Unravelling changes in soil fertility of agricultural land in The Netherlands

Keywords: Soil fertility, soil test, phosphorus, soil organic matter, soil organic carbon, fertilization recommendation, data base, the Netherlands On fertile soils, high-yielding crop production systems can be built which are indispensable both for profitable farming and for feeding the steadily increasing world population. With its high soil fertility, agriculture in the Netherlands is one of the most productive in the world. The high soil fertility is partly inherited from sea and rivers, partly it is men-made through manure and fertilizer applications. However, from the 1980s manure and fertilizer applications are limited through governmental regulations. This thesis aims to increase the understanding of spatial variations and changes over time in soil fertility of farmers’ fields in the Netherlands during the last century. More specifically, it addresses the following research questions: i) which changes have taken place in soil organic matter (SOM) and soil phosphorus (P) contents in the period 1970 to 2000s, ii) will mean soil P status develop towards the optimal agricultural range, with a small standard deviation when virgin soil is cultivated with high craftsmanship?, iii) how did herbage quality respond to changes in mean soil fertility in dairy farming, iv) what are farmers’ perceptions and concerns regarding soil fertility?, and v) how to improve the usability of fertilization recommendations, using new knowledge? A large data base of a laboratory for routine soil, manure, herbage tests (BLGG) was analysed statistically, and a questionnaire was conducted. Results show that the mean SOM content of mineral soils remained stable during the last decades, despite worldwide reports about declining SOM contents, and concerns expressed by farmers. Restrictions on the use of animal manure did not yet have an effect on dairy farms; soil P status of grassland and mineral content of herbage remained within the optimal range during the last few decades, but crude protein decreased. Soil P status on arable land increased until the 2000s, partly to above agronomical optimal ranges. Risk avoidance seems a decisive factor for pursuing these higher statuses, stressing the need for improved recommendations. Since implementing new insights proves hard, a three-step schedule for incorporating results of novel soil tests into fertilization recommendations is suggested. Farmers endorse the importance of soil fertility and SOM, and P status should be monitored, including anticipated information about soil structure and soil life.