By Universidad Catolica agronomist and UC Davis doctoral candidate Sebastián Saa Silva

The nutritional analysis of leaves is widely used tool to monitor the nutrition of fruit orchards. For most fruits, leaf collecting is recommended in mid-summer and then the nutritional values obtained are compared to critical values that indicate levels of deficiency or adequate nutrition for every element.

However, in 2007 a survey covering approximately 500 almond growers and consultants in California concluded that growers would need to sample their trees earlier in the season, and that it was necessary to improve the way foliar analysis results were commonly interpreted.

Leaf sampling is usually done through the summer for practically all fruit species due to the concentration of nutrients in leaves that in general is more stable at the start of the season. However, this sampling turns out to be too late to make use of the results obtained. For example, in almonds, foliar analysis results tend to be available when most of the fertilization plan (approximately 80%) has already been applied, which leaves growers with little time to adjust their nutrition plan if it's necessary.

Another important problem related to leaf sampling is that the interpretation of results does not incorporate the spacial variability of orchards. Usually, the interpretation consists of simply comparing nutrient concentrations from an analysis of critical values estaablished for the species, and cataloguing the orchard as deficient or adequate. However, as the sample collected is the average of various trees, it doesn't represent the nutritional variability that can exist between trees in the same orchard. That's why it is necessary to have an estimation of the orchard's variability to determine the percentage of trees with deficient, adequate or excessive nutrient levels.

At UC Davis, after five years of work we have managed to develop substantial improvements in foliar analysis in the case of almonds and the interpretation of results. Today, Californian almond growers have the possibility of sampling early in the season - approximately three months earlier than the old sampling method - and to use models that allow them to predict nitrogen concentration and the percentage of trees that would be at adequate levels in mid-summer.

These new practices are the result of analyzing the spacial and seasonal distribution of nutrients in four Californian orchards for three years, and later validating the results in another six orchards. The models created up until now utilize information about various nutrients to reduce nutritional instability early in the season, and in turn increase the certainty of predictions.

The almond industry's reception to the results of this project has been very encouraging as it has been a strong dissemination of information from the Almond Board of California, growers and field consultants that have started implementing this new monitoring strategy.

As a researcher, I hope what has been learned in almonds and other species could be applied in the future, thus perfecting a technique that is very widely used but requires a better and greater implementation.

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