Horticultural crops include a large number of botanical species with very variable soil and climate. Some of these crops are typical of the coldest months, such as artichoke or cauliflower, while others are grown in the warmer months, such as melon, watermelon or tomato.
The production and quality of horticultural crops are influenced by levels of availability of macro and micronutrients in the soil, especially when these levels are out of certain range. Nitrogen is the nutrient that most often limits production, although in other cases the limiting factor may be the availability of phosphorus and potassium or some micronutrient. The influence that every nutrient can have on the quality of horticultural products depends on every crop. For example, an excess of nitrogen raises the nitrate content in lettuce and spinach and this increase can affect their commercial value.
The nutrients dose to be applied in each case depends mainly on crop extractions, nutrient content in the soil and their efficiency of use by the crop. Extractions of nutrients depend on production, while efficiency of use (especially in the case of nitrogen) depends fundamentally on the root system, fertilizers management and irrigation system efficiency.
Below are the basics of calculating the fertilizer doses for the main three nutrients: nitrogen, phosphorus and potassium.
Nitrogen dose = (N plant uptake + Minimum mineral N content in the soil at the end of the crop cycle) - (Contribution by crop residues + N mineral content in the soil at the beginning of the crop cycle + Soil organic material mineralization + Organic amendments mineralization + Irrigation water contribution)
In this simplified formula leaching losses, gaseous losses and immobilization are not included so it is advisable to increase the fertilizer dosages calculated by 10-20%.
Phosphorus dose = P plant uptake - (Available nutrient reserves contribution + Contribution by crop residues + Organic fertilizers contribution + Irrigation water contribution)
Potassium dose = K plant uptake - (Available nutrient reserves contribution + Contribution by crop residues + Organic fertilizers contribution + Irrigation water contribution)
N, P and K fertigation distribution is little by little: between 20-30% in the first third of the crop cycle, 50-60% in the second third and 10-30% in the last third of the cycle.
- At early stage, the nutrient requirements are low but if there is a nitrogen deficit the effects on the growth process may be irreversible.
- During phenological stages such as flowering, setting and bulb formation, excessive nitrogen applications should be avoided
- At final stage, the application of N should be small, as it may have negative impact on the quality
Suggestions that can help improve fertilizers efficiency:
- In winter crops, it is appropriate to apply moderately phosphate fertilizer (50% restoration dose), even in soils with high levels of assimilable phosphorus.
- Good irrigation management reduces water losses by deep percolation and, therefore, reduces nitrate losses by leaching.
- Uniform distribution of fertilizers and irrigation water increases efficiency.
- If there is significant rainfall that can cause leaching of nitrate, it may be necessary to apply some additional nitrogen fertilizer in order to compensate these losses.