Leaf Colour Change to Diagnose Fertilizer Needs
4.20 - Leaf colour change to diagnose fertilizer needs
Justin Huber, University of Guelph, Canada
Suggested citation for this chapter.
Huber, J. (2022) Leaf colour change to diagnose fertilizer needs. In Farmpedia, The Encyclopedia for Small Scale Farmers. Editor, M.N. Raizada, University of Guelph, Canada. farmpedia.org
Introduction to Nutrient Deficiency Keys
Nutrient deficiency keys use photos to show a leaf with various nutrient deficiencies plotted next to one another, so a farmer can see the differences among nutrient deficiencies and compare the images to their own crops, helping diagnose fertilizer needs. For example, a woman growing maize can realize that yellow discolouration in the margins of her maize leaves may mean the soil is deficient in nitrogen (Tucker, 1984). Conversely, such photos can help smallholder farmers add only the fertilizers they need, preventing waste and improving incomes. Figure 1 is an example of a basic nutrient deficiency key.
Access to fertilizer varies significantly between developed and developing nations. In 2018, Canadian farmers applied an average of 85 kg/ha and 144 kg/ha of nitrogen fertilizer for wheat and maize respectively, while Senegalese farmers in Africa applied only 18 kg/ha for wheat and 25 kg/ha for maize (Ludemann et al., 2022). In recent years, the average rate of nutrients applied to sub-Saharan African crops has been about 16 kg/ha, only just over a quarter of the 2016 goal set at the 2006 Africa Fertilizer Summit of 50 kg/ha (Vanlauwe & Dobermann, 2020). Since the 2006 Africa Fertilizer Summit, fertilizer prices have increased by 130% due to heightened petroleum prices (Chianu, Chianu, & Mairura, 2012), and in 2022 fertilizer prices increased even further (Crespi et al., 2022).
Advantages of Nutrient Deficiency Keys
Variation in soil types at the local and continental level makes diagnosing nutrient deficiencies difficult; furthermore, soil testing is not available to many smallholder farmers, and a lack of access to government extension results in inadequate farmer training, which is amplified by low levels of literacy (Sinyolo & Mudhara, 2018). Nitrogen, potassium and phosphorus are the most common types of fertilizers (Chianu et al., 2012). However, as there are 15 nutrients required by crops, a more comprehensive addition of fertilizers, especially in extensively degraded soils (very common in Africa), may be needed to increase yield; but micronutrient fertilizers (e.g. zinc, boron) can be more expensive and difficult to diagnose, since they are less well known (Chianu et al., 2012). Nutrient deficiency keys offer a way for smallholders to identify nutrient deficiencies that do not require the help of an expensive soil testing kit or reliance on a government extension agent, and the production of keys specifically for micronutrients could help smallholder farmers use a more comprehensive approach to nutrient deficiencies. Knowledge about the diagnosis of nutrient deficiencies, specifically in rural areas, is significantly lacking (Chianu et al., 2012). The use of photo-based nutrient deficiency keys can help educate smallholder farmers with minimal interference caused by a language barrier.
Cost vs. Benefit Analysis
The most significant cause of decline in per capita food production in sub-Saharan Africa is the decline in soil fertility (Sanchez et al., 1997), and therefore it is evident that improving fertilizer usage is vital for an increase in smallholder farmer income. Evidence of this was shown in Malawi, where farmers who used fertilizer experienced a 105% increase in yield and a 21-42% increase in profit. Smallholder farmers experience yields that are rarely above 0.5 t/ha, while commercial farms and research station trials with greater knowledge and access to fertilizer can achieve yields of 6-8 t/ha (Chianu et al., 2012), equivalent to a 1100-1500% increase in yield. The cost to produce the deficiency keys would be minimal. About $5 USD would be needed for language translation of a nutrient deficiency key; however, when significant numbers are being purchased, this cost would be negligible on a per-farmer basis. Printing is relatively cheap, with the addition of colour adding costs of about 5 cents per key. Clear sheet covers to protect the nutrient deficiency keys from weathering would add about 10 cents to production. In total, the nutrient deficiency keys could easily be sold for less than 25 U.S. cents each, and with the use of fertilizer increasing yields by over 100% in some cases, the potential benefits of a nutrient deficiency key would help it pay for itself rapidly.
Distribution
Getting nutrient deficiency keys to smallholder farmers could be difficult, specifically in rural parts of Africa, as well as in politically unstable regions that may not be safe. However, there are existing methods used for other minimal-cost products that help distribute goods to rural areas. Women's cooperatives can be an effective method of distribution, as they can have many members coming from many villages. For example, a women's cooperative supported by UN Women in Ethiopia included about 2,000 direct beneficiaries and almost 32,000 community members who benefited indirectly (United Nations, 2018). This extensive outreach would be an excellent way to both distribute and spread the word about nutrient deficiency keys.
Critical Analysis and Alternate Ways to Increase the Efficiency of Fertilizer for Subsistence Farmers
There are several factors that can render nutrient deficiency keys less useful. Some nutrient deficiencies can still decrease yields while showing minimal to no symptoms. For example, manganese deficiency usually does not show any symptoms unless it is severe (Schmidt, Jensen, & Husted, 2016). Some nutrient deficiencies also have very similar symptoms. Molybdenum is used as a cofactor for nitrate reductase, meaning it is needed to break down nitrate into a form of nitrogen a plant can use (Zimmer & Mendel, 1999). So, even if there were excess nitrogen available to a crop in the form of nitrate, insufficient molybdenum would cause the plant to show symptoms identical to a nitrogen deficiency (Cox, 1992). Similarly, cobalt and iron deficiencies can be very difficult to differentiate, as they both show chlorosis of younger leaves as their main symptom (Rathour, 2022; Anderson, 2008).
