Nitrogen management – essential for organic cultivation of horticultural crops and potatoes

Availability of adequate amounts of nitrogen at the right time was identified as the biggest challenge in terms of nutrient supply to organically grown horticultural crops. This was made clear in a survey that EPOK has performed among advisors in organic farming.

Nitrogen management is important for field vegetables and potatoes, fruit and berries, as well as for greenhouse cultivation. A late release of nitrogen from organic fertilizers can also give an unbalanced ratio between nitrogen and potassium, which affects the balance of the plant's vegetative and generative development.

Difficulties in monitoring the balance between nitrogen and potassium restrict the grower’s opportunities to control the plant. For the organic cultivation of indoor plants, where the plant's appearance is crucial, the difficulties in controlling the development of cultures is a major obstacle for many growers to convert to organic production.

Also for potatoes it is hard to get good enough quality if much nitrogen is released late in the growing season. Imbalances have also been observed between nutrients other than nitrogen and potassium. Fertilizers that are high in potassium can cause problems with magnesium deficiency. Several of the advisers had observed micro- nutrient deficiency associated with high pH. When grown in limited amounts of substrate in greenhouses, accumulation of salts can be a problem.

Right timing and balance of nutrients
To apply nutrients in quantities and rates that is tailored to the plant's needs is important both to achieve optimum yield and to avoid nutrient losses. Losses of nitrogen and phosphorus by surface runoff and drainage water can lead to problems with eutrophication and algal blooms. Several advisers mentioned the problem that phosphorus sometimes accumulates in organic farming. Many organic fertilizers, such as manure, contain too much phosphorus in relation to plant needs of nitrogen.

To get enough nitrogen growers often add phosphorus in excess. Too high phosphorus content in the soil increases the risk of phosphorus loss. Examples of measures used by growers to reduce plant nutrient losses are to adjust the timing for ploughing of ley and manuring, to mulch the fertilizers, to use cover crops and to select of fertilizers low in phosphorus.

More knowledge demanded
The advisers called for more knowledge on the organic fertilizers nitrogen availability and how to optimize the management of plant nutrients in actual cultivations. How big are the effects of preceding crops, of various green manuring and cover crops? What is a good crop rotation, customized for optimal nutrient utilization?

More knowledge is needed also on rational management and applying of organic fertilizers. What fertilizers can be spread by manure mixers or spreaders? For more accurate plant nutrient supply there is a need for good fertilizers that can be used for top dressing during the growing period. Suitable fertilizers for drip irrigation were called for by the fruit and houseplants growers. There were also requests for simpler, practical demonstration experiments where nutrient problems can be illustrated.

Plant protection is also important
Many advisors emphasized the importance of analysing the soil and plant content of nutrients in order to better control both the fertilizer amounts and fertilization time during the culture period. It was also emphasized that the plant protection measures is essential to achieve good growth, high yields and thus an optimum nutrient utilization.