susceptible regions, increased temperatures have already resulted in diminished water availability. Precipitations in both western Africa and southern Asia have decreased by 7.5% between 1900 and 2005.
Most of the major deserts in the world including the Namib, Kalahari, Australian, Thar, Arabian, Patagonian and North Saharan, are likely to experience decreased amounts of precipitation and runoff with increased warming. In addition, both semiarid and arid areas are expected to have a decrease and seasonal shift in flow patterns. If increased temperatures cause an intensification of the water cycle there will be more extreme variations in weather events, as droughts will become prolonged and floods will increase in force.