Using a Lysimeter Set-up to Study Seasonal Nutrient Uptake by Cocoa

Shahar Baram, Uri Yermiyahu, Ellen R. Graber

This project is carried out as a collaboration between various companies and organizations.

Background

Today, the majority of cacao (Theobroma cacao L.) is cultivated in tropical near-equatorial regions in West Africa and Indonesia, far from its origins. Cacao producing nations rely on cacao for a large proportion of their foreign currency. Despite its economic importance, cacao is predominantly grown in low input and low output systems. Today, there are more than 5 million smallholder family farms producing about 4.5 million tons of cocoa beans per year (World Cocoa Foundation, 2018). While production has tripled between 1961 and 2016 in response to rising global demand for cocoa products, severe production constraints, including depletion of soil fertility on cacao farms, lack of adequate nutrient managements, damage due to diseases and pests, and expanding labor costs, limit the sustainability of cacao production (Zhang and Motilal, 2016). Instead of increasing yields in existing producing regions through the adoption of advanced farming practices (e.g., fertigation, pest management etc.), the predominant factor responsible for meeting rising global demands for cocoa products has been expansion to new production regions. As such, the average cocoa yields in West Africa to date are about 10% from the achievable yields (i.e., ~400 kg/ha vs. 5,000 kg/ha, respectively).

Various lines of research related to mineral nutrition of cocoa have contributed to current understanding. However, to date there is still a lack of qualitative and quantitative understanding of the mechanisms underlying the constraints to cocoa yield and the trees response to nutrients uptake (van Vliet et al., 2015). In other words, exact nutrient requirements of cocoa are still unknown (van Vliet and Giller, 2017) with contradicting results in published literature. Several trials have shown that cocoa productivity can be more than doubled when fertilisers are applied, while others reported large variability in yields and fertilizer response  (van Vliet et al., 2015). Such contradicting reports could have derived from soil nutrient availability, water availability and other unmanaged environmental conditions. Recently, more controlled fertilizer experiment showed that increased fertilizer inputs increase the number of flower cushions, pollen germination and pollen viability per tree per season (Krishnamoorthy et al., 2015).  Despite these findings, the percentage of flowers that set pods in commercial cocoa orchards is usually small at 0.5-5% (Almeida and Valle, 2007), and cherelle wilt may be as high as 81% of the young pods (Asomaning et al., 1971). These poor field results demonstrate the huge knowledge gap in commercial cacao orchards, and exacerbate the lack of sound protocols by which the nutritional status of the trees can be evaluated. Accordingly, knowing the nutritional demands of cacao and maintaining leaf nutrient concentrations at optimal levels are some of the key issues that needs to be resolved in order to sustain profitable orchards, and meet the rising global demand for cacao. Yet, to date, there is no agreement on a standard foliar sampling procedure (i.e., timing, leaf age and location) on which the nutritional status of cacao trees can be evaluated (van Vliet and Giller, 2017), and farmers that rely on leaf samples rely on outdated studies (e.g., Verlière, 1981). As such, leaf analysis cannot be used to derive fertiliser recommendations for cocoa, although it may be useful to give an indication of prevalent nutrient deficiencies.

One of the methods by which such knowledge gaps in plant nutrition can be overcome, are lysimeter experiments. Lysimeter-grown tree provides a major advantage as it enables direct measurement of plant water and nutrients uptake at high temporal resolution during successive growth stages (Sperling et al., 2019). Although the root volume and water stress sensitivity of a lysimeter-grown tree differ from those of a field-grown tree (Goldhamer et al., 1999), lysimeter setup is the optimal tool to learn about plant water and nutrients uptake during successive growth stages. For instance, recent work by Silber et al., (2012, 2013, 2018) studied resembling phenomenon’s (inefficient pollination and fruit abscission) in “Has” Avocado. Using a lysimeter system, Silber et al. (2012) found that fruitlet abscission in Avocado was the outcome of a multifaceted process starting few months before it occurred, rather than a sudden and abrupt event as one could imagine from the visible aspects of this phenomenon. The causes for fruitlet abscission were attributed to malfunction of embryo or seed induced by water and\or nutrient deficiency during the early period of fruit development.

Research Plan

Objectives and significance of the research

Based on all of the above, a nutrition lysimeter experiment will initiate in Israel, where cacao trees will be grown in a controlled greenhouse environment. The main objective of this research is to assess the seasonal water and nutrients uptake of Cocoa. The specific objectives of the lysimeter experiment are:

1. Assess the seasonal water and N uptake by a chosen cacao cultivar (CCN-51) grown from seed, especially during leaf flushing, jorquetting, flowering and the early stage of pod development, which may later affect the abscission of young pods (“cherelles wilt”) and determine crop yield.
2. Determine diagnostic leaf and leaf sampling timing.
3. Evaluate the effect of P and K deficiencies on the plant.