Potato (Solanum tuberosum L.) response to nitrogen forms and phosphorus sources in different soil types

Abstract

Potato (Solanum tuberosum L.) is one of the most important tuber crops globally and is classified amongst the most crucial food crops in Africa. South Africa has a very vibrant potato industry, producing about 2.5 million tonnes every year, with quantities bettered only by Algeria and Egypt. Potato production is very expensive (R150 000 ha-1), with fertilizers contributing 20%. Potato is highly reliant on steady nutrient supply and any deficiencies result in poor yield. Potato fertilizer demand is higher than that of other crops such as cereals and it has a very unique demand for phosphorus (P), which is vital from its early development to maturity. In addition, potato has a very shallow root system, which compromises P uptake, making most potato cultivars ineffective in nutrient uptake. Therefore, high P fertilizer rates are applied of which <20% is utilized by plants within a few days after application and about a further 4% within the next 10 days, mostly due to fixation. The production of P fertilizer, such as super phosphate (SP) is energy-consuming, costly and emits fluorine. There is also a risk of cadmium (Cd) accumulation in soils and plants due to the heavy fertilisation, posing a risk to human health, animals and aquatic life. Runoff phosphorus leads to eutrophication of water bodies. In addition, P fertilizer production is severely threatened by declining rock phosphate (RP) reserves, expected to hit a low by 2200. This will result in a hike in P fertiliser prices as miners move to low concentration ores. The high demand of P in potato, the environmental and human health risks, the high costs and declining reserves, all call for prudent and sustainable management of P in potato production. Nitrate and ammonium results in contrasting plant metabolism and growth. Most importantly through rhizosphere modification where ammonium supply results in reduced soil pH while nitrate results in increased soil pH. The pH reduction in ammonium supplied soils increases P dissolution and availability while the opposite is noted in nitrate. Most of the studies in this phosphorus-nitrogen interaction have been conducted on tree species, grasses and cereal crops with little done on tuber crops. In addition, the application of RP directly to plants could help cut the emissions, processing costs and environmental contamination associated with chemical P fertiliser production. There is, therefore, an urgent need to develop P fertilizer management systems to effectively manage this finite resource by improving its use efficiency for maximum yield at optimum application rates. To attain this objective, two experiments were conducted, namely a laboratory study to investigate the interaction between nitrogen forms and phosphorus sources in soil columns without a test crop, and a glasshouse pot trial to investigate the same interaction with potato as test crop. The column study treatments comprised of two soil types, N supplied as ammonium or nitrate and three P sources (SP, RP and a P0) to give 12 treatments that were replicated four times to give 48 columns. Mechanical dry packing method was used. The columns were leached with one pore volume over four watering events (1, 21, 42 and 63 days) and terminated on day 90. The leachate was collected in glass bottles at the column bases and analysed for pH, phosphorus, calcium, potassium and magnesium contents. A glasshouse pot trial was set up at the University of Pretoria Experimental Farm with potato cultivar Mondial as the test crop over two seasons, with a high and low initial soil P in season one and two, respectively. One minituber was planted per 10 litre pot. Watering was done using a pressure compensated drip irrigation system. Data was collected at tuber initiation (TI) and at the end of the season (ES). Parameters assessed included plant height, dry masses, number of tubers initiated, yield, leaf tissue and soil P status. Significant phosphorus-nitrogen interactions occurred on most assessed parameters in both trials. The exceptions were pH, potassium, phosphorus, calcium and magnesium levels, at some stages of the column study. Significant phosphorus-nitrogen interactions were noted at all watering events for both soil and leachate pH, phosphorus, potassium, calcium and magnesium concentration. In the pot trial, significant phosphorus-nitrogen interactions were noted for most of the plant measurements at both the TI and ES assessment periods with a few exceptions. Ammonium + SP produced the highest tuber initiation rate and final yield, as well as highest tissue and plant available P levels in both seasons. In the leachate and soils at the end of the column study, as well as at both stages assessed in the pot trial, ammonium treatments tended to have higher P contents. In the pot trial, ammonium treatments gave taller plants, but with lower dry mass compared to nitrate. Nitrate treatments had higher soil and leachate pH compared to ammonium treatments in both trials. Plants supplied with SP tended to have longer haulms and roots, higher haulm and root biomass and higher yield compared to treatments with RP and P0. The findings of these trials indicated that ammonium results in higher phosphorus dissolution (with or without a crop) and uptake by plants due to increased soil acidity. The resulting effect on potato crop is an increase in the number of tubers initiated and higher yields. However, the positive effect of ammonium was mostly achieved in combination with superphosphate. Rock phosphate, despite the increased yields, compared to treatments without P, gave inferior plant performance and is therefore not a worthy substitute for superphosphate.