Plants generally have an increased demand for N at various stages of development, particularly during leaf development and flowering;therefore, this element is frequently applied as a mineral fertilizer.In our study, vegetables cultivated in the conventional way were supplied with this nutrient several times during the growing season, mainly in the form of ammonium sulphate. Nevertheless, our results indicate that the concentration of this element is higher in organic crops which correspond with the higher concentration of N in the soil.Na is considered to be of secondary importance in the soil and its uptake depends mainly on the plant species as well as the K level of the soil, rather than the concentration of Na extractable from the soil.
Colla et al. in a two-year study demonstrated that the soil Na content did not significantly differ among organic, conventional and low input farming systems. The same authors observed that the Na content in the crops changes over time and one year they recorded a significantly higher level of this element in conventionally grown tomatoes but the following year there were no differences among farming systems.The meta-analysis by Worthington showed that organic crops on average have about 20% more Na than conventional ones. In our study, no important differences in the soil Na content between two growing systems were observed.However, the concentration of Na in organic vegetables was significantly higher in 4 out of 8 vegetables analysed.
The total content of K in soil ranges from 8000 to 25,000 mg∙kg−1 and it depends on the clay fraction and soil mineral content. The highest concentration of K is found in heavy soils, where the clay content represents more than 3%. In loose sandy soils, K concentration usually does not exceed 0.4% . The monitoring of the soils, plants, agricultural products and foodstuffs in 2000 reported that the average concentration of available K in the soils of Poland was184 mg∙kg−1. The average for the south-west region, where both farms are located is, however, much higher and represents 270 mg∙kg−1. Generally, it was reported that mineral fertilizers increase the content of K in the soil to a larger extent than organic ones. Though, it is not entirely true in our study, where the content of available K in the soil was slightly higher in the organic samples compared to conventional ones.
A lower concentration of K in the conventionally treated soil might be the result of a slightly more acidic pH which might lead to the higher loss rate . Plants absorb K ions from the soil solution and usually contain more of this element than other minerals such as Ca, Mg or P. The concentration of K in plant cells exceeds significantly the content of this element in the surrounding environment. This indicates that K is actively transported by plants, despite the concentration gradient. Our results clearly confirm this process.Due to many factors, the content of K in the plant can vary from 1245 to 33,190 mg∙kg−1. It was observed that the roots contained less K than the above ground parts of the plant , which can be clearly observed in the obtained results for the root and leaves of parsley.Analysis of K concentration revealed that organically grown vegetables in our study had generally higher content of this element compared to conventional ones. The difference was significant in the case of celery, carrot, potato and onion.
It is in agreement with the results obtained by Worthington which showed that organic crops contain on average 10% more of K than conventional ones. On the other hand, Warman and Havard demonstrated that the superior level of K in organic crops is not conclusive. Some vegetables grown in a conventional way, such as carrot, had a higher content of K over organic ones,but in the case of cabbage, it was opposite.The total content of P in the soil varies from 200 to 1500 mg∙kg−1 and a significant amount of this element comes from soil organic matter . The soil monitoring program revealed that the average concentration of this element in Poland is 274 mg∙kg−1 and that the average for the south west region is 352mg∙kg−1. Furthermore, Lemanowicz and Koper reported that arable soils usually contain more of this element and in Poland; it is on average 478 mg∙kg−1.In our study, the P content of the soil was generally higher than in Poland and in this region. It was observed that the organically treated soil was twice richer in P compared to the conventional one.
Lemanowicz and Koper , in their studies,showed that the use of organic fertilizers in the form of manure significantly increases the amount of available P, while mineral fertilizers in the form of ammoniumnitrate reduce its amount in the soil. hey showed that among the different types of fertilizations,organic ones such as compost contributed to the highest increase of soil P content. This may explain the results obtained in our study, where an elevated amount of this element was found in the organic soil probably due to the regular application of compost and cow manure.P is taken up by plants from the soil solution as soluble or thophosphates at a soil pH between 6 and 7.