On-the-spot information in real time can be useful to agricultural advisors in any part of the world, but it is especially critical in developing countries where the infrastructure for delivery of information through sampling and traditional laboratory –based analysis is poorly developed and unreliable. And in Africa these times are the rule rather than the exception… When sending samples off to a soil and plant laboratory and waiting for the results in order to make recommendations to a farmer or solvea problem just won’t do. In many third world countries the results from lab may take months to come back, if they ever do. In the meantime, the chance to build a rapport with the farmer and interest him or her in solutions has been missed. So more and more as I’ve been working with smallholder farmers in countries all over the third world during the past decade or so I have tried to solve problems on the spot. Of course, each farmer and each farm is different and the terrific variation in soils and crops and practices and personalities as part of what makes working with smallholders both exciting and challenging. Although I have not developed-and do not much believe in-cookbook recipes for solving complex problems, I have been able to distill what seems to work into four basic steps, as shown here. They usually work best in the order shown, but circumstances don’t always permit that. Often, the agronomist/problem solver has been summoned at the request of the farmer or the farmer’s advisor.That is, it’s good to begin by talking to the farmer as well as, of course, asking permission to walk in the fields and take samples. Once in the field, I believe it’s important to make an overall assessment of the crop condition and the soil variability within the entire field and the surrounding landscape. These observations of in-field variability are often very useful in guiding the next two steps: analyzing the soils and analyzing the plants.
Talking to the farmer is critical. The farmer, or farmers as shown here in front of their irrigated potato field, are a tremendous source of information and ideas. Although smallholders rarely have written records, it is amazing how well their memory has recorded all of the practices performed and any material supplied to a particular field. Farmers also often have an idea of what they think is wrong or what they think is causing a problem. They may not have sufficient scientific insight to have the correct idea, but it’s always a good place to start. I find that rather than sitting in the house are under shade tree where it may be more convenient to take notes, it is much more useful to interview farmers while standing in the field itself and looking at the crops (there’s no better time to try to diagnose a problem then when the problem is manifesting itself in crop performance). Finally, I am not very enthusiastic about the information gathered via surveys and I believe that a gentle but healthy dose of skepticism is always a good policy. In the words of President Reagan “ trust but verify”. People, especially smallholder farmers and many third world countries being interviewed by an outsider or government agent, will tend to say what they think the interviewer wants to hear. So it’s always a good idea to make some measurements, do some counts, and dig around in the soil for evidence. I sometimes ask a farmer to show me-essentially to reenact-what he or she did with regard to a certain practice such as applying fertilizer or incorporating manure.
Soils are inherently and spatially variable. Superimposed on that inherent variability management related variability often also exists. In smallholder fields usually only a fraction of the hectare the variability and spot to spot in both soil properties and crop performance can be dramatic. This variability within fields is a fact of life, but I have become more and more convinced that it (the variability) can be used to the farmers advantage. … By which the infield variability can be utilized is through comparisons between areas of good and poor crop performance. Small holder fields, and I’m thinking especially of those in Africa, often have small irregularly shaped areas in which crop growth and yields far outperform the field as a whole. The question then becomes what makes the good spots so good? A comparative analysis of plant sap or tissue for soil properties in the good spots compared to the rest of the field often reveals the answer. In the lives of differences are telling even if critical levels in an absolute sense are not known. These series of samples can be taken from the good and the port is in the field and analyses beef performed was the result at the parameters that are distinctly better in the good in areas and probably hold the key to what is lacking in the poor areas. Of course, it is possible that the difference in crop performance is resulted from some factor that is not measured in the samples. To make careful and broad observations of the soil and crop conditions in the contrast in areas to detect assistance is as water content, aeration status, compaction issues, insect pests, crop diseases and differences in agronomic practices such as plant spacing or planting date. In conjunction with such observations quantitative analysis of soil and can often reveal the nature of the problem and suggest solutions.The second way the in which farmers can take advantage of in-field variation in soil properties is to recognize that soils with dramatically different products in the potentials should be managed differently. As basic as this principle seems, is my observation that very few small holder farmers appreciate this fact. In almost every field that I’ve observed farmers plant their crops using the same practices across soil boundaries the mark in zones of extremely different potential for activities. That is, one often sees a zone of extremely stunted plants cutting across the smallholder field and upon examination week and be seen that the of stunted plants are growing on soil that is either much more sandy or more shallow or underlain with gravel or more waterlogged or far more eroded than the rest of the field.
