Soil fertility and management

For farming to expand northward, and be carried out sustainably and adaptively, one must first well define the physical, chemical and biological context of the soil systems, and integrate the realities of the locally relevant resources to establish the knowledge bases for an informed development of northern cropping. Locally relevant solutions such as use of biochar, rock crushing, and means to re-use of diverse organic waste streams (including fishery wastes); as to the latter, a main focus of the work is to contrast conventional and non-conventional means to maximize utility while reducing management steps and thus costs. Adoption is critically dependent on the adaptability and accessibility of the proposed solution. Local knowledge is central to all assessments; most of the work is carried out on farmers’ fields.

Nitrogen in boreal acid soils

In a natural state a forested Podzol has a relatively tight nitrogen cycle where nitrate and ammonia are taken up by plants at rates matching mineralization with very little leaching. When land is converted from a natural state a series of increases in temperature and oxygen results in an increased mineralization rate by an enhanced microbial activity. Nitrogen in a Podzol can be associated with carbon where the largest pools can be found in dead decaying matter. Smaller pools involve dissolved total N which can be associated with dissolved organic carbon found in illuvial layers. To adequately understand the drivers of the biogeochemical cycles it is important to look at how the mechanisms of mineralization and immobilization are influenced by the soil’s parameters.

Phosphorus in boreal acid soils

In a Podzol the concentration of organic phosphorus can be assumed to be 4% of the total P where the remaining 96% of the total P may be inorganic. Phosphorus is available to plants in forms of phosphate ions, and its uptake occurs directly through roots and hyphae via mycorrhizal associations. Organic phosphorus can be available to plants after mineralization, usually carried out by soil microorganism. Mineralization of organic phosphorus mediates the availability to plants of common soil organic P compounds such as inositol hexaphosphate, lecithin, nucleic acids, nucleotides and glycerophosphate. Overall availability to the plant ultimately relies on the P stores in the soil. If a soil has a low P retention capacity, then the availability of added P would be high. Conversely soils with high P retention capacity limit availability of P. This also affects the availability for P to soil microorganisms that may mineralize organic forms of P. The low availability of organic P to biological transformation may be a result of sorption as well as fixation by soil colloids. Phosphate ions are very reactive and form insoluble complexes with metallic anions including Fe and Al. Nevertheless, under natural conditions P reaches soils mainly in organic forms. The interest in the organic fraction of P in soils is due to the increased mobility compared to inorganic P. The quantification of organic phosphorus is also important to track the balance of P fixation as inorganic P is assimilated into organic P (by microorganisms), while mineralization of organic P from dead OM and from organic compounds excreted by microorganisms and shed at roots’ surfaces . The combination of metallic cations and low pH impact the form of inorganic phosphorus through immobilization making P availability in soils an issue of concern when converting to agricultural land.

Locally relevant fertilizer sources

Commonly, significant amounts of organic materials are added to soils to enhance the fertility of newly converted boreal lands. For example, we assessed the organic waste streams across Newfoundland, Canada identified that wastes contain sufficient nutrients to cover between 50 to 100% of the N and P fertilizer required at the current intensity of agricultural production (figure below, TN=total nitrogen, TP=total phosphorus in local organic waste; image from here).

There is a need to understand how the soil layers that might be variably mixed during LUC, and thus variably contribute to the new plough layer, respond to the addition of locally relevant fertilizers. This will impact agronomic management and the general capacity of the new farmlands to sustain agriculture. Given the current push to expand northern agriculture, it is essential to understand how LUC affects soil biogeochemistry and thus, the chemical speciation and fluxes of plant essential nutrients.

BMP’s; the need for locally calibrated management recommendations

Fertilizer recommendations must be locally calibrated; Podzols have a strong capacity to specifically adsorb phosphorus on iron and aluminium oxides. This may lead to precautionary over-application of phosphorus, but that can rapidly lead to excessive availability as soon as the soils storage capacity for phosphorus is satisfied.