| Soils, Crops and Fertilizer Use |
|Chapter 7: Evaluating a soil's fertility|
Well-run fertilizer trials can be very helpful. There are 3 kinds:
• Test strips
• Field experiments (farm experiments)
• Field tests (field trials, result tests)
Running test strips through a field is a quick and easy way to test crop response to different fertilizer rates and nutrient combinations. Test strips have less statistical significance than formal trials, but they do allow farmers to conduct some research on their own farms, which can be very useful. A researcher's test of statistical significance is usually a 95 or 99 percent likelihood that the results were due to the treatment applied rather than to chance. Farmers (and most of us) would probably settle for a much less stringent figure and try a new practice, even if there were only a 75 percent chance of getting a response.
To reduce the influence of soil variations, each treatment tested should consist of several strips 2-3 rows wide placed in different parts of the field. The soil should still be uniform visually and in terms of past management. Don't rely on just one season's results, since weather and pests can influence yields.
Fertilizer rates for test strips: Consult your extension office and Chapter 9 of this manual.
Field Experiments (Farm Experiments)
These are designed to be statistically valid and require much more effort and care to set up and manage. They are designed to determine the most profitable kind and amount of fertilizer needed for a given crop and soil. Suppose you want to try 3 different rates of fertilizer. It's not simply enough to mark out one test plot and 3 fertilized plots. Each of these 4 "treatments" needs to be replicated 3-4 times and laid out within a bloc in a randomized manner. Each of the 12-16 plots are only a few rows wide and several meters long. Plot size, plant population, and fertilizer rates have to be carefully measured, along with yield differences. It's a good idea to repeat a trial for 2-3 years to take into account weather variations.
Formal experiments require much time, skill, attention to detail, and scientific discipline. They are not something that you should do on your own. However, you can play a very useful role in a well conducted and ongoing fertilizer testing and demonstration program.
Field Tests (Field Trials, Result Tests)
This type of research tests the best fertilizer type and rate determined from the farm experiments above, but this time under actual farming conditions. There are normally just 2 plots: the "control" plot (traditional practice) and the "treatment" plot (improved practices). Rather than rely on randomization and replication of the plots on each farm, the field test gets its statistical validity from being conducted on a number of farms with fairly similar conditions. In most cases, fertilizer will be only one of several improved practices making up the treatment plot. The plots should be large enough so that realistic farming methods can be used. To be valid, the farmer and other usual labor should carry out the practices themselves with some initial instruction and supervision by the extension worker.
Experiments/Trials vs. Demonstrations
The experiments and trials above seek to develop fertilizer recommendations for local conditions that will be the most affordable and profitable for the farmers involved. Don't succumb to the natural and prevalent temptation to use such tests as demonstrations. After all, a demo is designed to provide farmers with "living proof" of the benefits of a new practice (or package of practices) - one that has first proven its worth under local conditions. This syndrome of promoting without adequate prior testing has cost many an extension worker an irreparable loss of credibility. Testing is always the first stage; promoting comes later.
NOTE: For more information on experiments, trials, and demos, refer to the Peace Corps/ICE manual, Traditional Field Crops (M-13).