Electrical conductivity (EC) can be easily measured in both soil and water. For measurements in water, such as water to be used for irrigation, the sensing tip of the meter need only be dipped into the liquid. The conductivity of the water is primarily a function of the activity of the dissolved ions it is measured but passing a small current between two metal electrodes separated by a gap. The meter maid of the doubt in units of either total dissolved solids (TDS) or deciSiemens per meter (dS/m). Although the conversion depends somewhat on this specific ions present, the rule of thumb conversion is that TDS equals 0.5 * dS/m. Most EC meter is are quite sensitive an accurate so the choice of the dir is largely determined by such features as the size and diameter of the sensing probe and the price. For humid region, non-saline soils, the EC of a 1:1 soil:water suspension is often generally indicative of the level of soil fertility as most the dissolved ions are nutrient ions, especially nitrate, potassium and calcium. In Africa, the EC and these ions are typically associated with fire ash, either from vegetation burned in place or fuelwood in cooking fires (usually in soils adjacent to the homestead). (Right). The drop in soil pH after adding a sufficientsalt to the suspension is called the "delta pH" and is indicative of exchange acidity and cation or anion exchange capacity. The added salt cations (e.g. K or Ca) exchanges with H+ (Al3+). Since EC (expressed as dS/m) is about 2x total dissolved solids (in parts per thousand), an EC of 2.1 is about equivalent to 0.01 molar CaCl2. This is not enough salt to completely exchange with exchangeable acidity, so as to give a true “delta pH”. Thus, as can be seen in the data, increased salt (increased EC) caused a greater drop in pH (expressed here as an increase in the delta pH). So the size of the CaCl2 granule does make some difference on the “delta pH”.
To test the repeatability of the field determinations with the ion specific meters, I ran two soil samples, six times each. Both the pH (left) and the K and NO3 ions (right) were distinct for the two soils and the measurements were quite repeatable. The Tropept from the upper 30 cm of a mountain terrace in Mgeta village was much lower in nutrient ions, but higher in pH than the more weathered OxicTropustult from a field that was in fallow after a well fertilized maize crop on the SUA farm.
This data suggests that the specific ion (K and NO3) electrode sensors are not bothered by suspended particles (muddy samples) and give essentially the same reading whether the sample used is clear or muddy.
Simple quantitative and qualitative field test for diagnosis of small holder farm problems
Ray R. WeilUniversity of Maryland
Test Plants Test Soil • Petiole/stem Visually assess sap Field • pH / EC • FoliarTalk to the variability • Nutrients symptomsFarmer • Crop growth • Physical • Cut open• Trust but • Soil colors tests stem verify • Topography • Dig up roots
Talk to the farmer• What’s the problem?• What was done?• What was applied?• Trust but verify
In-field variability What makes the “good spots” so good?Can we make the whole fieldas good as the “good spots? Maize field in Tigray
Clear sample after settling Muddy sample after stirring
Clear vs. muddy samples for K and NO3 meters 70 70 NO3-N using clear solution, mg/L 60 K NO3K+ using clear solution, mg/L 60 50 50 40 40 30 SOIL SOIL Tropept- ter 30 20 Ustult-fal Tropept- ter Ustult-fal 20 10 0 10 0 10 20 30 40 50 60 70 10 20 30 40 50 60 70 K+ using muddy suspension, mg/L NO 3-N using muddy suspension, mg/L
1: 10 soil: Mehlich3 extraction solution Mehlich3 extractable K+ Tests for P and S use Mehlich3 extractable PO4 2- light beam and so require filtering. Mehlich3 extractable SO4-S
Labile carbon (indicative of SOM, degradation)2.5 g dry soil in 20 mL 0.02 M KMnO4 (method of Weil, 2003)
Earthworms and channels Termites, ants and nests Dung beetles and other fauna Plant roots, nodules, mycorrhizae Fungal mycelium /fruiting bodies. Bio-interpretation of nitrate and labile C tests
Flat-tip pH meter on moistened soil Direct-reading EC probe inserted into moistened soil• Presence of free carbonates (10% HCl drops)• Reduced iron (α-α diperidil)• Mn oxides (H2O2 drops)• Physical observations (e.g. water, density, colors, nodules)
Example of soil profile limitations:Guambeh brothers, Nimba County, Liberia
Guambeh brothers farm in Nimba, Liberia “Poor” “Good area ” area
Test Plants• Petiole/stem sap• Foliar symptoms• Cut open stem• Dig up roots