Too many macronutrients in soil often interfere with micronutrient availability; thus, a balance is very important. Soil nutrient analyses must be conducted before and during all cultivating period to evaluate nutrient possible uptake from the plants. More specifically:

•  NO3 - and NH4 + provide nitrogen which plays many roles in plants, such as among other, affects leaf development and chlorophyll production.
• Phosphorus is an essential nutrient both as a part of plant structure compound, and as a key player in biochemical reactions.
• Potassium is taken up in significant amounts by crops and is essential in almost all processes needed to sustain plant growth and reproduction.

For the determination of nitrates and ammonium concentrations, each sample of sieved soil was extracted using a KCl solution as described by Keeney and Nelson (1982).

The nitrate and ammonium concentrations in the sample extracts were determined by applying the vanadium chloride and the indophenol blue methods, respectively (Keeney and Nelson, 1982) as described in SOP09 using a microplate spectrophotometer.

Phosphorus, potassium and other nutrients (eg. Ca, Mg, Fe, Zn, Cu & Mn) are extracted by Mehlich III method. The concentrations of these elements in the soil extracts are determined through the methods described in SOP09.