An evaluation of whey, compost and mineral fertilizers used in an organic farming system : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Soil Science, Massey University, Palmerston North, New Zealand

Abstract

An evaluation of whey, compost and mineral materials as fertilizers for an organic farming system, was undertaken in an ongoing 3 year old field trial at the MAFTech Levin Horticultural Research Centre. Sweetcorn (var. Honey and Pearl) was grown as a summer crop (1989-1990) and responses to the three fertilizer forms were measured using fresh cob, plant dry matter and dried grain yield as production criteria. Plant uptake of N and K was measured along with soil inorganic and total N and mineralizable N.

The whey fertilizer gave the highest sweetcorn cob yield averaging 12 t/ha. All fertilizer forms at low and medium levels increased fresh cob yield above the control in the order of whey mineral compost. The increase in yield averaged over all levels was 26% for whey and 21% for compost. The mineral treatment at the high level gave a significant depression (-20%). Whey fertilizer also increased sweetcorn N and K uptake to a higher level than compost and mineral fertilizers. Nitrogen and K weed uptake, which was measured only on the control and high level of fertilizer addition, was considerable (26-46 kg N/ha and 83-143 kg K/ha).

Apparent plant N recovery from whey and compost treatment levels were low, 3-13 % for compost and 12-22 % for whey. Apparent plant K recovery ranged from 4-15% for whey rates, 10-43% for compost rates and 0-27% for the mineral rates.

Soil inorganic N levels, 20 DAS, relate well to plant N uptake which also showed a good relationship with plant K uptake. The mineralizable N potential of the soil associated with various treatments was measured by 3 methods. The anaerobic incubation appeared to relate well to N uptake by sweetcorn. At all levels, the whey treatment mineralized at a faster rate than either the compost or mineral treatments. It appeared that the N component of whey and compost was mainly responsible where yield increases were measured although, the P and K component of the fertilizers may have

contributed in some situations.

Some suggestions are made regarding the design and conduct of future trials i.e. use of plant nutrient analysis to monitor nutrient status and a treatment eliminating annual fertilizer application from part of the main treatment to allow measurement of the residual effects from previous applications.

Some guidelines for organic growers using whey, compost and mineral fertilizers were suggested. These include the continual/annual monitoring of the soil's nutrient status, the measurement of nutrient losses in produce, the construction of a simple nutrient balance for each crop and the suggestion that the fertilizer forms used could be altered when some soil nutrients are considered to be in excess of requirements.