Nutrient deficiencies in trees can also be difficult to diagnose with a nutrient deficiency key. This is because, first, an important part of a nutrient deficiency key is whether newer or older leaves are affected, and second, because the height of trees would make a key difficult to use.
Symptoms very similar to those of a nutrient deficiency can also be present for other reasons. For example, Verticillium dahliae is a pathogenic fungus that causes the yellowing of older leaves (Hanson, 2000). Nitrogen deficiency has this same symptom (Tucker, 1984). If a farmer were to use nitrogen fertilizer on such a crop, it would not help get rid of the fungus, potentially wasting fertilizer.
While nutrient deficiency keys will certainly help increase yields on subsistence farms, as well as educate smallholder farmers about the use of fertilizer, increased knowledge and access to fertilizer is ultimately critical for dealing with the decline in soil fertility in Africa. This issue is mainly due to minimal policy and institutional support concerning fertilizers (Chianu et al., 2012). Success of such programs and policies is not lacking evidence: Malawi has a specialized, policy-led agricultural extension program concerning the use of fertilizer, which caused more smallholder farmers to use more mineral fertilizers, resulting in a food surplus in a country with previous food security problems (Chianu et al., 2012). The educational differences between women and men are another factor contributing to the lack of access to fertilizer; Igbo women in Nigeria typically have little to no knowledge of fertilizer, compared to just 25% of men who lack this knowledge (Chianu et al., 2012). Additionally, despite Africa having large deposits of raw materials, it produces only 13% of its own fertilizer, with the rest imported, which is part of the reason nutrients can be so expensive for African farmers (Chianu et al., 2012). Zimbabwe has provided evidence that domestic production of fertilizer can be very beneficial: it produces much of its own fertilizer and, as a result, uses the most fertilizer per hectare in sub-Saharan Africa except for South Africa (Chianu et al., 2012).
Conclusion
In summary, nutrient deficiency keys can be very helpful for smallholder farmers in diagnosing fertilizer needs for crops. However, there is no use for them if farmers have no access to fertilizers and the knowledge needed to apply them. An increase in government extension programs, women's education and supportive policy can all be vital to helping Africa cope with its degrading soils.
Picture Based Lesson to Train Farmers
Links to Useful Resources
References
1. Anderson, W. B. (2008). Diagnosis and correction of iron deficiency in field crops – an overview. Journal of Plant Nutrition, 5(4), 785-795. Link
2. Chianu, J. N., Chianu, J. N., & Mairura, F. (2012). Mineral fertilizers in the farming systems of sub-Saharan Africa: A review. Agronomy for Sustainable Development, 32, 545-566. Link
3. Cox, D. A. (1992). Foliar-applied molybdenum for preventing or correcting molybdenum deficiency of poinsettia. HortScience, 27(8), 894-895. Link
4. Crespi, J. M., Hart, M., Pudenz, C., Schulz, L. L., Wongpiyabovorn, O., & Zhang, W. (2022). An examination of recent fertilizer price changes. Center for Agricultural and Rural Development. Link
5. Hanson, L. E. (2000). Reduction of verticillium wilt symptoms in cotton following seed treatment with Trichoderma virens. Journal of Cotton Science, 4(4), 224-231. Link
6. Ludemann, C. I., Gruere, A., Heffer, P., & Dobermann, A. (2022). Global data on fertilizer use by crop and by country. Scientific Data, 1(9), 501. Link
7. Rathour, S. K. (2022). Cobalt: 18th essential nutrient for plant growth? Just Agriculture, 2(7), 1-4. Link
8. Sanchez, P. A., Shepherd, K. D., Soule, M. J., Place, F. M., Buresh, R. J., Izac, A. N., Mokwunye, A. U., Kwesiga, F. R., Ndiritu, C. G., & Woomer, P. L. (1997). Soil fertility replenishment in Africa: An investment in natural resource capital. In R. J. Buresh, P. A. Sanchez, & F. Calhoun (Eds.), Replenishing Soil Fertility in Africa (pp. 1-46). Link
9. Schmidt, S. B., Jensen, P. E., & Husted, S. (2016). Manganese deficiency in plants: The impact on photosystem II. Trends in Plant Science, 21(7), 622-632. Link
10. Sinyolo, S., & Mudhara, M. (2018). Farmer groups and inorganic fertilizer use among smallholders in rural South Africa. South African Journal of Science, 114(5/6), 60-69. Link
11. Tucker, T. C. (1984). Diagnosis of nitrogen deficiency in plants. In R. D. Hauck (Ed.), Nitrogen in Crop Production (pp. 247-262). Link
12. United Nations. (2018). Women's cooperatives boost agriculture and savings in rural Ethiopia. UN Women. Link
13. Vanlauwe, B., & Dobermann, A. (2020). Sustainable intensification of agriculture in sub-Saharan Africa: First things first. Frontiers of Agricultural Science and Engineering, 7(4), 376-382. Link
14. Zimmer, W., & Mendel, R. (1999). Molybdenum metabolism in plants. Plant Biology, 1(2), 160-168. Link