12/03/2019
SOIL FERTILITY Soil fertility is a complex feature and is crucial to the productivity of agricultural soils. It deals with the ability of soil to provide nutrients for crop production drawing both from its own reserves and external applications.
In greenhouses, the management of soil fertility is of utmost importance for optimizing crop nutrition on both a short-term and a long-term basis to achieve sustainable crop production. It is related to the greenhouse climate and the complex interaction involving the many factors contributing to the biological, chemical and physical properties of the soil:
•Biological factors can be both beneficial (microbial population, mycorrhizal fungi, Rhizobium bacteria) and harmful (soil-borne pathogens).
•Physical properties of importance for greenhouse production are soil texture and structure, the soil volume that can be explored by the roots, and its water-holding capacity.
•Chemical factors contributing to soil fertility include nutrient status and soil organic matter, soil pH, and cation exchange capacity.
GAP recommendation.
Cropping systems and fertilizer recommendations differ between regions but some general rules are valid:
•Plant nutrients are absorbed by the roots and eventually assimilated in order to sustain plant metabolism and growth.
•Seasonal variation in nutrient requirements depends on crop ontogeny. Nutrient uptake ratios change according to developmental stage as a result of variation in the growth rate of different plant organs, which have different mineral compositions.
•The rate of mineral uptake is related to the rate of crop growth and tends to decline with plant age, as does the critical nutrient concentration of plant tissues.
•Soil N-P-K supply derives from the mineralization of old organic matter and the addition of fresh material (e.g. crop residues, organic manure). Therefore fertilizer recommendation systems need to estimate this input.
•When growth is stimulated by increasing irradiation and water uptake, it is important to supply more minerals.
•Remobilization of nutrients stored in old or adult leaves makes little contribution to the mineral requirements of growing organs in greenhouse crops; the synchronization of fertilization to actual mineral uptake is crucial for optimal crop growth.
•Overfertilization results in excessive consumption and has many drawbacks.
•Optimal fertilizer supply ensures unrestricted growth and a yield that is close to potential, while minimizing losses to the environment.
•Vegetable quality is at its best when fertilizer supply matches crop requirement over the length of the growing season.
Therefore: Estimation of the crop’s nutrient requirement is very important for both economic and environmental reasons. Thus:
•Knowledge of expected crop yield is crucial to estimate total nutrient requirements and avoid overfertilization.
•Knowledge of actual crop water uptake (or transpiration) may provide a useful tool for real-time estimation of crop growth and, hence, nutrient requirements.
•Prevention of nutrition imbalance is important: visually monitor crop mineral status day-by-day and analyse, at regular intervals, soil and leaf tissues and, in soilless culture, growing medium and nutrient solution.
•Soil mineral status at the start of the growing season is a good starting point for fertilizer recommendations.
•Soil tests, carried out before planting, provide an estimate of the amount of fertilizer to be applied; plant analyses can then be used to monitor crop nutritional status throughout the growing season, allowing adjustment for errors in fertilization.
•For assessment of soil nutrient concentration, follow specific sampling procedures for both pattern of sampling and sample size to give adequate spatial coverage.
•For assessment of plant nutrient concentration, follow standard recommended procedures for sampling whole plant or plant parts. Collect the right plant part at the right time (within the day or the growing season) for the specific nutrient to be assessed. Consider:
•Splitting the supply of fertilizers throughout the growing season greatly increases the opportunity to match total supply to actual requirement for fertilizer, lessens the nutrient loss from agricultural land, and reduces risk of soil salinization. It is therefore crucial to enhance the efficiency of fertilizer applications:
•Apply at least part of the fertilizers during crop growth in accordance with crop nutritional status.
•Apply fertilizers via fertigation.
